University of California Water Resources Center

Estuaries are highly productive ecosystems that support many endangered and commercially important species. In most estuaries, nitrogen limits primary productivity. However, if present in excess, nitrogen leads to eutrophic conditions, which adversely affects water and habitat quality. Southern California’s estuaries have highly developed watersheds resulting in high loads of nitrogen from anthropogenic sources and eutrophication. Our overall goal was to understand the role of two biogeochemical processes, N-fixation and denitrification, that affect processing of N in estuaries and to investigate their response to increased N loads from the watershed. Nitrogen fixation transforms elemental nitrogen (N2) into ammonium ions (NH4+) that can be used by primary producers, and therefore is a “new” source of nitrogen. Denitrification transforms nitrate (NO3-) into atmospheric nitrous oxide (N2O) or nitrogen gas (N2), and is therefore a loss of nitrogen from aquatic ecosystems. As these processes either contribute or remove nitrogen from the system, they have the potential to affect water quality and ecosystem health. However, little is known about nutrient dynamics in southern California estuarine environments.

We measured spatial and temporal variability of nitrogen fixation and denitrification and investigated relationships with abiotic/biotic factors in an eutrophic southern California estuary through field surveys and experiments in Upper Newport Bay Ecological Reserve, a large estuary in Orange County, California. Field surveys were conducted in two seasons (wet and dry) over two years and included sampling in 10 sites. Nitrogen fixation and denitrification rates were measured, as well as water and sediment nitrogen and Phosphorus, and sediment grain size, organic content, and chlorophyll a. Rates of N-fixation varied greatly spatially and temporally, with spatial variability more prevalent than seasonality. N-fixation correlated negatively with salinity and positively with nutrient (NO3) concentration, and did not relate to any sediment characteristics, suggesting that short-term responses to rain events were driving patterns. N-fixation was not related to chlorophyll a, suggesting that the majority of fixation was by bacteria rather than blue-green algae. Overall, denitrification was low or immeasurable across all sites and seasons.

Three experiments identified biotic and abiotic factors affecting these rate processes. In a common garden field experiment, N-fixation found to be driven by the characteristic of the experimental common site rather than the transplanted sediment, also suggesting that short-term water column factors are controlling fixation. In a laboratory mesocosm experiment, we determined that water column nitrate greatly accelerated denitrification, but inhibited rates of N-fixation. However, even the enhanced rates of denitrification in our experiment remained far lower than the increases in nitrate supply typical of our highly eutrophic estuaries. Thus, denitrification, at best, removes a small portion of nitrogen supplied from the watershed. The third experiment found no effects of the prolific macroalgal mats that dominate upper Newport Bay on sediment nitrogen fixation in the field.

Our research has contributed to basic understanding of nitrogen cycling in these unique, understudied, and heavily impacted ecosystems. Unfortunately, we did not find that denitrification is an effective tool to ameliorate accelerating loads of N to our coastal ecosystems; rather reduction in loads appears to be the only viable solution.