the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 30 Aug 2024
Bivalve monitoring over French coasts: multi-decadal records of carbon and nitrogen elemental and isotopic ratios (δ13C, δ15N and C:N) as ecological indicators of global change
Abstract. Recent changes in climate and environment, influenced by both global and local factors, have had profound impacts on coastal ecosystem functioning and trajectories. By examining archived samples from ongoing ecological monitoring efforts, particularly focusing on bivalves like mussels and oysters, we gain a valuable long-term perspective on how ecosystems are responding at various scales. We conducted analyses on carbon and nitrogen elemental and isotopic ratios (C:N, δ13C, δ15N) of mussel and oyster soft tissues collected annually at 33 sites along the French coast from 1981 to 2021. This extensive dataset (https://doi.org/10.17882/100583, Liénart et al., 2024a) offers a comprehensive view spanning multiple decades and ecosystems, allowing to track how coastal ecosystems and marine species record changing climate, physical-chemical environments and organic matter cycles. Additionally, these data are crucial for establishing isotope baselines for studying food webs. Ultimately, this data set provide valuable information for more effective ecosystem conservation and management strategies in our rapidly changing world.
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CC1: 'Comment on essd-2024-364', Camilla Liénart, 16 Sep 2024
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Note that the authors will homogenize Table 1 and Table 2 by replacing 'NA' with ' - ' in Table 1 and replacing the only 'NA' occurence in Table 2.
Citation: https://doi.org/10.5194/essd-2024-364-CC1 -
RC1: 'Comment on essd-2024-364', Anonymous Referee #1, 23 Sep 2024
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The article ‘Bivalve monitoring over French coasts: multi-decadal records of carbon and nitrogen elemental and isotopic ratios (δ13C, δ15N and C:N) as ecological indicators of global change’ presents an extensive long-term study of the carbon and nitrogen isotopes in mussels and oysters over 33 sites along the French coast from 1981 to 2021. The study has used this extensive historic sample set to monitor changing coastal environments, and to establish potential implications for trophic transfer of nutrients into mollusc tissues. This study has been able to use mussel and oyster samples collected by Réseau d'Observation de la Contamination CHimique as bioindicators for chemical contamination to study the nutrient changes over decades using mussel and oyster tissue isotopes. This is an innovative use of existing historic samples to understand environmental change and implication for ecosystem cycling. This is a really nice study and important for considering future environmental change with climate change induced extreme events, increased rainfall, warming all influences the C/N at the coast and uptake for the molluscs living in those environments. The importance of the study refers to using the C/N isotopes to monitor food webs for effective management in a changing world. I would have liked to have seen more discussion on the influence of freshwater inputs at the coastal sites to alter C/N and implications for algal blooms under climate change. Please see below specific points where this could be expanded.
Some questions to the authors:
Results
Line 174 ‘it is impossible to determine if the differences observed in elemental and isotopic values between the three species of bivalves and between the three sea facades are due to species effect or spatial differences (see 2.4 and details hereafter).’ Can the authors confirm how many of each species were analysed for C/N? As this can vary between individuals. Could this be a factor in the variability? It is known from other studies examinng environmental change e.g. Mele et al., (2023), Lee et al., (2021), Fitzer et al., (2019), Lu et al., (2018) that C/N does differ between species of mussel and oyster and again will differ due to environmental change, i.e. freshwater input at coastal regions and change to algae species can reduce by upto –5 per mil.
Could the authors please discuss this difference in species and environment and can refer to examples to support this. E.g. in reference to previously published data to support this, e.g. ~-20 per mil C for mussels Mytilus edulis (Mele et al., 2023, Lee et al., 2021; Lu et al., 2018), oysters <-21 C. gigas (Mele et al., 2023), Saccostrea glomerata ~-21 - -26 (Fitzer et al., 2019). Nitrogen also differs lighter ~<7 per mil for mussels Mytilus edulis and > ~8 per mil for oysters C. gigas (Mele at al., 2023).
Line 207-208 It is not unsurprising that the lowest carbon isotopes are in those environments close to riverine input where the DIC would be influenced by the freshwater inputs reducing by ~-5 per mil. See studies examining environmental change e.g. Fitzer et al., (2019).
The above points need expansion into the discussion (~lines 240) of the importance of coastal environmental change for C/ N and what this tells us about the algal availability. This directly influences the C/N uptake into the tissues form environment and food source. How is this likely to change under climate change? What does this mean for the food web and availability of food for mussels and oysters in these coastal regions? This is key to support statements in the abstract relating to food web baseline data and management in a changing world.
Lines 253-255 Figure 3 nicely presents the downwards shift in δ13C at all sites, noticeably in locations near the Seine plume. Can the authors comment on changes to precipitation in the catchment of the Seine historically and how this relates to the downward shift in carbon isotopes? I.e. climate change, this could be expanded to support the findings and statements of the conclusions.
Citation: https://doi.org/10.5194/essd-2024-364-RC1 -
AC1: 'Reply on RC1', Camilla Liénart, 16 Oct 2024
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1) General reply :
We thank the reviewer for their valuable comments and suggestions which have helped improve the quality of our article. As this article is a data paper, in-depth discussion of the processes associated to the data was not our primary objective. However, we believe it is important to provide key messages and possible interpretations, which we have included accordingly. Therefore, we have expanded the manuscript with a few sentences, particularly addressing the influence of freshwater inputs, as suggested. These data, along with additional environmental data, will be thoroughly discussed in a forthcoming publication that will include environmental databases and examine the effects of global climate and regional factors like river inputs and food availability), as well as the potential effects of bivalve species and physiology on the isotope signal.
2) Regarding comments on Line 174 and the possible difference in elemental and isotope values between bivalve species :
This dataset presents the C and N elemental and isotope values for three bivalve species: the oyster Crassostrea gigas, the mussel Mytilus edulis, and the mussel Mytilus galloprovincialis. The sampling strategy is described in section 2.1, with details regarding the size and number of individuals sampled for each species are provided in lines 84 to 87 : “A minimum of 50 mussels or 10 oysters was required to constitute a representative pooled sample accounting for inter-individual variability”. Since the sampling protocol was standardized across sites and years, and each sample is a composite of multiple individuals, inter-individual variability is integrated withing each sample. Therefore, we considered this approach sufficient to minimize intra-species variability in elemental and isotope signals.
Additionally, we acknowledge the intrinsic differences in isotope values between mussels and oysters. The differences observed between species in N isotopes and in the C:N elemental ratio may be attributed to species effects (discussed in section 3.1), which were tested for the Atlantic coast as explained in lines 185-189. We recommend that data users be cautious when making comparisons between species. However, for each species, the selected stations represent a range of environmental conditions (e.g., near river mouths, littoral zones, bays), allowing for comparisons of elemental and isotope values and enabling the analysis of environmental changes on bivalve isotope values.
The sentence on lines 174-175 will be modified as following: “It is important to note that, due to the nature of the dataset (with one species present for each sea façade), it was not possible across all our study sites, to determine whether the differences observed in elemental and isotopic values between the three bivalve species and across the three sea façades were due to species effects or spatial differences. However, this could be assessed at the scale of the Atlantic façade (see sections 2.4 and 3.1).
An introductory sentence will be added to section 3.1 on line 177: “Different species may exhibit different C and N elemental and isotopic values under the same environmental and growth conditions (Mele et al., 2023), and within a single species, varying environmental conditions can lead to difference in these values (e.g. Magni et al., 2012; Briant et al., 2018).
3) Regarding comments on Line 207-208 and the effect of freshwater influence and environmental variability under climate change :
We agree with the reviewer. We will add the following sentences on line 243: 'δ13C values of dissolved inorganic carbon (DIC) are lower in riverine/freshwater ecosystems compared to marine environments (Mook and Rozanski, 2000), resulting in more negative δ13C values in phytoplankton, which may be ingested locally by bivalves. Additionally, δ13C of particulate organic carbon (POC) from terrestrial sources tends to be more negative (Liénart et al., 2017) and can be consumed by bivalves. However, whether terrestrial POC is a significant food source for bivalves remains debated (Malet et al., 2008; Marín Leal et al., 2008; Dubois et al., 2014). Similarly, the δ15N signal of freshwater and marine phytoplankton differs due to the distinct nitrogen sources in these two environments. Therefore, it is not surprising that the δ15N signal in bivalves varies along a gradient related to proximity to river mouths.
4) Regarding comments on Line 253-255 about the Seine river inputs :
Based on existing data (Hydroportail ; EauFrance), over the period 1981-2021, there is no significant increase or decrease in the Seine river flows. As mentioned above, in depth discussion the effects of environment on bivalves elemental and isotope signal is not the object of this data paper. Bivalve elemental and isotope data, along with additional environmental data, will be thoroughly discussed in a forthcoming publication that will include environmental databases and examine the effects of global climate and regional factors (like river inputs and food availability), as well as the potential effects of bivalve species and physiology on the isotope signal.
Citation: https://doi.org/10.5194/essd-2024-364-AC1
Data sets
Bivalve multi-decadal (1981-2021) records of carbon and nitrogen elemental and isotopic ratios (δ13C, δ15N and C:N) of 33 coastal sites of France C. Liénart et al. https://doi.org/10.17882/100583
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