This paper examines the effect of oxygen on nitrous oxide (N 2 O) concentrations in estuarine waters. N 2 O has been measured year-round in the Schelde estuary, a high-nitrogen, lowoxygen macrotidal system. N 2 O concentrations were above atmospheric equilibrium levels indicating that this estuary represents a source to the atmosphere. The distribution of N 2 O showed consistent and systematic relationships with distribution patterns of ammonium, oxygen, nitrite and nitrification activities. A controlled laboratory experiment with a natural bacterial community from the Schelde estuary revealed maximum N 2 O production to occur at oxygen concentrations of about 5 µM. This production was inhibited by acetylene, a nitrification inhibitor. Maximum N 2 O concentration in the field occurred at oxygen concentrations below 35 µM. The difference in the oxygen concentration that results in maximum N 2 O may have arisen because low-oxygen environments present in the estuary were destroyed by stirring in our laboratory experiment. It appears that low oxygen concentrations in estuarine water trigger enhanced N 2 O production if ammonium is present in sufficient amounts. This conclusion is further illustrated by data from the Thames, Loire and Gironde estuaries.
In two-stage continuous cultures, at bacterial concentrations, biovolumes, and growth rates similar to values found in Lake Vechten, ingestion rates of heterotrophic nanoflagellates (HNAN) increased from 2.3 bacteria HNAN-' h-' at a growth rate of 0.15 day-' to 9.2 bacteria HNAN-' h-' at a growth rate of 0.65 day-'. On a yeast extract medium with a C/N/P ratio of 100:15:1.2 (Redfield ratio), a mixed bacterial population showed a yield of 18% (C/C) and a specific carbon content of 211 fg of C Fm-3. The HNAN carbon content and yield were estimated at 127 fg of C ,um-3 and 47% (C/C). Although P was not growth limiting, HNAN accelerated the mineralization of PO4-P from dissolved organic matter by 600%. The major mechanism of P remineralization appeared to be direct consumption of bacteria by HNAN. N mineralization was performed mainly (70%) by bacteria but was increased 30% by HNAN. HNAN did not enhance the decomposition of the relatively mineral-rich dissolved organic matter. An accelerated decomposition of organic carbon by protozoa may be restricted to mineral-poor substrates and may be explained mainly by protozoan nutrient regeneration. Growth and grazing in the cultures were compared with methods for in situ estimates.
We studied seasonal variation in heterotrophic nanoflagellates (HNAN) and bacterial densities at different depths in the sediment of two freshwater littoral stations. Station 1 was in a reed bed of Phragmites australis; station 2 was outside the reed zone in open water. Benthic HNAN abundances ranged from 0.7 x lo3 to 167.9 x lo3 cells cm-3, with small cells (~5 pm) being the most abundant. Bacterial densities were high, ranging from 1.5 to 16.4 x lo9 cells cm-3 sediment. HNAN and bacterial densities showed significant time, depth, and station effects. At station 1, the bacterial standing stock increased during the course of the observation period, probably because of an increase in macrophyte biomass. At station 2, bacterial densities in the upper 0.5 cm of the sediment fluctuated little and without a clea:-seasonal pattern. In the top layer of both stations, HNAN densities were high in winter and lower in midsummer and early autumn. Density ratios between bacteria and HNAN were very high in this layer, ranging from 22.4 x lo3 to 2.0 x lo6 bacteria HNAN-I. Contrary to our expectations, these ratios suggest that the impact of benthic HNAN on the bacterial dynamics is minimal in this littoral zone. We hypothesize that throughout the year, a limited access to bacteria prevented HNAN from reaching high abundances. Seasonal HNAN density fluctuations probably were caused by epi-mciobcnthos grazing on HNAN in summer and early autumn.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.