Phytoplankton seasonality in a highly stratified karstic estuary (Krka, Adriatic Sea)

The abundance of phytoplankton and the composition of nutrients were analysed at three stations in the highly stratified karstic Krka Estuary (east Adriatic coast), in the period March 2000-February 2001. The phytoplankton assemblages were analysed by cluster analysis (average linkage), applied to the Bray-Curtis dissimilarity index computed on density data. Seven groups were identified, which can be combined into two major groups. The spring period was influenced by high riverine water inflow and characterised by the development of cocolithophorids and autotrophic nanoplankton, and small diatoms in winter. In the summer-autumn period domination of one large group occurred, comprising dinoflagellates (micro and nano) and colonial diatoms. Three small groups were also present in that period, each with a specific phytoplankton composition. Variance explained by the Canonical Correspondence Analysis (CCA) of the time-position data was low but corresponded with clustering of samples. CC analysis revealed temperature and salinity as the most important indicators of riverine water impact that influenced species composition and phytoplankton seasonality. The nutrient regime was highly influenced by river/seawater exchange, except in the case of orthophosphates, which showed no correlation with salinity.