Damir Viličić

Phytoplankton population density and volume as indicators of eutrophication in the eastern part of the Adriatic Sea

Population density and volume-biomass of microplankton, nanoplankton and total phytoplankton were analyzed at 14 stations along the eastern part of the Adriatic Sea. Based upon data frequency distribution, four categories of ecosystems were established in the investigated area. Criteria for ecosystem categorization and biological evaluation of eutrophication are proposed.

Vertical distribution of phytoplankton in a stratified estuary

We present a fine vertical distribution of physico-chemical parameters and phytoplankton density around the halocline (freshwater/seawater interface) in the Krka estuary on the East coast of the Adriatic Sea. Suspended matter, surfactant activity, surface active aggregates, particulate organic carbon and phaeophytin all show largest concentrations in the 20–50 cm thick interface layer. The highest concentration of chlorophyll-a is found on the upper boundary of the interface and it is a consequence of majority of viable freshwater phytoplankton cells that accumulate there. The phytoplankton accumulates selectively in the interface: larger size fractions accumulate faster than smaller ones. In particular, nanoplanakton accumulates the least. The visible interface is also populated by dead phytoplankton cells. Most of the freshwater phytoplankton dies and decomposes in the interface, as revealed by a small chlorophyll-a/phaeophytin ratio, and it represents the main source of surface active dissolved and particulate organic matter. Marine flagellates migrate and divide in the interface, while some species likeProrocentrum micans andSyracosphera sp. are also found in the upper freshwater layer.

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.

Phytoplankton blooms in the oligotrophic open South Adriatic waters

Abstract The phytoplankton bloom recorded in April 1987 in the offshore south Adriatic waters was compared with a less extensive bloom in April 1986. Differences between 1986 and 1987 hydrographic data occurred because of different hydrometeorological conditions. April 1987 was a period of a pronounced advection of eastern Mediterranean water (S > 38.65%, T>14°C) into the Adriatic, and the extensive phytoplankton bloom was probably induced by earlier upwelling. The difference between the situations in 1986 and in 1987 can be attributed to the measurements being made at two different stages of the phytoplankton blooms. In 1986, the measurements were made during the first phase, whereas in April 1987 they were made during the late phase of the bloom. The complex dynamics of the south Adriatic waters probably involve upwelling/downwelling and advection.

Microphytoplankton in the Strait of Otranto (eastern Mediterranean)

The Strait of Otranto is the connection between the Adriatic and Ionian Seas. Low nutrient concentrations, high transparency, and low phytoplankton cell density and biomass reflect the oligotrophic character of the area. Enrichment of the euphotic layer with nutrients is mainly due to discharge of Albanian and Greek rivers, as well as mixing and upwelling in winter/early spring. Following phytoplankton bloom in April, a progressive decrease of phytoplankton cell density is due to the consumption of nutrients throughout the proceeding summer and autumn. Nitrogen was a strong limiting factor for phytoplankton growth in summer. Deep biomass maxima were detected in the 50 to 100 m layer and corresponded mostly to cells smaller than 20 μm. The eastern part of the strait is mostly influenced by the northerly inflowing current from the Ionian Sea, and the western part by the southerly outflowing current from the Adriatic Sea. This typical circulation could be disturbed by inertial oscillations in the current field, generated by the strong oscillating winds and cyclonic eddies. The type of circulation determined the distribution of thermohaline characteristics, abundance, biomass, as well as taxonomic composition of phytoplankton, across the strait. Ecological characteristics of the water masses on two sides of the strait were significantly different during the formation of a longitudinal thermohaline front in May 1990.

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.

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.

Subsurface Gonyaulax polyedra bloom in a stratified estuary

A subsurface bloom of the marine dinoflagellate Gonyaulax polyedra Stein was observed on October 19, 1988 in the central part of the Krka Estuary (east Adriatic coast) as a 6-m-thick red-brownish layer, located below the halocline. The bloom might have been initiated by excystment in shallower parts of the estuary, where temperature near the bottom exceeded 20°C. The autumn of 1998 was exceptionally dry, river inflow was small and the brackish upper layer was 2 m thick and transparent. The exchange rate of water in the marine layer was at its lowest in the year. The supply of nutrients for growth came from sinking and decomposition of freshwater phytoplankton at the halocline and the marine layer.

Transport of silica in a stratified estuary

The enrichment of dissolved silica found in the marine layer near the head of the highly stratified Krka estuary is attributed to sinking of freshwater diatoms through the halocline and subsequent decomposition of biogenic silica. In the marine layer, freshwater diatoms sink and order of magnitude slower than in the brackish layer. As a consequence their dissolution path is shorter. Freshwater phytoplankton species contributing to the majority of silica enrichment are Cyclotella sp. and Synedra acus. Their sedimentation rate in the brackish and in marine layers is estimated.

The Heterotrophic Ebridian Microflagellate Hermesinum adriaticumZach. in the Adriatic Sea

Summary Hermesinum adriaticum Zach . is a rare non-photosynthetic flagellate with an internal siliceous skeleton. It is of uncertain taxonomic position and was, recently classified within dinoflagellates (Ebriales). Subsurface accumulation of H. adriaticum Zach . (up to 380,000 cells - I−) was found around the brackish/seawater interface (BSI), in a small, saline coastal lake (Lake Rogoznica, central Adriatic coast), from March to November 1994. Dense populations of this species may be found only in stratified ecosystems with a well defined oxic/hypoxic (anoxic) - H2S enriched boundary layer. In Lake Rogoznica such conditions coincided with the BSI. The increasing gradient of phosphate, nitrate, ammonia and silicate concentrations provided evidence of a sharp chemocline below the BSI. In winter, an advective transport of H. adriaticum population was evident in the southeastern Adriatic; by an inflowing northerly current from the eastern Mediterranean.