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.

Long-term declines in chlorophyll a and variable phenology revealed by a 60-year estuarine plankton time series

“…It is essential to note that chl a concentration represents a balance of growth and mortality and many of the environmental factors measured at NBPTS affect phytoplankton growth but not mortality ( 3 , 6 ). Sources of phytoplankton mortality include predation by zooplankton, ingestion by filter feeders, viral activity, and parasitism ( 55 – 58 ). For example, in NBay, microzooplankton grazing removes an average of 96% of phytoplankton primary production annually ( 53 ).…”

Sub-lethal and lethal effects of chronic and extreme multiple stressors on a critical New Zealand bivalve under hypoxia

“…During MMEs, proliferation of bacteria and pathogens as well as increases in ammonia concentrations can further stress and damage the soft tissues of surviving animals which can result in a positive feedback on mortality rates (de Zwaan et al 2002). In these circumstances within enclosed bays and inlets the residence times of water, clearance rates of suspension feeders and uptake rates of nitrogenous wastes by phytoplankton (Jones et al 2017, O'Connell-Milne et al 2020) play significant roles in how bivalve populations respond to combined heat and hypoxic stressors (Diaz & Rosenberg 1995).…”

“…Austrovenus stutchburyi (Bivalvia: Veneridae) (Wood, 1828), the endemic cockle or Tuaki (hereafter Austrovenus), is a critical infaunal suspension feeding bivalve and ecosystem engineer in coastal bays and inlets throughout New Zealand and subject to both prehistoric and modern day changes in environmental conditions (Tallis et al 2004, Wells et al 2019, O'Connell-Milne et al 2020. Austrovenus is an important benthic−pelagic coupler (e.g.…”

“…Austrovenus is an important benthic−pelagic coupler (e.g. Marsden 2004), influencing biogeochemical cycling in estuarine environments and trophic pathways (Thrush et al 2006, Karlson et al 2021 as well as supporting customary, re creational and commercial harvest (O'Connell-Milne et al 2020). Behavioural responses by Austrovenus to low oxygen include reduced burrowing depths and extending siphons above the sediment surface (Marsden & Bressington 2009).…”