Catchment modifications influence the composition of basal organic matter supporting suspension-feeding bivalves

Abstract

Marine primary production is sensitive to multiple environmental stressors from land-based and maritime anthropogenic activities. Stressors may alter the distribution, abundance and composition of primary producers and thereby directly influence the fluxes of organic matter into marine food webs. To investigate these effects, we measured the relative contributions of basal organic matter derived from macroalgae and phytoplankton to five suspension-feeding bivalve species (Pecten novaezelandiae, Atrina zelandica, Tucetona laticostata, Perna canaliculus and Mytilus galloprovincialis) along a gradient in catchment modification and marine farming activity in the Marlborough Sounds, Tasman Bay and Fiordland, on the South Island of New Zealand. Stable isotope analyses (δ15N and δ13C) were conducted on samples of basal organic matter and muscle tissue of bivalves collected from multiple sites within each region during 2018 and 2019. Mass balance mixing models revealed significant differences in the composition of basal organic matter supporting individual bivalve species among sites and regions. The proportion of macroalgal-derived organic matter supporting the subtidal bivalve community was negatively associated with the area of modified land use in the surrounding catchments. These results support strong linkages between land use in catchments and the composition of basal organic matter sources supporting bivalve communities. Values of δ15N among four of the bivalve species were consistently higher at sites associated with salmon farming, indicating inputs of allochthonous nitrogen from the farms. These findings highlight the intimate connection between anthropogenic stressors and the trophic dynamics of shellfish in the coastal zone.