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.

We thank the reviewer for the valuable comments and suggestions, which have helped to improve the quality of our article.

Lines 35–37: We modified the sentence on lines 35-37, which now includes examples of integrative and sensitive ecological indicators. “Yet, only a few ecological indicators are both integrative (i.e. measured in long-living species tissues) and sensitive enough (e.g. measured at molecular level such as metallothionein used as a biomarker of metal exposure, Amiard et al., 2006) to various disturbances while also exhibiting predictable responses with a low variability in its response (Dale and Beyeler, 2001; Niemi and McDonald, 2004).”

Line 54: We refined this section as following : The C:N ratio is mostly an indicator of bivalve condition and physiology, reflecting the balance between organisms’ requirements and elemental availability in the environment (i.e., ecological stoichiometry sensu Elser et al., 2003; N content increases (thus C/N ratio decreases) as protein content increases whereas C content increases (thus C/N ratio increases) as lipid or carbohydrate content increases). 

Lines 63–64: We slightly modified the sentence lines 63-64 and added the following sentence about using such data in models in Section 5. “Ultimately, it could provide valuable input for developing predictive models of bivalve physiology (Emmery et al., 2011) or trophic ecology (Marín Leal et al., 2008) explaining ecosystem response to future environmental changes and possibly forecast potential impacts of climate change and human activities on coastal ecosystems.”

Lines 84–85: We agree that this information would have been interesting. Unfortunately, there is no such information in the ROCCH database. Note that all the individuals are pooled in a single meta-sample (see below). Thus, the only usable value would be the mean size.

Line 106: We agree that sample homogeneity is crucial for stable isotope analysis and we were careful about it when analyzing the archived sampled. Archived sampled were already pooled, grounded and homogenized before long-term storage as described in section 2.1. lines 85-94. We modified a few words in section 2.1. for more clarity.

Line 110: Indeed, caffeine is not certified for nitrogen analysis, but calibration was performed with internal standards. We added information about the standard values and calibration on lines 110-114 :

• Casein (δ13C -23.30 ‰; δ15N 6.30 ‰)
• Glycine (δ13C -45.20 ‰; δ15N 3.00 ‰)
• Caffeine (IAEA600: δ13C -27.77 ‰, δ15N 1.00 ‰) not certified for N
• Graphite (USGS24: δ13C -16.05 ‰)

Line 161: We used linear models to calculate the slopes. We added missing packages and function names on lines in section 2.4 and the corresponding references to the manuscript.

Figure 2 and text: We modified Figure 2 and add the C% and N% data and specified the units in section 2.3.

Line 229: We agree that C and N% are also valuable and we included C and N% data description in the entire manuscript.

Section 3.3.1: It was an oversight. We corrected all section’s titles.

Lines 289–290: We added the visualization of C and N% in figures 2 and 3 and included its description in the ms.

Figure 5: We added C and N % data in Figure 2 and as a new Figure 6 and 7 (similar to Figures 3, 4 and 5).