Blooms of nitrogen fixing cyanobacteria (NFC) occur in many freshwater and marine systems, including the Baltic Sea. By fixing dissolved nitrogen, they circumvent general summer nitrogen limitation, while also generating a supply of novel bioavailable nitrogen for non-diazotrophic primary producers and ultimately supporting secondary production. The thesis elucidates trophic links between primary consumers and NFC is essential for understanding role of these blooms for secondary production.

The results provided evidence that NFC contribute to feeding and reproduction of zooplankton during summer and create a favourable growth environment for the copepod nauplii. Further using field observations on zooplankton and phytoplankton development during a growth season in the northern Baltic proper, we found that NFC nitrogen is assimilated and transferred to zooplankton via both direct grazing and indirectly through grazing on small-sized phyto- and bacterioplankton. Finally, these and other findings emphasizing the importance of NFC for Baltic Sea secondary production during growth season were synthesized to show that diazotrophic nitrogen enters food webs already at bloom initiation and is transferred via multiple pathways to pelagic and benthic food webs and, ultimately, to fish.

Nisha H. Motwani defends her thesis. Faculty opponent is Catherine Legrand from Linnaeus University.

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