Water Temperature, Not Dissolved Oxygen, Linked to Nitrite Accumulation in Estuaries

Nitrification, a two-step process carried out by two distinct guilds of prokaryotes, ammonia- and nitrite-oxidizers, is a key reaction linking the reduced and oxidized sides of the nitrogen cycle. The two steps are typically tightly coupled, with the primary intermediate product, nitrite, rarely accumulating in the environment. Transient spikes of nitrite accumulation in estuaries have been attributed to stratification and the development of hypoxia; however, regular, mid-summer peaks of nitrite (>10 mM) are seen in well-mixed, southeastern USA coastal waters. These spikes coincide with water column blooms of ammonia-oxidizing organisms (>10⁴-fold increases in abundance), suggesting: 1) that the two steps of nitrification are decoupled during these blooms; and 2) that nitrite accumulation may serve as an index of similar events in other systems. Here we analyze data from 272 stations in temperate estuaries and show a strong relationship between elevated water temperature (>20 °C) and nitrite accumulation. Dissolved oxygen concentration (DO) covaries with temperature in this data set, but the distribution of high-nitrite events suggests that DO does not exert primary control of nitrite accumulation. The two steps of nitrification appear to be decoupled at elevated temperatures as a consequence of differential growth responses of ammonia- and nitrite-oxidizing organisms. Increasing estuarine water temperatures thus have the potential to result in higher ammonia oxidation rates; in more frequent and more severe decoupling of nitrification; and to extend the phenomenon to more northerly environments. Potential consequences include enhanced loss of fixed N and altered ratios of dissolved inorganic nitrogen species that can affect phytoplankton species composition.