Does the Size Structure of Venerid Clam Populations Affect Ecosystem Functions on Intertidal Sandflats?

Abstract

Estuaries are hotspots for biogeochemical cycling, with bivalves integral to many ecosystem functions. Anthropogenic activities often cause reductions in biomass and abundance of key bivalve species in estuaries worldwide. Large bivalves are particularly vulnerable due to their slow growth, low replacement rates and propensity for being overharvested; however, there is a paucity of studies on how declines in bivalve size and density affect ecosystem functions. An in situ manipulative experiment was conducted on a New Zealand intertidal sandflat in late summer (March to May 2017) to investigate changes in ecosystem functions (sediment nutrient regeneration, primary production, community metabolism and microbial activity) across a biomass gradient (0–4 kg wet weight m−2) of small (shell length (SL) 30 mm) venerid clams (Austrovenus stutchburyi). Bird predation reduced the biomass of small clams, so ecosystem functions were normalised to per kilogramme of wet weight. Small clams significantly increased gross primary production by ~ 3× and net primary production by ~ 7× compared with large clams. Small clams also doubled activity rates of microbial enzymes associated with nutrient cycling and organic matter breakdown (leucine aminopeptidase and alkaline phosphatase). By contrast, nitrate/nitrite flux was significantly greater with large clams. Macrofaunal species diversity and mud content also influenced benthic nutrient cycling, possibly due to increased sediment reworking that alters solute flux rates. Results demonstrate how different size classes of venerid clams influence complementary ecosystem functions. Accordingly, a skewed size class distribution of bivalves will reduce the productivity and functioning of intertidal sandflats.