Zrinka Burić

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.

Composition and annual cycle of phytoplankton assemblages in the northeastern Adriatic Sea

Phytoplankton abundance and seasonality in the northeastern Adriatic Sea were directly related to the annual regime of the Po River discharge in the period 2002–2007. In 2005, 2006, and 2007, there was a shift in the timing of the annual Po River maximum discharge and of phytoplankton maxima from spring to autumn. In addition, the eastern coastal area was greatly influenced by oligotrophic karstic rivers and the Eastern Adriatic Current. Dominant phytoplankton taxa were defined on the basis of abundance and frequency of appearance. Such an approach may help in the comparison of phytoplankton communities within different environments. The dominant taxa were the diatoms Cerataulina pelagica, Chaetoceros socialis, Chaetoceros vixvisibilis and Pseudo-nitzschia spp., which appeared at maximum abundances )106 cells l-1 in more than 20% of samples. Among other phytoplankton, the most common was the coccolithophorid Emiliania huxleyi (maximum abundance of 2=10 5 cells l -1 in 50% of samples), and the dinoflagellate Prorocentrum minimum (maximum abundance of 3=10 5 cells l -1 in 12% of samples). The present state of the phytoplankton community may provide valuable information for evaluation of possible future environmental changes in the shallow Mediterranean ecosystem.

PSEUDO-NITZSCHIA BLOOMS IN THE ZRMANJA RIVER ESTUARY (EASTERN ADRIATIC SEA)

The spatial and temporal distribution of the diatom genus Pseudo-nitzschia was investigated in the small, highly stratified Zrmanja River estuary (eastern Adriatic Sea) from 1998 to 2004. During this period, Pseudo-nitzschia spp. mostly dominated the phytoplankton assemblages (abundance close to 1.3 × 106 cells L−1, appeared in 51% of the samples). The maximum abundance of Pseudo-nitzschia spp. was during mid-winter (February 1999), when it contributed up to 85% of the total diatom abundance. Pseudo-nitzschia bloom was significantly correlated with phosphorous concentration and temperature, whereas there was no significant correlation with total inorganic nitrogen, silicate and salinity. The very broad range of environmental factors over which Pseudo-nitzschia spp. were observed suggests there was more than one Pseudo-nitzschia species in the diatom assemblage. Morphological analysis has shown that the dominating Pseudo-nitzschia species corresponded to Pseudo-nitzschia calliantha. Two other species were encountered; one identified as Pseudo-nitzschia heimii, while other has yet to be identified. This study was the first to find P. heimii in the Adriatic Sea and the first to record the presence of a potentially toxic diatom P. calliantha in an Adriatic estuary.

The occurrence and ecology of the centric diatom Cyclotella choctawhatcheeana Prasad in a Croatian estuary

The centric diatom Cyclotella choctawhatcheeana Prasad has been found for the first time in the Adriatic Sea. It develops dense populations in the highly stratified, oligotrophic, karstic estuary of the Zrmanja River (eastern Adriatic Sea, Croatia). Cyclotella choctawhacheeana contributed to the phytoplankton community in the estuary with high abundance of up to 2 x 10(6) cells L-1. Maximum abundance was found in the marine layer, below the halocline, at a salinity range between 13-28, but it was able to develop over a very wide salinity range. The maximum development appeared in the temperature range of 8-18 degrees C. It was present in 52% of samples throughout the year. The significant negative correlation with silica, nitrates and total inorganic nitrogen showed its preference for a marine, oligotrophic environment. We consider C. choctawhacheeana to be a significant constituent of the phytoplankton of the east Adriatic coast. It can be assumed that its succes is based on species wide tolerance to fluctuations of environmental factors.

Rogoznica Lake (Croatia), a Unique Euxinic Seawater System on the Adriatic Coast

The characteristics outlines above (a well- studied hydrography-since 1994 ; a stable and constant oxic/anoxic interface ; and a stratified system that has persisted long enough to produce steady-state conditions relative to alkalinity and sulfide concentrations) make Rogoznica lake ideal sites to examine the processes that control the biogeochemistry of anoxic environments. The small size and physically stable nature of the lake allows researchers to examine small scale spatial and temporal variability as well as longer term processes. All our data show how particular sequences of meteorological events, some of which can be regarded as extreme, have affected the internal processes in the Lake. An integration of all results (since 1994 up to now) reveals an interesting possibility that this environment may well potentially serve as valuable sentinels of climate change