Dynamics of Marine Meso-Zooplankton at Seili, Northern Baltic Sea, in 1967–1975

Abstract

Abstract Long-term (1967–1975) dynamics of meso-zooplankton population densities at Seili (Northern Baltic Sea) are described. The data were obtained from quantitative net hauls taken at 10 day intervals during the open water season and once a month in winter. Water salinity and temperature were recorded concurrently. Altogether 34 taxa (species, species groups, instars) were frequent enough for time series analysis. The most abundant rotifers were Synchaeta species (S. baltica and S. monopus for the most part). Among c1adocerans Podon polyphemoides and Eubosmina longispina maritima were numerically dominant, while Acartia spp. and Eurytemora affinis hirundoides were the most abundant copepods. The nauplii of Balanus improuisus were the most frequent and abundant meroplankton. In winter meso-zooplankton biomass is dominated by copepods (nauplii), in spring rotifers increase, followed by c1adocerans and then by copepod copepodids and adults in autumn. From 20 to 100% of the total meso-zooplankton biomass is attributable to copepods. There is considerable year-to-year variation in meso-zooplankton composition. For example, biomass in the summers of 1968 and 1974 was dominated (40-60%) by rotifers, while in 1967, 1970 and 1973 rotifers were less important (10-15%) in the biomass. The salinity at Seili rose substantially during 1970–1971, followed by another inflow of saline water in 1973. These salt water intrusions kept the salinity at an elevated level to the end ofthe sampling period. The meso-zooplankton taxa responded to the salinity change: 11 out of 34 taxa increased in numbers (seasonal effect removed), while most (23) of the taxa decreased in numbers. However, in most cases these changes were not distinguishable from random fluctuations. Presently it is very difficult to assess the significance of other factors (human-caused eutrophication, or biotic interactions) to the documented abundance changes.