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

Planktonic carbon and nitrogen cycling off northwest Spain: variations in production of particulate and dissolved organic pools

“…Such an increase was primarily caused by the enhanced production of phytoplankton. In this way, measurements of short-term release rates of both DOC (Teira et al 2001a, Morán et al 2002a,b, Teira et al 2003b) and DON (Varela et al 2003b) showed a close relationship with the production of particulate matter during upwelling conditions. However, the fraction of recently fixed C or N that is released as DOC or DON was not constant, and PER values from the low productivity winter and autumn period were higher than those from the upwelling period (Teira et al 2001a, Morán et al 2002a,b, Teira et al 2003b,Varela et al 2003b).…”

“…Thus, shortterm released organic matter in the study area derives mainly from phytoplankton, whatever the liberation processes implied. In contrast, a previous study of 4 selected oceanographic situations in coastal and oceanic waters off the southern shelf of Galicia found a low correlation between DOC and DON release, and the corresponding slope of the regression line was far from the Redfield value (Varela et al 2003b). Also, C:N ratios of DOC and DON stocks are often much higher than the Redfield ratio (Doval et al 1997, AlvarezSalgado et al 1999.…”

“…During active upwelling, particle sedimentation near the coast accounts for a small fraction of total carbon export (Bode et al 1998, Schmidt et al 2002). Nevertheless, dissolved organic matter accumulation near the coast during the productive upwelling season was reported by studies of DOC and DON concentrations (Doval et al 1997, Alvarez-Salgado et al 1999, Bode et al 2001, Varela et al 2003a) and short-term release rates (Teira et al 2001a, Varela et al 2003b. Such accumulations are caused by the relatively low bacterial consumption rates of DOC (Morán et al 2002a, Varela et al 2003a) during the productive upwelling season, which translates into a time lag between DOC release and consumption, as reported for other temperate marine ecosystems (e.g.…”

“…Short-term DOC release rates are high during phytoplankton blooms, as those caused by the upwelling between March and October (Teira et al 2001a, Morán et al 2002a,b, Teira et al 2003b, and the same occurs with DON release (Varela et al 2003b). Such release, along with relatively low bacterial uptake rates (Barbosa et al 2001, Morán et al 2002a, Valencia et al 2003, Varela et al 2003a, produce accumulations of DOM, which reach a maximum at the end of summer.…”

“…Moreover, if DOM is released by cell breaking, as through lysis (Agustí et al 1998) or zooplankton sloppy feeding (Hasegawa et al 2000b), it can be expected that its composition would be similar to that of the intact cells, with a C:N ratio close to the Redfield value (Redfield et al 1963). However, only few studies allowed for a comparison of DOC and DON release in natural plankton communities and the results showed either low C:N values because of DON accumulation (Bronk et al 1998) or a low correlation between both release rates and high variability in C:N ratios (Varela et al 2003b). Therefore, from these previous data it appears that DOC and DON release fluxes are generally uncoupled at both short and long time scales.…”