Long-term changes in the soft-bottom macrofauna of the Gulf of Riga

A comparison of Gulf of Riga soft-bottom macrofauna in 1993-1996 with earli er studies shows that significant changes have taken place. Successively hi gher biomass values were found during the 1960s-1980s. This change reflects the changes of nitrate concentrations in the water. The total biomass in t he 1990s was slightly higher than during the 1970s, but only about half the value of the 1980s. Abundance and biomass of Monoporeia affinis have decre ased by over 50% between the 1980s and the 1990s. The decrease was extreme in deeper areas, while in shallower areas the species increased. For Macoma balthica, there were higher abundance and biomass values in the 1980s than in the 1970s. Since the 1980s abundance has increased slightly, while the biomass has nearly halved. During the 1970s and 1980s, annelids made up les s than 25% of the abundance and 1% of the biomass. The arrival of Marenzell eria viridis in 1988 changed the picture drastically. This is now the most abundant species; it is found all over the gulf, and its share in abundance and biomass has increased to 42 and 12%, respectively. Annual production o f macrozoobenthos was about twice as high during the 1980s as during the 19 90s. Community annual turnover rates were estimated at approximately 0.5 fo r both periods. (C) 1999 International Council for the Exploration of the S ea.