Vera Istvánovics

Updating water quality targets for shallow Lake Balaton (Hungary), recovering from eutrophication

The paper presents an overview about recovery of shallow Lake Balaton from eutrophication by assessing quantitative and qualitative changes in phytoplankton, zooplankton, and chironomids as a function of load reduction. The aim was to update the present water quality targets. The proposed targeting scheme supplements the existing one with a range of lake-specific ecological criteria. We conclude that simple targets (desired phytoplankton biomass and permissible load) are the best choice during the initial stage of eutrophication management, but more complex schemes including ecological criteria are needed to trace recovery when re-organization of the ecosystem takes place.

Factors influencing lake recovery from eutrophication : The case of Basin 1 of Lake Balaton

Lake Balaton is a large, shallow, and calcareous lake that was subject to a rapid eutrophication during the 1970s. Management measures taken from the mid-1980s decreased the phosphorus load to the lake from 0.5 to 0.3 g P m-2 yr-1. Using long-term load and water quality data, we analyse the response of the formerly hypertrophic Basin 1 of the lake by the means of simple empirical models. Several factors that are commonly neglected during studies of lake recovery modified the apparent settling velocity of total P and consequently, the biomass of the phytoplankton. These factors included the loads of calcium and suspended solids, the loading ratio of the dissolved to particulate phosphorus, and blooming of the dominant cyanobacterium, Cylindrospermopsis raciborskii. Due to the rapid immobilisation of the mobile phosphorus in the surface sediments, moderate reduction (45-50%) in the external load resulted in a surprisingly fast and significant improvement of the water quality in the hypertrophic southwestern basins of the lake.

Phytoplankton dynamics in relation to connectivity, flow dynamics and resource availability—the case of a large, lowland river, the Hungarian Tisza

Large lowland rivers with sufficient hydrological storage capacity are capable of supporting primary production, but the dynamics of the advecting phytoplankton is poorly understood. Our study aimed at exploring how longitudinal versus lateral connectivity, flow dynamics versus resource availability and continuous versus discontinuous environmental gradients shaped the species composition of phytoplankton. Samples were taken from February to October 2000 along the Hungarian Tisza River (HTR) and in its main tributaries. Longitudinal and seasonal patterns were related to resources (light and nutrients) availability and flow dynamics derived from a 1D hydrodynamic model. The HTR was autotrophic during the study period, but tributary input considerably exceeded net autochtonous production. The Szamos River was the major source of both phytoplankton and nutrients in the HTR. Chryso- and euglenophytes were flushed into the main river from floodplain oxbows during high discharge. Imported algae experienced discontinuity in environmental gradients when entering the main river. The merged impact areas of two dams (IAD) that separate the two large meandering patches of the HTR disrupted the longitudinal profiles of both physico-chemical variables and attributes of algal assemblages (biomass, species composition, richness, similarity between adjacent sampling sites). Hydraulic storage along the IAD selectively favoured the recruitment of cryptophytes that, however, could not compensate for the enhanced sedimentation of diatoms in terms of biomass. Although the meandering patches presented several small-scale differences in major environmental gradients, both patches supported the growth of planktonic diatoms. Changes in algal biomass were decoupled from nutrient availability. We conclude that various measures must be applied in various lowland rivers within the same catchment to control their trophic status as a component of the ‘good ecological status’ defined in the Water Framework Directive.

The role of biota in shaping the phosphorus cycle in lakes

Abstract Although phosphorus is an abundant element on Earth, its low availability often constrains the growth and/or biomass of aquatic biota. Introducing large quantities of available P into the biosphere, humans have opened up the relatively closed biogeochemical cycle of P, resulting in the eutrophication of many types of aquatic ecosystems worldwide. A thorough understanding of the P cycle is needed, therefore, to both understand the structure and functioning of aquatic ecosystems and to preserve the quality of our aquatic resources. In this review, we deal first with the often misused concept of ‘nutrient limitation’. The rather general use of P uptake kinetics as an indicator of nutrient deficiency requires a discussion on methodology. Since metabolic rates and nutrient demands scale with the size of organisms, coexistence of aquatic osmotrophs relies on unique adaptations and is controlled by the whole network of ecological interactions. Some of these adaptations and interactions are reviewed, with a focus on P cycling. Finally, a case study demonstrates that the complicated P cycle must be simplified to extremes to predict eutrophication-related changes in a shallow lake.

Efficiency of nutrient management in controlling eutrophication of running waters in the Middle Danube Basin

Nutrient emission dropped significantly during the last two decades in the Danube Basin. To assess the effect of reduced nutrient loads on the trophic status of running waters, this regional study analyzed the relationships between nutrients (P and N) and suspended chlorophyll (Chl) using long-term monitoring data in Hungary. Including the upstream catchments of trans-boundary rivers, the study covered an approximate area of 400,000xa0km2, equivalent to the half of the entire Danube catchment. Decadal median Chl was unrelated to P and N concentrations in the whole data set and weakly related to total P (TP) at natural-moderately polluted (N-MP) sites, which were distinguished from highly polluted (HP) sites by using cutoff values for chloride, chemical oxygen demand and TP. At both the N-MP sites and most of the HP sites, Chl increased with channel length. This indicated that water residence time was a more important determinant of Chl than nutrients. Nutrient concentrations showed a significant downward trend in time at half of our sites. With a nearly equal frequency, a parallel trend might or might not occur in Chl. The apparent efficiency of nutrient management was expressed as the quotient of the slopes of linear trends in Chl and nutrients. At sites within 150xa0km from source, this efficiency was marginal. In larger rivers, efficiency improved steeply. The highest efficiency was observed in the downstream reach of the Danube (upstream length >1,300xa0km) where P availability might frequently limit algal growth. The results suggest that eutrophication management in rivers should be based on Chl response functions, rather than universal nutrient criteria. Four Chl response classes were identified based on the observed longitudinal P and Chl gradients.

Management measures and long-term water quality changes in Lake Balaton (Hungary)

A reduction in the external and internal P loadings along with the removal of benthivorous cyprinids resulted in a significant improvement of the water quality in the hypertrophic western part of Lake Balaton by 1996, some ten years following first management measure. Enhanced internal P loading after the reduction in external nutrient load is clearly an important factor explaining the duration of recovery in shallow lakes. Phytoplankton responded quite rapidly to reductions in nutrient loading and fish stock, whereas the effect on both the structure and abundance of the crustacean plankton was less pronounced. The role of top-down control of phytoplankton by crustaceans following nutrient reduction is not clearly understood in shallow lakes. Grazing did not play a significant role in the overall reduction of algal biomass in Lake Balaton. Since high concentration of suspended mineral particles from both sediment resuspension and lime precipitation causes permanent food limitation of daphnids, the top-down effects were negligible in Lake Balaton following fish stock reduction. Fish community structure did not respond to the lower external and internal TP loads cyprinids being dominant in both the pre- and post-management periods.

Stability and change of phytoplankton communities in a highly dynamic environment—the case of large, shallow Lake Balaton (Hungary)

Time series data of key environmental variables (water temperature, global radiation, vertical light attenuation, internal P load) and biomass of four colour classes of photosynthetically active algae were collected during 2003 and 2004 with daily resolution. Using these data, seasonal patterns of phytoplankton were analyzed as a function of the dynamic environment. Abstraction of the environmental state as a point in multi-dimensional space was used to identify habitat templates of bloom-forming groups and derive an indicator of environmental stability/physical disturbance. These templates were synthesized into a simple threshold model that sufficiently simulated development and collapse of various blooms. Blooms were, however, rare events related to specific environments with strong, unidirectional forcing. Tentative quantification of disturbance and compositional stability/community change allowed discriminating disturbance-driven changes and autogenic succession with reasonable success. The two processes were found to be equally important in shaping the composition and biomass of phytoplankton.