Ecological water quality problems are frequently connected to increment of phytoplankton productivity and overdominance of some phytoplankton species. Metrics that show monotonously increasing or decreasing tendencies along stressor gradients is recommended for ecological state assessment. Diversity metrics are influenced by various physical disturbances and show high within-year variability; thus, there is no agreement on the usefulness of these metrics as state indicators.To test the usefulness of phytoplankton diversity in ecological state assessment we investigated the productivity-diversity relationships for lakes and rivers in the Carpathian Basin (Hungary). We demonstrated that the shape of productivity-diversity relationship depends on the investigated water body type. Regarding lakes, hump-shaped relationship was found for all computed metrics. Parallel with the increase in phytoplankton productivity values, diversity metrics showed monotonously increasing tendencies in rhithral and decreasing tendencies in large potamal rivers. We found no systematic relationship in the case of small lowland rivers. Changes of diversity metrics calculated for species and functional groups showed similar tendencies within the types, only the slopes of regression lines differ each other. The use of diversity metrics as ecological state indicators should be restricted to water body types where diversity decreases or increases monotonously with phytoplankton biomass. Regarding the lakes the use of diversity metrics is not recommended for ecological state assessment. In rhithral and large potamal river assessment, application of diversity metrics should be strongly considered. We demonstrated that diversity metrics can be useful components of multimetric indices proposed to use by the Water Framework Directive.

Key-words:bigheaded carp, gill raker morphology, filter-feeding, food size, plankton Bigheaded carps and especially silver carp have been considered as an effective biological control for algal blooms, thus were introduced to several countries in the last decades, including Hungary. Our aim was to explore the feeding habits of bigheaded carps in Lake Balaton (Hungary), where the stock consists mainly of hybrids (silver carp × bighead carp). We examined the relationship between filtering apparatus (gill raker) morphology and size-distribution of planktonic organisms in the food. We failed to find any significant relationship between gill raker parameters and plankton composition in the filtered material. Bigheaded carps with various types of gill rakers consumed food within the same size-spectrum, independently of the rate of hybridization. However, the linkage between the proportion of different planktonic size classes in the water and in the diet of fish was detectable in case of both phytoplankton and zooplankton consumption, suggesting that the seasonally variable availability of different food items was an important factor in determining the food composition of bigheaded carps. We can deduce that bigheaded carps consume high amounts of zooplankton to meet their energy requirements, and the diet overlap among bigheaded carps and other planktivores may exert negative effects on native fish populations. RÉSUMÉRelation entre la morphologie des branchiospines et les habitudes alimentaires d'hybrides de carpes à grosse tête (Hypophthalmichthys spp) Mots-clés :carpe à grosse tête, La carpe à grosse tête et en particulier la carpe argentée ont été considérées comme un moyen de contrôle biologique efficace contre la prolifération des algues, ainsi elles ont été introduites dans plusieurs pays dans les dernières dé-cennies, dont la Hongrie. Notre objectif était d'étudier les habitudes alimentaires des carpes à grosse tête dans le lac Balaton (Hongrie), où le stock se compose principalement d'hybrides (carpe argentée × carpe à grosse tête). Nous avons

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25While phytoplankton studies on large potamal rivers have increased in number in recent years, upper 26 river sections have received considerably less attention. However, in order to better understand 27 processes that govern the development of dominance of euplanktonic elements in the lower river 28 sections, detailed studies of the upstream areas are necessary. We studied the composition, diversity

19The question of how species richness depends on the area is one of the most intensively 20 studied subjects in biogeography. Many studies reported this pattern for terrestrial and 21 macroscopic taxa, however microscopic and aquatic communities received much less 22 attention in the literature. The aim of our study was to reveal the relationship between the 23 habitat size and the richness of freshwater benthic diatom assemblages. We hypothesized that 24 if the size of studied water bodies covers wide spatial scales the species-area relationship 25 (SAR) could be described by a sigmoid model. Benthic diatom assemblages were investigated 26 in pools, ponds and lakes of various sizes (10 -2 -10 8 m 2 ). We demonstrated that although the 27 SAR in the log-log space can be described by linear model, the linear breakpoint regression