Distribution and reproduction of the Arctic ctenophore Mertensia ovum in the Baltic Sea

Species identification based on morphological characteristics has caused misidentifications and led to twisted views of abundances and roles of ctenophores. Based on extensive field studies from 2007 to 2010, the occurrence of the arctic ctenophore Mertensia ovum was genetically verified in the southern, central and northern Baltic Sea, and its egg production, distribution and abundance were studied in relation to physical factors. Genetic analyses indicate that M. ovum is by far the most abundant small ctenophore in the Baltic Sea. Specimens from a 20 yr old ctenophore collection were also genetically identified as M. ovum, contrary to their previous morphological identification as another ctenophore species, Pleurobrachia pileus. Thus, earlier reports on P. pileus in the Baltic Sea may actually refer to M. ovum. The abundance of M. ovum was regulated by both salinity and temperature, with highest abundances found in sea areas and water layers at temperatures 5.5 and oxygen levels >4 ml l-1. During summer, the highest abundances of ctenophores and their eggs were found near the halocline, while the distribution was more uniform throughout the water column during winter. Only ctenophores >3.5 mm (oral-aboral length) produced eggs in the experiments, with an average rate of 2.2 eggs ind.-1 d-1. Finally, comparison with published data from the 1980s (assuming that those data refer to M. ovum) indicates that the present-day ctenophore abundance is ~80% lower in the north and ~55% higher in the southern parts of the Baltic Sea, due to reasons yet to be established

Ecosystem impacts of the widespread non-indigenous species in the Baltic Sea: literature survey evidences major limitations in knowledge

Invasion of non-indigenous species (NIS) is acknowledged as one of the most important external drivers affecting structure and functions of marine ecosystems globally. This paper offers literature-based analysis on the effects of the widespread (occurring in at least 50% of countries) and currently established NIS on ecosystem features in the Baltic Sea. It appears that out of the 18 NIS taxa studied, there are no published records on 28% of NIS for any of the seven impact categories investigated. When ecological impacts are known, laboratory experimental evidence dominates over field studies. Combined observations on impact strength, information type and confidence level suggest that the two benthic invertebrates, the polychaete Marenzelleria spp. and the zebra mussel Dreissena polymorpha (Pallas 1771) exert the highest ecosystem impact. Despite continuously accumulating information on the NIS effects, however, the confidence of findings is still low. Thus, we still understand very little on both the direction and magnitude of the effects of even the most widespread NIS on the structure and dynamics of the Baltic Sea ecosystems. In order to increase reliability of such assessments, future research should be targeted towards spatially-explicit field surveys and experimenting of multitrophic systems, together with modelling of ecosystem impact

Impacts of changing climate on the non-indigenous invertebrates in the northern Baltic Sea by end of the twenty-first century

Biological invasions coupled with climate change drive changes in marine biodiversity. Warming climate and changes in hydrology may either enable or hinder the spread of non-indigenous species (NIS) and little is known about how climate change modifies the richness and impacts of NIS in specific sea areas. We calculated from climate change simulations (RCO-SCOBI model) the changes in summer time conditions which northern Baltic Sea may to go through by the end of the twenty-first century, e.g., 2-5 A degrees C sea surface temperature rise and even up to 1.75 unit decrease in salinity. We reviewed the temperature and salinity tolerances (i.e., physiological tolerances and occurrence ranges in the field) of pelagic and benthic NIS established in-or with dispersal potential to-the northern Baltic Sea, and assessed how climate change will likely affect them. Our findings suggest a future decrease in barnacle larvae and an increase in Ponto-Caspian cladocerans in the pelagic community. In benthos, polychaetes, gastropods and decapods may become less abundant. By contrast, dreissenid bivalves, amphipods and mysids are expected to widen their distribution and increase in abundance in the coastal areas of the northern Baltic Sea. Potential salinity decrease acts as a major driver for NIS biogeography in the northern Baltic Sea, but temperature increase and extended summer season allow higher reproduction success in bivalves, zooplankton, amphipods and mysids. Successful NIS, i.e., coastal crustacean and bivalve species, pose a risk to native biota, as many of them have already demonstrated harmful effects in the Baltic Sea