Lake Hiidenvesi Research Articles

Aminopeptidase and phosphatase activities in basins of Lake Hiidenvesi dominated by cyanobacteria and in laboratory grown Anabaena

1. Extracellular enzyme activities were examined in freshwater basins representing a transition from hypertrophy to mesotrophy and in axenic cyanobacterial cultures to evaluate the ecological role of extracellular enzyme activities of cyanobacteria.2. Aminopeptidase activity was related to the trophic status of the lake basins. The activity was highest in the most eutrophic basin and decreased in the less nutrient‐rich basins. Cyanobacteria were the most important autotrophic organisms and aminopeptidase activity was positively associated with cyanobacterial biomass.3. In an axenic Anabaena batch culture, nitrogenase activity was several orders of magnitude higher than leucine aminopeptidase activity. Nitrate did not have an effect on aminopeptidase activity or growth, but significantly reduced the rate of nitrogen fixation. A high phosphorus concentration at the beginning of the Anabaena batch‐culture experiment resulted in reduced phosphatase activity.4. In Lake Hiidenvesi, aminopeptidase activity probably originated mostly from attached bacteria and less so from cyanobacteria.

The seasonal dynamics and distribution of Chaoborus flavicans larvae in adjacent lake basins of different morphometry and degree of eutrophication

1. Seasonal dynamics, spatial distribution and population size of the phantom midge Chaoborus flavicans in different parts of the eutrophic Lake Hiidenvesi (30.3 km2) were studied.2. Density of larvae was low in the shallow, most eutrophic parts of the lake, while the deep Kiihkelyksenselkä basin was inhabited by a dense population. In the deepest part of Kiihkelyksenselkä (33 m) density was 13 989 ± 3542 m–2 in May, declined to 1102 ± 274 m–2 in July and recovered to 7225 ± 1314 m–2 by October. In spring and autumn the majority of larvae were benthic while, during high summer, few larvae were found in the sediment.3. Horizontal distribution fluctuated seasonally. On 3 June < 5% of the population inhabited areas shallower than 10 m. On 6 July the limnetic fraction was still restricted to regions deeper than 10 m, but 43% of benthic larvae were found between 6 and 10 m depths. In October both limnetic and benthic larvae were concentrated in areas deeper than 20 m.4. Within the lake, distribution was mainly regulated by stratification characteristics, degree of eutrophy being less important. The seasonal horizontal movements were probably induced by food shortage. Larvae could not meet their energetic demands in stratified areas and dispersed to shallower water, reducing predation risk by use of the benthic habitat.

The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia.2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m−2 of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m−2, respectively.3. With the resuspended sediment, 39.4 mg P m−2 day−1 was brought to the water column outside the stand, 22.4 mg P m−2 day−1 at the edge and 13.4 mg P m−2 day−1 within the stand.4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.

Langmuir circulations disturb the low‐oxygen refuge of phantom midge larvae

The previously unknown effects of wind‐induced Langmuir circulations on the distribution of phantom midge larvae (Chaoborus flavicans) were studied by echosounding. The study was carried out in the eutrophic Lake Hiidenvesi, where chaoborids use a metalimnetic oxygen minimum as a daytime refuge against fish predation. At a wind velocity of 8—9 m s−1, the upwelling water circulations snatched clouds of chaoborids from the metalimnion (12‐15 m depth) to the more oxygenated epilimnion. The average density of C. flavicans in the clouds was 790 ind. m−2, whereas, elsewhere in the epilimnion, it was 380 ind. m−2. Planktivorous fish (smelt Osmerus eperlanus) were actively aggregated in the upwellings. When the wind velocity decreased to 3 m s−1, chaoborids disappeared from the epilimnion. The results suggest that Langmuir circulations may affect the abundance of chaoborids by disturbing their low‐oxygen refuges.

Seasonal development of the cladoceran assemblage in a turbid lake: the role of emergent macrophytes

In eutrophic Lake Hiidenvesi, the water is turbid and the biomass of submerged macrophytes consequently low. In such lakes, due to low refuge availability, the highest zooplankton biomass usually occurs in the beginning of the growing season. In L. Hiidenvesi, however, the highest biomasses are found in late July-early August. To clarify the role of emergent macrophytes, we investigated the seasonal development in cladoceran community structure and biomass in different zones of an emergent macrophyte bed (Typha angustifolia). Total cladoceran biomass in the different zones did not differ, but the biomass of free-swimming filter feeders was significantly higher within the emergent stand than at the edge or outside the stand. This was mostly due to the higher biomass of Bosmina longirostris in the inner zone. Mean size of Ceriodaphnia quadrangula was also highest within the stand, The biomass of free-swimming filler feeders peaked on 7 July at 40 μg C/l and declined rapidly thereafter, whereas at the edge and outside the stand the biomass remained below 5 μg C/l throughout the study period. The observed seasonal development of the cladoceran community was probably a consequence of concomitant changes in the predation pressure and refuge availability. At the edge and ouside the stand, the estimated consumption by free-swimming filter feeders remained below 10 % of phytoplankton biomass, while in the inner zone it reached 70 %. Outside the emergent stand, the estimated grazing pressure by the plant-associated Sida crystallina was higher than total consumption of the free-swimming species. The result demonstrated that also nymphaeids (which were found outside the emergent stand) may be of great importance for herbivorous zooplankton, especially plant-associated species.

Movement of age‐1 pikeperch under the ice cover

Massive movement of age‐1 pikeperch under ice was observed in northern Lake Hiidenvesi. The size‐distributions of age‐0 and age‐1 pikeperch in different basins showed the fish movement, either passive or active, c. 3 km from one basin to another.

A metalimnetic oxygen minimum indirectly contributing to the low biomass of cladocerans in Lake Hiidenvesi - a diurnal study on the refuge effect

The diurnal vertical migrations of smelt (Osmerus eperlanus), larvae of phantom midge (Chaoborus flavicans) and cladoceran zooplankton in eutrophic Lake Hiidenvesi were studied in order to clarify the factors behind the low zooplankton biomass. In the study area, an oxygen minimum occurred in the metalimnion in the 10–15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above the oxygen minimum. C. flavicansconducted a diurnal migration. During the day, the majority of the population inhabited the 12 – 15 m depth just in the oxygen minimum, while during darkness they were found in the uppermost 8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around 23:00. During daytime, the majority of smelts inhabited the depth of 7–9 m, where the water temperature was unfavourably high for them (18 °C). Smelts thus probably avoided the steep oxygen gradient in the metalimnion, whereas Chaoborusused the oxygen minimum as a refuge against predation. Those smelts that were found in the same water layers as Chaoborusused the larvae as their main prey. The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladoceran zooplankton.

The food, growth and abundance of five co-existing cyprinids in lake basins of different morphometry and water quality

The food, growth and abundance of five co-existing cyprinid fish species in the eutrophic Lake Hiidenvesi were studied. The diet overlaps within the cyprinid community in shallow lake basins were compared with those in a deep basin, where littoral resources are less available. Roach, bleak and white bream inhabited both the shallow and the deep parts of the lake. Their growth rate was slow, probably due to the low availability of animal food, indicated by the increasing proportion of detritus and plant material in the diets towards the end of the summer. In the deep basin, roach and bleak, contrary to white bream, did not forage on the very abundant invertebrate Chaoborus flavicans, explained by the migration behaviour of C. flavicans. Blue bream, unlike other cyprinids, utilized copepods and had a relatively fast growth rate, but was mostly restricted to the shallow areas. The condition of the bream stock was weak both in terms of growth and abundance. The availability of zoobenthos was low and bream was not able to compete for zooplankton with the more specialized planktivores.

Seasonal succession of phytoplankton and zooplankton along a trophic gradient in a eutrophic lake - implications for food web management

The water quality and seasonal dynamics of phytoplankton and zooplankton in Lake Hiidenvesi were explored in order to evaluate the possibilites to improve the water quality by mass removal of fish. The four sampling stations were situated in basins representing a transition from hypertrophy to mesotrophy and maximum depths from 3 m to 33 m. The maximal phytoplankton biomasses ranged from 5 to 33 mg l-1 and the maximal zooplankton biomasses from 160 to 520 μg C l-1. Cyanophytes dominated the phytoplankton biomass in summer and the copepod biomass was higher than that of cladocerans at all the stations. No spring peak in the zooplankton biomass occurred and the average size of daphnids was below 1 mm. At present, the top-down control of phytoplankton biomass is thus ineffective. However, the water quality and morphometry of L. Hiidenvesi seem favourable for biomanipulation. The nutrient concentrations and external loading fall in the limits that facilitate successful restoration and the morphometry of the lake is favourable for the recolonization by submerged macrophytes. However, since stations 2 and 3 receive their nutrient loading mainly from station 1, restoration of station 1 may be crucial for the whole lake. This may not be achieved through food web management alone, but requires a reduction of the nutrient flow from River Vihtijoki.