Nitrogen cycling rates and light effects in tropical Lake Maracaibo, Venezuela

Isotope dilution experiments with 15NH4/+ 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 μaM as compared to particulate N concentrations of 9-29 μM N. Chlorophyll (Chl) levels ranged from 2.5 to 22 μg liter-1. Numbers and biomass of bacteria ranged from 1.0 to 9.1 X 106 cells ml-1 and 45 to 138 μg C liter-1 and those of heterotrophic nanoflagellates (HNAN) ranged from 0.5 to 3.5 X 103 cells ml-1 and 2.3 to 17.5 μg C liter-1, respectively. Highest Chl concentration and microbial abundance occurred in a region affected by sewage discharge from the city of Maracaibo. Potential ammonium uptake rates in near-surface waters ranged from about 1 μM h-1 to 8 μM 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 μM 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 nutrient supply process in the lake. Incubation bottle characteristics (e.g., light intensity, spectral quality, or possibly headspace 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%).