Characterizing Nitrogen Dynamics, Retention and Transport in a Tropical Rainforest Stream Using an in situ super(15)N Addition

This study was part of the Lotic Intersite Nitrogen eXperiment (LINX); a series of identical super(15)NH sub(4) tracer additions to streams throughout North America. super(15)NH sub(4)Cl was added at tracer levels to a Puerto Rican stream for 42 days. Throughout the addition, and for several weeks afterwards, samples were collected to determine the uptake, retention and transformation pathways of nitrogen in the stream. Ammonium uptake was very rapid. Nitrification was immediate, and was a very significant transformation pathway, accounting for over 50% of total NH sub(4) uptake. The large fraction of NH sub(4) uptake accounted for by nitrification (a process that provides energy to the microbes involved) suggests that energy limitation of net primary production, rather than N limitation, drives N dynamics in this stream. There was a slightly increased super(15)N label in dissolved organic nitrogen (DON) the day after the super(15)NH sub(4) addition was stopped. This DO super(15)N was < 0.02% of DON concentration in the stream water at the time, suggesting that nearly all of the DON found in-stream is allochthonous, or that in-stream DON production is very slow. Leptophlebiidae and Atya appear to be selectively feeding or selectively assimilating a very highly labelled fraction of the epilithon, as the label found in the consumers became much higher than the label found in the food source. A large spate (>20-fold increase in discharge) surprisingly removed only 37% of in-stream fine benthic organic matter (FBOM), leaves and epilithon. The fraction that was washed out travelled downstream a long distance (>220 m) or was washed onto the stream banks. While uptake of super(15)NH sub(4) was very rapid, retention was low. Quebrada Bisley retained only 17.9% of the added super(15)N after 42 days of super(15)N addition. Most of this was in FBOM and epilithon. Turnover rates for these pools were about three weeks. The short turnover times of the primary retention pools suggest that long-term retention (>1 month) is minimal, and is probably the result of N incorporation into shrimp biomass, which accounted for < 1% of the added super(15)N.