EPA Science Advisory Board
Effects of natural light on nitrogen dynamics.
Verh Internat Verein Limnol 27:64-73
Subgroup 2: Characterization of Nutrient Fate-Transport-Sources
Abstract: Isotope dilution experiments with $^15 NH_4^+$ were conducted in Lake Maracaibo, Venezuela, to examine potential N turnover rates and light effects and to examine the hypothesis that nutrient dynamics are biologically driven in this tropical, hypereutrophic lake. Ammonium and nitrate concentrations were both $<1 \muM$ as compared to particulate N concentrations of 9-29 $\muM$ N. Chlorophyll (Chl) levels ranged from 2.5 to 22 $\mug liter^-1$. Numbers and biomass of bacteria ranged from $1.0 to 9.1 \times 10^6 cells ml^-1$ and 45 to 138 $\mug C liter^-1$ and those of heterotrophic nanoflagellates (HNAN) ranged from 0.5 to $3.5 \times 10^3 cells ml^-1 and 2.3 to 17.5 \mug C liter^-1$, respectively. Higest Chl concentration and microbial abundance occurred in a regio affected by sewage discharge from the city of Maracaibo. Potential ammonium uptake rates in near-surface waters ranged from about 1 $\muM h^-1 to 8 \muM h^-1$. Chl-specific uptake rates were highest in central regions that were dominated by chroococcoid cyanobacteria. Ammonium regeneration rates ranged from near detection to 2 $\muM h^-1$ and correlated significantly with the ratio of HNAN to bacterial biomass, likely reflecting the degree of bacterivory. The high ratio of potential turnover rates to ambient ammonium concentrations suggests that internal recycling is a major nutreitn supply process in the lake. Incubation bottle characteristics (e.g., light intensity, spectral quality, or possibly headspqce differences) apparently affected potential uptake rates, which were relatively low in polystyrene bottles and in quartz tubes. Uptake rates were lower in screened polystyrene bottles than in screened polypropylene syringes, with different spectral characteristics, even though total light attenuation was similar (45% vs. 53%).