Effects of Combined Nitrogen on Baltic Sea Diazotrophic Cyanobacteria
(English)Manuscript (preprint) (Other academic)
We used planned variations in the discharge of combined nitrogen and the depth of outfall discharge from a sewage treatment plant (STP) to study the regulation of nitrogen fixation by filamentous cyanobacteria in Himmerfjärden Bay, northern Baltic Sea Proper. Using stable nitrogen isotopes, we found nitrogen fixation to be almost insensitive to combined nitrogen, with similar low δ15N-values found in cyanobacteria (mostly Aphanizomenon sp.) at all stations. This was in spite of significant gradients along the bay in both δ15N in seston (<10 µm), used as a proxy for δ15N in combined nitrogen, and total nitrogen. Only late in the productive season, when cyanobacterial biomass was declining or already low, did δ15N increase slightly in filaments, indicating some uptake of combined nitrogen. This coincided with an increase in the proportion of Anabaena spp. in the diazotrophic biomass and may indicate that Anabaena spp. has higher uptake of combined nitrogen than Aphanizomenon sp.. Overwintering, heterocyst-free filaments of Aphanizomenon sp. had as low δ15N as during maximum growth in summer, indicating no uptake of combined nitrogen from the water even in winter, when supplies were ample. These results show that almost all nitrogen used for growth by nitrogen-fixing cyanobacteria in the study area comes from nitrogen fixation, and very little from uptake of dissolved combined nitrogen. δ15N in seston had a similar seasonal signal at all stations, declining from spring to a minimum around week 33 followed by an increase, which lagged an increase in δ15N in diazotrophs by two weeks, presumably mirroring a reduced nitrogen fixation. During this increase, the abundance of Anabaena spp. increased at all stations.
Baltic Sea, cyanobacteria, nitrogen fixation, Aphanizomenon, eutrophication, sewage, waste water treatment, Nodularia, stable isotopes
Research subject Marine Ecology; Marine and Brackish Water Ecology
IdentifiersURN: urn:nbn:se:su:diva-90101OAI: oai:DiVA.org:su-90101DiVA: diva2:622791
ProjectsHimmerfjärden eutrophication study