PLoS ONE
Ocean acidification decreases grazing pressure but alters morphological structure in a dominant coastal seaweed
DOI 10.1371/journal.pone.0245017 , Volume: 16 , Issue: 1
Article Type: research-article, Article History
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### Notes

Abstract

Ocean acidification driven by anthropogenic climate change is causing a global decrease in pH, which is projected to be 0.4 units lower in coastal shallow waters by the year 2100. Previous studies have shown that seaweeds grown under such conditions may alter their growth and photosynthetic capacity. It is not clear how such alterations might impact interactions between seaweed and herbivores, e.g. through changes in feeding rates, nutritional value, or defense levels. Changes in seaweeds are particularly important for coastal food webs, as they are key primary producers and often habitat-forming species. We cultured the habitat-forming brown seaweed Fucus vesiculosus for 30 days in projected future pCO2 (1100 μatm) with genetically identical controls in ambient pCO2 (400 μatm). Thereafter the macroalgae were exposed to grazing by Littorina littorea, acclimated to the relevant pCO2-treatment. We found increased growth (measured as surface area increase), decreased tissue strength in a tensile strength test, and decreased chemical defense (phlorotannins) levels in seaweeds exposed to high pCO2-levels. The herbivores exposed to elevated pCO2-levels showed improved condition index, decreased consumption, but no significant change in feeding preference. Fucoid seaweeds such as F. vesiculosus play important ecological roles in coastal habitats and are often foundation species, with a key role for ecosystem structure and function. The change in surface area and associated decrease in breaking force, as demonstrated by our results, indicate that F. vesiculosus grown under elevated levels of pCO2 may acquire an altered morphology and reduced tissue strength. This, together with increased wave energy in coastal ecosystems due to climate change, could have detrimental effects by reducing both habitat and food availability for herbivores.

Kinnby, White, Toth, Pavia, and Cornwall: Ocean acidification decreases grazing pressure but alters morphological structure in a dominant coastal seaweed

## Introduction

Ocean acidification (OA) is the decrease in pH caused by the absorption of atmospheric CO2 into the surface of the oceans [1]. The majority of dissolved CO2 concentrates above the thermocline, generating an estimated drop in pH to 7.7 [2] or 0.4 units [1,3,4] by year 2100 in open ocean surface waters and the entire water column in the shallow coastal waters [5]. Thus, coastal ecosystems and the organisms that live there are expected to be among the most impacted by OA. Seaweeds are key habitat-forming primary producers that support high biodiversity in coastal areas [6] and therefore their responses to OA may have impacts throughout the ecosystem. Seaweeds primarily use CO2, and most species also use HCO3- , for carbon fixation and growth, and may therefore benefit from the increase in available carbon caused by OA [7]. A growing number of studies have, however, shown that OA can have positive, neutral, or negative direct effects on basic performance traits such as growth and photosynthesis of seaweeds [e.g . 7,8], and these effects may differ between life stages of a species, as well as between closely related species [911].

Aside from effects on basic performance traits, OA can also impact both primary and secondary metabolism in seaweeds, sometimes resulting in higher carbon to nitrogen (C:N) and carbon to phosphorous (C:P) ratios [but see 8,1214], which indicate a change in the nutritional content of the seaweed tissue. Increase in carbon availability for seaweeds generally results in decreased protein content [e.g . 1519], and either increased [17,18] or decreased [20] levels of fatty acids. Furthermore, the content of secondary metabolites, such as the grazing deterrent dimethylsulfoniopropionate (DMSP) in green seaweeds, has been shown to increase in response to elevated pCO2 levels [15]. In brown seaweeds phlorotannins (polyphenolic compounds) are ubiquitous metabolites that can occur in high concentrations, especially in fucoid species (Fucales). Phlorotannins have multiple functions e.g . as defense against UV-radiation and defense compounds against gastropod grazing [21,22]. To our knowledge, only two studies have investigated the effects of OA on phlorotannin production in brown seaweeds, with mixed results [9,11]. Olischläger et al . [9] found no effect on phlorotannin production in the kelp Laminaria hyperborea when grown under 700 μatm, while Swanson & Fox [11] found increased phlorotannin production in Saccharina latissima but not Nereocystis leutkeana when exposed to 3000 μatm pCO2.

The nutritional and defensive characteristics of seaweeds are critical traits in ecological interactions since they affect the growth and fecundity of herbivores [23,24]. Therefore, apart from direct effects on the physiology and biochemical content, OA may also have indirect effects on macroalgae through interactions with grazers. A decrease in the nutritional value and increase in deterrent defense metabolites under OA may lower the palatability of macroalgae to grazers [e.g . 16,25,26]. This may, however, also lead to an increase in the per capita grazing pressure through compensatory grazing if less nutritious food is available [e.g . 16]. Grazing may also be altered by direct effects of OA on the herbivore, e.g. through changes in respiration or behavior [e.g . 27,28]. Bibby et al . [27] showed that the snail Littorina littorea had noticeable reductions in both metabolic rate and induced defense (shell formation), which increased the avoidance behavior of the snails and could in turn affect their interactions with other species. Additionally, Young et al . [28] found that the grazing rate of a snail (Lacuna vincta) decreased when it was exposed to elevated pCO2, regardless of the effects of pCO2 on the seaweeds (Ulva spp.) that the snail was grazing on.

In temperate coastal ecosystems, fucoids are dominant habitat-forming seaweeds that provide shelter, habitat, and food for other organisms [29]. The presence of fucoids is associated with a local increase in species abundance and diversity [30], but there is no consensus how OA will affect the adult stage of associated species [but see e.g . 31 for effects on early life-stages]. Since many fucoids have an active uptake of bicarbonate [32], which is abundant in seawater (up to 91% [7]), it has been suggested that they should not increase growth in response to increased pCO2 since they may not be carbon limited [33]. We are only aware of two studies that investigate potential indirect effects of OA on fucoids through changes in interactions with herbivores [34,35]. One of these studies found that the herbivore Littorina obtusata consumed more of Ascophyllum nodosum under OA conditions, albeit this difference was not statistically different [35]. In contrast, the other study showed no effect of decreased pH on the interaction between herbivores and F. vesiculosus [34]. This relative lack of literature is surprising, considering the abundance of fucoids. Given the high phlorotannin content found in many fucoids, the effect of OA on phlorotannin production may also alter the interaction between seaweed and herbivore, but this has, to our knowledge, not yet been investigated.

The overall aim of the present study was to examine potential direct effects of OA on the fucoid F. vesiculosus, as well as indirect effects on the seaweed through changes in its interactions with the gastropod grazer L. littorea, both common species along coasts in the North Atlantic. We conducted manipulative experiments to determine how the growth rate, photosynthesis, carbon and nitrogen content, as well as chemical defense (phlorotannin content), and breaking strength of F. vesiculosus will be affected by increasing pCO2 levels in the future. Furthermore, we also tested the effect of elevated pCO2 on consumption, feeding preference, and condition index of L. littorea.

## Materials and methods

### Experimental design

Sixty individuals of F. vesiculosus were collected from the west coast of Sweden, in July 2018 and kept at Tjärnö Marine Laboratory (TML, 58°52’36.4”N 11°6’42.84”E) under ambient conditions (Table 1) for 7 days to acclimatize. Due to the small tidal range in the area, F. vesiculosus in western Sweden can be submerged for long time periods depending on prevailing weather conditions (personal observations, A. Kinnby), hence the algae were kept under water throughout the experiment. The experiment was performed in a greenhouse with natural lighting (natural light cycle 18:6 h, L:D). After the acclimation period, each seaweed was split into one experimental thallus and one control thallus, placed in separate 1L aquaria (a total of 120 aquaria, i.e. n = 60) with constant seawater flow from header tanks (4 per treatment, n = 15). Control thalli were maintained at ambient pCO2 (400 μatm) while experimental thalli were exposed to gradually increasing (~100 μatm/day) CO2 until a pCO2 of 1100 μatm was reached 7 days later (corresponding to the projected value at the end of this century [36]). 360 individuals of similar sized L. littorea were also collected and exposed to the same conditions as the seaweed, i.e. 180 snails were exposed to ambient water and 180 snails were exposed to treatment water in separate tanks from the seaweed thalli (these snails were used in a grazing experiment described below). The header tanks were aerated with either ambient atmospheric air (pCO2 of 400 ppm) or CO2-enriched air controlled by solenoid valves and pH-computers (Aqua Medic) to provide a final pCO2 of 1100 μatm. The pCO2 was monitored daily with LI-850 CO2/H2O Gas Analyzer (Li-COR). The CO2 analyzer was calibrated with custom mixed gas, 970 ppm (Linde Gas AB, Sweden). Filtered seawater (5 μm) flow was constant at 0.3 L/min in each aquarium throughout the experiment. Salinity, temperature, pCO2, and pHNBS were measured in the 1L aquaria. pH was recorded using HANNA instruments pH electrode HALO probe (HI-1102) calibrated with NBS pH 4.01, 7.01, and 10.01 standards (HANNA instruments) before each measurement. Total alkalinity was estimated from salinity using long-term salinity:alkalinity relationship data for this location (r = 0.94; data obtained from SMHI https://www.smhi.se/data/oceanografi/datavardskap-oceanografi-och-marinbiologi/sharkweb) [37] and pHT was calculated from the temperature, salinity, pCO2 , and total alkalinity using CO2calc [38; Table 1].

Table 1
Seawater chemistry of experimental treatments; partial pressure of CO2 (pCO2), pHNBS, salinity, and temperature were measured twice a week.
pCO2 (μatm)pHNBSpHTAT (μmolkg-1)Salinity (PSU)Temperature (°C)
Ambient400 ± 478.04 ± 0.038.05225832 ± 0.815 ± 1
Treatment1100 ± 617.64 ± 0.047.66225832 ± 0.815 ± 1
Total alkalinity was estimated from salinity using long-term salinity:alkalinity relationship data for this location (r = 0.94) and pHT was calculated from the temperature, salinity, pCO2, and total alkalinity using CO2calc. Data are averages (SD), n = 8.

All seaweed thalli were weighed fresh (n = 60) and photographed (for area (n = 60) measurements using Image J [39]) at the beginning and end of the 30-day experiment. At the end of the experiment the efficiency of photosystems II (Fv/Fm and P- index, n = 60 for both measurements) were measured in the new tissue formed during the experiment. The tissue was dark-adapted for 10 minutes, the fiber optics were held at a fixed 10 mm distance from the algae, and measurements were taken with a PAM (pulse amplitude-modulated fluorometer; Walz, Effeltrich, Germany). Breaking strength was measured by securing the seaweed to a dynamometer (Lutron FG-5020; Taiwan) such that only one apical tip was being strained, and increasing the strain until the thallus broke (n = 10). Thus, measuring breaking strength on tissue that was formed during the experiment. Following this, apical tissue samples were frozen (-60°C) for further elemental and phlorotannin analysis, the remaining tips were used in the consumption and preference experiment with L. littorea (see below).

### Phlorotannin analysis

For phlorotannin analysis, the frozen samples (n = 60) were freeze-dried, homogenized to a fine powder, and 10 mg of each sample was extracted in 60% acetone. Total phlorotannin content was quantified colorimetrically using the Folin-Ciocalteu method [40], with phloroglucinol (1,3,5-trihydroxybenzene, art. 7069; Merck, Darmstadt, Germany) as a standard. Results are presented as % dw (dry weight).

### Elemental analysis

For the determination of carbon (C) and nitrogen (N) content the frozen seaweed tissue was freeze-dried and homogenized to a fine powder and weighed to the nearest 0.01 mg. The total tissue C and N content, as well as δ13C and δ15N of the samples (n = 60) were analyzed with an elemental analyzer (ANCA-GSL, Sercon Ltd., Crewe, UK) coupled to an isotope ratio mass spectrometer (20–22, Sercon Ltd., Crewe, UK).

### Consumption and feeding preference of Littorina littorea

The palatability of the F. vesiculosus thalli grown in ambient and elevated pCO2 during 30 days was measured in two-choice feeding trials using starved L. littorea as the grazer. The feeding experiment was performed using a total of 120 containers (200 mL) with constant seawater flow of ambient pCO2 (400 μatm). In each container two similarly sized apical pieces of F. vesiculosus were placed (0.50 ± 0.019 g mean ± SD), one piece from the ambient pCO2 treatment and one from the elevated pCO2 treatment; as both pieces came from the same thallus they were genetically identical. Six individuals of L. littorea, exposed to either ambient or elevated pCO2 were placed in half of the containers (n = 30, i.e. a total of 60 containers with herbivores). To control for autogenic changes in mass (i.e. growth) during the experiment that was not caused by the grazing of the snails, each container with seaweed pieces and herbivores were paired with an identical control container without herbivores containing similarly sized apical pieces from the same genetic individual of seaweed. The wet weight of all seaweed pieces was determined at the start and at the end of the 24-hour experiment by using a standard blotting procedure, and the wet-weight change of each seaweed piece was calculated by subtracting the weight at the end of the experiment from the starting weight. The consumption of the snails exposed to different pCO2-levels was determined by calculating the total change in weight between initial and post-grazing weights for each container and subtracting the weight change in the autogenic control containers. To study feeding preference of herbivores exposed to different pCO2-levels, the difference between weight changes of the two seaweed pieces in each container was calculated by subtracting the wet weight change of the seaweed exposed to elevated pCO2 from the weight change of the seaweed piece exposed to ambient pCO2 [41].

### Condition index for Littorina littorea

Following the feeding preference experiment all snails were euthanized by freezing at -20°C. To assess whether the elevated pCO2 treatment had affected the physiological status of the snails a condition index was calculated, where a higher condition index is a sign of a healthier individual [35,42]. Snails were thawed and weighed whole; following this the shell was weighed alone. The dry weight of the soft tissue was obtained by weighing the body after drying for 48 hours at 50°C. The condition index was derived from the weights according to the following formula:

$=\left(100*dry\phantom{\rule{0.25em}{0ex}}tissue\phantom{\rule{0.25em}{0ex}}weight\right)/\left(whole\phantom{\rule{0.25em}{0ex}}weight-shell\phantom{\rule{0.25em}{0ex}}weight\right)$

### Data analysis

The seaweed response variables, i.e. growth (% increase in area and weight), breaking strength, efficiency of photosystem II (Fv/Fm and P-index), as well as phlorotannin, and nutritional content were all statistically analyzed with mixed model ANOVAs with pCO2 treatment as a fixed factor and header tank as a random factor nested within pCO2 treatment. However, since header tank was non-significant (p > 0.40 for all variables, the mean square for this factor was pooled with the residual mean square and paired t-tests were used to determine if there was a significant difference between the seaweed in ambient and elevated levels of pCO2. Paired t-tests were used because every treatment thallus was paired with a genetically identical control thallus. Before analysis the data for each response variable was checked and found to meet the assumptions of normality. To investigate if there was a difference in condition index between the snails exposed to ambient and treatment water a t-test was performed. The condition index data was not normally distributed, hence a Mann Whitney U-test was run. The consumption of herbivores exposed to ambient and elevated pCO2 was analyzed with a t-test. Preference for seaweed grown under the different pCO2 conditions was evaluated by comparing the difference in weight change between the seaweed pieces kept with the herbivores and their respective autogenic controls with two separate paired t-tests; one each for herbivores exposed to ambient and elevated pCO2 . A significantly lower difference in wet weight change for seaweed pieces kept with herbivores compared to autogenic controls will indicate a preference for feeding on the control seaweed [41]. All analyses were performed in RStudio (version 1.0.136).

## Results

The seaweed thalli exposed to elevated levels of pCO2 grew significantly more than the thalli exposed to ambient pCO2 when growth was measured as increase in surface area of the seaweed (Fig 1A; Table 2). On average, growth rates under elevated pCO2 were 34% higher than growth under ambient conditions. However, thallus weight did not differ significantly between the two treatments (Fig 1B; Table 2). We found a significant difference in the force needed to break the seaweed tissue in the control group compared to the seaweeds exposed to elevated pCO2 ; thalli from the treatment group were 57% weaker than those in the control group (Fig 1C; Table 2). We found no statistically significant differences in the efficiency of photosystem II measured as Fv/Fm and P-index (Fig 1D and 1E; Table 2).

Fig 1
Values are means ± 95% CI, n = 60 for a-j and n = 10 for k. Response variables measured as a) Growth measured % increase in area (n = 60), b) growth measured as % increase in weight (n = 60), c) breaking force (N) (n = 10), d) efficiency of photosystem II (Fv/Fm) (n = 60), e) efficiency of photosystem II (P index) (n = 60), f) Phlorotannin content (%dw) (n = 60), g) %Nitrogen (n = 60), h) δ15N (n = 60), i) C:N ratio (n = 60), j) %Carbon (n = 60), and k) δ13C (n = 60).Effects on response variables of Fucus vesiculosus grown under ambient (400 μatm) and elevated (1100 μatm) pCO2 for 30 days.
Table 2
Summary of effects of ambient (400 μatm) and elevated (1100 μatm) levels of pCO2 on Fucus vesiculosus measured as ten responses.
Response variablep-valuet-valueDfMean (400ppm)95%CI (400ppm)Mean (1100ppm)95%CI (1100ppm)
Growth: area (%)9.2e-07-5.4859147.56.54180.111.47
Growth: weight (%)0.767-0.3059136.05.91136.74.68
Breaking force (N)0.0322.33181.360.540.590.27
Photosystem II (Fv/Fm)0.407-0.83590.6950.0110.7010.007
Photosystem II (P index)0.147-1.47591.50.201.70.19
Phlorotannin content (% dw)0.0302.225910.60.5310.30.57
% Nitrogen0.3790.89591.320.0551.290.066
% Carbon0.8650.175936.040.4336.000.37
C:N0.236-1.205928.21.4429.31.80
δ13C4.762e-096.8559-11.770.32-13.330.41
δ15N0.315-1.01596.510.146.570.13
P-values and corresponding t-values and degrees of freedom of paired t-tests are reported for the analyses of all response variables as well as means and 95% confidence intervals. Values in bold denote statistically significant values.

The results of chemical analyses showed that there was a statistically significant decrease (3%) in phlorotannin content between the tips of thalli exposed to elevated levels of pCO2 and those being exposed to ambient levels (Fig 1F; Table 2). There were no statistically significant differences in C or N tissue content, nor in the C:N ratio. However, the stable carbon isotope (δ13C) content of F. vesiculosus was significantly reduced when exposed to elevated pCO2; the δ13C values decreased to -13% under elevated pCO2 levels. Tissue δ15N was not significantly changed when exposed to elevated levels of pCO2 (Fig 1G–1K; Table 2).

The mean condition index of L. littorea was 28.9% higher in snails that had been exposed to high pCO2-levels than those exposed to ambient pCO2 -levels (U = 13155, z-score = -2.60, p = 0.0093; Fig 2). Despite having a higher condition index, the herbivores exposed to high pCO2 levels consumed 37.5% less than the snails exposed to ambient pCO2 (t-test, t58 = 2.67, p = 0.0098, Fig 2). However, the herbivores did not show a statistically significant difference in preference based on the experimental treatment of the seaweed, regardless if the herbivores had been exposed to ambient pCO2 (paired t-test, t29 = -0.517, p = 0.609) or elevated pCO2 (paired t-test, t29 = -0.584, p = 0.564).

Fig 2
a) Consumption of the alga Fucus vesiculosus by the gastropod Littorina littorea, exposed to ambient (400 μatm) and elevated (1100 μatm) pCO2 for 30 days. b) Condition index for individuals of L. littorea following the grazing experiment. Values are means ± 95% CI.

## Discussion

Seaweeds play important ecological roles in coastal habitats and are often foundation species, with a key role for ecosystem structure and function. Hence, it is important to understand how seaweeds will be directly affected by changes in their environment, and also if these changes will alter seaweed interactions with other species. Here, we show that elevated pCO2-levels increased the thallus area, decreased the phlorotannin content, and reduced the breaking strength of F. vesiculosus. This may result in that the seaweeds become less robust in field conditions. This could lead to an overall loss of seaweed coverage which in turn is likely to affect all the organisms that live in, or consume, this seaweed. The condition index of the snails increased under exposure to elevated levels of pCO2, but the consumption decreased and we saw no significant effect of treatment on the palatability of F. vesiculosus thalli.

### Effects on growth

Increased pCO2 had no significant effect on the weight change of F. vesiculosus, but significantly increased the thallus surface area. Previous studies with F. vesiculosus have reported unaltered [43,44] or reduced [45] growth, measured as wet weight, under elevated pCO2-levels. Graiff et al . [46], however, reported a tendency (not statistically significant) of higher growth of F. vesiculosus, measured both as wet weight and length in apical tips, at elevated pCO2 levels. These different experimental results suggest that other factors interact with pCO2 to determine growth, e.g. seasonality or the life-cycle stage (age) of the seaweed, or genetic differences due to local adaptation among the populations used in the different studies. Such genetic differences in phlorotannin production and growth were recently demonstrated among F. vesiculosus populations at distances less than 100 km [47].

### Effects on breaking strength

The combination of a significantly larger thallus with no effect on the weight of F. vesiculosus under enhanced CO2 conditions found in the present study strongly indicates a decrease in tissue density, which is corroborated by the drastic (57%) decrease in breaking strength of the thallus. To our knowledge, this is the first time such an effect of increased pCO2 levels is reported for seaweeds, and it parallels findings in developing seaweed spores and terrestrial plants. For example, Guenther et al . [48] found that reduced pH delayed spore attachment in two different red algae, while Pretzsch et al . [49] documented an increase in growth rate, attributed to increasing CO2 levels, among tree species in central Europe between 1960 and 2014. They also showed that this increase in growth was coupled with a decrease in tissue density and/or strength. This leaves forests more vulnerable to the increased weather variability that is also associated with climate change [49,50]. Our results suggest that similar effects may also be present in coastal marine systems. The reduced breaking strength could make seaweeds more vulnerable to storms and wave action, which are projected to become more frequent as the climate changes [51,52]. Increased vulnerability will likely reduce the role F.vesiculosus plays in the nearshore ecosystem, with negative impacts on species that rely on this seaweed for food or habitat.

### Effects on photosynthesis

We observed no significant differences in the Fv/Fm ratio or P-index in our experiment, suggesting that F. vesiculosus does not increase the maximum quantum efficiency of photosystem II or sample vitality in response to elevated pCO2 . This follows the findings of Fernández and colleagues [53], who demonstrated that the increased carbon availability from increased pCO2-levels had no effect on photosynthesis. Seaweeds in general acquire carbon by passive diffusion of CO2 and active transportation of bicarbonate; as the concentration of CO2 rises the amount of passively diffusing CO2 potentially also rises, reducing the seaweed’s reliance on active transport proteins, and potentially allowing more energy to be allocated for growth [54]. Increased uptake of CO2 coincides with a decrease in tissue δ13 C [8]. In this study we found that the δ13C decreased from -11.77% to -13.33% when seaweeds were exposed to increased levels of CO2, which indicates a transition away from active intake of bicarbonate towards passive uptake of CO2. A similar change was previously documented in both Gracillaria sp. and Ulva sp.,[54], as well as in Lomentaria australis where an increase in growth and decrease in δ13 C were hypothesized to indicate a transition away from a more costly CCM (carbon dioxide-concentrating mechanism) [8]. In our study, however, this did not translate to an increase in biomass as we did not find any significant differences in the weight gain of the seaweeds exposed to different levels of CO2. Fucus vesiculosus can use two parallel CO2 pathways for photosynthesis, both directly taking up carbon from their environment and also storing it as an organic intermediate for use when other carbon in less available [55], suggesting that under normal circumstances F. vesiculosus plants are most likely not carbon-limited. This, together with the fact that F. vesiculosus used in our experiment were constantly submerged (which is common due to the low tidal range along the Swedish west coast) may explain the lack of effect from increased pCO2 on growth measured as weight gain.

### Effects on elemental and phlorotannin content

In terrestrial plants, increased atmospheric CO2 has been shown to increase C:N ratios as well as lead to an accumulation of phenolic compounds, such as tannins, affecting the consumption and growth rates of grazers [56,57]. However, we found no significant differences in C or N tissue content, nor in the C:N ratio. In contrast, Gutow et al ., [45] showed that elevated levels of pCO2 (700 μatm) decreased the C:N ratio of F. vesiculosus. Studies on the effects of increased CO2 on phenolic compounds in marine macrophytes are few and only one previous study on kelp species found that elevated CO2 leads to increased levels of phlorotannins [11]. By contrast, marine vascular plants have been shown to reduce phenolic acid production under increased CO2 conditions [58], which aligns with our results on F. vesiculosus showing slightly lower phlorotannin content in apical tips exposed to elevated pCO2-levels.

### Effects on interactions with a grazer

Despite finding a somewhat lower phlorotannin content in seaweeds exposed to elevated pCO2, we did not find a difference in grazing preference of the gastropod L. littorea between seaweeds from the different treatments. However, snails exposed to elevated pCO2 generally consumed less than those exposed to ambient conditions, regardless of which food type they were offered. Reduced consumption by snails at increased pCO2 levels could indicate easier ingestion and digestion of food, or decreased activity of the grazer (and therefore decreased caloric requirements), in line with previous research on L. littorea [27] and other marine invertebrates [59] showing reduced metabolic rates at increased pCO2 levels. The snails exposed to elevated levels of pCO2 had a higher condition index than snails exposed to ambient conditions. This combination of results is surprising, as a decreased consumption would be expected to result in a drop in condition index. Increased condition index, i.e. a higher dry to wet weight ratio of the soft tissue, could indicate more accumulation of tissue, i.e. increased growth, but also possibly failure to osmoregulate or other associated physiological problems. In summary, the results from the feeding experiment in the present study suggest that there are direct effects of increased pCO2 on herbivores and their consumption of seaweeds, but any indirect effects mediated by changes the palatability of the seaweeds are harder to discern.

### Conclusion

In conclusion, our study shows that under OA conditions the habitat forming seaweed F. vesiculosus increases growth by thallus area, reduces reliance on active carbon uptake, shows a slight decrease in phlorotannin content and a drastic reduction in breaking strength. At the same time the herbivore L. littorea seems to tolerate increased pCO2 with an increased condition index even as they reduce their consumption of seaweeds. Reduced consumption for the herbivore suggests that the seaweed could gain some ecological benefits under OA. However, our most unanticipated finding–that the seaweed could become more vulnerable to physical forces under OA because of a significantly reduced breaking strength–could result in loss of seaweed biomass due to increased storm events that are associated with climate change. This might in turn have implications for the future community structure of shallow coastal areas under OA.

## Acknowledgements

We are grateful to Gunnar Cervin (University of Gothenburg) for help with the experiments and Kerstin Johannesson (University of Gothenburg) for valuable comments on the manuscript.

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23 Oct 2020

PONE-D-20-25580

Increased thallus area and decreased breaking strength in a habitat forming seaweed under ocean acidification

PLOS ONE

Dear Alexandra Kinnby,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

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Christopher Edward Cornwall, Ph.D.

PLOS ONE

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I have received the reports of two different reviewers back. They both support the work and offer minor comments that should be addressed in the revisions. I have also read the manuscript, and I consider that the authors should pay particular attention to the comments from reviewer 2 regarding the numbers of tanks etc. This is extremely important in determining the validity of the replication here. Please indicate how many header tanks, experimental tanks etc. per treatment, and how any interdependence was dealt with in the model. I cannot currently see this clearly described.

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

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5. Review Comments to the Author

Reviewer #1: Kinnby et al exposed Fucus vesiculosus and Littorina littorea to ocean acidification conditions predicted for the year 2100 to look at the direct and indirect effects on organismal response and grazer interaction, respectively. Kelp growth, breaking strength, phlorotannin production were affected by OA, while grazer preference was unaffected but grazers decreased consumption and improved condition index. The manuscript provides new information on ecologically important species performance and interaction in future oceans. Overall, the manuscript is presented and written in a straightforward manner. However, more details are needed in the methods section. As the authors point out, there are discrepancies in the OA literature, much of which likely comes from different experimental setups and techniques, so it is imperative to provide as much information as possible. Provided the authors amend the methods section, I think this manuscript would be a good addition for Plos One and add to our understanding of OA effects on different species. I have performed the review with track changes in Word, pointing out missing details that should be added, also editing some text to help clarity or flow, which the authors can use at their discretion.

Reviewer #2: The manuscript provides interesting and useful data on the response of habitat forming seaweeds to ocean acidification. With some minor improvements i consider the manuscript warrants publication. The main issue i have is the discussion in which i think the interesting results could be better placed in the context of the broader literature and structured to better highlight the impacts of the key findings.

Abstract

Line 34

change “available” to “availability”

Introduction

Line 41

in surface and coastal waters? A little confusing with the wording, I think you mean to suggest that its surface of the open ocean but the coastal waters will be all the water column due to it being shallow. Might help to have a reword for clarity.

Line 42-43

The sentence that says coastal organisms the most impacted. I suggest saying one of the most impacted.

Line 55-56

Line 83

Change “Temporal” to “Temperate”

Line 90-94

This sentence is a little confusing – suggest a slight reword.

Line 94-95

I have noticed a little bit of repeatability about the importance of fucoids/seaweeds in general in coastal systems – e.g. Lines 83-84 and 43-44. I suggest going through the introduction and reducing repetition of this point wherever possible.

Line 96-99

Suggest framing this point as important rather than interesting as it is both interesting and important!

Line 101-107

Are both these species common globally or just in the region you work? As written, it suggests globally.

Methods

Line 128-129

Could you provide frequency and total number of measurements? Also what equipment was used to measure these parameters? This info needs to be included.

Table 1

Please provide standard error in the table for the measured parameters. Was AT estimated from only pCO2 and salinity or also pH and temp? Needs to be clarified in the caption/text. Was pH measured on the NBS scale only? Ideally seawater pH should be measured on the Total scale. For comparison to other studies could the pH also be reported on the Total scale (if not measured) using estimates from CO2Calc and this stated?

Line 135

All seaweed thalli? Same question for phlorotannin and elemental analysis and breaking strength. Please clarify and include n per treatment for each response variable.

Line 140

Could you include make and model of the equipment.

Line 148

Change to “as a standard”.

Line 157-177

I found it a little hard to follow whether the snails exposed to different pCO2 were placed in separate containers and then also if not how this was accounted for in the data analysis? Some more clarity would be useful.

Line 179

For readers not familiar with condition index could you provide a brief description of whether higher or lower CI is good or bad?

Line 196

Could a transformation of the data allow you to use a t-test also?

Line 200-202

I don’t quite understand what you are testing here or why the results are stated here? Could you please clarify.

Results

Firstly – could both figures be converted to higher resolution – a little blurry at the moment and hard to read

Line 230-231

Could you please say how much it was reduced here – given the high replication you are able to detect subtle effects but including the magnitude of the effect (as you have done for other factors such as growth) provides a clearer picture.

Discussion

The results of the study are quite clear and important. However, I think the discussion could do with some reworking in general to better put these results in the greater context of the effects of climate change in the oceans. Firstly, there tends to be a description of the findings and then a comparison with other studies for each response but not much discussion of what these findings might mean for the ecology of seaweed communities. Secondly, each response variable is mostly discussed in isolation, with little attention given to the findings as a whole. I consider that the manuscript (which has very interesting results) will substantially benefit by some reworking of the structure of the discussion and the better framing of the results in the broader context of the climate change impacts in the ocean.

My suggestions are as follows:

1. I would suggest a slight rephrasing of the first paragraph to highlight the implications of the findings – this is sort of done by the first two sentences in a general sense but it would be good to see the key results and the specific implications of them.

2. Discuss the key findings of increases in surface area, with no increase in weight but decreased resistance to breakage. This is mostly done in between lines 261-283 but it could be a little more concisely written and better placed in the context of the ecology by mentioning briefly what increased loss of seaweeds due to breakage could mean for coastal systems – e.g. habitat loss, food loss etc.

3. Discuss the findings related to carbon use to be more focused on your findings and what they mean – presently they are a little bit too focused on what others have found and don’t place your results in the context of the broader literature and are not used to explain any of your other results - e.g. an increase in growth due to CCM down-regulation.

4. Place some sub-headings for the discussion so that it is easier to follow.

Line 285

Photosystem 1? Does Fv/Fm not only measure PSII? Also it may be important to state that measuring PSII may not reflect photosynthetic rate per se as only getting half the story.

Line 292

RuBisCo is the site of carbon fixation rather than an active transport protein, can this be reworded to be more accurate.

**********

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If you choose “no”, your identity will remain anonymous but your review may still be made public.

Reviewer #1: Yes: Conall McNicholl

Reviewer #2: No

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19 Nov 2020

Dear Dr. Cornwall,

We would like to thank you for the editorial feedback on our submission PONE-D-20-25580. Based on your feedback and the reviewer’s comments, we have made changes to the manuscript and believe it to be much improved.

We have prepared a detailed point-by-point response letter for you (with our replies in red) where we have addressed all comments from Reviewer 2 and what we believe to be non-editorial comments from Reviewer 1. Rather than refer to specific changes in the response letter for the editorial comments from reviewer 1, these have been addressed in the track-changes copy of the manuscript.

We look forward to your response to our revised version of the manuscript and hope we have satisfactorily addressed all previous issues.

Sincerely,

Alexandra Kinnby (on behalf of all authors)

Editor:

I have received the reports of two different reviewers back. They both support the work and offer minor comments that should be addressed in the revisions. I have also read the manuscript, and I consider that the authors should pay particular attention to the comments from reviewer 2 regarding the numbers of tanks etc. This is extremely important in determining the validity of the replication here. Please indicate how many header tanks, experimental tanks etc. per treatment, and how any interdependence was dealt with in the model. I cannot currently see this clearly described.

Reviewer #1: Kinnby et al exposed Fucus vesiculosus and Littorina littorea to ocean acidification conditions predicted for the year 2100 to look at the direct and indirect effects on organismal response and grazer interaction, respectively. Kelp growth, breaking strength, phlorotannin production were affected by OA, while grazer preference was unaffected but grazers decreased consumption and improved condition index. The manuscript provides new information on ecologically important species performance and interaction in future oceans. Overall, the manuscript is presented and written in a straightforward manner.

However, more details are needed in the methods section. As the authors point out, there are discrepancies in the OA literature, much of which likely comes from different experimental setups and techniques, so it is imperative to provide as much information as possible. Provided the authors amend the methods section, I think this manuscript would be a good addition for Plos One and add to our understanding of OA effects on different species. I have performed the review with track changes in Word, pointing out missing details that should be added, also editing some text to help clarity or flow, which the authors can use at their discretion.

Response: We thank reviewer 1 for the thorough review, which has greatly improved the clarity and flow of our manuscript! All editorial comments have been addressed using track changes. We have further detailed our experimental set-up in the methods section using comments provided in the track changes file from reviewer 1 as a guide and hope that we have clarified this. We have chosen to include our responses to specific comments below:

Line 189-196 & Table 1

How was pH calibrated? NBS? Proper standard OA TRIS buffers?

Response: pH was calibrated using NBS buffers, information about frequency of measurements as well as how alkalinity and pH was calculated has been added together with standard deviations in table 1.

Line 256-260

This is a little confusing. So, were these apical pieces of the same individual? Of the same size? Or the remaining individual? And performed at the same time or after the grazing experiment?

Response: The autogenic control containers where identical to the containers with herbivores, except for the herbivores. Since the seaweeds are alive and will grow during the experiment, it is customary to include autogenic controls in feeding preference experiment with seaweeds. We have now clarified this in the manuscript.

Line 313

How was this calculated? I am getting ~30% higher based on the means provided

Response: Thank you for pointing this out. This was calculated from the raw data, by using the cm2 value of growth (final area - initial area for each individual). The percentages are presented as a statistical tool to standardise the results by starting weight and were calculated from the same raw data. This number has now been changed in the manuscript.

Line 352-353

Was feeding quantified during the 30 d? Perhaps they ate more during the 30 d and grew more but the snails from high CO2 were stressed since the feeding experiment was performed at ambient CO2? And so during that 24 hr it seems they ate less.

Response: Feeding was only quantified during the 24 h feeding experiment and not during the 30-day exposure to CO2, so we cannot say if herbivores ate more or less during this time. It could be as the reviewer suggest, but from the design of our experiment we cannot tell.

Line 460-461

Is there anything in the literature showing low pH reduces metabolic rates?

Response: We have added a reference that shows that metabolic rate reduces at low pH in other marine invertebrates.

Line 463

Again, I would be interested in how the 30 d exposure potentially affected the shell. How would the index be affected if there was greater tissue and lower shell density? I know for some calcifying algae there has been increased organic tissue and lower CaCO3 content, not sure about for inverts

Response: The shell weight is a component of the condition index (see formula in the manuscript). We analyzed the shell weight data but found no statistically significant difference due to exposure to CO2. Therefore, we decided not to include these data in the manuscript.

Line 488-491

Reword and perhaps tone down. Avoid words such as "striking". And "substantial loss" for things that were not quantified.

Response: We agree with the reviewer and have deleted the words striking, much and substantial from the sentence.

Reviewer #2: The manuscript provides interesting and useful data on the response of habitat forming seaweeds to ocean acidification. With some minor improvements i consider the manuscript warrants publication. The main issue i have is the discussion in which i think the interesting results could be better placed in the context of the broader literature and structured to better highlight the impacts of the key findings.

Abstract

Line 34

change “available” to “availability”

Response: This change has been made.

Introduction

Line 41

in surface and coastal waters? A little confusing with the wording, I think you mean to suggest that its surface of the open ocean but the coastal waters will be all the water column due to it being shallow. Might help to have a reword for clarity.

Response: This sentence has been reworded for clarity.

Line 42-43

The sentence that says coastal organisms the most impacted. I suggest saying one of the most impacted.

Line 55-56

Response: This reference has been added.

Line 83

Change “Temporal” to “Temperate”

Response: This change has been made.

Line 90-94

This sentence is a little confusing – suggest a slight reword.

Response: This sentence has been reworded for clarity.

Line 94-95

I have noticed a little bit of repeatability about the importance of fucoids/seaweeds in general in coastal systems – e.g. Lines 83- 84 and 43-44. I suggest going through the introduction and reducing repetition of this point wherever possible.

Response: This has been addressed, we have tried to minimize repetition as much as possible.

Line 96-99

Suggest framing this point as important rather than interesting as it is both interesting and important!

Response: This has been reworded.

Line 101-107

Are both these species common globally or just in the region you work? As written, it suggests globally.

Response: This has been clarified in the manuscript.

Methods

Line 128-129

Could you provide frequency and total number of measurements? Also what equipment was used to measure these parameters? This info needs to be included.

Response: This information has been added to the manuscript. The pCO2 was checked daily, however the values in table 1 are based on biweekly measurements (n=8).

Table 1

Please provide standard error in the table for the measured parameters. Was AT estimated from only pCO2 and salinity or also pH and temp? Needs to be clarified in the caption/text. Was pH measured on the NBS scale only? Ideally seawater pH should be measured on the Total scale. For comparison to other studies could the pH also be reported on the Total scale (if not measured) using estimates from CO2Calc and this stated?

Response: This information has been added in the caption, the table, and in the text. Total alkalinity was estimated from salinity using a long-term salinity:alkalinity relationship for this location (r = 0.94). This method has been shown to generate very low uncertainties in estimates of carbonate system parameters (uncertainties ≤±0.006 pHNBS and ±0.08 ΩAr) (for more details see reference 38 in the manuscript, Eriander et al., 2016). pHT was calculated in CO2Calc using temperature, salinity, total alkalinity, and pCO2 data.

Line 135

All seaweed thalli? Same question for phlorotannin and elemental analysis and breaking strength. Please clarify and include n per treatment for each response variable.

Response: This information, including sample sizes, has been clarified.

Line 140

Could you include make and model of the equipment.

Response: This information has been added to the manuscript.

Line 148

Change to “as a standard”.

Response: This has been changed.

Line 157-177

I found it a little hard to follow whether the snails exposed to different pCO2 were placed in separate containers and then also if not how this was accounted for in the data analysis? Some more clarity would be useful.

Response: Information has been added to this section for clarity.

Line 179

For readers not familiar with condition index could you provide a brief description of whether higher or lower CI is good or bad?

Response: Information has been added for clarity.

Line 196

Could a transformation of the data allow you to use a t-test also?

Response: Transforming the data does not make it normally distributed. However, this data has now been analyzed with a Mann-Whitney U-test as this seems more fitting. Nevertheless, this does not change the results and conclusions.

Line 200-202

I don’t quite understand what you are testing here or why the results are stated here? Could you please clarify.

Response: This text is not a statistical test or describing results; it is included to make it easier for the reader to interpret the results that are presented in the results-section. Depending on how the difference between wet weight changes is constructed, the results will mean different things. The way we constructed the difference, a result that show a significantly lower difference in wet weight change for seaweed pieces kept with herbivores will mean a preference for feeding on control seaweeds. If we had constructed it the other way around, it would have meant a preference for the treated seaweeds. We have now clarified this point. For more information on how to analyze feeding preference experiments, please refer to the publication by Peterson and Renaud (1989).

Results

Firstly – could both figures be converted to higher resolution – a little blurry at the moment and hard to read

Response: The figures have been converted, hopefully to satisfactory resolution.

Line 230-231

Could you please say how much it was reduced here – given the high replication you are able to detect subtle effects but including the magnitude of the effect (as you have done for other factors such as growth) provides a clearer picture.

Response: This has been added to the manuscript.

Discussion

The results of the study are quite clear and important. However, I think the discussion could do with some reworking in general to better put these results in the greater context of the effects of climate change in the oceans. Firstly, there tends to be a description of the findings and then a comparison with other studies for each response but not much discussion of what these findings might mean for the ecology of seaweed communities. Secondly, each response variable is mostly discussed in isolation, with little attention given to the findings as a whole. I consider that the manuscript (which has very interesting results) will substantially benefit by some reworking of the structure of the discussion and the better framing of the results in the broader context of the climate change impacts in the ocean.

My suggestions are as follows:

My suggestions are as follows:

1. I would suggest a slight rephrasing of the first paragraph to highlight the implications of the findings – this is sort of done by the first two sentences in a general sense but it would be good to see the key results and the specific implications of them.

Response: This paragraph has been altered with this suggestion in mind.

2. Discuss the key findings of increases in surface area, with no increase in weight but decreased resistance to breakage. This is mostly done in between lines 261-283 but it could be a little more concisely written and better placed in the context of the ecology by mentioning briefly what increased loss of seaweeds due to breakage could mean for coastal systems – e.g. habitat loss, food loss etc.

3. Discuss the findings related to carbon use to be more focused on your findings and what they mean – presently they are a little bit too focused on what others have found and don’t place your results in the context of the broader literature and are not used to explain any of your other results - e.g. an increase in growth due to CCM down-regulation.

Response: We have made changes to this paragraph with this comment in mind.

4. Place some sub-headings for the discussion so that it is easier to follow.

Line 285

Photosystem 1? Does Fv/Fm not only measure PSII? Also it may be important to state that measuring PSII may not reflect photosynthetic rate per se as only getting half the story.

Response: Thank you for pointing this out. The manuscript has been modified accordingly.

Line 292

RuBisCo is the site of carbon fixation rather than an active transport protein, can this be reworded to be more accurate.

Response: RuBisCo has been taken out from this sentence.

16 Dec 2020

PONE-D-20-25580R1

Ocean acidification decreases grazing pressure but alters morphological structure in a dominant coastal seaweed

PLOS ONE

Dear Dr. Kinnby,

Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process.

I have received both reviews back. Only one reviewer indicates that there are minor revisions required, while they other indicated accept as is. Please make these minor adjustments and resubmit your manuscript. After these are made I will accept the manuscript.

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We look forward to receiving your revised manuscript.

Kind regards,

Christopher Edward Cornwall, Ph.D.

PLOS ONE

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Reviewer #1: (No Response)

**********

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Reviewer #2: Yes

**********

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Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: Yes

**********

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Reviewer #2: Yes

**********

6. Review Comments to the Author

Reviewer #1: Kinnby et al. have added more details to the method section and cleared up other uncertainties as requested. I just have some minor suggestions for further improvement of the article. I think with these edits the manuscript will be ready for publication.

14: insert “projected to be 0.4 units lower..”

46: recommend changing “some” to “many” or even most

66: insert pCO2 at the end of the sentence to give reference to the concentrations you are talking about

72: here and a couple other places, OA is spelled out

87: remove “very”

88: since they may not be carbon limited

117: You should report the light levels if measured and type of light? LED? Natural? This is important since you discuss photosynthetic implications

128: space between number and units

141: change sd to SD

176: add parentheses after SD & change all sd to SD

251-252: Combine these two sentences… “The stable carbon isotope content of F. vesiculosus was significantly reduced to -13% when exposed to elevated pCO2.”

276 -278: reword, “which” is used twice in this sentence

276-281: These last sentences could use some work, its difficult to read. Maybe switch the last one around, “While the condition index increased…there was no change in preference and consumption actually decreased”

291: But since you used genetically identical samples, how does this new information add to our existing understanding?

295: See Guenther et al 2017 “Macroalgal spore dysfunction” I believe they show weakened attachment strength – which could add to your story

325: CCM - write out in full first time

327: Species names should be written out in full at the beginning of a sentence

371: reword – suggest "Reduced consumption for the herbivore…”

373: Was it really unanticipated?

Reviewer #2: (No Response)

**********

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Reviewer #2: No

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18 Dec 2020

Response to reviewers:

Reviewer #1: Kinnby et al. have added more details to the method section and cleared up other uncertainties as requested. I just have some minor suggestions for further improvement of the article. I think with these edits the manuscript will be ready for publication.

14: insert “projected to be 0.4 units lower..”

Response: The word lower has been inserted.

46: recommend changing “some” to “many” or even most

Response: Some has been changed to most.

66: insert pCO2 at the end of the sentence to give reference to the concentrations you are talking about

Response: pCO2 has been added to the end of the sentence.

72: here and a couple other places, OA is spelled out

Response: Thank you for pointing this out, this has been changed here and throughout the manuscript.

87: remove “very”

Response: Very has been removed from the sentence.

88: since they may not be carbon limited

Response: “Since they may not be carbon limited” has been added to the end of the sentence.

117: you should report the light levels if measured and type of light? LED? Natural? This is important since you discuss photosynthetic implications

Response: The experiment was performed in a greenhouse with natural lighting (natural light cycle 18:6 h, L:D).

128: space between number and units

Response: A space has been added.

141: change sd to SD

Response: sd has been changed to SD

176: add parentheses after SD & change all sd to SD

Response: A parentheses has been added and sd has been changed to SD throughout the manuscript.

251-252: Combine these two sentences…”The stable carbon isotope content of F. vesiculosus was significantly reduced to -13% when exposed to elevated pCO2.

Response: These two sentences have been merged to one.

276-278: reword, “which” is used twice in this sentence

Response: This sentence has been modified.

276-281: These last sentences could use some work, it is difficult to read. Maybe switch the last one around, “While the condition index increased…there was no change in preference and consumption actually decreased”

Response: These last sentences have been reworked.

291: But since you used genetically identical samples, how does this new information add to our existing understanding?

Response: This sentence has been altered for clarity. We want to clarify to the reviewer that we used the same individual in the treatment and its control, but there were not genetically identical samples for the entire experiment.

295: See Guenther et al 2017 “Macroalgal spore dysfunction” I believe they show weakened attachment strength – which could add to your story

Response: Thank you for bringing this article to our attention, we have added it to the text.

325: CCM – write out in full first time

Response: “carbon dioxide-concentrating mechanism” has been added to the sentence

327: Species names should be written out in full at the beginning of a sentence

Response: The full species name has been written out.

371: reword – suggest “Reduced consumption for the herbivore…”

Response: This has been reworded.

373: Was it really unanticipated?

Response: Yes, we weren’t expecting to find that the seaweeds would break more easily after exposure to elevated levels of pCO2. To our knowledge the literature on fleshy algae is pointing towards a positive effect of OA on these species since it generally enhances growth. As far as we know this is the first study that suggests that despite OA having a positive effect on growth there is a synergistic effect resulting in the algae becoming more vulnerable. Based on the available literature we did not expect to find this effect, and so it was unexpected for us. We appreciate that the reviewer brought our attention to Guenther et al. (2017) who suggest a similar pattern for spore attachment in red seaweeds.

Reviewer #2: (No Response)

21 Dec 2020

Ocean acidification decreases grazing pressure but alters morphological structure in a dominant coastal seaweed

PONE-D-20-25580R2

Dear Dr. Kinnby,

We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements.

Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication.

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Kind regards,

Christopher Edward Cornwall, Ph.D.

PLOS ONE

6 Jan 2021

PONE-D-20-25580R2

Ocean acidification decreases grazing pressure but alters morphological structure in a dominant coastal seaweed

Dear Dr. Kinnby:

I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department.

If we can help with anything else, please email us at plosone@plos.org.

Thank you for submitting your work to PLOS ONE and supporting open access.

Kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Christopher Edward Cornwall