Dynamics and sources of suspended particulate organic matter in the Marennes-Oléron oyster farming bay: Insights from stable isotopes and microalgae ecology
Introduction
Distinguishing between the different sources of organic matter (OM) in complex aquatic environments such as rivers (Raikow and Hamilton, 2001), lakes (Grey et al., 2001), estuaries and coastal areas (Peterson et al., 1985, Kwak and Zedler, 1997, Cloern et al., 2002) remains an important issue. For instance, OM suspended in the water column of estuaries comprises a mixture of allochthonous sources, such as terrigenous river input and marine phytoplankton, and autochthonous sources, such as coastal phytoplankton and intertidal mudflat biomass.
Techniques using stable isotopes allow identification of OM sources from coastal areas and estuaries (Simenstad and Wissmar, 1985, Cifuentes et al., 1996). Carbon isotope ratios distinguish 13C-depleted terrestrial OM from the more 13C-enriched marine OM; thus, they are widely used in studies of estuarine food webs (Fry and Sherr, 1984, Gearing et al., 1984). In addition, studies carried out from the head to the mouth of estuaries have identified terrigenous and marine endmember contributions (Shultz and Calder, 1976, Cai et al., 1988, Cifuentes et al., 1988, Middelburg and Nieuwenhuize, 1998, De Brabandere et al., 2002). However, it remains difficult to determine the complex mixing processes involved when there are more than two different OM sources (Cloern et al., 2002). The consideration of other factors may reduce this ambiguity, allowing the suspended particulate organic matter (SPOM) source contributions in estuaries and bays to be determined. Stable isotopes are frequently associated with C/N ratio (Thornton and McManus, 1994, Middelburg and Nieuwenhuize, 1998, Hellings et al., 1999, Cloern et al., 2002), POC/Chl a (Cifuentes et al., 1988) or organic biomarkers (Canuel et al., 1995, Goñi et al., 2003). Characterization of the microalgae community in the water column may help to distinguish the different SPOM components related to benthic or pelagic compartments. The most frequently used method involves taxonomic analysis (Lucas et al., 2001). This considers, for instance, that all pennate diatoms are derived from the benthic component. The microphytobenthic contribution to the water column is thus well referenced in ecosystems with large intertidal areas (Shaffer and Sullivan, 1988, de Jonge and Van Beusekom, 1992, de Jonge and Van Beusekom, 1995).
The Marennes-Oléron Bay is one of the most important oyster farming areas in Europe with an annual oyster Crassostrea gigas production of 40,000 tonnes (Goulletquer and Le Moine, 2002). Several earlier studies focused on the dynamics of organic matter available to cultivated oysters in the whole bay (Héral et al., 1984, Raillard and Ménesguen, 1994, Soletchnik et al., 1998). Seston dynamics are strongly influenced by advection of the different water masses (Bacher, 1989) and by wind- and tide-induced resuspension of mudflat sediment (Héral et al., 1984, Prou et al., 1994). Consequently, autochthonous primary production by phytoplankton in the Marennes-Oléron Bay is very limited in both space and time due to the high turbidity of the water column (Raillard and Ménesguen, 1994), and a considerable amount of chlorophyll (Chl) a in the water column is derived from resuspended microphytobenthos (Zurburg et al., 1994, Guarini et al., 2004). However, our knowledge of the origin of OM sources and their contribution to cultivated oyster diet is still incomplete (Riera and Richard, 1996, Richard et al., 1997, Riera and Richard, 1997, Malet et al., 2007). Riera and Richard (1996) suggest that resuspended microphytobenthos is of major importance to oysters living on intertidal mudflats. Microphytobenthos production varies both spatially and temporally (Guarini et al., 1998); thus, one would expect the contribution of resuspended microphytobenthos to cultivated oyster diet to be site- and time-dependent. This was demonstrated in recent studies performed on other oyster farming areas (Decottignies et al., 2007, Riera, 2007).
The main objective of this study was to further determine the temporal variability of OM sources and their contribution to the resuspended particulate organic pool in the Marennes-Oléron Bay. We performed our investigations on the southern mudflats of the bay, where the dynamics of cultivated oysters stocks have been well studied (Soletchnik et al., 1998). We characterized SPOM by measuring a number of variables (i.e. chloropigments, C/N and POC/Chl a), natural stable isotope ratios and the relative contributions of benthic and pelagic phytoplankton species. Seasonal and inter-annual variability of primary production, together with a major effect of the tidal regime on resuspension events within the bay (Prou et al., 1994, Raillard and Ménesguen, 1994), has also been reported. Therefore, field investigations were based on a biannual cycle and bimonthly sampling was performed to examine the different time scales of SPOM variability and accurately assess the contribution of different OM sources to the Marennes-Oléron Bay SPOM pool.
Section snippets
Study area
The Marennes-Oléron Bay is located north of the Gironde estuary on the French Atlantic Coast and is composed of 60% of intertidal mudflats (Fig. 1). Of the two rivers flowing into the bay, the Charente River has the largest output (10 to 470 m3 s−1), whereas the discharge of the Seudre River is much lower, ranging from 0 to 40 m3 s−1 (Soletchnik et al., 1998). During winter, when river discharge is high, low salinity waters from the Gironde estuary plume enter the bay either through the wide north
Environmental factors
In 2003, there were nine months with a mean monthly air temperature higher than the monthly average over—with both June and August having average monthly temperatures four degrees higher—whereas mean monthly air temperatures in 2002 were similar to the monthly average (Fig. 2a). The salinity ranged from 28 in winter to 34.5 during summer and fall, with an average of 32.5 in spring. The lowest salinity (ca. 22 in March 2002 and 2003) was caused by heavy rainfall during the sampling days (Fig. 2
Characterization of SPOM sources
In the Marennes-Oléron Bay, allochthonous SPOM inputs are a combination of terrigenous inputs carried by both the Charente River (Riera and Richard, 1996) and the Gironde estuary plume entering the bay through the Maumusson Pertuis (Fontugne and Jouanneau, 1987). Our findings revealing correlations between δ13C values and salinity or river discharge are consistent with this spatial feature. Indeed, depleted stable isotope ratios were associated with low salinity river inputs or high river
Conclusion
Temporal variability in chloropigments and the associated biomarkers, δ13C and δ15N ratios, together with microalgae ecology, provided consistent information on the origin and mixture of the OM sources collected from an intertidal mudflat in Marennes-Oléron Bay. One unexpected result of this study is the difference in δ13C of SPOM during the summer–fall period between 2002 and 2003. The enhanced enrichment of 13C in 2003 was attributed to decaying intertidal inputs with both resuspended
Acknowledgments
The authors thank F. Mornet and M. Breret for chlorophyll a analysis and water sample filtration. Thanks are also due to Professor McManus, P. Soletchnik and anonymous reviewers for their valuable comments on the manuscript. This research was funded by CNRS, IFREMER and Région Poitou-Charentes. N.M. was supported by a PhD grant from Conseil Régional de la Charente-Maritime.
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2017, Progress in OceanographyCitation Excerpt :The estimates of phytoplankton δ13C, δ15N and C:N ratios range usually from −24‰ to −20‰, from 2‰ to 8‰, and from 5 to 8 mol mol−1, respectively (Table S3). These values are consistent with the data reported in the literature for the temperate coastal systems (Brzezinski, 1985; Fry and Wainright, 1991; Savoye et al., 2003) and especially in the literature dedicated to the studied systems (e.g. Dubois et al., 2012, 2014, Arcachon Lagoon; Harmelin-Vivien et al., 2008, Gulf of Lion; Malet et al., 2008, Marennes-Oléron bay; Savoye et al., 2003, Bay of Seine). In contrast, the high POC:Chl a ratios of the Gironde Estuary (Fig. 2), indicate that the phytoplankton is a low contributor to the POM pool there.