Monoporeia affinis. Photo: Nicklas Wijkmark/Azote.

The small bottom-living mussels, bristle worms and crustaceans of the Baltic Sea provide important information about the environmental status of the sea. The species composition of bottom living (benthic) fauna is an important part of Sweden's national environmental monitoring program to quantify effects from eutrophication.

In the new study, researchers from Stockholm University combined information from several environmental monitoring programs to investigate which environmental factors affect bottom communities. Particular focus was on identifying what affects the abundance of the small crustacean Monoporeia affinis

– Monoporeia largely determine the status classification in the Baltic Sea because it is so sensitive to oxygen deficiency. Where it occurs in high densities, there is often a good status classification, says Caroline Raymond, PhD student at the Department of Ecology, Environment and Plant Sciences (DEEP), and research engineer in the environmental monitoring program for benthic macrofauna.

"The benthic community may respond to things that we do not measure"

Caroline Raymond, researcher at the Department of Ecology, Environment and Plant Sciences at Stockholm University.

Salinity, temperature, sea depth and levels of polycyclic aromatic hydrocarbons, PAHs, are the main regulating factors of the benthic community’s structure, according to the researchers’ results. Of the factors that control how much Monoporeia are present in the bottom sediments, the levels of PAHs stand out as particularly important.

– PAH levels in the sediments turned out to be the best and only explanatory variable for variations in densities of Monoporeia, says Caroline Raymond.

This new knowledge may indicate that results have been misinterpreted in previous assessments, where Monoporeiahas been used to assess how affected areas are by eutrophication and oxygen-poor conditions.

– We use the benthic community mainly to assess eutrophication status in the Baltic Sea. But the benthic community may respond to other things that we do not measure. We need to keep that in mind as we look at these issues, says Caroline Raymond.

Relates to other monitoring programs

Agnes Karlsson, assistant professor in marine ecotoxicology, the Department of Ecology, Environment and Plant Sciences at Stockholm University. Photo: Isabell Stenson.

Combining information on sediment living fauna with data from other environmental monitoring programs is difficult. Each country around the Baltic Sea has its own set of environmental monitoring programs and in Sweden alone almost 50 different programs are implemented at local, regional and local level. These are rarely coordinated with each other, as some programs lie under the Swedish Marine and Water Management Agency, others under the Swedish Environmental Protection Agency, and in addition, county administrative boards and municipalities handle some sampling and various recipient control programs.

For example, there is monitoring that looks specifically at toxins in sediments. But in the Baltic Proper, six of the eight stations lie below the halocline (the horizontal stratification layer between salt and fresh water in the Baltic) where usually no macrofauna is present.

– This makes it difficult for us to link this data with our own from the monitoring of benthic macrofauna. In this study, we benefited from archived sediment from the environmental monitoring program for biological effects of contaminants, says Agnes Karlson, assistant professor in marine ecotoxicology at DEEP, and initiator of the study.

– We also took advantage of the big database of monitoring data at the Baltic Sea Centre where water quality data collected within many different programs throughout the year could be linked with our data, not only the oxygen concentrations measured when the benthic sampling is carried out in May every year.

PAHs lead to poorer growth of Monoporeia

Elena Gorokhova, professor at the Department of Environmental Science, Stockholm University.

The results of the study are also important for the monitoring program for biological effects of pollutants, where the deviation in embryonic development of Monoporeia is used as an indicator of exposure to hazardous substances.

– Concentrations of PAHs in sediments have earlier been identified as an important factor for embryonic malformations that are fatal, and this study support that this can lead to low population growth. Integrating indicators of eutrophication and environmental pollution would provide a comprehensive status assessment of the Baltic Sea benthic system, says Elena Gorokhova, professor at the Department of Environmental Science, Stockholm University.

According to the authors, this study shows that there is a great value in co-analyzing environmental monitoring data from different programs and in a larger perspective they state that levels of pollutants in the Baltic Sea are so high that even stations far from point sources of emissions have an impact on benthic fauna communities.

Text: Isabell Stenson