the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Nov 2025
Ocean acidification alters phytoplankton diversity and community structure in the coastal water of the East China Sea
Abstract. Anthropogenic CO2 emissions and their continuous dissolution into seawater lead to seawater pCO2 rise and ocean acidification (OA). Phytoplankton groups are known to be differentially affected by carbonate chemistry changes associated with OA in different regions of contrasting physical and chemical features. To explore responses of phytoplankton to OA in the Chinese coastal waters, we conducted a mesocosm experiment in a eutrophic bay of the southern East China Sea under ambient (410 μatm, AC) and elevated (1000 μatm, HC) pCO2 levels. The HC stimulated phytoplankton growth and primary production during the initial nutrient-replete stage, while the community diversity and evenness were reduced during this stage due to the rapid nutrient consumption and diatom blooms, and the subsequent shift from diatoms to hetero-dinoflagellates led to a decline in primary production during the mid and later phases under nutrient depletion. Such suppression of diatom-to-dinoflagellate succession occurred with enhanced remineralization of organic matter under the HC conditions, with smaller phytoplankton becoming dominant for the sustained primary production. Our findings indicate that, the impacts of OA on phytoplankton diversity in the coastal water of the southern East China Sea depend on availability of nutrients, with primary productivity and biodiversity of phytoplankton reduced in the eutrophicated coastal water.
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Status: final response (author comments only)
- CC1: 'Comment on egusphere-2025-5218', Li Gang, 31 Dec 2025
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RC1: 'Comment on egusphere-2025-5218', Anonymous Referee #1, 15 Mar 2026
Reviewer Report
This manuscript aims to understand the effects of ocean acidification (OA) on marine phytoplankton communities in coastal waters of the East China Sea. Using surface seawater collected from a highly eutrophic coastal region, the authors conducted an in situ 3000 L mesocosm experiment to determine primary productivity, as well as phytoplankton growth rate and composition. They found that the effects of OA on phytoplankton diversity and primary productivity depend on nutrient availability. Since both the diversity and biomass of phytoplankton decreased under OA and nutrient-depleted conditions, the authors propose that diversity will be lost and primary production will decrease in future oceans. The premise of this study is meaningful, as the effects of OA on marine phytoplankton remain controversial. This in situ mesocosm experiment makes the conclusions straightforward and reliable, so my comments on this manuscript are quite minor as follows:
Minor revisions
Lines 26-27, as I understand it, and as shown in Figs. 5, 6, the smaller phytoplankton became dominant after the diatom-to-dinoflagellate succession and concurrently with enhanced organic matter remineralization, but these three events look like to happen simultaneously, please check it.
Lines 59-61, Does this sentence describe the same thing (i.e., the availabilities of nutrient can modulate the effects of OA) as that in line 66 ‘On the other hand….’?
Lines 123-128, more information is needed on the nutrient analyses, especifically the methods used and the standard curve information. More importantly, does ‘0.45 mm’ refer to the diameter or pore size of the membrane?
Line 162, lugol’s iodine or acidic lugol’s iodine? Was the statically time sufficient for cells sedimentation? The method should be described in more detail.
Line 176, TYPO: analyses.
Lines 190, 194 and elsewhere, the description of ‘day’ and ‘Day’ should be consistent through the manuscript.
Line 267, Figure 4. It is unclear why primary production increased, yet primary productivity decreased. For example, from day 4 to day 6, primary production increased with increasing Chl a, but primary productivity decreased. Please add more explanation.
Lines 282-283, this sentence “Diatom density was lower in the HC than in the AC mesocosms, though the difference was not statistically significant (p = 0.259, Fig. S7 a)” should be revised to “There was no statistically significant difference in diatom density between the HC and AC mesocosms (p = 0.259, Fig. S7a), although the value was lower in the former than in the latter treatment.”.
Line 313, Fig 6, according to figure legend, the authors classified diatoms and dinoflagellates as microphytoplankton, and classified Cyanobacteria, Chlorophyta, Cryptophytes, Euglenophyta as nanophytoplankton. Is that correct? I disagree with this classification, since some diatoms and dinoflagellates are smaller than 20 μm, as indicated in lines 329-332. I acknowledge that the phytoplankton groups mentioned as small (Cyano, Chlo, Cryp, Eugl) are generally smaller than diatoms and dinoflagellates, but this statement, as well as the legend and caption in Fig. 6 should be clarified more carefully.
Lines 326-329, which shift was suppressed? Indeed, as seen in Figs. 5 c, S7 b, transition from diatoms to autotrophic dinoflagellates was suppressed. However, as shown in Fig. S8 b, elevated pCO2 only had a negative effect on community diversity after day 24. Before that day, elevated pCO2 led to higher diversity under diatom dominance and partially compensated for the negative effects of subsequent reduced nutrient availability on community diversity (as described by the authors in lines 344-345 and 402-403). Therefore, I believe the authors should substantially revise this part of the discussion.
Citation: https://doi.org/10.5194/egusphere-2025-5218-RC1 -
RC2: 'Comment on egusphere-2025-5218', Anonymous Referee #2, 15 Apr 2026
In this manuscript, Rao et al. conducted a mesocosm experiment in a eutrophic bay of the southern East China Sea under ambient (410 µatm, AC) and elevated (1000 µatm, HC) pCO₂ levels to investigate the responses of phytoplankton to ocean acidification (OA) in Chinese coastal waters. The study documents fluctuations in phytoplankton growth and primary production during the experiment and reveals that elevated pCO₂, together with the natural decrease in surface water temperature and declining nutrient availability, altered the structure and diversity of the phytoplankton community.
Overall, the methods and analyses are sound, and the interpretations are generally appropriate. The manuscript is well written and adequately referenced. In principle, this is an excellent study. However, several points should be addressed before acceptance.
Minor comments:
- Line 26: Please clarify the phrase “such suppression of diatom-to-dinoflagellate succession.” Additional context is needed to help readers understand this statement.
- As the authors stated, the experiment was conducted in autumn, during which water temperature naturally decreased. Is there a way to estimate the influence of temperature on phytoplankton growth and community structure? Additionally, could temperature interact with ocean acidification? Please elaborate on this point in the Discussion, potentially incorporating insights from your previous study (Huang et al., 2021; cited in Line 395).
- Line 185: In the Results section, the order of figures should be consistent with their citation in the main-text. For example, after Figure 1 is mentioned (Line 193), Figure 3, rather than Figure 2, is cited (Line 194). Please revise the figure order in both the main text and the supplementary materials.
- Line 192: Please explain why total alkalinity (TA) did not differ significantly between the HC and AC treatments.
- In the Discussion section, it is generally unnecessary to cite figures and restate results already presented in the Results. Please streamline this section accordingly.
- Line 200: If data are available, please include a figure showing diel fluctuations of pCO₂ under HC and AC conditions during the early, middle, and final stages of the experiment.
- Figure 6: The bottom panel should be labeled “AC,” not “LC,” to maintain consistency with the rest of the manuscript. Moreover, please define the terms “micro” and “nano” in the figure legend.
- Please consider adding a figure that further resolves the phytoplankton composition presented in Figure 6. Such a figure could illustrate temporal changes in dominant groups (e.g., diatoms, autotrophic dinoflagellates, and heterotrophic dinoflagellates) throughout the experiment.
- To improve readability, consider including a table of abbreviations.
Citation: https://doi.org/10.5194/egusphere-2025-5218-RC2
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- 1
Reviewer Report
The authors present an interesting manuscript aimed at understanding the effects of OA (ocean acidification) on marine phytoplankton communities in coastal waters. Using surface water collected from highly eutrophic coastal region in the East China Sea, they performed an in situ mesocosm experiment to measure primary productivity, as well as phytoplankton community growth and composition. They observed the effects of OA on phytoplankton diversity and primary productivity depend on availability of nutrients. Since both phytoplankton diversity and biomass decreased under OA and nitrient-depleted conditions, the authors proposed that community diversity will be lost and primary production will decrease in the future ocean.
The premise of the study is of interest to the scientific community, as the effects of OA on marine phytoplankton remain controversy. The authors utilized in situ mesocosm experiment making the results straightforward and reliable. For these reasons, my comments on the manuscript are quite minor and based on the authors’ line numbers as follows:
Lines 26-27, as I understand it, and as shown in Figs. 5, 6, the smaller phytoplankton became dominant after the diatom-to-dinoflagellate succession and concurrently with enhanced organic matter remineralization, but this sentence looks like these three incidents happened simultaneously, please clarify.
Lines 59-61, Doesn’t this sentence describe the same thing (i.e., the availabilities of nutrient can modulate the effects of OA) as what’s described after line 66 ‘On the other hand….’?
Line 84, I am not in favor of the repeated mentions of Chinese coastal regions, shouldn’t the results of this work have some inplication that reflect a global perspective?
Lines 123-128, more information is needed on the nutrient analyses, specifically the actual methods used and the standard curve information. More importantly, does ‘0.45 mm’ refer to the diameter or pore size of the membrane?
Lines 139 and 299-301, What’s the point of measuring POC and PON in different size fraction? If 20 μm is the threshold of micro- and nano-sized phytoplankton, the discussion of the correlations between POC/PON in different size an phytoplankton species of corresponding size classes was not sufficient.
Line 162, lugol’s iodine or acidic lugol’s iodine? Was the statically time sufficient for cells sedimentation? The actual method should be described in more detail.
Line 176, TYPO: analyses.
Lines 190, 194 and elsewhere, the discription of ‘day’ and ‘Day’ should be consistent through the manuscript.
Line 267, Figure 4. It is unclear why primary production increased, yet primary productivity decreased? For example, from day 4 to day 6, primary production increased with the increasing Chl a, but primary productivity decreased.
Line 313, Fig 6, as I understand from the figure legend, did the author classifed diatoms and dinoflagellates as microphytoplankton, and classfied Cyanobacteria, Chlorophyta, Cryptophytes, Euglenophyta as nanophytoplankton? I disagree with the classification, since some diatoms and dinoflagellates are smaller than 20 μm, which is acknoledged by the author in lines 329-332. I admit that the phytoplankton groups mentioned as being small (Cyano, Chlo, Cryp, Eugl) are generally smaller than diatoms and dinoflagellates, but this statement, as well as the legend and caption in Fig. 6 should be clarified more carefully.
Lines 326-329, which shift was suppressed? Indeed, as seen from Figs. 5 c, S7 b, transition from diatoms to autotrophic dinoflagellates was suppresed. However, as shown from Fig. S8 b, elevated pCO2 only had a negative on community diversity after day 24. Before that, elevated pCO2 led to higher diversity under diatoms dominance and somehow compensated for the negative effects of subsequent reduced nutrient availability on community diversity (as described by the authors in lines 344-345 and 402-403). Therefore, I believe the authors should substantially revised this part of discussion.