Detritivorous Fish Increase N-fixation in a Tropical Stream.

In nitrogen-(N) limited ecosystems, N-fixing cyanobacteria are an important source of primary production that is independent of N limitation. In an N-limited Venezuelan piedmont stream, we measured N-fixation in the presence and absence of the migratory benthic feeding fish, Prochilodus mariae. Prochilodus clear the sediment from the substratum when stream discharge is low, which may alleviate light limitation for attached cyanobacteria and increase N-fixation. We split a 230-m reach of stream longitudinally, where one side allowed movement of Prochilodus in and out of the reach, and the other side excluded the fish. The acetylene reduction method was used to measure N-fixation of the stream substrate. In the presence of Prochilodus, N-fixation was nearly 3 times higher than in the absence of this benthic feeding fish (9.6 mg of N/m2/day versus 3.4 mg of N/m2/day). This increase in N corresponds to one fourth of the daily ammonium uptake found in the split stream N budget. N-fixation is a substantial contributor to overall N cycling in this tropical stream, and Prochilodus facilitates the input of new N, potentially increasing the productivity of the river.