Variable effect of a large suspension-feeding bivalve on infauna: experimenting in a complex system

In soft-sediment habitats there are many examples bf species that modify th eir habitat and thus can be expected to have an important influence on macr obenthic community structure. The large, suspension-feeding pinnid bivalve Atrina zelandica adds complexity to soft-sediment habitats by protruding in to the water column and altering boundary-flow conditions and by providing predation refuges and substrates for epifaunal settlement. To investigate e ffects of A. zelandica density on macrobenthic community composition, we co nducted a density manipulation experiment in 4 different habitat types in a nd around Mahurangi Estuary, New Zealand. Our experiment incorporated a com paratively large spatial and temporal scale: each habitat was separated by at least 1.75 km, and was sampled 3 times over 16 mo. Based on previous wor k, we predicted that macrofaunal community responses would differ between s ites and would be stronger at sandy sites than muddy sites, and that variab ility in site hydrodynamic and sediment characteristics would help explain differences in benthic community responses to the density manipulation. Whi le these predictions were supported, there was considerable temporal variat ion in the response. We also made predictions of the response of different aggregate macrofaunal groups to the A. zelandica manipulation (i.e. total n umbers of individuals and taxa, suspension feeders. deposit feeders, top 2 cm dwellers, and mobile, short and long-lived species). Whether these predi ctions were supported varied spatially as well as temporally. To be able to generalise results, larger scale experiments. conducted at more than 1 sit e and at more than 1 time, are generally considered preferable. Although ou r A. zelandica manipulation experiment has these attributes, the results ha ve demonstrated that the influence of this large suspension feeder on the a ssociated macrofaunal community is not simple (except perhaps in sandy, rel atively non-tidal environments), and illustrates our limited success in 're ducing' the complexity of this system using a field experiment. However, we were able to demonstrate that interactions between A. zelandica, site hydr odynamic conditions and sediment characteristics were all important in infl uencing macrofauna, rather than there being a simple A. zelandica density-m acro-fauna relationship. Thus, where multi-species interactions, indirect e ffects, non-linear biotic/abiotic interactions and threshold effects play a n important role, specific experiments may not always lead to generalisable results, simply because the system is too complex.