Forest Lakes In Southern Finland Research Articles

An evaluation of gillnet CPUE as an index of perch density in small forest lakes

Gillnets are commonly used for monitoring fish communities and assessing the ecological quality in European lakes. However, the relationship between gillnet CPUE and fish abundance is not linear and depends on several factors affecting the catchability of gillnets. We investigated the effects of high water colour, naturally anoxic hypolimnion, water temperature and sex ratio on the catchability of perch Perca fluviatilis in gillnets. Data were collected during 10 years in nine forest lakes in southern Finland. Such lakes are very common in Finland. The density and size structure of perch populations were estimated by mark and recapture method during spring and standard gillnet fishing (CEN, 2005. Water quality Sampling of Fish with Multimesh Gillnets. European Committee for Standardization, Brussels (EN 14757)) in late summer. Perch CPUE was positively but non-linearly dependent on the perch density. Catchability increased with water colour, water temperature, and the volume of the anoxic hypolimnion. The sex ratio was also important because late summer gillnetting overestimates the proportion of female perch. If the factors that affect the catchability of perch in gillnets are taken into account, gillnet CPUE using standard methods can be used as rough index of fish density in small forest lakes.

Intensive fishing can mediate stronger size-dependent maternal effect in pike (Esox lucius)

In pike E. lucius L., evidence on maternal effect on reproductive output is mixed. We studied whether older and larger pike females produce eggs and larvae of higher quality (weight, starvation resistance) in three forest lakes in southern Finland. Later, the study lakes were subjected to intensive experimental pike fishing, which we assumed would increase resource availability and lead to higher maternal investment (larger egg size). Length of female pike was positively correlated with the dry weight of eggs and larvae but this relation was dependent on female age. In old females, the effect of female length on egg weight was lower or even negative. Survival analysis showed a positive effect of female length on larval survival time indicating that larvae from larger females are less vulnerable to starvation during the early stage of life. After the intensive pike fishing, the positive effect of female length on egg weight was stronger in all age classes probably due to the released resources. Based on the high quality and amount of reproductive products in large (but not very old) females, they are important for the reproduction of pike populations. This should be considered in fisheries management.

Short- and long-term patterns of 137Cs in fish and other aquatic organisms of small forest lakes in southern Finland since the Chernobyl accident

We summarize the patterns of 137Cs activity concentrations and transfer into fish and other biota in four small forest lakes in southern Finland during a twenty-year period following the Chernobyl accident in April 1986. The results from summer 1986 showed fastest accumulation of 137Cs into planktivorous fishes, i.e. along the shortest food chains. Since 1987, the highest annual mean values of 137Cs have been recorded in fish occupying the highest trophic levels, for perch ( Perca fluviatilis) 13,600 Bq/kg (ww) and for pike ( Esox lucius) 20,700 Bq/kg (ww). At the same time, activity concentrations of 137Cs in crustacean zooplankton and Asellus aquaticus have ranged between 1000 and 19,500 Bq/kg (dw). In 2006, 5–28% of the 1987 137Cs activity concentration levels were still present in perch and pike. Since 1989 their 137Cs activity concentrations in oligohumic seepage lakes have remained significantly higher than in polyhumic drainage lakes due to the increased transfer of 137Cs into fish in the seepage lakes with lower electrolyte concentrations, longer water retention times and lower sedimentation rate.

Perch production in forest lakes: the contribution of abiotic and biotic factors

Horppila J, Olin M, Vinni M, Estlander S, Nurminen L, Rask M, Ruuhijärvi J, Lehtonen H. Perch production in forest lakes: the contribution of abiotic and biotic factors. Ecology of Freshwater Fish 2010: 19: 257–266. © 2010 John Wiley & Sons A/SAbstract – The biomass, growth and production of perch (Perca fluviatilis (L.)) in five humic forest lakes in southern Finland were estimated and the abiotic and biotic factors contributing to the between‐lake differences were explored. The average perch biomass in the study lakes varied from 13.1 to 45.6 kg·ha−1 and annual production from 6.0 to 16.5 kg·ha−1. The highest biomass and production were found in Lake Majajärvi with highest water colour and nutrient concentration. Variability in fish density rather than in individual growth rate seemed to be the main factor behind the between‐lake differences in perch production. No dependence of perch production on water transparency was found, because transparency had a stronger effect on individual growth than on fish density. The length increment of perch in their second year increased significantly with water transparency but at older ages no such dependence was found. At age of 4 years, the growth rate of perch decreased with perch density, indicating intraspecific competition for benthic food resources. At age of 7 years, the growth rate increased with increasing roach (Rutilus rutilus (L.)) abundance, suggesting that roach served as a resource for piscivorous perch. The effects of predatory top‐down control on perch populations were probably obscured due to the variable size and water quality of the study lakes.

Stable Isotope Analysis of Zooplankton Carbon Nutrition in Humic Lakes

Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12 small forest lakes in southern Finland during summer 1997. The lakes were selected to provide a wide range of water colour (12-365 mg Pt l-1) but minimum variability in other limnological characteristics. POM and DOM showed similar stable isotope ratios (δ 13 C around - 28‰, comparable with values commonly reported for terrestrial C3 vegetation) and these showed no relationship to water colour. Zooplankton were consistently depleted in 13 C relative to the bulk POM on which they might feed. This relative 13 C depletion increased significantly with water colour. Therefore the zooplankton must have been feeding selectively on isotopically light food sources and doing so to a greater extent in the more humic lakes. Although phytoplankton were 13 C-depleted relative to POM, their abundance and their variability between lakes did not appear to explain adequately the observed trend in zooplankton stable isotope ratios. We propose that some zooplankton were grazing on methanotrophic bacteria which utilise isotopically light methane derived from allochthonous organic matter decomposed under anaerobic conditions. This process would be more pronounced in coloured lakes with a higher loading of allochthonous organic matter and greater development of hypolimnetic anoxia.

Effects of catchment deforestation and burning on the limnology of a small forest lake in southern Finland

(1993). Effects of catchment deforestation and burning on the limnology of a small forest lake in southern Finland. SIL Proceedings, 1922-2010: Vol. 25, No. 1, pp. 525-528.

Effects of liming on crustacean zooplankton in an acidified, artificially divided forest lake in southern Finland

Effects of liming on crustacean zooplankton in an acidified, artificially divided forest lake in southern Finland

The occurrence and bioavailability of radioactive 137Cs in small forest lakes in Southern Finland

Following the Chernobyl accident in late April 1986, central Finland was subjected to considerable radioactive fallout. The radioactive isotope of caesium, 137Cs, was potentially the most harmful isotope in the fallout because of its long half-life (30 years). 137Cs activities remained unexpectedly low within the biota (algae, zooplankton, aquatic moss, fish) of some humic forest lakes in southern Finland compared to the clear water lakes within the same district. This observation suggested that humic substances, by binding 137Cs chemically, may reduce its bioavailability in lake water. Our laboratory experiments (throughflow systems; gel chromatography) demonstrated that binding of 137Cs by humic substances was negligible in untreated lake waters; only when most of the alkali metals had been removed with an ion exchange resin was any binding of 137Cs by low molecular humic fractions apparent. Instead, the concentrations of cations (especially potassium) were of overwhelming importance for the bioavailability of 137Cs within the lacustrine ecosystems. The concentrations of 137Cs within food chains are expected to stay at a high level for many years especially in lakes with long water renewal times.

Changes in the density, population structure, growth and reproduction of perch, Perca fluviatilis L. in two acidic forest lakes in southern Finland

The density of the perch populations decreased rapidly during the 1980s in two acidified lakes in southern Finland. In an acid-humic lake the density of the perch population decreased from 240 to 60 ha −1 and in an acid clearwater lake from 150 to 20 ha −1. In both lakes the rapid decrease was mainly due to failures in reproduction, although post-spawning mortality of adult fish was also recorded. The perch spawned annually in both lakes but according to the field exposures of egg strands, most of the eggs died before hatching. Nevertheless, some new recruits appeared in both lakes. These perch, born in 1983–1986, grew faster than those of the more abundant year-classes of the late 1970s, indicating a clear density-dependent growth response. According to the mortality estimate and the age distribution of perch, the latest year-classes that produced more than 100 adult perch ha −1 were born in 1978 or 1979 in both lakes. It was calculated that the density of both populations was >500 ha −1 in the early 1980s.

Phosphorus transformations in the epilimnion of humic lakes: biological uptake of phosphate

SUMMARY. 1. The hypothesis that dissolved humic material (DHM) stimulates bacterial involvement in phosphorus transformations and may thus lead to decreased accessibility of phosphorus to algae was investigated by studying three small forest lakes in southern Finland representing a wide range of concentrations of DHM.2. Other chemical differences between the three lakes were slight, although the most humic lake exhibited higher concentrations of total phosphorus and of molybdate‐reactive phosphorus. Bacterial biomass did not differ significantly between the lakes, but algal biomass was significantly lower at higher DHM concentrations. Consequently the ratio of algal biomass to bacterial biomass was significantly lower in the most humic lake.3. Uptake of phosphorus from added 33PO4 was partitioned between algal and bacterial size fractions by differential filtration. No significant variation between lakes was found in the proportion of particulate 33P recovered from the algal fraction.4. Turnover times for phosphate were significantly longer in the most humic lake and also showed lower variability. In general turnover times were long in comparison with values reported from many other lakes. Only briefly in mid summmer did turnover times in two of the lakes shorten to values which would indicate that demand for phosphate was outstripping supply.5. Short‐term storage of samples from the most humic lake stimulated biological incorporation of 33P, but additions of nitrogen and iron had little effect on phosphate uptake.6. In these small forest lakes it is probable that no single nutrient consistently limits plankton development. Since no evidence was found that DHM shifts the balance of plankton phosphate uptake away from algae towards bacteria, the influence of DHM on phosphorus transformations may rather be through chemical regulation of free phosphate availability.

Heat uptake and resistance to mixing in small humic forest lakes in Southern Finland

The annual heat budgets and maximum thermal stabilities achieved by small Finnish forest lakes are considerable for lakes of their size. Heat uptake is rapid during the brief period of vernal circulation, with the latent heat of fusion of ice contributing substantially to this. Heating then slows, with maximum heat contents and thermal stabilities occurring around early August. Absorption of solar radiation by the dystrophic waters, effective shelter from wind-induced turbulence, and considerable relative depths all combine to prevent mixing of heat much below the surface 2 m of most study lakes. Birgean wind-work values therefore remain low, and cold hypolimnetic waters occupy much of the lake volume. The lakes also respond quickly to meteorological change, which causes considerable year-to-year variation, and some short-term fluctuation, in their heat budgets and maximum stabilities. The lakes' small size and shallow thermal stratification may contribute to this.

Spring development of a Chlamydomonas population in Lake Nimet�n, a small humic forest lake in southern Finland

Biomass development and vertical distribution of a Chlamydomonas population in a small humic forest lake was followed by daily sampling in May-June, 1984. Chlamydomonas dominated the phytoplankton spring bloom, forming 71% of the maximum phytoplankton biomass on 18 May. In early May the outflow rate was high and during the 24 hour period when the maximum rate of surface runoff was recorded (8–9 May), 43% of the Chlamydomonas biomass was flushed out of the lake, which delayed the onset of biomass increase. When surface runoff had slowed down Chlamydomonas biomass started increasing and during wax of the population most cells were 10 µm in diameter.

Analysis of organic and inorganic sulfur constituents and 34S-isotopes in dated sediments of Forest Lakes in southern Finland

A modification of the Jonson-Nishita digestion-distillation apparatus was used to determine the S constituents of sediments from three forest lakes (Munajarvi, Orajarvi, and Sorvalampi) in southern Finland. Lakes Munajarvi and Orajarvi had undergone marked acidification during the past 30 yr or so presumably due to acid deposition as revealed from sedimentary diatom and chrysophycean remains. Organic S constituted a major part of the total S, and during the recent years the portion of C-bonded S increased in the Lakes Munajarvi and Sorvalampi more strongly than the portion of sulfate esters. In the case of Lake Orajarvi the concentration of total S increased by a factor of 2.3 from its preindustrial level which seems to be due to an increase of both C-bonded S and SO4 −2. Coincidentally with the up core increase in total S, the sediment cores of Orajarvi and Munajarvi were strongly depleted of the 34S-isotope (r = −0.83 in Orajarvi n = 15, and r = − 0.84 in Munajarvi n = 10). This implies that the recent changes in sedimentary S are related to the increasing input of airborne pollutant S since about 1850 in Orajarvi and 1950 in Munajarvi provided that the 210Pb dating of the cores is correct and diagenetic changes have not greatly altered the vertical distribution of S in these sediments.

The importance of bacterial utilization of released phytoplankton photosynthate in two humic forest lakes in southern Finland

Bacterial utilization of photosynthetically fixed dissolved organic carbon (PDOC) released from natural phytoplankton assemblages was studied in two small, extremely humic, forest lakes in southern Finland. Bacterial activity (measured as uptake of 14C‐glucose) and phytoplankton photosynthesis (measured as light uptake of 14CO2) could be most effectively separated using Nuclepore filters of pore size 1–2 μm. Released PDOC was 10–67% of total phytoplankton carbon fixation during in situ experiments, and represented about 0.1% of total DOC. Net uptake of PDOC by bacteria was found to be about 20% during 24 hour laboratory incubations, although about 40% of PDOC present at the start of an experiment could be utilized by bacteria during a 24 hour period. PDOC does not provide a quantitatively important substrate supply for bacterial respiration in humic forest lakes.

The predation on Asellus aquaticus (L.) by perch, Perca fluviatilis (L.), in a small forest lake

Asellus aquaticus (L.) was the most important benthic food item for perch, Perca fluviatilis (L.), in a small, extremely humic forest lake in southern Finland. The proportion of A. aquaticus in the diet of perch varied according to the former's availability, which, in turn, depended on its life cycle. Perch 11.0–12.9 cm (total length) most frequently fed on A. aquaticus; smaller and larger perch fed more frequently on crustacean zooplankton and aquatic insect larvae, respectively. The high density of the perch population, the importance of A. aquaticus in the benthos of the study lake and the general high activity level of the prey resulted in a high predation (0.1–1.8% per day). The population of A. aquaticus was also limited by the scarcity of macrophytic vegetation and the small area of oxygenated littoral in the lake.

Light penetration and some related characteristics in small forest lakes in Southern Finland

Penetration of total PAR was measured in 14 small forest lakes during early August at the time of most intense stratification of the water columns. The lakes were chosen to cover the widest possibl...

Autumnal and vernal circulation of small forest lakes in Southern Finland

Autumnal and vernal circulation of small forest lakes in Southern Finland

Primary production and phytoplankton in two small, polyhumic forest lakes in southern Finland

Primary production and phytoplankton in two small, polyhumic forest lakes in southern Finland