Gábor Paulovits

The role of the Kis-Balaton Water Protection System in the control of water quality of Lake Balaton

Abstract Since the 1960s the nutrient load of Lake Balaton has increased significantly, causing a clear decline in water quality. To retain the nutrients from the lake, the Kis (small)-Balaton Water Protection System (KBWPS) was designed on the lower part of the main inflow, the River Zala. The first part of the KBWPS started to function in July 1985. After a few years the system became hypertrophic. According to our results, approximately 80 000 t of suspended solids, 300 t of total phosphorus (TP), 250 t of phosphate-P, 850 t of total nitrogen (TN), and 2450 t of nitrate-N were retained between 1986 and 1997 by the first part of the system. At present, the KBWPS retains about half of the suspended solids, more than one-third of TP, more than two-thirds of phosphate-P, only one-tenth of TN, but more than half of nitrate. A 16 km 2 area of the second part has been operating experimentally since 1992. This part retains approximately 75% of suspended solids (mostly of phytoplankton origin) coming from the first phase, but the phosphorus retention is low due to release of phosphorus from sediments. Until 1991 there were no significant differences in monthly means of the chlorophyll-a content from the western part of Lake Balaton. Since that time the chlorophyll-a content has been decreasing in the lake. This decrease could have been the result of the combined effects of weather conditions (temperature, rainfall) and nutrient availability.

Biomass dependent interactions in pond ecosystems: responses of lower trophic levels to fish manipulations

The effects of mature benthivorous cyprinid fish and theirrecruitment on sediment resuspension, turbidity, phyto- andzooplankton, and benthic macroinvertebrates were studied in fourexperimental ponds. The ponds were stocked with bream (Abramisbrama L.), white bream (Blicca bjoerkna L.), roach (Rutilus rutilus L.) and wild carp (Cyprinus carpio L.) of3+–5+ age classes at standing crop biomass varying from 0 to500 kg ha-1. Cyprinids caused an increase in sedimentresuspension and in turbidity, in proportion to their biomass. Meancrustacean biomass did not significantly affect phytoplanktonbiomass due to intense grazing by fish during spring. Ponds withhigh fish stocks showed reduced midge biomass and vegetation coverand increased biomass of predatory invertebrates.

Biomass of planktonic crustaceans and the food of young cyprinids in the littoral zone of Lake Balaton

The littoral zone of Lake Balaton and its periphyton-zooplankton-fish communities have been investigated intensively in recent years. Total average number of crustacean plankton varied from 36 to 126 ind l−1, their biomass from 0.49 to 1.86 mg ww l−1 month−1 at different areas of the littoral zone. In general, these values for the above parameters were higher in hypertrophic areas. 23 fish species occurred in the littoral zone with cyprinids dominating. The seasonal food spectra of Y-O-Y roach (Rutilus rutilus), white bream (Blicca bjoerkna) and bream (Abramis brema) were based mainly on planktonic crustaceans and benthic/periphytic invertebrates. According to the frequency of occurrence of crustaceans and other invertebrates, the food composition of young cyprinids differed significantly in the NE and SW-basins of the lake.

Indirect effect of different fish communities on nutrient chlorophyll relationship in shallow hypertrophic water quality reservoirs

Effects of different fish communities on the proportion of different nitrogen and phosphorous forms and the amount of phytoplankton (chlorophyll a) were examined in two consecutive years (1992–1993) in three Hungarian shallow water reservoirs (Cassette and outer reservoir of the Kis–Balaton Water Protection System, and Marcali reservoir). Possible interactions between nutrient concentrations and the amount of phytoplankton in these reservoirs were also examined. Considerable differences in the proportions of different nutrient forms were observed between the three test sites, which could be explained by the presence of different fish stocks in these reservoirs. In the Cassette, the fish biomass necessary for a water quality improvement was around 50 kg ha−1. Phytoplankton biomass was controlled by the zooplankton, consequently chlorophyll a concentrations decreased considerably, while those of dissolved nutrients significantly increased. In the outer reservoir, phytoplankton was controlled bottom-up, since the 250 kg ha−1 fish biomass was larger than the critical value due to the high proportion of planktivorous species. Chlorophyll a concentrations were high, and nutrients were mainly in particulate form (in algal cells). In the Marcali reservoir, the recently introduced silver carp population could not control fully the phytoplankton. The biomass of phytoplankton decreased only slightly, while its composition changed considerably. Although biomanipulation with silver carp is suitable for ceasing cyanobacterial blooms, reduction of the amount of planktivorous fish seems to be a more adequate method for increasing water transparency, rather than introduction of phytoplankton feeding fish.

Age determination and growth of eel, Anguilla anguilla (L.), in lake Fertő Hungary

Abstract Ground and polished otolith sections, 0.2 mm thick, were produced from both sagittae of each fish, and repeated readings of the growth marks resulted in more accurate estimates of fish age. Supernumary zone formation in eel otoliths was a common phenomenon. The growth in length of the eels was accurately described by Bertalanffys model using back-calculated lengths. Values of L ∞ = 1459 mm, K = 0.058 and t 0 = −0.869 years were determined. Mean incremental growth was calculated as 5–9 cm per year, showing that length increases rapidly with age. However, the growth of the eel population in Lake Ferto has become stunted, probably due to overstocking since 1975.

The food of bream (Abramis brama L.) in two basins of Lake Balaton of different trophic status

Contrary to earlier observations the food composition of bream (Abramis brama L.) in Lake Balaton has changed in parallel with eutrophication. These changes were probably caused by increased population density and connected with density dependent growth as well as sharpened inter- and intraspecific competition in the cyprinid community. According to the frequency of occurrence, zooplankton followed by benthic invertebrates comprised the majority of the breams food. By weight, however, benthic food dominated in both basins. Size-related dietary changes were pronounced, but statistically not always significant, suggesting the overall importance of the most abundant and available prey types. Bream did not strongly select any of the zooplankton groups, however, seasonal changes in food composition alternated in parallel with the trophic gradient along the longitudinal axis of the lake.

The effect of different fish communities on the cladoceran plankton assemblages of the Kis-Balaton Reservoir, Hungary

In 1995 the authors studied the effect of different fish communities on the structure of the cladoceran plankton in a shallow hypertrophic lake. After a fish kill of 1991, different fish communities developed in the Kazetta and the outer area of the Kis-Balaton reservoir. In the outer area of the reservoir, the densities of plankton feeding fish species were considerably higher than in the Kazetta. These differences induced changes in the structure and dynamics of the cladoceran plankton. The biomass of small-bodied cladocerans (mainly Bosmina longispina) was higher and the biomass of the large-bodied cladocerans (D. hyalina, D. magna) was lower in the western and eastern part of Kis-Balaton reservoir than in the Kazetta. A peak in cladoceran biomass in the Kis-Balaton reservoir was observed during the summer, close or during a bloom of filamentous cyanobacteria, whereas in the Kazetta a peak was observed during the spring, before the bloom of cyanobacteria. The adult females of D. hyalina were larger and produced more eggs in the Kazetta than in the outer area of the reservoir.

Management of fish communities and its impacts on the lower trophic levels in shallow ecosystems in Hungary

This study shows that multiyear control of phytoplankton by grazing is possible in otherwise cladoceran dominated ecosystems at low cyprinid fish stocks (around 100 kg ha−1) and where piscivore populations, following manipulation measure, can be sustained at the biomass ratio >15%. This reinforces the idea that fish community structure may be a key to the stability of trophic structures that suppress phytoplankton in ecosystems where otherwise cladoceran plankton dominates. Experimental ponds with lower fish biomass (<150-kg ha−1) had less chlorophyll-a concentration per unit TP than those with higher fish biomass. Regressions of chlorophyll-a vs. total phosphorus in the ponds and Major Lake were not significant at lower fish biomass. However, at higher fish biomass the bottom-up processes dominated across all types of ecosystems studied and the regressions were significant. The biomass of herbivorous Cladocera was significantly higher in ponds with a higher percentage of piscivores.

Regulation of plankton by omnivore cyprinids in a shallow lake in the Kis-Balaton Reservoir System

A long-term food web manipulation experiment was started in 1999 with monitoring in the eutrophic shallow Major Lake (area 11 ha, mean depth 1.1 m). In 2000, studies were continued with removal of 204 kg ha−1 of fish and restocking with 134 kg ha−1 of cyprinids. The removal of about 50% of cyprinid had a quite considerable impact on almost all of the observed parameters. Summer mean chlorophyll a values in 2000 decreased by 43%, phytoplankton community changed from being dominated by filamentous blue-greens causing blooms in 1999 to a much more diverse community with, e.g., green algae, dinoflagellates and cryptomonads in the year following fish removal. The zooplankton community showed the typical composition of lakes with very high predation pressure of omnivorous cyprinid fish leading to a decrease in both mean body length and fecundity of daphnids.

Risk assessment of non-native fishes in the catchment of the largest Central-European shallow lake (Lake Balaton, Hungary)

The Fish Invasiveness Screening Kit (FISK) has proved to be a useful tool for assessing and screening the risk posed by potentially invasive fish species in larger risk assessment (RA) areas (i.e. country or multi-country level). In the present study, non-native freshwater fishes were screened for a smaller RA area, the closed and vulnerable but economically important drainage basin of Lake Balaton (Hungary). Receiver operator characteristic analysis of FISK scores for 26 fish species screened by four assessors identified 21 species with scores of ≥11.4 to pose a ‘high risk’ of being invasive, with five species ranked as ‘medium risk’ and none as ‘low risk’. The highest scoring species were gibel carp Carassius gibelio and black bullhead Ameiurus melas, with three Ponto-Caspian Gobiidae identified as amongst the species posing the potentially greatest threat to the catchment. The results of the present study indicate that FISK can be applied to risk assessment areas of smaller geographical scale.