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

Occurrence, composition and biolability of total dissolved nitrogen (TDN, i ncluding dissolved inorganic and organic N ([DIN and DON]) were examined in May and July 1996 in the northern (Koiguste transect) and southern part (S aulkrasti transect) of the Gulf of Riga. In the Saulkrasti transect, the Da ugava 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 ope n gulf. DON made up 80 to 90 % of the TDN, but on May 16 a nitrate-rich riv er plume lowered the proportion of DON to 65 %. Riverine DIN and DON stimul ated the biological activity along the transect in May, but apparently not in July. In the Koiguste transect, concentrations of TDN were 0.5-fold lowe r than at Saulkrasti. In May, input of terrestrial nutrients to the inner p art of the transect probably increased the biological production. In both t ransects, the labile DON fraction, defined as bacterial DON degradation ove r a week, was estimated at 4 to 29 %, with an average of 13 %. Dissolved co mbined amino acids (DCAA) made up 10 to 30 % of the DON. A high DON labilit y coincided with a large proportion of DCAA, relative to TDN, or a high alg al biomass. The L/D ratio of especially dissolved combined glutamic acid ap peared to be a potential indicator of DON lability. DCAA, as well as dissol ved 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 u rea. The effect of solar radiation on DON biolability was tested, but no co nsistent evidence for a Light effect on transformation or bacterial utiliza tion of recalcitrant DON was found. Our results show that there is an activ e microbial cycling of DON in the gulf, driven in part by input of N from t he 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 la bile DOM, as compared to a ratio of 4 in May. The microbial N dynamics in t he Gulf of Riga are comparable to low-eutrophic estuaries and do not indica te direct eutrophication effects.