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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.

The embryonic development of the deposit-feeding Baltic amphipod Monoporeia affjnis (Lindstrom) was studied from fertilization to hatching. Severe types of aberrant embryonic development are described and the normal variation of the aberrations is estimated. Between 2 and 6% malformed, 0 and 5 % undifferentiated and 0 and 6% dead eggs and embryos were observed at different sites in the coastal area of the northern Baltic proper and the Bothnian Sea without local contaminant &scharges. The embryonic development of M. affinis was described to facilitate the use of the embryo variables in field studies of sediment toxicity. The merits of the different embryo variables as biomarkers in effect monitoring were evaluated. Gravid females were sampled in the receiving waters of 2 different types of pulp mills and an aluminum smeltery. Fecundity (eggs per female) and frequency of malformed, enlarged, undifferentiated and dead embryos per female were determined. Frequency of malformed embryos was higher at industrially affected stations as compared to reference areas (p < 0.0001, p < 0.0001 and p = 0.003 respectively). The highest frequencies (15.3%) were recorded close to the alurninum smeltery. In reference areas the frequency vaned between 1.5 and 4.9. The number of enlarged embryos with oedema was higher near the pulp m d s (p = 0.03, p > 0.0001), particularly the pulp mill with a bleaching stage, where 25 to 40% embryos per female were affected. Background frequencies of enlarged embryos (0.7 to 3.5%) were observed outside the alurninum smeltery. UnMferentiated eggs and dead eggs did not correlate to the exposure situation, suggesting these variables are of lunited value in effect monitoring of xenobiotics. The frequency of malformed embryos was increased also when fecundity, reproduction success and developmental stage of embryos were unaffected. Thus, the variable is suggested to be particularly toxicant-sensitive and by analysing the number of malformed embryos of M. affinis an early warning of affected populations could be provided.

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The significance of black carbon (BC) for the bioavailability of polycyclic aromatic hydrocarbons (PAHs) was examined by using historically contaminated intact sediment cores in laboratory exposure experiments with the deposit-feeding amphipod Monoporeia affinis. Log values of amphipod biota-sediment accumulation factors (BSAFs) were significantly related to log BC, whereas log BSAFs were related to log octanol-water partition coefficients only in background sediments containing less BC. In the background sediments, the BSAF for polycyclic aromatic hydrocarbons (PAHs) was 1 to 2 for phenanthrene, with lower values for more hydrophobic PAHs, indicating an increase in nonequilibrium conditions with increasing PAH molecular size. For the near-equilibrated phenanthrene and fluoranthene, higher BSAFs were measured during exposure to background sediments, with BSAF decreasing to <0.1 in contaminated sediments in the Stockholm waterways. In situ caged mussels (Dreissena polymorpha) exhibited field BSAF values (relative to sediment-trap-collected suspended matter) for polychlorinated biphenyls (PCBs) of 0.1 to 0.4, but for PAHs of similar hydrophobicity and molecular size, the field BSAFs were much lower and in the range 0.002 to 0.05. This PAH-PCB dichotomy is consistent with recently reported much stronger binding to diesel soot (a form of BC) for PAHs than for PCBs of equal hydrophobicities. Lower BSAFs for the near-equilibrated PAHs (phenanthrene and fluoranthene) in the urban sediments relative to the background sediments were consistent with the larger presence of BC in the urban sediments. This study provides the first linked BSAF-BC field data that supports a causal relationship between strong soot sorption and reduced bioavailability for PAHs.