Globally, estuarine ecosystems stressed by anthropogenic activities such as increased nutrient and sediment loading. The ecological processes that drive the uptake of nutrients and responses to increased turbidity are inherently non-linear, and so minor changes to loading can drive rapid and unpredictable changes in ecosystem dynamics. Here we explore one of the processes that mediates benthic productivity in a key intertidal species, the deposit-feeding bivalve, Macomona lilliana, using a simple non-linear model. M. lilliana feed on microphytobenthos (MPB) at the sediment surface, and through the exhalent siphon excrete nutrients into porewaters at depth. This behaviour pressurises the sediment, and effectively pumps limiting porewater nutrients back to the surface making them available to MPB. We modelled this system where MPB are controlled by the activities of M. lilliana (i.e. grazing rates and pumping capacity which supplies limited nutrients) and light (controlled by water column turbidity) using published field and lab measurements. Pressurisation caused by the pumping activity is superimposed on a background pressure generated by the tide and waves, which are used to generate upward flow using Darcy’s law. The model shows that slow variations in forcing parameters like light can cause a rapid biomass response, associated with a shift between a MPB-dominated system to a M. Lilliana-dominated system. The model is critically dependent on assumed mortality rate, which was used as a tuning parameter, to bring the MPB and M. lilliana biomass into observed ranges.

Primary Presenter: Conrad Pilditch, University of Waikato (karin.bryan@waikato.ac.nz)

Authors:

Taylor Thomson, University of Waikato  (taylort42@gmail.com)

Giovanni Coco, University of Auckland  (g.coco@auckland.ac.nz)

Conrad Pilditch, University of Waikato  (Conrad.pilditch@waikato.ac.nz)