Anja Lehtovaara

Migrations of haemoglobin-rich Daphnia longispina in a small, steeply stratified, humic lake with an anoxic hypolimnion

Migrations of Daphnia longispina were studied in a small humic lake with an exceptionally shallow oxic epilimnion. Horizontal distributions showed clear avoidance of the shoreline, which might be explained by the lower density of predators (Chaoborus sp. and Notonecta sp.) in the central parts of the lake. In early summer all size classes of D. longispina exhibited upward nocturnal vertical migration, descending to the upper hypolimnion in daytime. Later in summer, when the nocturnally migrating Chaoborus sp. had grown large enough to graze on small Daphnia, the latter seemed to shift towards twilight migration. However, large Daphnia individuals showed no synchronized migration; rather their bimodal vertical distributions suggested asynchronous vertical migration. Large individuals showed a particular tendency to concentrate near to the oxycline, close to the dense phytoplankton and bacteria populations in the upper part of the anoxic hypolimnion. According to vertical trap experiments, large D. longispina visited the anoxic hypolimnion and might harvest its abundant food resources. The high haemoglobin content of large individuals seems a specific adaptation to allow access to low oxygen water and hence to maximize grazing potential, in both epi- and hypolimnion, and minimize predation pressure. By staying predominantly in cooler water near the oxycline, Daphnia might also minimize its energy consumption to adjust to low food availability while sustaining a sufficiently high population density to exploit those unpredictable short periods with abundant food which are common in small headwater lakes. It is suggested that migrations of zooplankton are a complex behavioural adaptation which may not be explained by any single factor. In humic lakes with shallow stratification, vertical migrations seem to offer particularly high potential advantages, because of the short distances between dramatically different environments in the water column. In further studies more emphasis should be placed on migrations of individuals rather than populations, and migrations should be considered as a dynamic part of the structure and function of the whole planktonic ecosystem.

Vertical distributions of bacteria and algae in a steeply stratified humic lake under high grazing pressure from Daphnia longispina

The vertical distributions of bacteria and algae in a steeply stratified, highly humic lake were studied during three 24 h periods in summer. The highest bacterial and algal densities and biomasses were recorded in the anoxic hypolimnion. The bacterial biomass in the hypolimnion was composed mainly of photosynthetic green sulphur bacteria (Chlorobium) which occurred at very low light intensity (< 1.5 µmol m-2 s-1). The numbers and biomasses of bacteria, both in the epilimnion and hypolimnion, were mostly higher at night than during the day, indicating possible asynchrony between the production and loss of bacteria. Because of vertical migration, the diurnal vertical distribution of algae was more variable than that of bacteria. Particularly in July and August, when cryptomonads were abundant, the biomass of algae was much higher in the epilimnion during the day than at night. The flagellated chlorophytes, Chlamydomonas spp. and Scourfieldia cordiforrnis, stayed mainly in the upper hypolimnion close to the oxic-anoxic boundary zone where only a small proportion of Daphnia longispina was continuously present. Unpalatable Mallomonas chrysophytes with silicified plates and bristles, and small, presumably heterotrophic, flagellates stayed in the oxic epilimnion together with a dense (up to 300 ind l-1) population of D. longispina. The results indicated that, besides the physical and chemical properties of the water column, grazing pressure by Daphnia longispina strongly affected the vertical distribution of microorganisms in this polyhumic lake.

Carbon, phosphorus and nitrogen budgets of the littoral Equisetum belt in an oligotrophic lake

Stores and flows of carbon, phosphorus and nitrogen in a littoral Equisetum stand were studied in 1978–1980 in the oligotrophic, mesohumic lake Pääjärvi, southern Finland. The major carbon and nutrient stores were sediment and Equisetum. The seasonal cycle of the macrophyte vegetation had a profound influence on the whole littoral ecosystem. In spring, when only dead remains of Equisetum were present above ground, there were few differences in nutrient, chlorophyll a and zooplankton concentrations between the littoral and the open lake; phytoplankton and epiphytes were the major producers.In early June, when new shoots of Equisetum reached the water surface, water exchange between the littoral and the open lake started to diminish, and the characteristic features of a closed macrophyte zone gradually developed: by August the P, Chl a and zooplankton concentrations in the littoral were 5–10 times those in the open lake. From late June until autumn Equisetum was overwhelmingly dominant both in biomass and in production.The measured total primary production and respiration values indicated a high rate of internal cycling of carbon and nutrients. The daily P requirements of plant growth exceeded the total P stored in the water by a factor of 2–4, and also exceeded the release of nutrients in excretion. High N:P ratios in the water (total 10–64, inorganic 18–171) suggested that P was probably always the limiting nutrient.The P content of the annual production of Equisetum in Pääjärvi was 2.3% of the mean annual P load, and 5.3% of the mean total P storage in the water volume of the lake.

Recovery of the fish community and changes in the lower trophic levels in a eutrophic lake after a winter kill of fish

A severe oxygen deficit induced a fish kill in the eutrophicated two-basin Lake Äimäjärvi in southern Finland during winter 2002–2003, resulting in cascading effects on the lower trophic levels of the lake. Pikeperch disappeared from the lake and bleak and white bream decreased to low numbers. The recovery of the populations of other species started immediately when strong year-classes of roach and perch appeared in summer 2003 and onwards. A sharp increase in the growth of perch and roach was recorded, and perch became the dominant fish species during 2004–2006. Consequent responses after the fish kill included increased Secchi depth, expansion of submerged macrophytes, decreased nutrient concentrations and reduction of Cyanophyta from the more eutrophic northern basin of the lake, and a temporary increase in the size of Daphnia in the early summer 2003. However, the ecosystem of the lake returned gradually to the earlier structure and level of eutrophication. Bluegreen algae have returned since 2005, the water has become more turbid, macrophytes declined in 2007 and the fish community was again dominated by small cyprinid fishes in 2008.

Littoral-pelagial interchange and the decomposition of dissolved organic matter in a polyhumic lake

The small, polyhumic lake, Mekkojarvi (southern Finland), is bordered by a moss vegetation zone (Warnstorfia and Sphagnum species) which provides a habitat-rich and productive environment for many planktonic and periphytic animals. Impacts of moss on the metabolism of bacterioplankton, phytoplankton and zooplankton in polyhumic water were investigated in laboratory throughflow systems. Growing Warnstorfia (together with epiphytic algae and bacteria) suppressed the production of planktonic algae but had no clear effect on leucine uptake, and hence bacterial production, or on the decomposition of humic substances. Phenol uptake and mineralization rates, however, were lower in the littoral water than in the pelagial water. Excretion of organic carbon by Warnstorfia algae or Daphnia longispina (the predominant crustacean in the pelagial water) provided only a minor contribution to bacterial production; therefore, a major contribution had to be from humic substances. A bacterial production efficiency of 31–38% could account for the microbial respiration in the water. The results indicated that bacterial, or detrital matter (originating largely from the littoral zone), could not obviate the need for algal food, and that a great deal of particulate matter in the water was poor or useless food for Daphnia. In all, the bulk of dissolved organic matter in Lake Mekkojarvi was biochemically highly recalcitrant. Our results indicate that humic substances (from watershed or littoral area) which, through bacterial degradation, enter the planktonic food web of the lake are mainly lost through respiration by microorganisms.

Responses of zooplankton and fish to restoration in eutrophic Lake Tuusulanjärvi in southern Finland

Lake Tuusulanjãrvi ( surface area 6 km2, mean depth 3.2m, maximum depth !Om) is the largest lake of theVantaa river basin, located in a densely populated area o f southernmost Finland, 30 km north o f H e isinki. Symptoms of eutrophication of this originally slightly dystrophic lake were first recorded in the 1930s, and since the mid-1950s rapid eutrophication has taken place due to increased nutrient loading ftom settlement and agriculture. Winter aeration of the lake was started in 1972, and sewage discharge stopped in 1979, but the lake remained hypertrophic due to high diffuse pollution and interna! loading (PEKKARINEN 1990). After positive experiences from some restoration projects of large Finnish lakes, such as L. Vesijãrvi (HORPPILA et al. 1998) and L. Kõyliõnjãrvi (SARVALA et al. 2000), biomanipulation by means of mass removal of planktivorous fish in Tuusulanjãrvi was started in 1997. For details of methodology, see SAMMALKORPI (2000). The annual mass removal catches during 1997-2003 were 24-ll3metric tons, 40-190 kg ha-1, totalling ei ose to 420 metric tons and 700kg ha-1 (Table l).

Zooplankton and fish communities in Finnish lakes of different trophic status: responses to eutrophication

Eutrophication is a major environmental problem in Finnish lakes. Although the external nutrient loading has been reduced, it is still high, especially in agricultural and densely inhabited areas. In many lakes the internalloading is also important (ÜLIN et al. 1998). In comparison to the neighbouring countries, Norway and Sweden, the total phosphorus concentrations are higher, especially in sourhern and western Finland (MANNIO et al. 2000). Consequently, the frequency of lakes with eutrophication-induced cyprinid-dominated fish communities is highest in the same area (TAMMI et al. 1999). During the last l O years, lake restoration by mass removal of cyprinids, especially roach, Rutilus rutilus, has become a common method in attempts to improve the water qualiry, recreational value and fisheries value of the eutrophied lakes in southern Finland. The increase in these activities has been encouraged by successful joint management and research projects, especially the cases of Lake Vesijarvi (HoRPPILA et al. I 998) and Lake Koylionjarvi (SARVALA et al. 2000). In these lakes, intensive removal of planktivorous fish resulted in decreased concentrations of total phosphorus and the decreased occurrence of cyanobacterial blooms. To solve the practical questions of assessing the quantitative need of fish removal and of measuring the effects of fish removal on the fish communiry, a large cooperative project was started in I 997 by the Finnish Game and Fisheries Research lnstiture, Universiry of Helsinki, Finnish Environment Insititute, and regional fisheries and environment administrations. In addition, the aim of this project was to study the limnological and ecological effects of restoration in eutrophicated lakes in southern Finland. In the present study the zooplankton and fish communities were examined in 12lakes and/or basins along a eutrophication gradient. The purpose was to characterize the qualitative and quantitative features of the communities before starting the restoration project and major fish communiry manipulations. lt was hypothesized that in the lakes with higher total phosphorus concentrations, the fish communities would be dominated by planktivorous cyprinids, the fish would be more abundant and, due to high predation pressure by the fish (LANGELAND 1982, CARPENTER et al. I 987, PERSSON et al. 1988, }EPPESEN et al. 2000), the zooplankton communiry would be dominated by small-sized cladocerans and copepods.

Effects of Biomanipulation on Fish and Plankton Communities in Ten Eutrophic Lakes of Southern Finland – Hydrobiologia (2006) 553: 67–88

The effects of biomanipulation were studied in ten Finnish lakes to determine responses in fish and plankton communities and water quality after mass removal of cyprinids. From 1997 to 2001, the fish communities shifted from the dominance of large cyprinids to an explosion of small cyprinids and a higher proportion of piscivores in effectively biomanipulated lakes (>200 kg ha 3 yr). The biomass of cyanobacteria decreased, and the duration of the blooms shortened and shifted towards the autumn. Decreased concentrations and slower cycling of nutrients and increased grazing by cladocerans probably affected the declined biomass of cyanobacteria. Less intensive sediment disturbance and increased phosphorus-retention in fast growing fish biomass may have turned the role of the fish assemblage from ‘nutrient recycler’ to ‘nutrient storage’. Increased potential grazing pressure, higher proportion of edible algae, and lower chlorophyll a:total phosphorus ratio indicated strengthened herbivore control. A high mass removal catch in relation to trophic state, low background turbidity, and bearable external loading favoured the successful biomanipulation, whereas intensive cyprinid reproduction, high nutrient loading and non-algal turbidity hindered the recovery. Three important issues should be noticed before biomanipulation in Finland: (1) careful selection of target lake, (2) well-planned, effective and long-lasting biomanipulation and (3) sustainable management of piscivores.