Fish and fisheries of the Prespa lakes

Twenty-three taxa of fish have been identified fromthe Prespa lakes. Eleven of these have been introducedor translocated, and 7 of the remaining 12 are endemicto the Prespa lakes. This high proportion of endemismrequires confirmation because the systematic positionof several of the species and subspecies remainsuncertain. The absence of a predatory fish in MikriPrespa, the occurrence of trout in Megali Prespa, thepresence of eels in both lakes and the occurrence ofmany rheophilous fish species typifies the fishcommunities of these two lakes. Monitoring of the fishpopulation in Mikri Prespa, from 1984 to 1994, showedapparent stability or a slight increase in catchesduring this period. Not all the introduced specieshave been able to establish populations and theirimpact on the indigenous populations is yet to bedefined. A spectacular re-establishment of the carppopulation, following the crash of the stock of thisspecies in the 1970s–1980s, was recorded in the springof 1991. The past and present situation of thefisheries in lakes Mikri and Megali Prespa isdescribed, and their productivity and yields areanalysed. Particular attention is given to atraditional fishing method unique to the Presparegion: pelaizia fishing. The factors involved inthe temporal trends in the fisheries are identifiedand analysed. Recommendations for the management ofthe fish populations and the fisheries are proposed,so that the sustainability of this resource and thesurvival of the endemic fish species can beensured.     

Lake Prespa,a European natural monument,endangered by irrigation and eutrophication?

The most interesting (Hutchinson, 1957), and at the same time oldest graben lakes of Europe, are Lakes Ohrid and Prespa. According to geologists and geographers, both originated during the Pliocene. They show karstic features and differ by volume, much less by area. At the occasion of an excursion to Lake Prespa in September 1994, an absence of oxygen in the hypolimnion between 17 m and the maximum lake depth, 48 m, was observed, for the first time since Stankovic' (1926) record. Moreover, a continuous decrease in lake level since the 1960s, the lowest transparency ever recorded from Megali Prespa, and high nutrient values, though only observed during a short part of the late stagnation period, are alarming signals, all suggesting eutrophication.     

The importance of Prespa National Park for breeding and wintering birds

The breeding avifauna of Prespa National Park is ofnational and international importance both due to itshigh richness and the internationally importantpopulations of rare species. The latter include theDalmatian (Pelecanus crispus) and Great White(Pelecanus onocrotalus) pelicans and the PygmyCormorant (Phalacrocorax pygmaeus). Populationsof national interest include Cormorant (Phalacrocorax carbo), Greylag Goose (Anseranser), Goosander (Mergus merganser), GreatWhite Egret (Egretta alba) and six otherspecies of ardeids. Among the 261 bird speciesobserved in the area of ca 250 km² since thelate 60s, 164 breed and 41 are very rarely observed.Passage migrants and winter visitors include 91species. Twelve breeding species became extinct in theNational Park during the last 25 years and others areendangered. Raptors and aquatic birds suffered moredue to habitat and land use alterations brought aboutby the construction of an irrigation scheme.Eighty-one species observed in the area are listed asin need of special protection in Annex I of the 79/409EEC Directive and 32 of them breed. Lake Mikri Prespais of virtually no importance for wintering waterfowlsince it is frozen every winter, but the Greek part ofLake Megali Prespa, which never freezes, concentratesamong other species the 20% and 35% respectively ofthe Great Crested Grebes and Tufted Ducks wintering inGreece.     

The Norwegian reference lake ecosystem, Øvre Heimdalsvatn

The subalpine lake, Øvre Heimdalsvatn and its catchment, situated in the mountains of southern Norway, has been the subject of extensive ecosystem studies over the last 50 years covering a wide range of disciplines. The lake is located at 1,088 m a.s.l., has an area of 0.78 km², a maximum depth of 13 m, and a catchment area of 23.6 km². It lies more or less on the limit of the birch forest, while the catchment extends up over 1,800 m a.s.l. and into the high alpine zone. The valley is little influenced by human activity and there is no permanent habitation, although the area is used for grazing of domestic livestock during summer. Otherwise the lake and its catchment have always been important for fish and game. The catchment also received major radionuclide fallout from the Chernobyl accident in 1986. The studies include Quaternary history, the development of algorithms for remote sensing of snow conditions, long-term changes in lake ice cover and temperature, the zooplankton and benthic communities, as well as the uptake of Chernobyl radionuclides and mercury by the biota. The consequences of the introduction of the European minnow (Phoxinus phoxinus) on the benthos, zooplankton and the brown trout (Salmo trutta) have also been investigated. The lake and its catchment are widely used in university teaching and thesis work. On account of the substantial knowledge base, Øvre Heimdalsvatn is well suited to long-term monitoring.     

A Limnological Reconnaissance of the Lough Erne System,Ireland

The Lough Erne System (Northwest Ireland) consists of two linked eutrophic lakes. Phosphorus input budgets suggested mean catchment exports of 30–62 kg total P km⁻² a⁻¹, resulting in a heavy P loading to the small upper lake. The export of soluble P is related to the human population density in the catchment. Although complex in outline, the upper lake has similar phytoplankton populations throughout, but the populations in the simpler shaped lower lake differ quantitatively and qualitatively from place to place. This is probably due to differences in mixed depth. A chlorophyll a map of the surface water of the lower lake, obtained by fluorometry, showed the relationship between algal biomass and depth of water.     

Relationships between morphometry,geographic location and water quality parameters of European lakes

I addressed the question how lake and catchment morphometry influences water chemistry and water quality over a large scale of European lakes, and developed the regression equations between most closely related morphometric and water quality indices. I analysed the data of 1,337 lakes included in the European Environment Agency (EEA) database, carrying out separate analyses for three basic lake types: large lakes (area ≥100 km², 138 lakes), shallow lakes (mean depth ≤3 m, 153 lakes) and large and shallow lakes (area ≥100 km² and mean depth ≤8 m, 35 lakes). The study revealed that in Europe, the lakes towards North are larger but shallower and have smaller catchment areas than the southern lakes; lakes at higher altitudes are deeper and smaller and have smaller catchment areas than the lowland lakes. Larger lakes have generally larger catchment areas and bigger volumes, and they are deeper than smaller lakes, but the relative depth decreases with increasing surface area. The lakes at higher latitudes have lower alkalinity, pH and conductivity, and also lower concentrations of nitrogen and phosphorus while the concentration of organic matter is higher. In the lakes at higher altitudes, the concentration of organic matter and nutrient contents are lower and water is more transparent than in lowland lakes. In larger lakes with larger catchment area, the alkalinity, pH, conductivity and the concentrations of nutrients and organic matter are generally higher than in smaller lakes with smaller catchments. If the lake is deep and/or its residence time is long, the water is more transparent and the concentrations of chlorophyll a, organic matter and nutrients are lower than in shallower lakes with shorter residence times. The larger the catchment area is with respect to lake depth, area and volume, the lower is the water transparency and the higher are the concentrations of the nutrients, organic matter and chlorophyll as well as pH, alkalinity and conductivity. The links between lake water quality and morphometry become stronger towards large and shallow lakes. Along the decreasing gradients of latitude, altitude and relative depth, the present phosphorus concentration and its deviation from the reference concentration increases.     

The second investigation of Lake Puma Yum Co located in the Southern Tibetan Plateau,China

Limnological features of Lake Puma Yum Co, a typical alpine lake located at the altitude of 5030 m in the Tibetan Plateau, China, are described based on the findings of the second expedition in September 2004. The lake surface is about 280km² and maximal depth is 65 m. Total inflow just after the rainy season was estimated to be about 860 000m³ day⁻¹, and the lake water was drained from a newly excavated channel at a rate of 960 000m³ day⁻¹. We may have to expect undesirable lowering of the water level by this new drainage, especially in the dry season. Thermocline developed from 20 to 30 m depth, and the euphotic zone reached the 50 m depth. Dissolved oxygen in surface water was supersaturated as in productive lakes, although there were no large point and nonpoint sources of nutrient in the catchment. Vertical distribution of phytoplankton biomass and primary production suggested the presence of photoinhibition. What should be noted about the flora and fauna is that a Chara zone and a shell zone were distributed at about 30 or 40 m of depth.     

Mollusc biodiversity in a European ancient lake system: lakes Prespa and Mikri Prespa in the Balkans

The spatial distribution of (endemic) biodiversity in ancient and potentially ancient lakes in Europe is poorly understood. Examples include Lakes Prespa and Mikri Prespa in the Central Balkans. Utilizing information of the most species-rich taxon in these lakes, the Mollusca, we therefore attempt to statistically assess and visualize the spatial distribution of biodiversity, to analyse biogeographical patterns, and to carry out a conservation assessment. We estimate that at least 40 (sub)species (29 gastropod and 11 bivalve taxa) occur in the lakes. For both lakes combined, 37.5% of the mollusc taxa are endemic. In general, the mollusc richness in Lake Mikri Prespa is lower than in Lake Prespa and less heterogeneously distributed. The highest species richness can be seen on the western and south eastern shores of Lake Prespa. Based on the presence/absence of genera, a minimum spanning tree analysis supports the sister lake relationship of both lakes, which, in turn, are most closely related to lakes in the western Balkans and not to nearby Lake Ohrid. The IUCN red list assessment revealed (A) a tendency towards mollusc faunal change, (B) a contemporary decline and potential loss of mollusc diversity, and (C) that all endemic species are of conservation concern.     

Paleolimnology of Slapton Ley,Devon, UK

Slapton Ley, a coastal lake in SW England, has been shown by a variety of paleolimnological studies, to have become increasingly eutrophic in the period since 1950 AD. Since that time, intensification of agriculture has resulted in increased erosion of topsoil from fields in the catchment of the Ley. Sediment accumulation rates, as estimated by ²¹⁰Pb-dating and multiple core correlation of peaks in whole core volume magnetic susceptibility, are equivalent to a catchment erosion rate of 13.4 t km^–2 a^–1, which figure agrees well with directly monitored data. Diatom and chlorophyll a analysis of the uppermost sediments shows that the Ley has recently experienced a major shift in its trophic status, changing from a clear water, macrophyte lake to one dominated by plankton in a hypertrophic system. This last point is further amplified in the paper by Heathwaite & O'Sullivan (1991).     

Prokaryotic aminopeptidase activity along a continuous salinity gradient in a hypersaline coastal lagoon (the Coorong, South Australia)

The relation between climatic parameters (relative air humidity) and the water activity of the Urmia Sea water determines the possible maximum evaporation of the lake. Using the Pitzer thermodynamic approach, the activity of the Urmia Lake water during evaporation was calculated and compared to the present relative air humidity above the water. Present climatic conditions allow the Urmia Sea water to evaporate down to water with activity of 0.55, corresponding to the lowest air humidity measured over the lake. This water activity falls in the range of halite precipitation, while carnalite precipitation starts at somewhat lower (a H₂O = 0.499) point. Our dynamic model predicts that for air humidity as low as 55% (reflecting present climate conditions), the Urmia Sea level may drop to as low as 1270 m (i. e., 1270 m above mean sea level). At that point, the lake water volume will have a volume of 11 km³. For the sake of comparison, at the beginning of 1990, the level of the lake was 1275 m, its volume was 25 km³, and its surface area was 5145 km².     

Eco-hydrological model and critical conditions of hydrology of the wetland of Erdos Larus Relictus Nature Reserve

Erdos Larus Relictus Nature Reserve is the only wetland of international importance to protect Larus Relictus in China. From the early part of this century, the impact of water use, vegetation construction and the climate of the catchment area increasing on the wetland of this Nature Reserve has brought about severe water shortage. Until now, no eco-hydrological model at home or abroad integrates the wetland ecological and hydrological processes. This paper considers the relationship between the food chains of the Larus Relictus and environmental elements, and establishes the eco-hydrological processes from Larus Relictus to damselfly + Chironomidae and the salinity and the area of Bojiang Lake. It also establishes a relationship among Larus Relictus breeding, the area of islands outcropped in the levels of the lake and hydrological season based on the relationship between Larus Relictus breeding and environmental elements. The eco-hydrological model is then constructed according to these relationships. In virtue of the constraints of the lake salinity, the suitable area of the lake is 9.01 km², and 4.88 km² for the smallest area. While in accordance with the area of islands outcropped needed for Larus Relictus breeding, the lake area is 11.02 km² in breeding season corresponding to critical high level.     

Eco-hydrological model and critical conditions of hydrology of the wetland of Erdos Larus Relictus Nature Reserve

Erdos Larus Relictus Nature Reserve is the only wetland of international importance to protect Larus Relictus in China. From the early part of this century, the impact of water use, vegetation construction and the climate of the catchment area increasing on the wetland of this Nature Reserve has brought about severe water shortage. Until now, no eco-hydrological model at home or abroad integrates the wetland ecological and hydrological processes. This paper considers the relationship between the food chains of the Larus Relictus and environmental elements, and establishes the eco-hydrological processes from Larus Relictus to damselfly + Chironomidae and the salinity and the area of Bojiang Lake. It also establishes a relationship among Larus Relictus breeding, the area of islands outcropped in the levels of the lake and hydrological season based on the relationship between Larus Relictus breeding and environmental elements. The eco-hydrological model is then constructed according to these relationships. In virtue of the constraints of the lake salinity, the suitable area of the lake is 9.01 km², and 4.88 km² for the smallest area. While in accordance with the area of islands outcropped needed for Larus Relictus breeding, the lake area is 11.02 km² in breeding season corresponding to critical high level.     

The Morphometry of Lake Palmas,a Deep Natural Lake in Brazil

Lake Palmas (A = 10.3km²) is located in the Lower Doce River Valley (LDRV), on the southeastern coast of Brazil. The Lake District of the LDRV includes 90 lakes, whose basic geomorphology is associated with the alluvial valleys of the Barreiras Formation (Cenozoic, Neogene) and with the Holocene coastal plain. This study aimed to investigate the relationship of morphometry and thermal pattern of a LDRV deep lake, Lake Palmas. A bathymetric survey carried out in 2011 and the analysis of hydrographic and wind data with a geographic information system allowed the calculation of several metrics of lake morphometry. The vertical profiling of physical and chemical variables in the water column during the wet/warm and dry/mild cold seasons of 2011 to 2013 has furnished a better understanding of the influence of the lake morphometry on its structure and function. The overdeepened basin has a subrectangular elongated shape and is aligned in a NW-SE direction in an alluvial valley with a maximum depth (Z_max) of 50.7m, a volume of 2.2×10⁸ m³ (0.22km³) and a mean depth (Z_mv) of 21.4m. These metrics suggest Lake Palmas as the deepest natural lake in Brazil. Water column profiling has indicated strong physical and chemical stratification during the wet/warm season, with a hypoxic/anoxic layer occupying one-half of the lake volume. The warm monomictic pattern of Lake Palmas, which is in an accordance to deep tropical lakes, is determined by water column mixing during the dry and mild cold season, especially under the influence of a high effective fetch associated with the incidence of cold fronts. Lake Palmas has a very long theoretical retention time, with a mean of 19.4 years. The changes observed in the hydrological flows of the tributary rivers may disturb the ecological resilience of Lake Palmas.     

Length–mass relationships for lake macroinvertebrates corrected for back-transformation and preservation effects

Coloured dissolved organic matter (CDOM) modifies the light penetration into water bodies due to stronger absorbance of UV and short wavelengths of light. Therefore, in natural waters with high CDOM concentration, the spectrum of sunlight is shifted towards brown, also referred to as brownification. Here, the relation between the spectrophotometrically measured water colour (CDOM) and landscape properties is examined. These properties explained at best > 40% of the CDOM variability among the study lakes larger than 10 km². The key “permanent” landscape variables were lake percentage (Lake_%) in the uppermost catchment area, and the peat land coverage (Peat_%) of the catchment, which indeed was strongly correlated with lake elevation above the sea level. High Lake % indicated low CDOM concentration, while high Peat_% indicated the opposite. Relative to the Peat_% of the catchment, the CDOM concentrations were, on average, slightly higher in medium-size lakes (area 10–100 km²) than in large lakes (area > 100 km²), while relative to Lake_% the concentrations declined more in medium-size lakes.     

Species composition and seasonal cycles of phytoplankton with special reference to the nanoplankton of Lake Mikri Prespa

The phytoplankton of Lake Mikri Prespa was studied atmonthly or biweekly intervals during the period May1990–September 1992. Its species composition,consisting of a great number of cyanophytes and a verysmall number of chrysophytes and desmids, may reflectthe eutrophic character of the lake. Moreover, themean annual biomass values (15.0 and 3.2 g m^–3 inthe two years, respectively) and the maximum biomass(38.1, 6.4 and 9.6 g m^–3), classify Mikri Prespaas a eutrophic lake. A tendency towards adouble-peaked pattern of biomass distribution in timewith one peak in autumn, composed mainly ofcyanophytes, and another in spring made up of diatoms,was observed. This pattern contrasts with the standardpattern in eutrophic, stratified temperate lakes,which exhibit a third biomass maximum in summer.Cyanophytes were the most important group in terms ofbiomass and were dominated by the species Microcystis aeruginosa, Microcystis wesenbergii,Anabaena lemmermannii var. minor and Aphanocapsa elachista var. conferta. Diatomsconstituted the second most important group, with main representative the species Cyclotellaocellata. Cyanophytes, diatoms, chlorophytes anddinophytes revealed annual periodicity whereas theother algal groups did not show any seasonality atall.The nanoplankton constituted an important part ofalgal biomass (38.9 and 49.9% in the two years,respectively) and revealed annual periodicity withmaximum values in winter and spring, mainly composedof diatoms and cryptophytes. Low temperature,increased rainfall and high DIN concentrations seemedto be the main factors influencing the seasonality.Although the percentage contribution of nanoplanktondecreased with the increase in total biomass,justifying the classification of Lake Mikri Prespaamong the eutrophic lakes, the nanoplankton biomassdid not correlate significantly with totalphytoplankton biomass.     

Lake Horowhenua: a computer model of its limnology and restoration prospects

Lake Horowhenua, a small (2.9 km²) shallow (< 2 m deep), coastal dune lake on the west coast of the North Island of New Zealand, receives the runoff from intensive agriculture within its catchment and, until 1987, the treated sewage effluent from the town of Levin. Consequently the lake is highly enriched but with an annual cycle of algal P-limitation in winter and N-limitation in summer. There have been several schemes proposed to accelerate the improvement of the lake's water quality for recreational use. A computer hydraulic and nutrient model of Lake Horowhenua was developed using rainfall, evaporation and nutrient data to describe the nutrient budget. To match the lake nutrient concentrations, terms for in-lake processes of sedimentation, seasonal sediment nutrient release, phytoplankton production, and denitrification were required. The computer model results indicated that denitrification was the major natural restoration process accounting for a net loss of more than 50% of the N from the lake each year. Application of the model also allowed lake managers to evaluate the potential effects of a number of proposed restoration schemes.     

Reaction of large and shallow lakes Peipsi and Võrtsjärv to the changes of nutrient loading

More than 20-year monitoring of Estonian rivers reveals that the loading of nitrogen to large shallow lakes Peipsi (3,555 km², mean depth 7.1 m) and Võrtsjärv (270 km², mean depth 2.8 m) decreased substantially in the 1990s. Phosphorus loading decreased to a much smaller extent than nitrogen loading. In L. Võrtsjärv both N and P concentrations followed the decreasing trends of loading, which show the high sensitivity of large shallow lakes to catchment processes. Our study showed a positive relationship between P content in sediments and the relative depth of the lake. Assumingly the resilience of a lake in responding to the reduction of nutrient loading decreases together with the decrease of its relative depth. In L. Peipsi the concentration of P has not decreased since the 1990s. Our data show indirectly that P loading from Russia to L. Peipsi may have increased. The N/P ratio has decreased in both lakes. Cyanobacterial blooms have been common in both lakes already at the beginning of the 20th century. The blooms disappeared during heavy nitrogen loading in the 1980s but started again in L. Peipsi in recent years together with the drop of the N/P ratio. In L. Võrtsjärv the N/P ratio is higher and the ecosystem is more stable although the share of N₂-fixing cyanobacteria increased from the 1990s. Reappearing cyanobacterial blooms in L. Peipsi have caused fish-kills in recent years. In L. Peipsi summer/autumn fish-kills during water-blooms are a straightforward consequence of reduced nitrogen level at remaining high phosphorus level while in L. Võrtsjärv the climatic factors affecting water level are more critical––at low water level winter fish-kills may occur. In L. Võrtsjärv nutrient loading has decreased and water quality has improved, present ecological status seems to be mostly controlled by climatic factors through changes of water level. The most important measure to improve water quality in L. Peipsi would be the reduction of phosphorus loading from both Estonian and Russian subcatchments.     

Limnological studies of Lake Wisdom, a large New Guinea caldera lake with a simple fauna

SUMMARY. Lake Wisdom is of interest because it is large and deep, has oxygen throughout, and yet, apparently as a result of its recent formation, its biota has some surprising lacunae. It is nearly circular and fills the central caldera of Long Island, Papua New Guinea (5° 20′ S, 147° 6′ E). Its maximum length is 13.4 km, and area approximately 95 km². A bathymetric map of the lake to depths of 300 m has been constructed from fathometer transects. Repeated soundings in the deeper parts gave depths of about 360 m. The surface of the lake is approximately 190 m above sea-level and water samples from the deepest part of the lake are fresh, so the basin is apparently sealed. The lake level shows annual fluctuations of c. 1.0 m. Limnological information has been collected during seven visits to Lake Wisdom over the period 1969–76. The surface temperature of the lake was constant at 28°C throughout this period and the temperature falls very gradually to 26–27°C at 60 m, except for a relatively rapid drop of approximately 1°C from 10 to 20 m. One of the most unusual features is the relatively high oxygen concentration in the deepest parts of the lake. Living chironomid larvae and molluscs were collected from the bottom in depths of 360 m. Light penetration in the lake varies greatly depending on the amount of rainfall and the volcanic activity of Motmot, a secondary cone within the lake. The biota of Lake Wisdom is rather simple. There is a low standing crop of phytoplankton. Benthic algae are abundant and diverse although they cover only a limited area because of the depth of the lake. There are no vascular aquatic plants. The pelagic fauna consists of two species of Cladocera and one species of notonectid. There is one species of sponge, four species of molluscs, and a small number of species of aquatic insects including Hemiptera. Odonata, and the larvae of chironomids, mayflies, a caddisfly, and a pyralid moth. Water birds, including ducks, grebes and waders, are fairly numerous. The lake also contains one or more crocodiles.