Abstract
The present thesis investigates the density-dependent effects of bivalve populations on carbon degradation rates, a key function in carbon cycling, in estuarine ecosystems. In particular, the study uses the Rapid Organic Matter Assay (ROMA) in situ to estimate carbon degradation rates along a natural density gradient of the suspension-feeding bivalve, Austrovenus stutchburyi (A. stutchburyi), at Waitati Inlet (Blueskin Bay), Otago. In the first study (Chapter two), carbon degradation rates were compared in the austral winter versus the austral summer. In the second study (Chapter three), the effect of a low dose of macroalgal detritus addition (Ulva spp.) was investigated to understand the effects of organic matter enrichment on bivalve-mediated carbon dynamics. In each study, three treatments were used on each ROMA plate (unmeshed: all fauna, 400 μm meshed: meiofauna and microbes, and 40 μm meshed: only microbes) to account for carbon degradation mediated by the different benthic components. Sediment characteristics and biological community data were collected along the bivalve density gradient to consider co variability of abiotic and biotic variables with organic matter processing rates. The research revealed complex and context-dependent interactions between A. stutchburyi density and carbon degradation rates. Higher average carbon degradation rates were observed in the summer across all treatments. In winter, carbon degradation rates were positively correlated with A. stutchburyi density (abundance) in surface sediments and negatively correlated in deeper sediments. In plots containing Ulva spp. detritus, A. stutchburyi density (biomass) positively correlated to carbon degradation rates in both surface and deeper sediment, with larger bivalves explaining more variance than smaller conspecifics. A. stutchburyi density (biomass) positively correlated with total macrofaunal abundance in plots containing Ulva spp. detritus. Further analyses found that large A. stutchburyi explained 28% of the variation in macrofaunal functional trait community composition, highlighting the indirect influence of large bivalves on ecosystem functions. The present thesis concludes that A. stutchburyi density had significant effects on rates of carbon processing in sediments, with seasonality and macroalgal detritus input having an effect on the relative rate of carbon decay, although the direction of response is context-dependent and varies with depth. Potential mechanisms for the contrasting relationships between A. stutchburyi density and carbon degradation rates at depth include negative effects such as spatial competition, grazing pressure, resource competition, and burrowing effects, and positive effects such as bivalve bioturbation and excretion activity stimulating benthic remineralization rates. The findings highlight the complex and context-dependent effects of bivalve beds on benthic ecosystem function dynamics, as well as the importance of considering bivalve size distribution and depth dependent carbon degradation processes. The research has important implications for management and monitoring in the context of bivalve overharvesting and restoration practices and the accelerating effects of eutrophication in estuarine ecosystems.