Interactive effects of community structure and environmental conditions on primary production in recirculating mesocosms

Ecosystem processes are the product of the expression of species functional traits and environmental gradients influence how species traits are expressed, interactions between species, and the strength of those interactions.  We examined the effects of a diverse guild of benthic filter feeders (unionid mussels) on primary production in recirculating mesocosms.  These bivalves link benthic and pelagic compartments by converting suspended material into excreted dissolved nutrients; excretion rates vary among species according to both environmental context (primarily temperature) and community structure (interactions with other species).  Our design included 34 species treatments manipulating richness and density across three sets of environmental conditions (spring, summer and fall).  Ammonia contributed to the water column by bivalves was highest in treatments with the greatest mussel densities.  Chlorophyll in the water column and on the sediment was higher in higher density treatments, and increased over time, because of a nitrogen fertilization effect.  Higher nutrient and chlorophyll levels were due primarily to novel species effects from a strong competitor that had higher ammonia excretion rates and decreased the condition of other species.