Cyanobacteria, otherwise known as blue-green algae, are oxygenic, photosynthetic prokaryotes. They occur naturally in many fresh, marine and brackish waters worldwide and play an important role in global carbon and nitrogen cycles. In their long history, cyanobacteria have developed structures and mechanisms that enable them to survive and proliferate under different environmental conditions. In the Baltic Sea, the mass development of cyanobacteria is compounded by a high level of eutrophication. The dominant species in the Baltic, the filamentous Aphanizomenon flos-aquae and Nodularia spumigena, can fix dissolved atmospheric N 2 , as a result of which they can outcompete other phytoplankton organisms. Heterocystous, filamentous cyanobacteria also make a significant contribution The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/ 294 H. Mazur-Marzec, M. Pliński to the internal nutrient loading in the Baltic. The blooms of N. spumigena are of particular concern, as this cyanobacterium produces nodularin (NOD), a hepatotoxic peptide. The concentration of the toxin in the sea is regulated mainly by dilution with uncontaminated water, photolysis, sorption to sediments and microbial degradation. The transfer of the toxin in the Baltic trophic chain through zooplankton, mussels, fish and birds has been reported, but biodilution rather than bioconcentration has been observed. Cyanobacterial blooms are thought to pose a serious threat to the ecosystem. Their harmful effects are related to the occurrence of a high biomass, oxygen depletion, a reduction in biodiversity, and the production of toxic metabolites.

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I n t e r n a t i o n a l J o u r n a l o f O c e a n o g r a p h y a n d H y d r o b i o l o g y by The Oceanological and Hydrobiological Studies is online at oandhs.ocean.ug.edu.pl pages (305-315) AbstractThe American panopeid crab species Rhithropanopeus harrisii (Gould, 1841) is listed as an non-native species in European waters. In Poland, it occurred in the 1950s at two sites at the Baltic Sea coast, the Dead Vistula River (DVR) and the Vistula Lagoon (VL). Almost 50 years later, two additional populations were identified in the Gulf of Gdańsk (GG) and its inner part, Puck Bay (PB). In the present study, we sequenced and analyzed part of the mitochondrial cytochrome oxidase subunit I gene of the four Polish populations of R. harrisii in order to assess their genetic diversity and connectivity. The analyzed sequences of a length of 989 base pairs revealed eight different haplotypes. The highest number of haplotypes (n=6) was observed in the population from GG, whereas the lowest (n=3) in VL. The most common haplotype was recorded in 37% of the analyzed individuals. Pairwise Φ ST values were mostly non-significant, with the exception of the comparison between DVR and VL (Φ ST = 0.267; P < 0.05) and between PB and VL (Φ ST = 0.194; P < 0.05), indicating a restricted gene flow or different sources of colonization.

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