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

Abstract

Isotope dilution experiments with IsN& 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, hypcreutrophic lake. Ammonium and nitrate concentrations were both Cl PM as compared to particulate N concentrations of 9-29 PM N. Chlorophyll (Chl) levels ranged from 2.5 to 22 pg liter-'. Numbers and biomass of bacteria ranged from 1.0 to 9.1 X lo6 cells ml-' and 45 to 138 pg C liter-' and those of heterotrophic nanoflagellates (HNAN) ranged from 0.5 to 3.5 X 10" cells ml-' and 2.3 to 17.5 pg C liter-l, 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 PM h-l to 8 PM h--l. Chlspecific uptake rates were highest in central regions that were dominated by chroococcoid cyanobacteria. Ammonium regeneration rates ranged from near detection to 2 PM h-I 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%).

Photochemical production of ammonium in the oligotrophic Cyprus Gyre (Eastern Mediterranean)

“…We found no evidence for light or dark NH + 4 incorporation into DON during or after our irradiations, in contrast to the results of some other studies (Thorn and Mikita, 1992;Gardner et al, 1998;Koopmans and Bronk, 2002). Net photoconsumption of NH + 4 has on occasion, been identified in both fresh and saline waters (Gardner et al, 1998;Koopmans and Bronk, 2002), while for other freshwaters NH + 4 concentration changes remained undetectable (Jørgensen et al, 1998;Bertilsson et al, 1999;Wiegner and Seitzinger, 2001;Koopmans and Bronk, 2002).…”

“…For example, Wang et al (2000) report pH control of photoammonification in river and estuarine waters, and Gao and Zepp (1998) observed that the presence of dissolved iron (Fe) enhanced photoammonification rates in river water. Net photochemical uptake of NH + 4 by DOM has also been observed both in C18 extracted dissolved organic matter (DOM) of marine- (Kieber et al, 1997) and in filtered brackish- (Gardner et al, 1998) and ground-waters (Koopmans and Bronk, 2002). Additional controls of photoammonification suggested in published work include intrinsic DOM properties, ambient oxygen levels, and prior radiation exposure (Koopmans and Bronk, 2002;Buffam and McGlathery, 2003).…”

“…These observations seem to suggest that the potential impact of photoammonification is highest in shelf waters and marginal seas. However, the role of photochemistry in open ocean nutrient budgets has received little attention, even though NH estuarine and coastal seawater (Bushaw et al, 1996;Gardner et al, 1998;Bushaw-Newton and Moran, 1999;Morell and Corredor, 2001;Koopmans and Bronk, 2002;Buffam and McGlathery, 2003), while rates of up to 4700 nmol L −1 h −1 (corrected for dilution during ultrafiltration), were observed in freshwaters (Bushaw et al, 1996;Gao and Zepp, 1998;Wang et al, 2000;Koopmans and Bronk, 2002;Buffam and McGlathery, 2003;Vähätalo et al, 2003). In addition to ammonium, photoproduction of nitrite (NO − 2 ) and low molecular weight organic nitrogen compounds such as dissolved primary amines and dissolved free amino acids were also observed (Spokes and Liss, 1996;Bushaw-Newton and Moran 1999;Kieber et al, 1999;Tarr et al, 2001;Wiegner and Seitzinger, 2001;Koopmans and Bronk, 2002;Buffam and McGlathery, 2003).…”

Nitrification and ammonium dynamics in Taihu Lake, China: seasonal competition for ammonium between nitrifiers and cyanobacteria

“…Due to high biological demand and fast turnover rates, NH + 4 often does not accumulate in the water column, resulting in low in situ concentrations. Ammonium regeneration is especially important to phytoplankton productivity in eutrophic systems (Gardner et al, 1998(Gardner et al, , 2017McCarthy et al, 2013). For example, water column regeneration was up to 6 times higher than sediment regeneration in Taihu Lake, China (McCarthy et al, 2007;.…”

DON as a source of bioavailable nitrogen for phytoplankton

“…Of these compounds, NH + 4 generally has the greatest observed rates of production (Bushaw et al, 1996;Gao and Zepp, 1998). Photochemical release has been observed in a number of locations including an estuary in Georgia (Bushaw et al, 1996;Gao and Zepp, 1998) and a humic-rich lake in Venezuela (Gardner et al, 1998). Photodegradation of DOM increases the availability of more labile N sources to phytoplankton and bacteria.…”