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The microphytoplankton in Lake Visovačko jezero was studied during one year period, on base of monthly sampling data covering period from April 1995 to March 1996. Changes in species composition and seasonality of the dominant species were investigated in relation to the environmental variables. Diatoms dominated the phytoplankton assemblage of the lenitic barrage area of karstic River Krka. The master species in Lake Visovačko jezero was Asterionella formosa Hass. The dinoflagellates (Ceratium hirundinella (O. F. Müller) Bergh) were generally the second most representative group in terms of abundance, where their annual development has been shifted to the late summer and autumn period. In the Lake Visovačko jezero, under mesotrophic conditions Reynolds groups B and C are present, with exception of late summer and autumn assemblages which were characterized by association of large dinoflagellate species known as Group LM. The effects of the short term dynamics of environmental factors on the algal growth in barrage lenitic area of calcareous river stressed several variables as important factors controlling annual microphytoplankton distribution. The temperature was the most valuable variable affecting the species abundance, changes in dominance and species composition (Primer 5, BIO-ENV). CCA analysis performed on abundance dataset and environmental variables confirm the importance of temperature but also highlighted total phosphorus, conductivity and silica.

The present paper is a revision of diatoms and dinoflagellates contributions to the algal assemblages in Croatian waterbodies concerning the spices number and diversity. The aim is to define diatoms and dinoflagellates flora in several freshwater and some brackish habitats in Mediterranean and continental part of Croatia. Study contains data sets of several investigations held at Krka River, Zrmanja River, Drava River and Plitvice Lakes. Analyses were based on relevant studies and available data during last decade. In the term of species number diatoms dominated mostly the assemblages same planktonic, periphyitic and benthic, in Croatian rivers, their barrage lakes, reservoirs and estuary. Since taxonomy is rapidly developing discipline, instead of conclusion, in the paper, several future objective are defined: (i) future taxonomical investigations of centric diatom species and dinoflagellates (ii) definition of annual development of species concerning species environmental requirements, and (iii) in the term of taxonomy, comparisons of similar and species sharing the same names, and concerning ecology, comparison of assemblages, with similar in neighboring countries.

Karstic lake ecosystems are specific and have great ecological significance. They provide unique habitats to numerous organisms, macroinvertebrates as well as planktonic organisms, the latter representing the reasons why it is important to understand how those complex and diverse ecosystems function. Dinoflagellates are an important part of phytoplankton community as they sometimes contribute significantly to the total phytoplankton biomass. Results in this paper were obtained during phytoplankton ecology research with the objective to document armored dinoflagellate species. The taxonomic interest for dinoflagellates started during project of comparative studies on centric diatoms and dinoflagellates of Hungarian and Croatian large rivers, reservoirs and lakes and continued with project of taxonomical comparative studies on dinoflagellates species on the same area. During the projects, phytoplankton samples collected in period from 1995 to 2006 on 21 localities in Croatia were reviewed. Preliminary results of first project were presented on Croatian Biological Congress in 2006 as first annotated check list of 20 free-living armored dinoflagellates of the Croatian freshwater systems. A main goal of the paper was to present detailed taxonomic morphological features of armored dinoflagellate taxa assigned to the genera Ceratium, Peridinum and Peridiniopsis in Croatian freshwater karstic systems. Taxonomical observations and general morphological structures are based on SEM and light micrographs. Results present data from scanning electron microscopy, sketchy drawings of main distinguishing features and species-specific ecological preferences and/or tolerance in karstic systems. Also, taxonomical issues of problematic species, such as Peridinium umbonatum/incospicuum group, are shortly discussed.

Phosphatase enzymes are capable of releasing phosphate through cleavage of phosphoester bonds. The seasonal importance of this process was examined by using a model substrate paranitrophenylphosphate and the Michaelis-Menten equation to estimate the release rate of PO4-P from phosphomonoesters. The seasonal occurrence of phosphomonoesters and acid phosphatase activity was used to estimate the velocity of phosphate release from these compounds. Filter fractionation of phosphatase activity demonstrated most activity (> 60 %) was in size fractions less than 0.45 μm. The release rates were highest in May and June (15 to 25 nmol L-1 min -1) during the Gonyostomum semen (Raphidophyta) bloom and decreased to less than 2 nmol L-1 min-1 in two weeks and remained low throughout the summer and the fall. Fractionation of 32P-H3PO4 labelled dissolved organic phosphorus showed this fraction to vary considerably through the year. Potential phosphate release declined through the summer and into the fall. Significance of the co-occurrence of phosphomonoesters and acid phosphatase activity maxima and Gonyostomum semen (Raphidophyta) bloom is discussed.

The population of an unknown naviculoid diatom from karstic, polymictic Lake Vrana in Croatia was identified as Navicula hedinii Hustedt, a species that was described in 1922 from a small lake in eastern Turkestan. The occurrence of Navicula hedinii in samples collected for the study of the relationship between phytoplankton and environmental variables in a lake, allowed a detailed study of the morphological features of this diatom species using light microscopy (LM), scanning (SEM) and transmission electron microscopy (TEM). Type material collected by Sven Hedin and kept at the Friedrich Hustedt Diatom Collection in Bremerhaven was also examined using light and scanning electron microscopy. These results, the possible conspecificity with the Croatian species and the implication for the correct taxonomical position of this species are discussed. Based on the ultrastructure of this species, it can no longer be included within the taxonomical concept of Navicula s.s. Bory. Due to the large similarity species Navicula pseudocrassirostris Hustedt certainly stands very near to Navicula hedinii and it is used in the comparison. Following a comparative morphological analysis of species with genera bearing similar characters (Adlafia, Veigaludwigia, Kobayasiella, Cavinula, Stenoneis, Climaconeis, Berkeleya, Sellaphora, Cosmioneis), a new genus, Envekadea Van de Vijver, Gligora, Kralj & Cocquyt gen. nov. is proposed. The genus name Envekadea is derived from the NVKD (Nederlands-Vlaamse Kring van Diatomisten), the association of the Dutchspeaking diatomists. The name was chosen to underline its relationship with Adlafia, a genus named after the Association of the French-speaking diatomists (ADLaF)

Dinoflagelati s 21 lokaliteta na području Republike Hrvatske istraživani su u razdoblju od 1995. do 2006. godine. Uzorci fitoplanktona za kvalitativne i kvantitativne analize uzimani su u krškim, baražnim jezerima, akumulacijama, rijekama, povremeno plavljenim područjima i malim stajačicama (lokve, bare). Provedena istraživanja po prvi puta daju sustavan popis slobodno živućih oklopljenih (tekatnih) vrsta dinoflagelata slatkovodnih ekosustava Hrvatske. Sustavni popis dinoflagelata za istraživano područje Hrvatske uključuje 20 vrsta (izuzevši podvrste, varijetete i forme) koje pripadaju rodovima: Ceratium, Peridinium i Peridiniopsis. Za istraživane lokalitete na području Hrvatske po prvi puta su zabilježeni nalazi 12 vrsta i to sedam novih vrsta roda Peridiniopsis /Peridiniopsis borgei Lemmermann, Peridiniopsis cunningtonii Lemmermann, Peridiniopsis elpatiewskyi (Ostenfeld) Bourrelly, Peridiniopsis kevei Grigorszky et Vasas, Peridiniopsis penardiforme (Linderman) Bourrelly, Peridiniopsis penardii (Lemmermann) Bourrelly, Peridiniopsis polonicum (Woloszyńska) Bourrelly/ i pet vrsta roda Peridinium /Peridinium lomnickii, Woloszyńska, Peridinium palatinum Lauterborn, Peridinium gatunense Nygaard, Peridinium umbonatum Stein, Peridinium volzii Lemmermann/. Rezultati istraživanja daju uz taksonomiju, temeljenu na fotodokumentaciji SEM i svjetlosnoj mikroskopiji, uvid u prostornu i sezonsku distribuciju te utjecaj fizičko-kemijskih čimbenika na ekologiju pojedinih vrsta.

Even though the hydrological and hydromorphological characteristics of the River Tisza could qualify it as a standard for international water quality assessment systems, it is among the less known streams due to the lack of systematic middle-and long-term examination programmes. The natural self-purification abilities of the river are often put to test by pollution conveyed by its tributaries. If we want to maintain, or even improve, the present ecological conditions of the Tisza, we have to take its whole drainage basin as an integrated ecological system, with all the advantages and disadvantages this means. According to algological and physico-chemical variables, the main channel of the River Tisza can be divided into three parts in Hungary. The first part, the Tiszabecs section, receives special attention, as this is the only “ upstream-type” large river in Hungary with all featurespertaining to such streams. The following section of the mean channel extends to Ároktő at the mean water stages, and to Tiszadob at low water stages. This is the most problematic section in terms of water quality. As a result of the slowed-down flow of the river, “ nutrients” arriving from both abroad and the Hungarian areas yield a high potentional trophic level. Out arriving from both abroad and the Hungarian areas yield a high potentional trophic level. Out of the Hungarian streams, the River Szamos and the Lónyai-channel have the most significant effect on the plankton composition of the Tisza, as both convey considerable amounts of pollutants into the Tisza. The tributaries have such a strong influence on the main channel – especially at low water periods – that they can entirely transform its structure of organisms. Owing to this, it is practically impossible to define a planktonic population pertaining to this section. Therefore, it is essential to properly detect the pollution load on the Tisza coming from its tributaries. The quite balanced lower section reaches from Ároktő at mean water periods and from Tiszadob at low water periods to Tiszasziget. The section below the Kisköre-reservoir to the entry of the River Hármas-Körös is characterised by a fairly rich plankton population. However, this abundance falls back between the entry of the Hármas-Körös and the collection point at Tiszasziget.

Centric diatom communities of were studied in a few dozen of different lakes in Croatia, Romania and Spain during the last years. Several lakes are situated on protected areas, National Parks, Natura 2000 sites of these countries. The aim of these investigations was first to determine of centric diatom communities of lakes, then to analyse the species composition from a nature conservation point of view. We elaborated a typology of habitats based mainly on substrate, mineralization, water regime and climatology. Then, within the resulting types of environments, we pretend to establish different categories of their ecological status based on nutrients and chlorophyll concentration and other variables related to nature protection. Several lakes were never investigated for centric diatoms, therefore new species was found for Romania for example in Danube Delta (Lake Erentsuk - Thalassiosira gessnerii). Several similarities were found in carstic lakes of Croatia (Lake Plitvice – dominant species: Cyclotella plitvicensis) and Spain (Lake Marquesado – dominant species: C. plitvicensis). We can mention Cyclotella hispanica, described from Lake Banoles (Spain), which is probably endemic species (never found in other countries in Europe). Several species were found which are rare in Europe and are listed in Red List of algae.

For avoiding the unfavorable environmental conditions several of the aquatic microorganisms capable of forming specialized resistance cells like akinets, hypnospores, statospores etc. Recognition of the important role of cysts in the life cycles of dinoflagellates increased the need to study their role in the ecology of phytoplanktons, and this, combined with the knowledge of chemical and biological characteristics of the water, may lead to a better understanding of the spatial and temporal dynamics of dinoflagellates. This paper reports on the effects of temperature, nitrogen, and phosphorus on the percentage of encystment of the dinoflagellate Peridinum cinctum Stein. The phosphorus content of the medium affected encystment only at the highest temperature applied (22°C). Nitrogen content and temperature were the most important factors controlling the encystment.

The centric diatom Cyclotella striata is a common planktonic species in temperate estuaries as well as in the eastern coastal Adriatic Sea. We determined seasonal appearance of the species in the eastern Adriatic Zrmanja River estuary in the period from June 1998 - February 2001. The estuary is highly stratified (salt wedge), with sharp halocline. The depth of the halocline is determined by the Zrmanja river inflow. In summer salinity was high due to low river inflow. The highest inflow (avg 65 m3 s-1) and lowest salinity was registered in the period December - April. Annual temperature varies from 6.7 - 26.55 º ; ; C (avg 16.39 º ; ; C), and the salinity from 0 - 38 psu (avg 18.97 psu). Concentration of nutrients was low and transparency high, implying the oligotrophy of the estuary. In the Zrmanja Estuary microphytoplankton was mostly composed of diatoms (avg 1.9 x 105 cellsL-1), while dinoflagelates (avg 103 cellsL-1) and cocolithophorids (1.2 x 102 cellsL-1) were less abundant. Diatom Cyclotella striata was abundant species in spring (March - May), providing maximum of 2 x 106 cellsL-1. It appeared mostly in short chains of cells with valve diametar of about 8µ ; ; m and pervalval axis length of about 5 µ ; ; m. Cyclotella striata is marine diatom, but prefers decreased salinity. It could be mostly detected in the salinity range between 13 and 30 psu, with maximum development within the range of 13 to 17 psu. It was most abundant in and below the halocline. Due to such termohaline preferences C. striata was mostly distributed in the middle reach of the Zrmanja Estuary (in the area of stations N1 and Z1). In the upper reach of the estuary, average abundance was much lower. Cyclotella striata preferred temperatures from10 - 17 ˚ C. Outer stations were characterized by more stabile ecological conditions and more constant appearance of C. striata.