Occurrence and bacterial cycling of dissolved nitrogen in the Gulf of Riga, the Baltic Sea

Occurrence, composition and biolability of total dissolved nitrogen (TDN, including dissolved inorganic and organic N ([DIN and DON]) were examined in May and July 1996 in the northern (Koiguste transect) and southern part (Saulkrasti transect) of the Gulf of Riga. In the Saulkrasti transect, the Daugava River was a major source or TDN as indicated by concentrations of up to 60 mu M TDN in the river plume, compared to about 20 mu M TDN in the open gulf. DON made up 80 to 90 % of the TDN, but on May 16 a nitrate-rich river plume lowered the proportion of DON to 65 %. Riverine DIN and DON stimulated the biological activity along the transect in May, but apparently not in July. In the Koiguste transect, concentrations of TDN were 0.5-fold lower than at Saulkrasti. In May, input of terrestrial nutrients to the inner part of the transect probably increased the biological production. In both transects, the labile DON fraction, defined as bacterial DON degradation over a week, was estimated at 4 to 29 %, with an average of 13 %. Dissolved combined amino acids (DCAA) made up 10 to 30 % of the DON. A high DON lability coincided with a large proportion of DCAA, relative to TDN, or a high algal biomass. The L/D ratio of especially dissolved combined glutamic acid appeared to be a potential indicator of DON lability. DCAA, as well as dissolved free amino acids (DFAA), were dominant nutrient sources to the bacteria, on average sustaining 77 % (range 8 to 136 %) of the bacterial N demand. In addition to amino acids, the bacteria incorporated or released DIN and urea. The effect of solar radiation on DON biolability was tested, but no consistent evidence for a Light effect on transformation or bacterial utilization of recalcitrant DON was found. Our results show that there is an active microbial cycling of DON in the gulf, driven in part by input of N from the Daugava River. In summer, availability of assimilable N appears to limit. the microbial activity as indicated by an estimated C/N ratio of 43 of labile DOM, as compared to a ratio of 4 in May. The microbial N dynamics in the Gulf of Riga are comparable to low-eutrophic estuaries and do not indicate direct eutrophication effects.