SummaryThis study shows the influence of eutrophication pressure on the phytoplankton community structure, abundance and biodiversity in the investigated bays with different hydromorphological features. Šibenik Bay is a highly stratified estuary of the karstic river Krka; Kaštela Bay is a semi-enclosed coastal bay, which is influenced by the relatively small river Jadro; and Mali Ston Bay is located at the Neretva River estuary, the largest river on the eastern part of the Adriatic Sea. All of the areas are affected by urban pressure, which is reflected in the trophic status of the waters. The greatest anthropogenic influence was found in Kaštela Bay while the lowest influence was found in Mali Ston Bay. In this study, the highest biomass concentration and maximum abundance of phytoplankton were recorded at the stations under the strongest anthropogenic influence. Those stations show a dominance of abundance compared to the biomass and a dominance of opportunistic species, which is reflected in the lower biodiversity of phytoplankton community. Diatoms were the most represented group of the phytoplankton community in all three bays, followed by the dinoflagellates. Diatoms that were highlighted as significant for the difference between the bays were Skeletonema marinoi in Šibenik Bay, Leptocylindrus minimus in Kaštela Bay and the genus Chaetoceros spp. in Mali Ston Bay. Dinoflagellates were more abundant at the stations under the strongest anthropogenic influence, and most significant were Prorocentrum triestinum in Kaštela Bay and Gymnodinium spp. in Šibenik Bay and Mali Ston Bay

In order to test the response of phytoplankton to anthropogenic pressure, data of chlorophyll a concentration, phytoplankton abundance, and composition are analyzed in relation to anthropogenic pressure gradient and environmental variables such as temperature, salinity and nutrients. Investigated sites encompassed wide tropic range according to a preliminary determination of anthropogenic pressure, quantified through the LUSI index. Statistical analyses indicated nitrates and silicates as proxies of freshwater influence, and phytoplankton single metrics such as concentrations of chlorophyll a and abundances as indicators of anthropogenic pressure. Boundary values for different water quality classes for coastal waters under indirect freshwater influence (Type II) are obtained according to gradient between concentration of chlorophyll a and pressure index (LUSI), which empirically fit to exponential equation. The response of phytoplankton diversity was not linear, as the highest diversity was observed in the area with intermediate disturbance level. CCA analysis identified Skeletonema marinoii, Scrippsiella trochoidea, Guinardia flaccida, Leptocylindrus spp., Prorocentrum spp., Proboscia alata, Eutreptiella spp., and Pseudonitzschia spp. as local eutrophication indicators, whose abundances increased with nutrients loads

Annual dynamics and ecological characteristics of the genus Dinophysis spp. and associated shellfish toxicity events were studied from 2001 to 2005 during monitoring fieldwork in the coastal waters of the eastern Adriatic Sea. Analysis of the seasonal occurrence of Dinophysis species identified D. acuminata and D. sacculus as typical spring species, D. caudata, D. fortii and D. rotundata as summer and late summer species and D. tripos as a winter species. The highest abundances occurred when there were large differences between surface and bottom temperatures and salinities. D. caudata, D. sacculus and D. rotundata abundances had significant relationships with Δt, while the highest abundances of D. acuta, D. fortii and D. tripos were associated with high Δs values. Much higher abundances of D. caudata and D. fortii in offshore compared to inshore waters of the northern Adriatic Sea and the significant inverse relationship of these species' abundances with salinity suggested the possibility of their transport by Italian river-influenced coastal waters towards the eastern Croatian coast during the summer season and under stratified conditions. Toxicity events occurred more frequently in the more eutrophicated northern Adriatic Sea than in the southern Adriatic Sea and mostly succeeded the rainfall periods. Diarrhetic shellfish poisoning (DSP) toxin profile analyses identified okadaic acid and yessotoxin as the main DSP toxins occurring in Croatian waters.

In September 2015, a massive occurrence of the Ostreopsis species was recorded in central Adriatic Kaštela Bay. In order to taxonomically identify the Ostreopsis species responsible for this event and determine their toxin profile, cells collected in seawater and from benthic macroalgae were analyzed. Conservative taxonomic methods (light microscopy and SEM) and molecular methods (PCR-based assay) allowed the identification of the species Ostreopsis cf. ovata associated with Coolia monotis. The abundance of O. cf. ovata reached 2.9 × 10⁴ cells L⁻¹ in seawater, while on macroalgae, it was estimated to be up to 2.67 × 10⁶ cells g⁻¹ of macroalgae fresh weight and 14.4 × 10⁶ cells g⁻¹ of macroalgae dry weight. An indirect sandwich immunoenzymatic assay (ELISA) and liquid chromatography–high-resolution mass spectrometry (LC-HRMS) were used to determine the toxin profile. The ELISA assay revealed the presence of 5.6 pg palytoxin (PLTX) equivalents per O. cf. ovata cell. LC-HRMS was used for further characterization of the toxin profile, which showed that there were 6.3 pg of the sum of ovatoxins (OVTXs) and isobaric PLTX per O. cf. ovata cell, with a prevalence of OVTXs (6.2 pg cell⁻¹), while the isobaric PLTX concentration was very low (0.1 pg cell⁻¹). Among OVTXs, the highest concentration was recorded for OVTX-a (3.6 pg cell⁻¹), followed by OVTX-b (1.3 pg cell⁻¹), OVTX-d (1.1 pg cell⁻¹), and OVTX-c (0.2 pg cell⁻¹).