Long-term changes in diatom communities of phytoplankton and the surface sediments in the Curonian Lagoon (Lithuanian part)

Abstract

1 - The aim of this study was to analyse spatial and temporal patterns among phytoplankton diatom communities for assessing water quality in the Curonian Lagoon, SE Baltic Sea. The study of diatoms in sediments was selected as a method to establish if these records are useful for reviewing the environmental conditions of a transitional water body because they represent a composite accumulation of species occurring throughout all the seasons. 2 - 137 phytoplankton diatom communities were analyzed seasonally in 1986–1987, 1991 and in summer 2003–2004. 34 diatom assemblages in surface sediments (0–5 cm) were studied in 1998–1999 in the Curonian Lagoon. 3 - 360 diatom species and 38 genera were identified. The high anthropogenic disturbance in the lagoon was reflected by the diatom communities in the phytoplankton. Species typical of eutrophic waters Stephanodiscus hantzschii, S. minutulus, S. binderanus, Cyclostephanos dubius, Cyclotella atomus, C. meneghiniana became dominant in 1986–1987 and 1991, accounting for more than 90% of total abundance. The dominant species of previous study periods have lost their advantage during the last two decades. 4 - Because of extensive dredging activities in the Klaipëda Strait, the brackish species Actinocyclus normanii, Chaetoceros spp., Skeletonema costatum, Thalassiosira spp. comprised a substantial part of the plankton diatom communities, especially during summer–autumn seasons also in the southern sites. 5 - Different Curonian Lagoon zones were defined according to the surface sediment diatom assemblages. Salinity was the main environmental factor regulating species occurrence. Predominance of brackish water planktonic diatoms was characteristic in the western part of the lagoon where salinity changes rapidly and irregularly. Freshwater benthic diatom taxa prevailed in the eastern part of the lagoon. 6 - Environmental reconstruction of the lagoon based on sediment diatom assemblages is problematic because they do not properly reflect species that are representative in water environments. Further investigations of the surface sediment diatom assemblages according to their habitat and salinity gradient in the lagoon are required in order to use them in the historical assessment of environmental changes in the transitional water.