Microplanktonic regeneration of ammonium and dissolved organic nitrogen in the upwelling area of the NW of Spain: Relationships with dissolved organic carbon production and phytoplankton size-structure

Abstract

Ammonium regeneration and dissolved organic nitrogen (DON) release were studied experimentally in the euphoric zone of shelf and oceanic waters of NW Spain in relation to coastal upwelling dynamics and the size-structure of phytoplankton communities. Incubations of plankton labelled with [15N] ammonium were made during four cruises, two of which also included size-fractionated determinations of chlorophyll a and primary production, and experimental determinations of production rates of dissolved organic carbon (DOC) using 14C. Inorganic nitrogen concentrations were mainly related to nitrate enrichment by upwelling pulses, while ammonium concentrations were generally low in all situations. Ammonium did not accumulate in the study area, suggesting a daily time scale coupling between regeneration and uptake. In contrast, DON largely exceeded inorganic nitrogen in all situations and generally increased from spring to autumn. Ammonium regeneration was positively correlated with DON release and both rates showed the largest variation in summer, with minimum values during active upwelling and maximum values when upwelling relaxed. Comparison of DON stocks and rates in different shelf areas suggests that DON release near the coast during summer was more persistent in the water than DON release in off-shelf and oceanic areas. The carbon:nitrogen ratio of DOC and DON release rates was highly variable, revealing a large excess of DOC compared with DON. This excess can be attributed to either an underestimate of total D0N release (as only release from ammonium was measured) or to an enhanced production of carbon-rich organic substances by diatoms in coastal areas. By considering a broad range of trophic situations, this study reveals a fundamental difference between short term release of DOC and DON by plankton. Physiological processes (such as carbohydrate exudation by diatoms) seem to be the cause of large DOC excess, whereas trophic processes (such as grazing) are more likely the cause of enhanced DON release.