Fast growth and turnover of chironomid assemblages in response to stream phosphorus levels in a tropical lowland landscape

We studied the response of a dominant primary consumer, larval Chironomidae (Diptera), to phosphorus (via a microbial response) across eight detritus-based streams spanning a gradient in soluble reactive phosphorus (range 0.2-8.6 (mu)mol L^-1) in a tropical landscape in Central America. We predicted that phosphorus would enhance food resources for larval Chironomidae and positively affect their production. A low-phosphorus stream was also experimentally enriched to isolate effects of phosphorus from other solutes present. We found considerable variation in daily growth rates (range 0.09-0.57 mg mg^-1 d^-1) and annual biomass turnover rates (range 66-126 mg mg^-1 yr^-1) among streams, both positively related to phosphorus levels. Larval biomass was replaced every 5 d under low-phosphorus and every 3.5 d under high-phosphorus conditions (including the experimentally phosphorus-enriched stream). Annual secondary production (range 793-9,346 mg m^-2 yr-1) was variable among streams and negatively related to benthic organic matter, potentially because of low dissolved oxygen. The increase in larval biomass turnover along the phosphorus gradient indicates that a gradient of increasing energy flow exists from detrital-based resources (e.g., fungi) to primary consumers. Larval chironomids matured more rapidly under high phosphorus conditions; this strategy results in (1) similar larval biomass among streams and (2) a potential gradient of increasing energy flow from primary consumers to upper trophic levels as phosphorus increases. Nutrientenhanced turnover rate of primary consumers can occur without increases in secondary production in stream ecosystems.