Taavi Virro

General description of Lake Peipsi-Pihkva

Lake Peipsi-Pihkva (3555 km2, mean depth 8.3 m), consisting of three parts, (L. Peipsi, L. Pihkva, L. Lämmijärv) is located on the border of Estonia and Russia. L. Peipsi belongs to unstratified eutrophic lakes with mesotrophic features, L. Lämmijärv has some dyseutrophic features, while L. Pihkva is strongly eutrophic. The total annual nutrient load is 15.57 tons N km−2 and 327 kgP km−2 with 74% of N and 39% of P originating from agriculture. The mean concentrations of total N and P in the lake are 876 mg m−3 and 46 mg m−3, respectively, both being the highest in L. Pihkva and the lowest in the northern part of L. Peipsi. Average pH is 8.14 and Secchi disk transparency 1.63 m. Diatoms and blue-green algae prevail in phytoplankton biomass. The blue-greens Gloeotrichia echinulata and Aphanizomenon flos-aquae dominate in summer causing the water-blooms. The concentration of Chla was the lowest in the northern part of L. Peipsi (mean 14.7 mg m−3) and the highest in the southern part of L. Pihkva (mean 47.9 mg m−3, median 16.3 mg m−3). An increase of Chla and decrease of Secchi depth could be noticed in 1983–1988, while in 1988–1994 the tendency was opposite. The long-term average primary production is 0.8 g C m−2 d−1. Zooplankton is remarkably rich in species, the average biomass in the vegetative period being 2–3 g m−3 and production 22 g C m−2. The role of rotifers in production is 53% followed by that of cladocerans (30%), copepods (16%) and Dreissena polymorpha larvae (1%). The total count of bacteria is 1–9 million cells per ml. Chironomus plumosus and Potamothrix hammoniensis are dominating in the profundal. The average abundance of macrozoobenthos (without big molluscs) 2617 ind. m−2, and their biomass 12.34 g m−2 are considered to be the highest among the large lakes of North Europe. Macroflora occupies a small percentage of the total lake area but is rich in species. Taxa forming communities are Potamogeton perfoliatus, Phragmites australis, Schoenoplectus laccustris, Potamogeton lucens, Eleocharis palustris, and Polygonum amphibium. Submerged vegetation occupies the first place amongst different growth forms, followed by emergent plants. The main commercial fishes are lake smelt, perch, ruff, roach, bream, pike, vendace and pikeperch. The stock of vendace has sharply decreased in the last years, while the amount of pikeperch has increased. Considering annual fish catches (9000–12000 tons or 25–34 kg ha−1), L. Peipsi-Pihkva exceeds all large lakes in North Europe.

Vertical Distribution of Zooplankton in a Strongly Stratified Hypertrophic Lake

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 103 ind m−3; in the epilimnion 2320 × 103 ind m−3, in the metalimnion 2178 × 103 ind m−3, and in the hypolimnion 237 × 103 ind m−3. The average biomass of metazooplankton was 1.75 g m−3; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m−3. The highest abundances – 19,136 × 103 ind m−3 and 12,008 × 103 ind m−3 – were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer Keratella cochlearis f. typica (Gosse, 1851) was the dominating species in abundance. In biomass, Asplanchna priodonta Gosse, 1850, among the rotifers, and Eudiaptomus graciloides (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000–2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, Chaoborus larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.

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.

Taxonomic composition of rotifers in Lake Peipsi

Abstract161 rotifer taxa 46 of which are new records for Estonia were identified in the north-western part of Lake Peipsi. The greatest number of taxa occurred in August (93), followed by September (79). In all, 19 families (Monogononta) are represented with 41 genera. The dominant families are Brachionidae (32 taxa) and Synchaetidae (16). The species of Anuraeopsis, Conochilus, Kellicottia, Keratella, Notholca, Polyarthra and Synchaeta are dominating. The taxonomic composition is analysed in the ecological aspect. Some species of zoogeographic interest, including a supposedly endemic Ploesoma peipsiense, are reported.

The structure of psammic rotifer communities in two boreal lakes with different trophic conditions: Lake Võrtsjärv and Lake Saadjärv (Estonia)

Psammon communities are still poorly studied worldwide. The aim of the present study was to establish the structure of psammic rotifer communities including their diversity and quantitative parameters. A total of 41 rotifer taxa were found in hydro-, hygro-, and euarenal zones of eutrophic Lake Võrtsjärv and mesotrophic Lake Saadjärv during the study carried out in Estonia in 2008, including 11 rotifer species new to Estonia. In L. Võrtsjärv, the predominant rotifer taxa were Bdelloidea and Lecane psammophila. L. psammophila dominated in July and August and bdelloids prevailed in the rest of the year. In L. Saadjärv, the most abundant taxa were Lepadella ovalis, Keratella hiemalis, and Cephalodella megalocephala. The psammic rotifer community of L. Saadjärv proved to be more diverse than that of L. Võrtsjärv. Microphagous rotifers were dominant in L. Võrtsjärv. Raptorial feeders dominated in L. Saadjärv in summer and autumn and were more abundant at sites with coarser sand and low plant density.

Testing effects of shore height level, sediment characteristics and vegetation cover on the seasonality of zoopsammon communities in the two boreal lakes differing in their trophic state

As compared to macroscopic invertebrates zoopsammon communities, i.e. fauna inhabiting interstitial spaces between sand grains are largely understudied and the factors affecting their spatiotemporal variability are not well known. In a field observation, we tested how seasonal variability in the taxonomic composition and density of zoopsammon communities at different shore heights is related to sand temperature, wave activity, sediment characteristics and vegetation cover in the two boreal lakes differing in their trophic state. The mesotrophic Lake Saadjärv had higher number of psammon taxa dominated by protists than the eutrophic Lake Võrtsjärv dominated by nematodes. Within each lake, vegetation cover and sediment characteristics explained the largest variability in the spatial and seasonal dynamics of zoopsammon communities. Seasonality differed among taxonomic groups and lakes. The seasonal variability differed slightly among the levels of shore heights with densities being highest at the water line in spring and early summer and upward the water line later on. Due to the presence of strong interaction of temporal and spatial variability in psammon communities, large covariation among environmental variables and a presence of many species-specific interactions with environmental factors, the future studies should test those impact attributes with experimental approaches at the highest taxonomic resolution possible.

Seasonal variability in the structure and functional diversity of psammic rotifer communities: role of environmental parameters

Data on the variability in structure and functioning of interstitial rotifer assemblages are rare; however, this knowledge is essential for understanding their role in the interstitial food web. In the present study, we characterized psammic rotifer communities in terms of dominance structure, trophic traits, taxonomic and functional diversity at a seasonal scale in freshwater lakes across Estonia and coastal beaches of the Baltic Sea. A total of 42 rotifer species were found from the coastal beaches and 66 species from the lakes. Functional indices did not exhibit smaller seasonal variability and neither did they respond better to changes in the environment compared to taxonomic indices. However, there were differences how environmental variables affected these two broad groups of response variables. The taxonomy-based indices of rotifer communities were primarily driven by seasonal temperature regime, sediment characteristics and anthropogenic stressors, whereas the trait-based indices were a function of ecosystem types (freshwater or brackish water). The functional indices of the psammic rotifer communities strongly distinguished between freshwater and brackish habitats indicating that rotifers have different functional roles in food webs in fresh and brackish water environments.

Seasonal trends in horizontal and vertical patterns of zoopsammon in the brackish Baltic Sea in relation to key environmental variables

Abstract Beach meiofauna in brackish waters has received very little attention compared to fauna in fully marine and fresh water habitats. This study explores for the first time the species composition and seasonal dynamics of zoopsammon communities in the Gulf of Finland, the brackish Baltic Sea, and relates the observed patterns to the key environmental variables. The study communities consisted of rotifers, nematodes, ciliates, testaceans, tardigrades, oligochaetes, polychaetes, small crustaceans, gastrotrichs, turbellarians, and insect larvae. Regardless of environmental conditions, ciliates, nematodes, and rotifers were the most abundant taxa in all sampling sites. The total population densities varied largely within site, among sites, and seasons with different sites showing different seasonal patterns. The BRT models reproduced reasonably well the spatial patterns and seasonal dynamics of psammon communities and identified environmental variables that affected the psammon seasonality (temperature, sand organic content, human trampling) and spatial patterns (sediment characteristics, porewater, and pH). Our study also shows that the majority of psammon taxa had species-specific responses to their environment. This provides a strong conceptual argument for enhancing expertise in the taxonomy of psammon, especially when constructing predictive models of psammon in the brackish-water habitats.

Food spectrum of the omnivorous rotifer Asplanchna priodonta in two large northeastern European lakes of different trophy

In this paper we examine the stomach contents of the omnivorous rotifer, Asplanchna priodonta, to evaluate possible trophic interactions between this rotifer and its potential prey in two large northeastern European lakes: moderately eutrophic Lake Peipsi and strongly eutrophic Lake Võrtsjärv. Our results show that the A. priodonta diet consisted of Bacillariophyta, Chlorophyta, Cyanobacteria, and Dinoflagellata. Ciliata were detected on rare occasion. Bacillariophyta were the most frequent food items in stomachs. There were no between-lake differences in the consumption of Chlorophyta and Cyanobacteria, whereas Bacillariophyta were consumed more in Lake Peipsi. However, neither Dinoflagellata nor Ciliata were found in stomach samples in Lake Võrtsjärv. We conclude that A. priodonta is an opportunistic feeder that is capable of influencing the phytoplankton community structure in large lakes.

The effect of channelization on the biological quality of lowland streams using macroinvertebrates as proxies

We studied whether and how channelization affects macroinvertebrate communities and biological quality in lowland streams of Estonia (Northern Europe). Two datasets of semi-quantitative samples (collected during 2001–2010) were used: (1) pairwise data (natural sites located upstream of the channelized sites; 22 streams, 24 pairs), and (2) background data (73 natural sites from 62 streams, 73 channelized sites from 62 streams). The mean stream order in both datasets was 3. Channelization had no significant effects on physicochemical parameters or on macroinvertebrates in the pairwise dataset where channelization was done decades earlier. However, it had several negative effects on macroinvertebrates in the background dataset where several areas had probably been maintained more recently. Richness of sensitive data, Shannon diversity, mean sensitivity, organic pollution index, and multimetric quality were significantly lower for the modified sites than for the natural sites. Only total taxon richness and score of hydromorphology were not significantly affected by channelization. Thus, several macroinvertebrate metrics that indicate water pollution, proved to be sensitive to channelization as well. The recovery conditions (such as stream type, channelization season, recovering duration, etc.) have been poorly studied and would deserve further attention.