Abstract

The degree to which biotic and abiotic factors affect the transport of information, nutrients, and matter is a central organizing feature of ecosystem structure and function. I quantified how variation in the behavioral activities of benthic-feeding fishes affected the timing and transport of particulate organic and inorganic matter and nutrient in a neotropical river. I measured diet patterns of nutrient transport, fish feeding, and tracer-particle transport distance during 1997-2002. I tested alternative hypotheses by quantifying diet patterns of stream discharge, algal sloughing, animal disturbance, and terrestrial inputs to the river. I experimentally tested how variation in fish behavior affected the diet periodicity of particle transport by comparing treatments with diurnal and nocturnal benthic-feeding and surface-feeding fishes. I performed a whole-stream manipulation of the migratory detritivorous fish, Prochilodus mariae, (Prochilodontidae), by dividing long sections of stream (110-256 m) down the center and selectively removing Prochilodus from one side and leaving the other side open to the entire fish assemblage. Combined with nitrogen stable isotope additions I used this large-scale experimental approach to measure the effects of removing Prochilodus, a harvested fish species throughout South America, on carbon flow, ecosystem metabolism, and nitrogen cycling.

There was a distinct diet periodicity of nutrient transport that was strongly linked to the activity of benthic-feeding fishes, particularly Prochilodus. Material transport was not explained by diet changes in discharge, sloughing of algae, wind-induced mixing, terrestrial inputs, or upstream disturbances by humans or other terrestrial animals. Removal of Prochilodus reduced the periodicity, organic content, and downstream transport, repealing that one fish species regulates most of the nutrient transport in the natural stream. Natural declines and experimental removal of Prochilodus decreased downstream transport of organic carbon and increased primary production and respiration. The nitrogen isotope addition revealed that Prochilodus altered the processing, storage, and downstream transport of nitrogen during the dry season, a time when hydrologic transport is reduced, and affected the cycling of nitrogen, a nutrient that limits primary productivity. Hence, besides its economic value, Prochilodus is a critical ecological component of South American rivers.