Filamentous, nitrogen-fixing cyanobacteria form extensive summer blooms in the Baltic Sea. Their ability to fix dissolved N2 allows cyanobacteria to circumvent the general summer nitrogen limitation, while also generating a supply of novel bioavailable nitrogen for the food web. However, the fate of the nitrogen fixed by cyanobacteria remains unresolved, as does its importance for secondary production in the Baltic Sea. Here, we synthesize recent experimental and field studies providing strong empirical evidence that cyanobacterial nitrogen is efficiently assimilated and transferred in Baltic food webs via two major pathways: directly by grazing on fresh or decaying cyanobacteria and indirectly through the uptake by other phytoplankton and microbes of bioavailable nitrogen exuded from cyanobacterial cells. This information is an essential step toward guiding nutrient management to minimize noxious blooms without overly reducing secondary production, and ultimately most probably fish production in the Baltic Sea.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-015-0660-x) contains supplementary material, which is available to authorized users.

Nitrogen‐fixing cyanobacteria (NFC) are important primary producers in many freshwater and marine systems, including the Baltic Sea. In this system, NFC circumvent summer nitrogen limitation, while also generating a supply of novel combined nitrogen and thus supporting food webs. Using field observations on zooplankton and phytoplankton development during a growth season in the northern Baltic Proper, we show that cyanobacterial nitrogen is assimilated and transferred to zooplankton via both direct grazing on NFC and indirectly through grazing on picoplankton, such as picocyanobacteria. The key findings supporting these conclusions are: (1) all zooplankton grazers were found to ingest NFC (Nodularia spumigena) and picocyanobacteria (Synechococcus spp.); (2) ingestion of both NFC and picocyanobacteria measured by quantitative polymerase chain reaction analysis was highly correlated with ambient stocks of the respective cyanobacteria; (3) consumption of NFC and picocyanobacteria translated into decreased δ15N signature of zooplankton indicative of diazotrophic nitrogen input; (4) growth and reproduction indices in zooplankters were significantly positively related to NFC and picocyanobacteria; and (5) zooplankton biomass was positively related to the increasing nitrogen content of particulate organic matter (POM < 10 μm) and was highest at low POM δ15N values; the latter reflected overlap in zooplankton production and diazotroph seasonal dynamics. These findings provide empirical evidence that both NFC and picoplankton are readily ingested and assimilated by zooplankton, albeit with differential effects on growth and recruitment.

Diel vertical migration (DVM) is often assumed to encompass an entire population. However, bimodal nighttime vertical distributions have been observed in various taxa. Mysid shrimp populations also display this pattern with one group concentrated in the pelagia and the other near the bottom. This may indicate alternative migratory strategies, resembling the seasonal partial migrations seen in birds, fishes and amphibians, where only a subset of the population migrates. To assess the persistence of these alternative strategies, we analyzed the nitrogen and carbon stable isotope signatures (as proxies for diet), biochemical indices (as proxies for growth condition), and genetic population divergence in the Baltic mysid Mysis salemaai collected at night in the pelagia and close to the bottom. Stable isotope signatures were significantly different between migrants (pelagic samples) and residents (benthic samples), indicating persistent diet differences, with pelagic mysids having a more uniform and carnivorous diet. Sequencing of the mitochondrial cytochrome subunit I (COI) gene showed genetic differentiation attributable to geographic location but not between benthic and pelagic groups. Divergent migration strategies were however supported by significantly lower gene flow between benthic populations indicating that these groups have a lower predisposition for horizontal migrations compared to pelagic ones. Different migration strategies did not convey measurable growth benefits as pelagic and benthic mysids had similar growth condition indices. Thus, the combination of ecological, biochemical and genetic markers indicate that this partial migration may be a plastic behavioral trait that yields equal growth benefits.