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Studies of Marenzelleria species were often hampered by identification uncertainties when using morphological characters only. A newly developed PCR/RFLP protocol allows a more efficient discrimination of the three species Marenzelleria viridis, Marenzelleria neglecta and Marenzelleria arctia currently known for the Baltic Sea. The protocol is based on PCR amplification of two mitochondrial DNA gene segments (16S, COI) followed by digestion with restriction enzymes. As it is faster and cheaper than PCR/sequencing protocols used so far, the protocol is recommended for large-scale analyses. The markers allow an undoubted determination of species irrespective of life stage or condition of the worms in the samples. The protocol was validated on about 950 specimens sampled at more than 30 sites of the Baltic and the North Sea, and on specimens from populations of the North American east coast. Besides this test we used mitochondrial DNA sequences (16S, COI, Cytb) and starch gel electrophoresis to further investigate the distribution of the three Marenzelleria species in the Baltic Sea. The results show that M. viridis (formerly genetic type I or M. cf. wireni) occurred in the Ö resund area, in the south western as well as in the eastern Baltic Sea, where it is found sympatric with M. neglecta. Allozyme electrophoresis indicated an introduction by range expansion from the North Sea. The second species, M. arctia, was only found in the northern Baltic Sea, where it sometimes occurred sympatric with M. neglecta or M. viridis. For Baltic M. arctia, the most probable way of introduction is by ship ballast water from the European Arctic. There is an urgent need for a new genetic analysis of all Marenzelleria populations of the Baltic Sea to unravel the current distribution of the three species.

The effects of the polychaetes Marenzelleria sp. (Polychaeta, Spionidae), nonindigenous, rapidly increasing species in the Baltic Sea, on benthic nutrient fluxes, denitrification and sediment pore water nutrient concentration were studied in laboratory experiments using a flow-through setup with muddy sediment from coastal regions of the Gulf of Finland. In addition, different forms of sediment phosphorus (P), separated by chemical fractionation, were studied in three sediment layers. At a population density corresponding to about half the highest measured in the northern Baltic Sea, Marenzelleria sp. increased the fluxes of P and ammonium to the water column. No effect could be recorded for denitrification. Since the previously dominant species of the area, Monoporeia affinis, can enhance denitrification and lower the amount of dissolved P in the pore water, the replacement of M. affinis with Marenzelleria spp. may lead to increased P flux to the water column and decreased denitrification, further increasing the ammonium flux to the water column. However, sediment reworking by Marenzelleria spp. also oxidizes the surface sediment in the long run, improving its ability to retain P and support nitrification. Therefore, the impact of Marenzelleria spp. on sediment nutrient release may not be as drastic as the initial reactions seen in our experiments suggest.

In the Helsinki Commission Red List project 2009-2012, taxonomic and distributional data of benthic (macro) invertebrates were compiled by the present authors in a comprehensive checklist of the Baltic Sea fauna. Based on the most recent and comprehensive data, this paper presents the diversity patterns observed among benthic invertebrates in the Baltic Sea. As expected, the total number of species per sub-region generally declined along the salinity gradient from the Danish Straits to the northern Baltic Sea. This relationship is well known from the Baltic Sea and has resulted in a general assumption of an exponentially positive relationship between species richness and salinity for marine species, and a negative relationship for freshwater species. In 1934, Remane produced a diagram to describe the hypothetical distribution of benthic invertebrate diversity along a marine-freshwater salinity gradient. Our results clearly indicated the validity of this theory for the macrozoobenthic diversity pattern within the Baltic Sea. Categorisation of sub-regions according to species composition showed both separation and grouping of some sub-regions and a strong alignment of similarity patterns of zoobenthic species composition along the salinity gradient.

ABSTRACT:The relationship between the low-frequency (annually averaged) salinity and oxygen concentrations in the last 30 yr in the Baltic Sea and the large-scale atmospheric circulation was statistically analyzed. It was found that at these time scales the evolution of salinity and oxygen is quite homogeneous in depth, i.e. deviations from the long-term mean tend to be of the same sign in the upper and deeper layers simultaneously. Moreover, the evolution of the salinity and oxygen concentrations are negatively correlated within each layer: lower than normal salinities tend to be observed simultaneously with higher than normal oxygen concentrations, and vice versa. The statistical analysis seems to indicate that the forcing by the atmospheric circulation may be responsible for this anticorrelation between salinity and oxygen. A stronger meridional sea-level-pressure (SLP) gradient over the North Atlantic, and therefore stronger westerly winds, causes positive rainfall anomalies in the Baltic Sea catchment area and increase run-off giving rise to decreased salinities at all depths. The mechanisms by which a stronger zonal atmospheric circulation enhances the oxygen concentrations may be related either to a weakened stratification through the reduced salinity (at long time scales), or by stronger or more frequent inflows of North Sea waters (at short time scales). The influence of the atmospheric circulation can explain at these time scales of the order of 60 and 40% of the variability of these hydrographic variables, respectively. The question of how salinity and oxygen in the Baltic Sea may respond to changes of the atmospheric circulation in a global climate change scenario is also addressed. For this purpose the intensities of the relevant atmospheric circulation patterns in a transient experiment with a coupled atmosphere-ocean general circulation model forced with increasing anthropogenic greenhouse gas concentrations between the years 1860 and 2099 were estimated. In this integration the change of the atmospheric circulation most relevant for salinity and oxygen in the Baltic Sea seems to be an enhancement of the meridional SLP gradient in the North Atlantic. Thus, under the assumption that the statistical relationship between SLP and the hydrographic variables also holds in the future, this trend in the atmospheric circulation would lead to decreased salinity and improved oxygen conditions in the Baltic Sea.