Mirna Batistić

Chaetognaths in the South Adriatic: vertical distribution and feeding

Feeding and vertical distribution of pelagic chaetognaths were studied at coastal and open-sea stations in the northern part of the South Adriatic Pit during five cruises from April 1993 to February 1995. Chaetognaths were more numerous in the upper 100 m, decreased conspicuously with depth, and were captured only rarely between 600 and 1000 m. Copepods were the main food for the most abundant species in all layers. Cannibalism was evident in all species. Feeding on fertilized chaetognath eggs by Pseudosagitta lyra and Mesosagitta decipiens is reported for the first time. The food containing ratio (FCR) and number of prey items per chaetognath (NPC) of Flaccisagitta enflata were highest during the reproductive period. Higher FCRs and NPCs for M. decipiens and P. lyra were noted below 100 m, where the contribution of older specimens increased with depth and the abundance of copepods decreased.

Phytoplankton in three freshwater lakes in the Neretva River delta (Eastern Adriatic, NE Mediterranean)

Phytoplankton composition and temporal distribution were studied in the lakes Modro Oko, Desne and Kuti in the Neretva River delta throughout the seasons in 1994 and 1995. Physico-chemical parameters did not differ among three lakes, except for NO3 concentrations. Seventy-six phytoplankton taxa were found in all three lakes combined, including 29 Bacillariophyceae, 4 Chrysophyceae, 15 Chlorophyceae, 9 Charophyceae (Zygnematales), 16 Cyanophyceae, and 3 Dinophyceae. Lakes Modro Oko, Desne and Kuti showed 18, 24 and 57 taxa, respectively. Asterionella formosa, Fragilaria ulna and Pediastrum clathratum were common to all three lakes. Similarity coefficients (Jaccard index) of Modro Oko and Desne, Modro Oko and Kuti, and Kuti and Desne were, respectively, 13.2%, 12.3% and 23.9%. The highest mean values of surface phytoplankton density in Modro Oko (695 cells L-1), Desne (4308 cells L-1), and Kuti (103810 cells L-1) showed no significant differences. Bacillariophyceae and Cyanophyceae mostly dominated phytoplankton cell density. The most abundant (>105 cells L-1) taxa, Microcystis aeruginosa, M. wesenbergii and Aphanizomenon flos-aquae, were found in Lake Kuti during summer. Six taxa were recorded for the first time in Croatia: Cyclotella striata, Botryococcus braunii, Pediastrum sturmii, Desmidium swartzii, Chamaesiphon cylindricus, and Woronichinia naegeliana.

Calycophorae (Siphonophora) in the open waters of the central and southern Adriatic Sea during spring 2002.

Twenty species of calycophoran siphonophores were identified from the central and south Adriatic Sea in spring 2002. Highest abundance and species diversity were noted at the deepest stations in the south Adriatic. Highest total abundance was found in the upper 100 m. The dominant species above 100 m were Lensia subtilis , Eudoxoides spiralis and Sphaeronectes gracilis , none of which showed diel migration. The first two species correlated significantly with the vertical abundance of microzooplankton, and the last with that of copepods. The most abundant species in the 100-400 m layer was Lensia meteori , whereas Lensia conoidea and Chlausophyes ovata were most abundant below 400 m.

Diet and diel feeding activity of juvenile pompano ( Trachinotus ovatus ) (Teleostei: Carangidae) from the southern Adriatic, Croatia

Food and feeding activity of juvenile pompano Trachinotus ovatus collected in the summer-autumn in 2003 in coastal shallow water bays in the southeastern Adriatic were examined. Juvenile pompano were exclusively daytime feeders and feeds in whole water column, from surface to bottom taking not only plankton, but benthic and terrestrial species as well. Of eighteen prey taxa identified, crustaceans, particularly copepods, were the major group, followed by benthic foraminiferans and insects. Feeding on benthic species is reported for the first time in this study.

Hydrography, nutrients, and plankton along the longitudinal section of the Ombla Estuary (south-eastern Adriatic)

Abundance and structure of phytoplankton and zooplankton, along with their relationship to hydrographic conditions were determined in the highly stratified estuary of the karstic Ombla River, south-eastern Adriatic. Sampling was carried out during 17 cruises within a one year period. River discharge lowered surface salinity and enriched the estuary with NO 3 and SiO 4 . Nutrient ratios suggested that PO 4 was the most likely limiting nutrient for phytoplankton growth in the estuary. Diatoms were present in low numbers and dominated the winter-early spring period. Dinoflagellates dominated from the end of May to August. Phytoplankton and zooplankton were composed mostly of marine species and their abundance decreased in seaward direction. Planktonic populations are controlled by the river runoff, temperature, salinity, nutrient concentrations and grazing. The results were consistent with the hypothesis that no algal blooms have been recorded due to short renewal time.

Rogoznica Lake, a Euxinic Marine Lake on the Adriatic Coast (Croatia) that Fluctuates Between Anoxic Holomictic and Meromictic Conditions

Rogoznica Lake is a typical example of euxinic marine lake, situated on the eastern Adriatic Coast, Croatia (43°32′ N 15°58′ E). It is a karstic depression with an area of 10.276 m2, filled with seawater with a maximum depth of 15 m. The lake is stratified during the spring–summer season, both thermally and in its salinity. There are oxic upper water layer and anoxic deeper layer. The mixolimnion varies seasonally in depth and thickness for about 1 m due to influence of meteorological conditions (temperature, wind, rainfall), which also affect the dynamics between the water layers. Rogoznica Lake can be considered as a system with both meromictic and holomictic conditions, which alternate seasonally and affect the lake’s ecology. Anoxic water is usually located below 10 m depth, and it is characterized by high concentrations of reduced sulphur species, nutrients and dissolved organic carbon. In anoxic water and laminated sediment, enrichment of trace metals, especially those that accumulate in sulphidic environment (Fe, Mo), is found. According to sedimentary enrichment of Mo (up to 81 mg kg−1), the lake can be classified as typical anoxic and meromictic environment. Extreme ecological conditions, which prevail in the lake, control the phytoplankton and zooplankton activities. Diatoms are the dominant microphytoplankton group, while oligotrich ciliates and copepods, the heterotrophic zooplankton organisms, play an important role and control the food web of the lake, especially in the post-holomictic–anoxic period. Ciliates by grazing on phototrophic sulphur bacteria at the chemocline transfer organic carbon to higher trophic levels, from anoxic to oxic conditions.

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

Fritillaria ragusina sp. nov., a new species of Appendicularia (Tunicata) from the Adriatic Sea

A new species of appendicularian, Fritillaria ragusina, is described from specimens collected between the surface and 300 m depth at three stations in the open south Adriatic Sea (north-eastern Mediterranean). Fritillaria ragusina is characterized by a broad trunk–tail connection created by a wide tail fin in the initial part of the tail, sloping shoulders of the tail fin, broad tail musculature, multiple fusiform amphichordal cells along the edge of the tail musculature and small size at sexual maturity (maximum recorded trunk length of 445 mm and maximum recorded tail length of 960 mm). Maximum abundance, 2.4 ind m-3, was found within the 0–50 m layer.