Hydrobiologia

Blenckner, Thorsten; Nõges, Tiina; Tranvik, Lars; Pettersson, Kurt; Naddafi, Rahmat

doi: 10.1007/s10750-010-0409-2pmid: N/A

Tuvikene, Lea; Nõges, Tiina; Nõges, Peeter

doi: 10.1007/s10750-010-0414-5pmid: N/A

Long-term data on phytoplankton species composition in large and shallow Lake Võrtsjärv indicated a sharp deterioration of the ecological status at the end of the 1970s. The more traditional water quality indicators, such as the concentrations of nutrients and chlorophyll a, phytoplankton biomass, and Secchi depth, failed to capture this tipping point or even showed an improvement of the status at that time. As the shift coincided with a large increase of the lake’s water level (WL), we hypothesized that direct effect of the changing WL on traditional water quality indicators might have blurred the picture. We removed statistically the direct effect of the WL and the seasonality from the traditional water quality indicators in order to minimize the effects of natural variability. The average of the standardised water quality indicators, used as a proxy for the ecological status, distinguished a period of fast eutrophication in the first half of the 1970s (not captured by the phytoplankton species index), a fast improvement at the end of the 1970s (when the species index showed deterioration) followed by a continuous deterioration trend (when the species index remained rather constant). The causes of this inconsistency are discussed in the light of the alternative stable states theory and the priority of biotic indicators stipulated by the EU Water Framework Directive.

Üveges, Viktória; Vörös, Lajos; Padisák, Judit; Kovács, Attila

doi: 10.1007/s10750-010-0396-3pmid: N/A

Benthic algal communities can play an important role in matter and energy flux of shallow lakes. Their contribution to total primary production of lakes has been largely unexplored. The aim of this study was to estimate the primary production of the epipsammic algal communities at different water depths in Lake Balaton (Hungary) with photosynthetic measurements performed in laboratory. The photosynthesis of the benthic algae of different origin was studied at nine different irradiance levels, in three replicates. The maximum photosynthetic rate (P max) was always higher in samples from the shallow parts than those from the deeper regions of the lake. Along the west–east longitudinal axis of the lake P max decreased in the southern part and increased in the middle of the lake as a consequence of differences in the chlorophyll-a concentrations. Knowing P max, I k, global radiation and extinction coefficient, the primary production (mg C m−2 day−1) of the epipsammic algal community was calculated at different water depths. In the shallow regions at 0.5 and 1 m water depth 75–95% and 60–85% of the production was attributable to the epipsammon. The percentage contribution of epipsammon was at 2 m water depth 20–65%. In the deeper pelagic region (>3 m) more than 85% of the primary production originated from the phytoplankton.

Jurvelius, Juha; Kolari, Irma; Leskelä, Ari

doi: 10.1007/s10750-010-0385-6pmid: N/A

All sampling methods give selective or biased estimates of fish species abundance, distribution and size structure. This creates problems, e.g. in regard to the Water Framework Directive of the European Union, which demands evaluation of the quality and status of fish stocks in lakes. We compared fish sampling by means of Nordic multimesh gillnets, seining, trawling and hydroacoustics in two Finnish lakes in summer 2007 and 2008. Sampling methods were used ‘as such’, i.e. no special design was implemented for method comparison. In the shallow eutrophicated lake the species’ composition of gillnet sampling and seining were very different. The biomass-% of percids dropped from gillnet (61%) to seining (9%) and that of cyprinids grew from 39 to 90%, respectively. In the deep pelagic area of the oligotrophic lake, vendace and smelt predominated in trawl catches. The number of fish caught by gillnetting in that area was too small to make any conclusions about the species composition. In the eutrophicated lake, the combined length distribution for all fish species differed significantly between gillnetting and seining. In the oligotrophic lake, the gillnet catches were too small for any comparison of fish size. The difference in the length distribution of fish between trawl and echosounding was significant in most analysed depth layers. In upper depth layers acoustics sampled larger fish than trawling, and in deeper layers smaller fish. Using a combined acoustic-trawl method, the pelagic fish biomass was estimated to be approx. 17 kg ha−1 in the deep and oligotrophic lake. We conclude that in large and deep-water areas, the use of active gear is enough in fish sampling to evaluate the quality and status of fish stocks. Gillnetting together with seining is an appropriate method to work out the quality and status of fish stocks in shallow and littoral areas of large lakes. Variation in the catch selectivity of fish sampling gear requires a discrete ecological classification for each type of gear.

Rask, Martti; Vuori, Kari-Matti; Hämäläinen, Heikki; Järvinen, Marko; Hellsten, Seppo; Mykrä, Heikki; Arvola, Lauri; Ruuhijärvi, Jukka; Jyväsjärvi, Jussi; Kolari, Irma; Olin, Mikko; Salonen, Erno; Valkeajärvi, Pentti

Ventelä, Anne-Mari; Kirkkala, Teija; Lendasse, Amaury; Tarvainen, Marjo; Helminen, Harri; Sarvala, Jouko

doi: 10.1007/s10750-010-0415-4pmid: N/A

Säkylän Pyhäjärvi (SW Finland) is an example of a large and shallow lake suffering from eutrophication. During the last 20 years, the quality and general usability of water in Pyhäjärvi have shown considerable variation driven by both a variety of human activities and climate-related factors such as wet and dry years. The lake has been thoroughly studied for decades and has been the object of comprehensive restoration activities both in the catchment and in the lake since the 1990s. Large variety of water protection measures like wetlands, sedimentation ponds and filtering systems have been implemented in the catchment area to reduce external nutrient load. Another important tool for Pyhäjärvi restoration is biomanipulation, done by local commercial fishermen in winter. Twenty-five percent of the annual phosphorus input is removed with fish catch. Currently, restoration work is facing new challenges: short or even nearly missing ice cover period and increased winter time nutrient load from the catchment. In the 2000s, there were 3 years with exceptionally short ice period, allowing only brief winter seining periods. Consequently, the biomanipulation catch was very low in 2007 and 2008 leading to observable food web effects. Phosphorus load was especially high in winters 2006/2007 and 2008/2009. On the basis of the data of 1987–2008, we have tested the hypothesis if climate-related winter time variables like phosphorus load, air temperature and precipitation would affect the water quality of the lake in following summer, here measured as chlorophyll a concentration in the lake water. A linear model has been used and a validation procedure has been performed to select the best variables. Our results indicate some of the linkages between climate-related catchment processes and the ecological status of the lake.

Salmaso, Nico

doi: 10.1007/s10750-010-0394-5pmid: N/A

Major focus in interpreting phytoplankton changes in specific typologies of waterbodies or in single lakes is directed towards nutrients and climatic dynamics. During the last 35 years, Lake Garda (Northern Italy; A = 368 km2, z max = 350 m, V = 49 km3) underwent a significant increase of phosphorus in the water column, from ca. 10 μg P l−1 to 18–22 μg P l−1. At the multi-decadal scale, the increase of the trophic status had a positive impact on the growth of Cyanobacteria (mainly Oscillatoriales) and, partly, diatoms, as demonstrated by the long-term ecological research carried out since the beginning of the 1990s in the deepest zone of the lake. Conversely, the increase of Peridiniales (mostly Ceratium hirundinella) in the recent years appeared also associated with the interannual variations of lake temperature. At the seasonal and annual scale, the development of the large diatoms and Oscillatoriales during the periods of their maximum growth (early spring, and summer and autumn, respectively) was strongly controlled by the extent of spring vertical water mixing and nutrient fertilization of surface waters, which, in turn, were negatively dependent on the air and water temperatures in winter and early spring. Therefore, contrary to the positive impact of milder winters on phytoplankton growth in many lakes of high latitudes, warmer winter temperatures in deep oligomictic lakes of lower latitudes can determine periodic shifts towards more oligotrophic conditions and a minor development of diatoms and specific harmful cyanobacterial groups (Oscillatoriales). The complex relationships between the explanatory and response variables were tested by applying Path Analysis (Structural Equation Modeling). This multiequational technique has great potential for studying causal relationships in temporally ordered variables. The results highlight the necessity to study the consequences of climatic fluctuations on the phytoplankton communities at different temporal scales and complexity, also including the indirect effects of climatic dynamics mediated by the morphometric, morphological and hydrological characteristics of lakes, and the possible synergic or opposite effects with other forcing variables, including nutrients.

Gerdeaux, Daniel

doi: 10.1007/s10750-010-0412-7pmid: N/A

In France, the Arctic charr (Salvelinus alpinus) is native in only two lakes (Lakes Bourget and Geneva), in the most southerly part of its distribution area. It is a profundal morph living at depths of between 30 and 100 m in Lake Geneva. Following considerable stocking of Lake Geneva with juvenile Arctic charr and some good results during the 1980s, catches are currently declining. Several hypotheses have been proposed to explain this decrease, but the potential influence of warming of the lake in the 1990s has not been considered. This article studies the relationship between the strength of the cohorts and the temperature in Lake Geneva from 1992 to 2002, and discusses the various potential causes of the population collapse observed. The findings reveal close correlation between water temperature and the strength of the cohorts. It is concluded that the recent warming of Lake Geneva may have a significant direct or indirect impact on the Arctic charr population.

Ginter, Kai; Kangur, Külli; Kangur, Andu; Kangur, Peeter; Haldna, Marina

doi: 10.1007/s10750-010-0393-6pmid: N/A

Pikeperch is a major predator in the pelagic zone of eutrophic waters, such as the large north-temperate lowland lakes Võrtsjärv (Estonia) and Peipsi (Estonia/Russia). The size and structure of the pikeperch population is strongly influenced by their success at the juvenile stage. Therefore, we investigated the diet and prey selection of pikeperch fry caught in the ice-free period in lakes Peipsi and Võrtsjärv in 2007 and 2008. We analysed the stomach contents of 635 pikeperch from Lake Peipsi and 202 pikeperch from Lake Võrtsjärv, and compared our findings with similar data from the 1950s (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961). Analysing 4–20 cm long fry, we studied differences in prey size, seasonal diet patterns and the ontogenetic diet shift. In both lakes, 0+ pikeperch feed mostly on large predatory zooplankters. However, in Lake Peipsi the stomach content weight and the average number of food items in stomach were higher, and the food spectrum was wider than in Lake Võrtsjärv. There was also a difference in the type of food that dominated fry’s stomach content (calculated by weight) in the two lakes. In Lake Peipsi, chironomids larvae, as well as zooplankters Daphnia galeata and Bythotrephes longimanus dominated, while in Lake Võrtsjärv zooplankters Mesocyclops leuckarti and Leptodora kindti. Seasonal analysis showed that cladocerans dominated in pikeperch fry stomach content in summer and at the beginning of September, but copepods were dominant in autumn and spring. In contrast to the studies carried out from 1952 to 1958 (Erm, About Biological and Morphological Differences of Pikeperch. Hydrobiological Researches II (in Estonian), 1961), the shift from planktivory to piscivory at the end of the first growing season was hardly ever observed during our investigation. We believe this is due to the lack of suitable prey fish as there was a collapse of the smelt, Osmerus eperlanus (L.), population in both lakes. The transition of pikeperch from planktivory to piscivory was delayed till the next summer.

Winfield, I.; Fletcher, J.; James, J.

doi: 10.1007/s10750-010-0397-2pmid: N/A

An invasive species is defined as an alien (or introduced or non-native) species whose establishment and spread threaten ecosystems, habitats or species with harm. Such threats to UK lake fish communities have long been appreciated and this review assembles case histories, including new data, from the largest lakes of Scotland, Northern Ireland, Wales and England to examine the hypothesis that at least some of these introductions have become invasive. Loch Lomond in Scotland has experienced six introductions [chub (Leuciscus cephalus), common bream (Abramis brama), crucian carp (Carassius carassius), dace (Leuciscus leuciscus), gudgeon (Gobio gobio) and ruffe (Gymnocephalus cernuus)], of which the most significant has been that of the percid ruffe, which has been implicated in a recent decline of the native coregonid whitefish (Coregonus lavaretus). In Northern Ireland, the introduction of the cyprinid roach (Rutilus rutilus) to Lough Neagh has apparently had a negative impact on some overwintering waterfowl, although the native coregonid pollan (Coregonus autumnalis) remains abundant. Llyn Tegid in Wales has received three introductions [rudd (Scardinius erythrophthalmus), ruffe and silver bream (Blicca bjoerkna)], although no impacts on the native whitefish or other fish populations have been observed. In England, individuals of at least 12 native and non-native fish species have been brought to Windermere for the purpose of live-baiting, although only those of the cyprinids roach and common bream have established abundant populations. At the same time, the native salmonid Arctic charr (Salvelinus alpinus) has declined markedly while the native esocid pike (Esox lucius) has shown changes in abundance, distribution and individual condition, although these developments have not been shown to be causally linked. None of these introductions were sanctioned by appropriate fisheries or other regulatory bodies and almost all of them probably arose from the release or escape of live-bait used by pike anglers. Of the 10 species introductions documented here, four (common bream, gudgeon, roach and ruffe) have established abundant populations and two of these (roach and ruffe) have apparently caused or currently threaten harm, supporting the hypothesis that at least some of these introductions have become invasive.