Analysis of the seasonal impact of three marine bivalves on seston particles in water column

Suspension-feeding bivalves are the keystone species that affect the abundance and composition of phytoplankton communities. This study compares the food selectivity of oysters Crassostrea gigas (Thunberg, 1973), mussels Mytilus galloprovincialis (Lamarck, 1819), and cockles Katelysia rhytiphora (Lamy, 197) on natural particle assemblages in the laboratory using water collected from Coffin Bay, South Australia, in spring, summer, autumn and winter. The phytoplankton community was dominated by Bacillariophyceae (diatoms), Dinophyceae (dinoflagellates), Synechococcus (cyanobacteria), and picophytoplankton (2–5 μm) from September 2016 to August 2017. Diatoms and picoplankton, including cyanobacteria, were dominant in spring and summer, while dinoflagellates were dominant in autumn and winter. Oysters and mussels selectively fed on large food particles (e.g., diatom, dinoflagellate and large picoplankton > 2 μm) regardless of season, but mussels could access a wider size spectrum of food particles compared to oysters. In contrast, cockles selected for both large and small food particles (e.g., Synechococcus and small picoeukaryotes < 2 μm) and fed more efficiently on small particles than both oysters and mussels. Bacteria were more abundant in warmer seasons, but mussels and cockles selectively reduced bacterial abundance. The abundance of virus-like particles was not affected by season nor by filtration of any molluscan species. This study demonstrates that oysters and mussels utilise similar food sources, whereas cockles consume a large range of food particles. Additionally, these three molluscan species do not necessarily compete for food; hence, stocking multiple filter-feeding species in an ecosystem could exploit a wider spectrum of food particle size and increase system productivity.