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

Sa O’Connell-Milne,Lc Wing,Nh Salmond,Sh Suanda,Ja Udy,Lm Durante,Sr Wing

doi:10.3354/meps13522

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.

Highlights

Driven exchanges of organic matter and nutrients between estuaries and the coastal ocean are influenced by a chain of vital processes that together comprise coastal ecosystem function
Wind stress and wave transport conditions varied in the coastal ocean during the 5 deployments, with strong onshore wind stress and wave transport observed during Deployments 1 and 5, and periods of offshore wind stress observed during Deployments 2, 3 and 4 (Fig. 2A,B)
The data and results of the present study demonstrated that Waitati Inlet was a net sink for particulate carbon from coastal phytoplankton and a net exporter of NH4 to the coastal ocean

Summary

Introduction

Driven exchanges of organic matter and nutrients between estuaries and the coastal ocean are influenced by a chain of vital processes that together comprise coastal ecosystem function. Odum's (1980) seminal paper detailing the ‘outwelling hypothesis’ provided a landmark in ecosystem ecology by focussing attention on material fluxes between adjacent environments. It was stipulated that estuaries can either have a net export, or import, of organic carbon and nutrients, depending on the influences of biogeochemical processes in sediments, geomorphology, trophic processes, tidal flushing and exogenous forcing Efforts to integrate total fluxes of organic matter and nutrients across a range of marine systems have further highlighted the pivotal role of estuaries in biogeochemical cycling in the coastal ocean (Nixon 1980, Childers et al 2000, Snelgrove et al 2018)

Methods

Results

Conclusion