Related Time Series:
Zooplankton monitoring by the Finnish Institute of Marine Science began in 1979 after the Helsinki Convention (HELCOM) initiated cooperative environmental monitoring of the Baltic Sea. Monitoring was divided into four subareas based on differing hydrographic environments, including the Baltic Proper, Gulf of Finland, Bothnian Sea, and Bothnian Bay. For the purposes of this report, only the Bothnian Bay and Bothnian Sea regions are summarized. Zooplankton were collected in August, the peak abundance period, using a WP-2 net (100 mm mesh). Zooplankton data for Bothnian Bay are an average of two stations, whereas data from the Bothnian Sea are an average of three stations.
Water temperature in Bothnian Bay is 1-2 C colder than in the Bothnian Sea. At both sites, water temperatures are lowest in February/March and warmest in August. Both regions have relatively low salinities, with Bothnian Sea surface salinity ranging from 4.5 psu to 6 psu, and Bothnian Bay salinity ranging from 2.5 psu to 4 psu. These differences in salinity influence the zooplankton community structure in each region, with taxa preferring higher salinity (e.g. Acartia spp.) being fairly abundant in the Bothnian Sea and nearly absent in Bothnian Bay. This is changing, however, as surface salinity in both areas has been decreasing since the 1960s. In the late 1970s, the Bothnian Sea zooplankton biomass was dominated by Eurytemora spp., Acartia spp., and Limnocalanus macrurus. Since 1979, the biomass of Acartia has been steadily decreasing, whereas the other two taxa and total biomass in the region have been increasing. This increase in L. macrurus or in total biomass is not evident in Bothnian Bay, perhaps because a salinity threshold has already been reached in this community.
The general Baltic-wide decrease in salinity is the result of warmer temperatures and increased precipitation/river run-off in the Baltic. The SST values in both regions have been above the 100-year average since the 1990s. Those in the Bothnian Sea have also been above the 100-year maximum since 2000, as have those in Bothnian Bay since 2005.