Date of Award

Winter 2007

Project Type

Thesis

Program or Major

Natural Resources: Water Resources

Degree Name

Master of Science

Abstract

An intensive study that was part of the Lotic Intersite Nitrogen eXperiment II (LINX II) project was conducted to determine nitrogen transformations in nine low-order streams with contrasting land use. Short term (24-hour) additions of K15NO3 and NaBr were performed on a gradient of anthropogenically impacted streams in Puerto Rico. Nitrate uptake was determined from longitudinal decline in 15NO3 and denitrification rates were determined from the longitudinal pattern of 15N 2 and 15N2O. Several physical, chemical, and biological variables were also measured to determine controlling factors. I performed these experiments to investigate: (1) the mechanisms for NO3 uptake and denitrification in tropical streams, (2) how tropical streams differ from temperate streams in their rates and controls on NO3 uptake and denitrification and (3) the functional responses of these streams as NO3 concentrations increase due to anthropogenic impacts.

Background nitrate concentrations ranged from 105 to 997 mug N L -1 and stream nitrate uptake length varied from 315 to 8480 m (median of 1200 m). Uptake length was mainly predicted by specific discharge (L s -1 m-1) and ecosystem respiration rate (multiple regression analysis; r2 = 0.71, p < 0.05). The other nitrate uptake parameters (Vf, cm/s and U, mug N m-2 s -1) were primarily predicted by gross primary production and respiration, indicating strong biological control on nitrate uptake.

Denitrification rates ranged from 0.01 to 2.20 mug N m-2 s-1 (median = 0.25) and the strongest predictors were respiration and fine benthic organic matter (r2 = 0.89, p < 0.05). Denitrification accounted for 1 to 97% of nitrate uptake with 5 of 9 streams having 35% or more of nitrate uptake via denitrification showing that denitrification is a substantial sink for nitrate in tropical streams.

In comparison to rates in other regions, nitrate uptake was low and denitrification was high. Whole stream nitrate uptake more closely followed Michealis-Menten kinetics than in other regions, indicating that high N streams are approaching nitrate saturation. The efficiency with which these streams assimilate and remove nitrate (through denitrification) generally declines with increasing nitrate concentrations and loading.

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