Differences in phosphorus demand among detritivorous chironomid larvae reflect intraspecific adaptations to differences in food resource stoichiometry across lowland tropical streams

Abstract

We tested whether assemblages of detritivorous chironomid larvae (Chironomidae: Diptera) varied in phosphorus (P) demand based on local nutrient conditions using a series of streams in lowland Costa Rica that exhibit a natural range in ambient dissolved P and detrital P due to inputs of solute‐rich groundwater. Chironomids collected from three high‐P streams had growth rates and P‐excretion rates that were similar to, or lower than, those of chironomids collected from four low‐P streams. Chironomids from a naturally low‐P stream that was experimentally amended with P over 8 yr showed an increase in P‐excretion rates but not growth rates, indicating an inability of these chironomids to use additional dietary P. Chironomids from a high‐P stream showed greater evidence of P limitation (lower growth rates, P‐excretion rates, and ribonucleic acid content) when fed low‐P detritus compared to chironomids from a low‐P stream. Our findings support the hypothesis that chironomid assemblages are adapted to local food quality in this heterogeneous landscape, thereby circumventing P limitation. Genetic analysis indicates that chironomid assemblages across the study streams are similar in species composition, suggesting that differences in P demand may be due to microevolution. The lower P demand among chironomids from low‐P streams appears to limit their ability to respond to nutrient‐enriched food, effectively stabilizing the food web in response to changes in nutrient availability.