Interactions between bivalve filter feeding and oceanographic forcing drive the fluxes of organic matter and nutrients at an estuarine-coastal interface

Abstract

Fluxes of nutrients and organic matter between estuaries and the open coast comprise an important component of ecosystem connectivity. Nevertheless, relatively little is known about how oceanographic processes, for example onshore retention of water in the coastal boundary layer, interact with major sinks for particulate organic matter such as bivalve filter feeding within inlets and estuaries. To investigate this interaction, total fluxes of water, nutrients (NH4, NOx and PO4) and chlorophyll a between Waitati Inlet on the wave-exposed coast of the South Island, New Zealand, and the coastal ocean were quantified over 40 tidal cycles. We found declines in total flux of phytoplankton and increases in flux of NH4 between flood and ebb tides. Net declines in phytoplankton biomass followed a Type II functional response curve, consistent with consumption by the large biomass of filter feeding bivalves within the inlet; however, an asymptote was not reached for the highest concentrations, indicating that feeding was likely limited by exposure time rather than concentration of food relative to biomass. An information-theoretic framework was then used to infer the most likely combination of environmental conditions influencing total fluxes of phytoplankton into the inlet. Onshore wind stress, wave transport and salinity explained 90% of the variation in flux of phytoplankton entering the inlet on flood tides. These results highlight the importance of the interaction between oceanographic forcing and bivalve filter feeding in modulating material dynamics and connectivity between estuaries and the coastal ocean.