ABSTRACT:

The functioning of ecosystems results from the interplay between living organisms and their surroundings. Therefore, our ability to predict ecosystem functioning depends on our understanding of interactions between dynamic biotic and abiotic variables. Biodiversity can be influenced by the changes in the environment and can also influence ecosystem functioning, but our understanding of how environmental changes influence biodiversity-ecosystem functioning relationships remains limited. In coastal marine systems, water flow is a crucial determinant of resource delivery for sessile suspension-feeding animals and can potentially influence secondary production and the rate at which suspended materials are removed from water, determining water clarity, light penetration, and other key abiotic variables. We experimentally investigated how the presence and absence of water flow influenced phytoplankton consumption by suspension-feeder communities of varying species richness and composition. Consumer species richness enhanced filtration to the same degree in both flow conditions, despite generally decreased filtration in flow and considerable variation in the response to flow by consumers in monoculture. Species pairs filtered more than expected from monoculture performance, and species traits helped explain variance in these trends. However, there was no evidence of prey size selectivity by any consumer species, ruling this out as a mechanism underlying diversity effects. Our finding that the shape of the relationship between biodiversity and water filtration was identical in both flow conditions suggests that complementarity in this system is robust to at least one type of environmental variation.