The ecological role of the suspension feeding bivalve, Austrovenus stutchburyi, in estuarine ecosystems

Suspension feeding bivalves are described as key species in many aquatic ecosystems, where they can influence primary productivity and nutrient dynamics, are food for higher trophic levels, and may be harvested in commercial or recreational fisheries. In many areas where bivalve populations have declined, substantial changes to ecosystem structure and function have occurred. High-density beds of the infaunal suspension feeding bivalve, Austrovenus stutchburyi, are a dominant feature on intertidal flats in many New Zealand estuaries, but populations are declining in some areas, likely due to anthropogenic stressors such as overharvesting and sedimentation. This thesis examines the influence of Austrovenus on estuarine ecosystem function using laboratory, field and modelling studies. The effects of bed density and flow speed on boundary layer dynamics and Austrovenus clearance rates (CR) were investigated in annular flumes. Bed shear stress increased with increasing bivalve density, and under certain conditions was also increased when Austrovenus were feeding, compared to when they were not. The bed roughness and siphonal currents generated by the bivalves may therefore influence food supply to the bed. Both individual and bed CR were significantly greater at the high flow speed. Bed CR did not scale proportionally with density, in part because individual CR decreased with increasing density, but also because the proportion of bivalves actively feeding decreased at high densities. Thus, scaling up individual CR obtained from animals at low densities will significantly overestimate population filtration capacity. The effect of Austrovenus on ecosystem function was examined at two sites, one sandy, the other composed of muddy-sand, to determine whether sedimentary environment alters this key species’ role. Gross primary production (likely corresponding to microphytobenthos production) and ammonium uptake were significantly increased, and denitrification potential was also elevated, when Austrovenus was added, relative to removed, at the sandy site. In contrast, there was no effect of Austrovenus on any of these variables at the muddy-sand site. These results reveal the importance of considering sedimentary environment when examining the effect of key species on ecosystem function, and suggest that even moderate levels of sedimentation may reduce the positive effect of Austrovenus on primary productivity. A food-web model was used to quantify the interactions between Austrovenus and other estuarine species, and to determine the effect of reducing Austrovenus biomass on ecosystem properties. The model revealed an estuarine system dominated by benthic species, characterised as a developing, rather than a mature ecosystem. Detritus production and export were high, while transfer efficiency and internal recycling were low. Decreasing Austrovenus biomass decreased system maturity, suggesting that reductions in Austrovenus populations may decrease ecosystem stability. The model also confirmed that within these estuarine systems Austrovenus and microphytobenthos are key species which impact greatly on all other trophic levels. This thesis demonstrates that Austrovenus populations exert considerable influence on benthic communities and processes, and play a key role in the functioning of estuarine ecosystems. Reductions in Austrovenus populations will likely negatively impact on microphytobenthos and higher trophic levels, and may also reduce ecosystem maturity and stability