Abstract
Estuarine food webs are generally considered to be supported by marine pelagic and benthic primary producers and by the import of dead organic matter from the open sea. Although estuaries receive considerable amounts of freshwater phytoplankton and organic compounds from adjacent rivers, the potential contribution of these living and dead matter to estuarine food webs is often assumed to be negligible and, therefore, not examined. Based on stable isotope analyses, we report the importance of freshwater suspended particulate organic matter (FW-SPOM) for fuelling estuarine food webs in comparison to estuarine SPOM and microphytobenthos. This previously neglected food source contributed 50–60% (annual average) of food intake of suspension-feeding bivalves such as cockles (Cerastoderma edule), mussels (Mytilus edulis) and Pacific oysters (Magallana gigas) at the Balgzand tidal flats, an estuarine site in the western Wadden Sea (12–32 psu). For these species, this proportion was particularly high in autumn during strong run-off of SPOM-rich freshwater, whilst estuarine SPOM (20%-25%) and microphytobenthos (15%-30%) were relatively important in summer when the freshwater run-off was very low. These findings have implications for our understanding of the trophic interactions within coastal food webs and for freshwater management of estuarine ecosystems.
Highlights
Coastal ecosystems and their food webs are under the influence of a variety of tidal and seasonally fluctuating factors in environmental conditions
In 2014, the chlorophyll-a concentrations within the Marsdiep tidal inlet peaked in March (14.1 μg L-1, Fig 2) after which a steady decline occurred until July (40% of maximum, Fig 2)
In 2014, peak chlorophyll-a discharge from Lake IJssel via the sluice at Den Oever was found in autumn (August 19.6 μg Chl a s-1 and October 21.4 μg Chl a s-1) and lowest values were observed in April (0.9 μg Chl a s-1, 4% of maximum, Fig 2)
Summary
Coastal ecosystems and their food webs are under the influence of a variety of tidal and seasonally fluctuating factors in environmental conditions (e.g. temperature, salinity and hydrodynamics, [1]). Long-term (interannual) variation in coastal community structures due to loss or gain of biodiversity (e.g. extinction/disappearance of species, invasions) and anthropogenic changes (e.g. input of nutrients, extraction of biomass by fisheries) occur [2,3,4,5,6,7,8,9]. These variations at different time scales will have consequences for the trophic transfer, isotopic niches and predator-prey interactions within the food web. This is true for marine organisms such as macrozoobenthos for which long-term trend analyses are based upon data that are gathered once a year during a particular season.
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