Dissolved organic nitrogen dynamics in coastal ecosystems

Abstract

Eutrophication threatens many coastal ecosystems throughout the world. Despite wastewater treatment, which led to dissolved inorganic nitrogen (DIN) reductions in some regions, productivity has not decreased as expected. This is often attributed to internal loading and ef?cient recycling of nitrogen, shifting the focus to dissolved organic nitrogen (DON). However, DON dynamics is still little understood. This study addresses some fundamental questions on DON dynamics in coastal systems. A spatio-temporal analysis of time series of DON measurements in the Dutch section of the North Sea revealed strong spatial gradients with high seasonally varying DON concentrations in the coastal zone, and low erratically changing DON concentrations with weak seasonality in the open sea. This erratic character of offshore DON concentrations suggests a close coupling between influxes and effluxes from the DON pool, attributable to DIN scarcity. A wavelet-coherence analysis also revealed a change in the seasonality around 1999-2000 in an offshore station, that coincided with increased DIN depletion. To examine this hypothesized quick DON utilization, several experiments were performed in a coastal bay dominated by macrophytes. N and C uptake from individual inorganic and organic compounds and from complex DON pools by various biotic compartments (macrophytes, epiphytes, phytoplankton, microphytobenthos, and bacteria) was investigated using dual stable isotope labeling (13C and 15N) and biomarker measurements (D-alanine and phospholipid-derived fatty acids: PLFA). The two seagrasses and one macroalga under investigation exhibited DON uptake capabilities. All primary producers took up nitrogen from inorganic and organic substrates within hours, both in the watercolumn and the sediment, supporting our hypothesized quick DON utilization. Microalgae were an important N sink on these short time scales (up to 24 hours). In the sediment ammonium and glycine N uptake rates were highest for all macrophytes. N uptake from individual amino acids decreased with increasing chemical complexity and C/N ratio in all biotic groups. Algae-derived complex DON in the watercolumn was preferred over bacteria-derived DON, illustrating the influence of DON origin and composition on bioavailability. Watercolumn DIC was predominantly used by the seagrasses, whereas the microbial community dominated complex DOC uptake. Carbon and nitrogen retentions in the sediment were related through the substrate C/N ratio, but carbon and nitrogen uptakes were generally not related. PLFA markers specific for particular groups of algae or bacteria showed different C uptake characteristics per group, indicating that they either preferred different substrates or received DIC after remineralization. This research has illustrated utilization of a highly dynamic DON pool by primary producers in a matter of hours. Carbon and nitrogen dynamics are largely uncoupled. This results in different behaviour of carbon relative to nitrogen under conditions of low DIN availability. The reported inconsistency about DON seasonality in literature are likely related to a balance between supply and demand of nitrogen. These findings support the role of DON as nutrient, particularly under low or decreasing DIN concentrations, which has important consequences for limitation of primary productivity and remediation of eutrophication effects.