Juta Haberman

Zooplankton as indicators in lakes: a scientific-based plea for including zooplankton in the ecological quality assessment of lakes according to the European Water Framework Directive (WFD)

With the implementation of the EU Water Framework Directive (WFD), the member states have to classify the ecological status of surface waters following standardised procedures. It was a matter of some surprise to lake ecologists that zooplankton were not included as a biological quality element (BQE) despite their being considered to be an important and integrated component of the pelagic food web. To the best of our knowledge, the decision of omitting zooplankton is not wise, and it has resulted in the withdrawal of zooplankton from many so-far-solid monitoring programmes. Using examples from particularly Danish, Estonian, and the UK lakes, we show that zooplankton (sampled from the water and the sediment) have a strong indicator value, which cannot be covered by sampling fish and phytoplankton without a very comprehensive and costly effort. When selecting the right metrics, zooplankton are cost-efficient indicators of the trophic state and ecological quality of lakes. Moreover, they are important indicators of the success/failure of measures taken to bring the lakes to at least good ecological status. Therefore, we strongly recommend the EU to include zooplankton as a central BQE in the WFD assessments, and undertake similar regional calibration exercises to obtain relevant and robust metrics also for zooplankton as is being done at present in the cases of fish, phytoplankton, macrophytes and benthic invertebrates.

Contribution of different zooplankton groups in grazing on phytoplankton in shallow eutrophic Lake Võrtsjärv (Estonia)

The grazing impact of different sized zooplankton on ‘edible’ and total phytoplankton biomass and primary production was measured in L. Vortsjarv during a seasonal study in 1998 and 2000. The organisms of 48–100 μm size class, composed of ciliates and rotifers, contributed significantly to the total grazing of zooplankton community throughout the study period (average 68%). The average daily filtering and grazing rate of the whole zooplankton community (micro- and macro-zooplankton) remained low, corresponding to a filtration of 44% of the water volume, 4% of the total phytoplankton biomass and 29% of primary production. However, a strong grazing pressure on small-sized phytoplankton (<30 μm) was estimated in most of the study period (average 44% d−1). Among size classes of ‘edible’ phytoplankton, the size range 5–15 μm was the most important algal food for the dominant zooplankton grazers (herbivorous ciliates, Polyarthra spp., Chydorus sphaericus and Daphnia cucullata) in L. Vortsjarv.

The role of cladocerans reflecting the trophic status of two large and shallow Estonian lakes

The role of pelagic cladoceran communities is discussed on the basis of a comparative study conducted in two Estonian lakes, the moderately eutrophic Lake Peipsi (Ntot 700, Ptot 40 μg l−1 as average of ice-free period of 1997–2003) and in a strongly eutrophic Lake Vortsjarv (Ntot 1600, Ptot 54 μg l−1). The cladoceran community was found to reflect the differences in the trophic state of these lakes. In L. Peipsi, characteristic species of oligo-mesotrophic and eutrophic waters co-dominated (making up 20% or more of total Zooplankton abundance or biomass), whereas in L. Vortsjarv only species of eutrophic waters occurred. In L. Peipsi, the dominant cladocerans were Bosmina berolinensis and Daphnia galeata, while Chydorus sphaericus was the most abundant cladoceran in L. Vortsjarv. The cladocerans of L. Peipsi (mean individual wet weight 25 μg) were significantly (threefold) larger than those of L. Vortsjarv (8 μg). The mean wet biomass of cladocerans was higher and total cladoceran abundance was lower in L. Peipsi compared to L. Vortsjarv (biomass varied from 0.133 to 1.570 g m−3; mean value 0.800 g m−3 in L. Peipsi and from 0.201 to 0.706 g m−3, mean 0.400 g m−3 in L. Vortsjarv; the corresponding data for abundances were: 8,000-43,000 ind m−3, mean 30,000 ind m−3 for L. Peipsi, 50,000–100,000, mean 52,000 ind m−3 for L. Vortsjarv). Based upon differences in body size, cladocerans were more effective transporters of energy in L. Peipsi than in L. Vortsjarv. Cladocerans proved to be informative indicators of the trophic status and of the efficiency of the food web in studied lakes.

Effect of winter conditions on spring nutrient concentrations and plankton in a large shallow Lake Peipsi (Estonia/Russia)

During the study period 1997–2007, the duration of the ice cover of Lake Peipsi, the largest freshwater lake in Estonia, was variable. The ice-break took place in mid-April instead of generally in May. Compared with data from 1960s onwards, a trend of shortening of the duration of ice-cover period is evident. In under-ice phytoplankton, diatoms, cryptophytes and flagellated chlorophytes predominated from March to June. In zooplankton, while thermophobic rotifers dominated in densities, copepods were dominant in biomass. The monthly sum of positive water temperatures correlated with the zooplankton in the following month and duration of the ice cover correlated with the biomass of all different zooplankton groups in May and June. The sum of water temperatures in April, May and June was positively correlated with the biomass of cladocerans and copepods, and negatively with the biomass of winter rotifers. Mild winters affected nitrogen and silica concentration positively and phosphorus concentration negatively.

Lake Peipsi: Changes in nutrient elements and plankton communities in the last decade

In the 1990s, as a consequence of a decline in agricultural production in the watershed and a decrease in the amount of waste water discharged into rivers, the nutrient load carried from the catchment area into large, shallow, eutrophic Lake Peipsi (area 3,555 km 2 ) decreased. The aim of the present study was to analyze the in-lake response of key physical and chemical variables and the biota to large-scale changes in the nutrient load. Yearly changes in water transparency, nutrient elements of surface water, chlorophyll a content, as well as in phyto- and zooplankton biomass were studied during the growth season of 1992-2000. A clear decline in total nitrogen, ammonium ion, and orthophosphate ions was revealed in the northern part of the lake over the studied years. Beginning from 1996-1997, a decreasing tendency was revealed for the nitrate ion and total phosphorus. A significant decline of the total nitrogen:total phosphorus ratio was also observed from 1992-2000. However, the biomass of phytoplankton (particularly cyanobacteria) and chlorophyll a concentration did not follow the dynamics of nutrients, but displayed an increasing trend. Concentrations of nutrients in the lake during the last decade were not so low as to limit phytoplankton growth directly. In Lake Peipsi, strong and long-lasting algal blooms were observed in recent years, despite a definite decline in the nutrient content of surface water. Weather conditions appear to be very important factors in causing algal blooms in Lake Peipsi.

The rotifers of Lake Peipus

In the northern part of Lake Peipus, 140 taxa of rotifers were identified, with species of Anuraeopsis, Conochilus, Keratella, Polyarthra and Synchaeta dominating. Two main periods of sexual reproduction occur, in the spring and autumn. Different life cycle patterns are represented. Rotifer number and biomass have two maxima between spring and early autumn. The contribution of rotifers to total zooplankton production varies from 13.6% (Oct.) to 89.8% (May). The average production of grazing rotifers is 485.1 kJ m−2, while that of predatory rotifers (Asplanchna) is 10.0 kJ m−2.

Dominant rotifers of Võrtsjärv (Estonia)

Rotifers form 71% of the zooplankton of the strongly eutrophic (total N 2 g m−3, total P 53 mg m−3) Vortsjarv (Estonia). Altogether 150 taxa of rotifers occur. Species characteristic of oligo- and mesotrophic waters have totally disappeared during the last 30 years, or are disappearing now. Species whose numbers and biomass reached 20% or more of total zooplankton were considered dominants. These were Anuraeopsis fissa, Keratella cochlearis, K. quadrata frenzeli, Polyarthra dolichoptera, P. luminosa, Synchaeta verrucosa and Trichocerca rousseleti. The contribution of dominant rotifers to total zooplankton and its biomass is as follows: S. verrucosa 25% and 39%, P.dolichoptera 34% and 25%, K. cochlearis 25% and 7%, K. quadrata frenzeli 9% and 7%, A. fissa 28% and 0.4%, T. rousseleti 20% and 0.6%, respectively.

Factors controlling the three-decade long rise in cyanobacteria biomass in a eutrophic shallow lake

We aimed at quantifying the importance of limnological variables in the decadal rise of cyanobacteria biomass in shallow hemiboreal lakes. We constructed estimates of cyanobacteria (blue-green algae) biomass in a large, eutrophic lake (Estonia, Northeastern Europe) from a database comprising 28 limnological variables and spanning more than 50years of monitoring. Using a dual-model approach consisting in a boosted regression trees (BRT) followed by a generalized least squares (GLS) model, our results revealed that six variables were most influential for assessing the variance of cyanobacteria biomass. Cyanobacteria response to nitrate concentration and rotifer abundance was negative, whereas it was positive to pH, temperature, cladoceran and copepod biomass. Response to total phosphorus (TP) and total phosphorus to total nitrogen ratio was very weak, which suggests that actual in-lake TP concentration is still above limiting values. The most efficient GLS model, which explained nearly two thirds (r2=0.65) of the variance of cyanobacteria biomass included nitrate concentration, water temperature and pH. The very high number of observations (maximum n=525) supports the robustness of the models. Our results suggest that the decadal rise of blue-green algae in shallow lakes lies in the interaction between cultural eutrophication and global warming which bring in-lake physical and chemical conditions closer to cyanobacteria optima.

Some notes on Brachionus rotundiformis (Tschugunoff) in Lake Palaeostomi

In 1933 a connection was formed between the Black Sea and Lake Palaeostomi (Georgia) after which the latter became a brackish-water lake with water salinity up to 13{‰}. Water salinity is variable, depending first of all on the direction and strength of the wind. Brachionus rotundiformis (Tschugunoff) (with 20% or more of total zooplankton abundance) was the dominant zooplankton species in the samples collected in 1977 (July, Aug) and 1996 (July) from this lake. The morphology and ratio of physiological types of female and the population density of dominant species were studied. The lorica length of females was 105–250 μm, its widest part 95–250 μm. The average abundance fluctuated between 5 and 2,600 × 103 ind m-3. A positive correlation was revealed between the occurrence of B. rotundiformis and water salinity (r= 0.7 P < 0.0001). B. rotundiformis formed 84% of the abundance of rotifers and 49% of the numbers of total zooplankton. Several types of egg bearing female were detected. The proportions of females with egg types (M + F), (F + M) and (D/F) were 0.13, 0 and 0.01% in 1977 and 0.14, 0.01 and 0.01% in 1996, respectively.

The status of the southernmost part (Lake Pihkva) of large Lake Peipsi: A purification pond or polluter?

Large Lake Peipsi (3555 km2) is divided between two countries, Estonia (44%) and Russia (56%). The southernmost part of the lake, Lake Pihkva, is almost entirely (99%) in Russia, and is continuously being polluted by the greatest inflow to Lake Peipsi, the insufficiently purified River Velikaya. Research on this lake (Lake Peipsi) in the growing season has been possible as a result of Estonian–Russian joint expeditions (2003–2010) in the month of August. Our study has demonstrated statistically significant variances in the hydrochemistry and zoo- and phytoplankton data between the lake parts. Several parameters used for characterizing the lakes ecosystem were considerably higher for Lake Pihkva than for the larger Lake Peipsi s.s. These include increases in TP concentration (3×); TN (2×); Chlorophyll a (3×); biomass of cyanobacteria (4×); Microcystis (4×); Aphanizomenon (5×); abundance of Chydorus sphaericus (5×); Keratella tecta (8×). However, several values were lower in Lake Pihkva than in Lake Peipsi: water transparency (3×); ratio between zoo- and phytoplankton biomasses (2×); cladoceran mean weight (3×); abundance of Eudiaptomus gracilis (1.5×); abundance of Kellicottia longispina (1.8×). Different natural conditions (topography, catchment area, relative depth) and different pollution loads in the lake parts have resulted in apparently different resistances in their ecosystems and different responses to human activity. At present, on the basis of TP data (up to 200 mg m−3), Lake Pihkva appears to act a polluter of adjacent lake part rather than a purification pond.