Influence of Austrovenus stutchburyi on the recovery and restoration of intertidal estuaries in Aotearoa New Zealand

Estuaries worldwide are experiencing accelerating degradation due to catchment-derived stressors such as increased nutrient and fine sediment inputs, leading to declines in biodiversity and disruption of key ecosystem functions. As a result, shellfish-based restoration has emerged as a promising strategy to mitigate eutrophication symptoms, enhance biogeochemical processing, and rebuild functionally important macrofaunal communities. In Aotearoa New Zealand estuaries, the suspension-feeding bivalve Austrovenus stutchburyi is a key candidate for such interventions due to its roles in sediment biogeochemistry, nutrient regeneration, and ecosystem engineering.

To evaluate how Austrovenus influences ecosystem functioning, we experimentally manipulated Austrovenus biomass in Jacob’s River estuary (Southland) on two sites differing in ambient abundance and mud content to assess how changes in population size affect macrofaunal community structure and nutrient cycling. These experiments demonstrated that responses were strongly context dependent: the greatest shifts in macrofaunal functional composition and nutrient regeneration occurred where the proportional increase in Austrovenus biomass was largest, and where sediment mud content and background community functional diversity modulated system behaviour.

In contrast, the study in Waitati Inlet (Otago) focused on the role of naturally occurring Austrovenus populations in driving alkalinity generation. Direct measurements of CaCO₃ dissolution (via δ₁₃C_DIC) and denitrification (via N₂ fluxes), together with estimated CaCO₃ production from calcification, revealed that Austrovenus enhances alkalinity production through bioturbation and organic matter processing, with higher rates in areas supporting greater natural densities. However, when metabolic alkalinity gains were compared against alkalinity consumption through calcification, Austrovenus beds were found to be small and variable net sources of alkalinity.

Together, these studies show that Austrovenus can improve ecosystem functioning and support resilient macrofaunal communities, but that outcomes depend on sediment characteristics, background community structure, and the magnitude of catchment-derived stressors. Integrating knowledge of bivalve-driven nutrient and carbon cycling with catchment management is therefore essential for designing effective and context-sensitive estuarine restoration strategies in New Zealand and beyond.