Nitrogen excretion by mesozooplankton in a coastal upwelling area: Seasonal trends and implications for biological production

Abstract

Nitrogen excretion by zooplankton was studied in the coastal upwelling area off Concepción (36° S) in central/southern Chile. Experiments were performed using numerically dominant copepods of the mesozooplankton community during two contrasting periods (winter and spring 2009). The experimental setup was aimed at characterizing the organic and inorganic nitrogen excretion compounds, such as dissolved organic nitrogen (including urea), and ammonium. Moreover, the seasonal variability of excretion products was studied and analyzed as a function of microphytoplankton and nanophytoplankton abundance, and nutrient concentrations (NH 4 +, NO 3 −, NO 2 −, PO 4 −3, Si(OH) 4). We tested the hypothesis that ammonium supplied by mesozooplankton (represented by copepods) can support a significant fraction of the nitrogen demand of phytoplankton. To do so, excretion rates were determined and the budget of each nitrogen compound was assessed. Results showed that among the organic and inorganic nitrogen compounds excreted by copepods off central Chile, total DON (i.e., ureic and non-ureic components) may be the most important excretion component, contrasting with the view of ammonium as the main excretion compound of this group. We conclude that total (organic and inorganic) nitrogen excretion per group of dominant copepods in the continental shelf could have a significant impact on the nitrogen requirements of phytoplankton communities, potentially sustaining up to 26.7% of the phytoplankton ammonium demand.