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Gulf of Riga Associated Investigators: Sampling Station 121 is located in the central part of the Gulf of Riga, approximately 50 km offshore, at a water depth of 55 m. Zooplankton samples for determination of abundance and wet-weight biomass were collected by vertical hauls from a depth of 50 m to the surface using a WP-2 net (100 µm mesh). Sampling was carried out at least three times a year, representing the most productive seasons, i.e. spring (May), summer (August), and autumn (October-November). For the first half of the year, water temperature in the Gulf of Riga is the main determinant of zooplankton abundance and biomass. Minimum copepod abundance and biomass values are found from January until the beginning of March, and correspond to the coldest water temperatures. In May, the spring zooplankton community is dominated by copepods and rotifers (Synchaeta spp.). From the end of May to the beginning of June, as water temperatures rise, thermophilic species, such as cladocerans and rotifers (Keratella spp.), begin to appear, reaching maximum abundance and biomass in August. When water temperatures fall in autumn, the thermophilic species disappear, reducing copepod abundance and biomass. Water temperatures in the Gulf of Riga have been above the 100-year average for the duration of the time-series, and they have been above the 100-year maximum since 2000 (Standard Long-term Comparison Plot, bottom, red dashed line). There has been an overall decreasing trend in zooplankton abundance since the beginning of the time-series. In recent years, biomass and abundance maxima have been observed more frequently in July than in August, due in part to this overall warming in the region and a subsequent shift in the timing of community development. Other possible reasons for the observed decline in zooplankton are: August predation pressure on copepods by herring, which have been increasing over the last ten years, and the increasing presence of the invasive cladoceran Cercopagis pengoi since the late 1990s. |