Gulf of Riga
Related Time Series:
Pärnu Bay is a shallow, semi-enclosed water basin in the northeast Gulf of Riga in the Baltic Sea. Its maximum depth gradually increases from 7.5 m in the inner part to 23 m in the southwestern part. The hydrological conditions of the bay are influenced by meteorological processes, river discharge, and water exchange with the open part of the Gulf of Riga. The salinity of Pärnu Bay is slightly lower than in the Gulf of Riga, with an average salinity of 5 psu. Pärnu Bay also suffers from heavy anthropogenic eutrophication, with nitrate and phosphate coming into the bay from the town of Parnu and the Parnu River.
Zooplankton samples are collected in the middle part of the bay, where water depth is 10 m, using a Juday plankton net (0.1 m2 mouth area, 90 µm mesh). The frequency of sampling has varied over the years, from at least once a month to several times a week during summer in some years. Peak copepod abundance occurs in the warmer summer months, after the spring chlorophyll peak and just before the summer temperature increase (Standard Co-sampled Variables Plot).
The diversity of zooplankton in Pärnu Bay is low; two species, Eurytemora affinis and Acartia bifilosa, constitute 99% of the total copepod abundance, and Bosmina coregoni maritima is the prevailing cladoceran. Although copepod abundance has been increasing slightly for the duration of the time-series, cladoceran abundance went from an increasing to a decreasing trend around 1989 (Standard Co-sampled Variables Plot). The 1989 period corresponds with a rise in both copepod abundance and water temperature from below-average to above-average levels. One reason for the decreased cladoceran abundance in the 1990s may be the introduced predatory cladoceran Cercopagis pengoi, which occurs in large numbers in Pärnu Bay during periods of warm water. Water temperatures in Pärnu Bay have been warmer than the 100-year average since 1989 and are currently warmer than the 100-year maximum (Standard Long-term Comparison Plot, bottom, red dashed line).