Lake Vesijarvi Research Articles

Extreme temperature differences in the city of Lahti, southern Finland: Intensity, seasonality and environmental drivers

Abstract The extremes of month-specific spatial temperature differences were studied for a first time in the high-latitude city of Lahti and its surroundings in southern Finland. During the 2-year observation period (6/14–5/16), the largest momentary temperature difference, 11.1 °C, was detected in February, and the smallest, 6.2 °C, in April. The impacts of various environmental factors during the extreme situations were estimated by site-specific analysis of the warmest and coldest observation sites and a stepwise multiple linear regression model including all the 8 observation sites. The extreme temperature differences were characterised by inversions especially in winter and spring, the warmest site being the hill-top location in Kivistonmaki. In summer the role of urban heating was more apparent, and the temperature was the highest in the relatively low-lying city centre. In autumn the heating impact of the relatively warm Lake Vesijarvi caused the largest temperature differences with harbour as the warmest site. The weather during all of the momentary extreme situations was calm and in the majority of the situations also clear. The impact of cloud cover was less critical than that of wind speed in reducing spatial temperature differences. The momentary extreme situations existed at night or at dawn, with one exception: only in January, during the cold weather period dominated by high pressure, the delayed break of inversion in the vicinity of Lake Vesijarvi caused the extreme temperature difference to exist in the afternoon, reflecting for its part the substantial stabilising impact of seasonal ice cover on Lake Vesijarvi.

Development of picoplankton during natural and enhanced mixing under late-winter ice

We studied the development of autotrophic picophytoplankton and heterotrophic bacterioplankton during the tran- sition from winter ice cover to open water under natural and manipulated mixing conditions in eutrophic Lake Vesijarvi. During the melting of the snow and ice cover, a convection layer developed which eventually met the che- mocline at the interface between the oxic and anoxic water masses. However, in the years with mechanically enhanced mixing, the whole water column remained well oxygenated and the deepening of penetrative convection was facilitated. Stochastic variations in weather, primarily the thickness of the snow cover, likely determined the timing of picophytoplankton growth. Mechanical mixing slightly increased the biomass of picophytoplankton, especially the eukaryotic fraction, probably because eukaryotic picophytoplankton coped well with changing light conditions. Bacterial biomass was not notably affected, although their mean cell size was significantly smaller in the oxic deep water than in anoxic conditions during the natural mixing regime. Our results show that, when snow melts and solar radiation increasingly penetrates into lake water, picophytoplankton biomass and numbers already start to increase weeks before ice-break and may reach abundances typical of early summer under the ice. A similar development of bacterioplankton suggests a trophic relationship between heterotrophic and autotrophic producers.

Cladoceran remains in lake sediments: a comparison between plankton counts and sediment records

To evaluate the comparability of neo- and paleo-limnology, we made year-to-year as well as seasonal comparisons of contemporary zooplankton data and cladoceran remains in thick (9–42 mm) annual laminations in sediment of Lake Vesijarvi, southern Finland. We calculated the expected annual exuviae production of nine planktonic taxa in the water column using contemporary zooplankton records, and compared the value to the observed net accumulation of their remains in deep sediment for 7 years. Although all of the species studied occur commonly in the lake pelagic zone, deposition of remains differed significantly among taxa. The observed accumulation of three Bosmina species and Chydorus sphaericus was similar to or exceeded expected values, suggesting good preservation of their remains as well as focusing of sediment into the deepest part of the lake. The accumulation of Limnosida and Leptodora remains exceeded expected values several fold, suggesting under-representation of these species in pelagic plankton samples, as well as efficient transportation and spatial averaging of their remains in the sediment. Daphnia, Diaphanosoma, and Ceriodaphnia were clearly under-represented in the sediment due to poor preservation of their remains and use of a 50-μm sieve in sediment processing. Thus, sieving should be taken into account as a possible source of bias. Correlation between relative abundances of all species in the sediment and in the plankton was weak (rs = 0.54, P < 0.001), but for well-preserved species, the correlation was strong (rs = 0.91, P < 0.001). Inter-annual variation in the deposition of remains suggested that resuspension and sediment focusing may vary between years, thus making it difficult to interpret absolute abundances, even though the deepest part of the basin shows clear varve formation and seasonality is well recorded. Detailed study of the uppermost lamination showed the seasonal succession of a cladoceran community within an annual varve as well as differences in the seasonality and intensity of ephippia production among the species. We recommend that results of cladoceran analyses be expressed in several ways (relative abundances, per unit dry weight, per unit organic matter, and as net accumulation values) before drawing final conclusions, as each approach may reveal a different aspect of the deposition process. In addition, sedimentation differences between epilimnetic and hypolimnetic species should be considered in stratified lakes.

The role of phosphorus in growth of phytoplankton in Lake Vesij�rvi, southern Finland ? a multitechnique approach

The role of phosphorus in phytoplankton growth was studied in Lake Vesijarvi, a large previously eutrophic body of water with a history of flourishing fishery. The study combined different approaches: long-term algal enrichment experiments with natural phytoplankton assemblages were carried out together with observations on nutrient and chlorophyll a concentrations, elemental ratios (N:P, C:P, C:N) of particulate matter, and analysis of P uptake using [33P]. None of the approaches revealed periods of P limitation, but some growth experiments as well as elemental ratios indicated slight deficiency in early summer. Concentrations of total dissolved phosphorus (TDP), which were usually 20–30 μg l–1, also indicated luxurious P resources. Thermal stratification was weak and the lake mixed twice during the study period; this was reflected in the phytoplankton biomass that increased up to 4-fold. Results of elemental ratios usually suggested the occurrence of nitrogen limitation, and in general these ratios were low for a lake. All size fractions >0.22 μm in the experiments with [33P] showed P uptake. In August most of the P was taken up by picoplankton, but when the lake turned over in September, the uptake of P by this fraction was absent. Thus, there was always a plentiful supply of P for phytoplankton, but the shortage of inorganic N may have affected those algae not capable of fixing N2. These conditions should have favoured the growth of heterocystous cyanobacteria, but these prokaryotes never attained high abundances. This may have been due to the weak stability of the water column, or the growth of cyanobacteria may have been limited by trace elements such as molybdenum or iron.

Resuspension in regulating sedimentation dynamics in Lake Vesijärvi.

The role of resuspension in sedimentation dynamics and internal phosphorus (P) loading was investigated in the Enonselka basin of Lake Vesijarvi, southern Finland. The gross sedimentation rate was measured at three sampling sites, where sedimentation traps were placed 2 m above the bottom (the depth of 28 m, 18 m and 7m) and at the two deepest sites also in the metalimnion (11 m depth). The total settling flux did not follow either the phytoplankton biomass development in the lake or the bacterial mineralization rate in the settled material. The gross sedimentation rate was much higher in the hypolimnion than in the metalimnion, indicating that resuspension increased the settling flux. This conclusion is supported by the fact that when the gross sedimentation rate increased, the C and N concentrations of settled material resembled those of bottom sediment. The proportion of resuspended material, estimated by three different methods, averaged 56-99 % of the gross sedimentation rate during the study periods. Contrary to the common assumption, the Enonselka basin provides an example of successful restoration by biomanipulation in lakes with high resuspension.

The magnitude of Si dissolution from diatoms at the sediment surface and its potential impact on P mobilization

The magnitude of the influence of diatom-derived dissolved Si on the mobility of P at the sediment-water interface was investigated in eutrophic Lake Vesijarvi, southern Finland. The spring bloom, which consisted of small centric diatoms, depleted the Si from the water column (<100 μg l -1 ) and was followed by a sharp increase in the diatom numbers at the sediment surface and by a peak of dissolved Si in the sediment interstitial water. The diatom cells disappeared from the surface sediment on a time scale of weeks (5 % d -1 ). Calculations based on the cell numbers and on the Si deficit in the water column indicated that the dissolution of diatoms in the sediment surface layer may produce Si pulses high enough to influence the mobility of P.

Direct and indirect mechanisms behind successful biomanipulation

Lake Vesijarvi is a relatively large (length 25 km; total area 110 km2), shallow (mean depth 6 m), but stratified lake in southern Finland. The Enonselka basin (26 km2), surrounded by the city of Lahti, received its sewage effluent, and changed from a clear water basin with flourishing fisheries from the 1940–50s to one of the most eutrophic lake systems in Finland thereafter. In 1976, the sewage effluent was diverted, resulting in a temporary recovery of water quality. However, in the 1980s, massive surface scums of cyanobacteria degraded the water quality and arrested the recovery of the lake. A restoration strategy providing an ecologically sound basis for the management of the lake was initiated in 1987. This strategy involved biomanipulation (mass removal of coarse fish) together with conventional pollution control measures on discharges to the lake. Biomanipulation was chosen instead of much more expensive chemical and/or technical methods, such as chemical treatment or dredging of the profundal sediment. The large-scale biomanipulation trial was carried out in the Enonselka basin during 1989–93. Following the mass removal of coarse fish (1000 metric tons of fish; mainly roach and smelt), the biomass of cyanobacteria collapsed concomitantly with a decline of total phosphorus concentration from 45 to 35 mg P m−3, and with an increase of Secchi depth from 1 m to 3.5 m. These observed improvements in the water quality were matched with a large decline in roach-mediated phosphorus movement from littoral to pelagial, from 100 mg P m−2 in 1989 to 15 mg P m−2 in 1993. Year-to-year variation within the littoral communities, and in the recruitment of fish, could in this way cause large oscillations in the whole ecosystem. The involvement of local people (fishermen, farmers etc.) in controlling non-point nutrient loading and fish stock development, is of prime importance for the long term success of lake restoration.

Hydroacoustic monitoring of the distribution, density and the mass-removal of pelagic fish in a eutrophic lake

A large scale biomanipulation of pelagic fish by trawling was started in the eutrophic Enonselka basin (26 km2) of Lake Vesijarvi to improve the water quality which had remained poor in spite of the termination of nutrient loading. The distribution and density of the fish were studied by hydroacoustics before and during the removal. The initial annual fish density varied between 13 000–21 000 fish ha−1 in the study area in August 1984–89. The mass-removal of the fish by pelagic trawling took place in 1989–1992. The catch varied annually between 64 and 92 kg ha−1. Roach (Rutilus rutilus (L.)) and smelt (Osmerus eperlanus (L.)) accounted for c. 85% of the weight of the catch. The mass-removal decreased the pelagic fish density in the Enonselka basin during the trawling. An increase in the density was observed after the trawling ceased in these years, and the initial density level was reached within one month. The density level after mass-removal remained high compared with oligotrophic lakes. The pelagic fish had a diurnal ascending trend with the decreasing light intensity in August, and the fish were significantly (p < 0.01) higher in the water mass in temperature non-stratified water in autumn than in stratified water in summer. The fish were somewhat deeper in the years of mass-removal than before it. Mass-removal did not affect the individual length of the echosurveyed fish. Fish smaller than c. 15 cm (TS < −44 dB; mainly smelt) were numerically dominant throughout the whole study period.

The diet and growth of roach (Rutilus rutilus (L.)) in Lake Vesij�rvi and possible changes in the course of biomanipulation

Previous studies have suggested that the roach Rutilus rutilus (L.) stock of Lake Vesijarvi is one of the main factors delaying the recovery of the lake after sewage diversion. This study is concerned with the documentation of the diet of roach in the lake. In total, 531 roach were examined. Both in the pelagial and in the littoral the roach had mixed diets in May and in September—October. The importance of zooplankton decreased and the importance of benthos and plants increased with increasing size of roach. In July, in the pelagic zone all sizes of roach fed exclusively on zooplankton (Bosmina spp.), while in the littoral zooplankton had the highest volume proportions only in the smallest (<130 mm) roach. The frequent use of plant food and slow growth rate of large roach indicate a low availability of animal prey. As the fish densities decrease due to the mass removal taking place in the lake, the percentage of plant food in the diets of roach will probably decrease and the growth of roach will increase. Additionally, the tendency of the roach to migrate into the pelagic zone in early summer may be reduced, which would decrease their predation on the zooplankton.