The annual reproductive cycle of the freshwater mussel Dreissena polymorpha Pallas in lakes

Summary The annual development of the gonads of Dreissena polymorpha was studied at three sampling sites in two lakes over 3 and 1 1/2 years, respectively. A resting stage occurred after the last spawning in summer/autumn. Oogenesis (accompanied by multiplying segmentation of the oogonia and early growth processes of its oocytes) restarted in specimens at least 1 year old at low temperatures (below 10° C) during winter and early spring. At one location (Fühlinger See) the onset of the spawning season was correlated with an increase of water temperatures above 12° C. At 2 m depth, two main spawning periods in May and August were normally recognized, the first at temperatures of 12–16° C, the second at 16–21° C. It was clearly demonstrated for the first time in Dreissena polymorpha that the oocytes became mature in successive cohorts within one gonad. A female mussel may spawn several times during the reproductive season. At 9 m depth, the onset of spawning also started at about 12° C; this occurred in late summer, with two spawning periods within 1 month at a temperature range of 12–16° C. At another location (Heider Bergsee) the size of the gonads and the oocytes was reduced during April of both years studied, when food supply was low simultaneously with rapidly rising water temperatures in this shallow lake. There was no spawning period during spring. The major spawning period was delayed until July (temperatures 19–22°C). This shows (1) the synchronizing influence of low winter temperatures on the annual reproductive cycle and (2) a temperature threshold of at least 12° C for the start of the spawning processes. The results are discussed with regard to the geographical limits of further spread of Dreissena polymorpha.     

The annual cycle of heat content and mechanical stability of hypersaline Deep Lake,Vestfold Hills,Antarctica

Deep Lake, a hypersaline lake of about ten times seawater concentration, rarely freezes and is characterized by a monomictic thermal cycle, Winter circulation, at c. –17 °C, lasts for two to three months. In summer, epilimnetic temperatures from 7–11 °C result in large vertical thermal gradients (21–26 °C) which combine with the enhanced rate of density change per degree Celsius, accompanying such high salt concentration, to produce a particularly stable density configuration in Deep Lake (Schmidt stability c. 8000 g-cm cm^–2; 0.785 J cm^–2). The Birgean annual heat budget (c. 24500 cal cm^–2; 102.7 10³ J cm^–2) is comparable to that of a temperate lake with a similar mean depth, despite the comparatively high ratio of Birgean wind work to annual heat budget (0.37 g-cm cal^–1). Deep lake retains c. 50% of the incident solar radiation during the short summer heating period; within the range estimated for first class lakes in North America. Extended daylight hours certainly contribute to the high maximum rate of heating in the lake (444 cal cm^–2 day^–1; 1.86 10³ J cm^–2 day^–1). Deep Lake cools at a rate less than half its average heating rate. Partitioning the total stability into thermal and saline components shows that salinity can contribute up to c. 20% of the maximum summer Schmidt stability. In early summer, the effect of small melt-streams is to increase stability by diluting the epilimnion. In autumn, evaporative water loss can overtake this effect, creating small de-stabilizing salinity gradients. The usually short-term stabilizing influence of snowfall and drift is less predictable, but is probably more common in winter when strong winds are most frequent.Hypersalinity has a profound effect on the physical cycle of Deep Lake, through freezing point depression and the increased rate of density change with temperature. These changes affect the lake's biota, both in relation to osmotic stress, and by effectively exposing them to a more thermally extreme environment. A comparison between Deep Lake and a smaller lake of similar salinity (Lake Hunazoko, Skarvs Nes), demonstrates that it is inappropriate to consider the biological effects of salinity in isolation. The smaller lake offers warmer epilimnetic conditions for at least part of the summer, which may explain the much greater limnetic algal production in Lake Hunazoko.     

Factors influencing the species composition,distribution and abundance of benthic invertebrates in the profundal zone of a eutrophic northern lake

Factors influencing the species composition, distribution and abundance of benthic invertebrates were determined in a eutrophic subarctic lake from April 1978 to April 1979. Collections were made at five stations located at depths of 4 to 13 m. The largest populations of up to 5 × 10³ animals m^–2 were found in the deepest part of the lake. of the 24 species recorded in this area, the chironomidsProcladius denticulatus, Dicrotendipes modestus, Chironomus decorus andGlyptotendipes barbipes were most common. The strong development of benthos in the profundal zone was attributed to a consistently large supply of food and warm (4 °C) winter temperatures on bottom. Slightly smaller populations (up to 4 × 10³ animals, m^–2), composed of 19–23 species, occurred in shallower water, a reflection of lower (1.5 °C) winter temperatures. In the anoxic northern part of the lake, only 4–8 species were found in low numbers (400–1 000 animals m^–2). This was likely due to low (<5% saturation) oxygen levels in water and high organic content (18.5%) of the sediments.     

Water quality of rivers in the drainage basin of Lake Peipsi

Water quality and general regularities that have taken place in the water quality of rivers in the drainage basin of Lake Peipsi have been studied. The lake is located in the basin of the Narva River. Several changes of the water quality of the lake in 1970–1990, caused by increasing human impact, have been observed. The water quality of Lake Peipsi depends on its pollution load whereby the main part of the pollution reaches the lake via rivers. In 1985–1989 an extensive research programme of the lake and its drainage basin was carried out. In this paper a part of these results as well as recent changes in the water quality of the rivers of the Estonian side are studied. In the last years an improvement of the general state of rivers in the drainage basin of Lake Peipsi has taken place; the content of organic substances and nutrients in the inflows of Lake Peipsi and in the Narva River (lake outflow) is decreasing.     

Seasonal water quality of shallow and eutrophic Lake Pamvotis, Greece: implications for restoration

Lake Pamvotis is a moderately sized (22 km²) shallow (z _avg=4 m) lake with a polymictic stratification regime located in northwest Greece. The lake has undergone cultural eutrophication over the past 40 years and is currently eutrophic (annual averages of FRP=0.07 mg P l^-1, TP=0.11 mg P l^-1, NH₄ ⁺=0.25 mg N l^-1, NO₃ ^–=0.56 mg N l^-1). FRP and NH₄ ⁺ levels are correlated to external loading from streams during the winter and spring, and to internal loading during multi-day periods of summer stratification. Algal blooms occurred in summer (July–August green algae, August–September blue-green algae), autumn (October blue-green algae and diatoms), and winter (February diatoms), but not in the spring (March–June). The phytoplankton underwent brief periods of N- and P-limitation, though persistent low transparency (secchi depth of 60–80 cm) also suggests periods of light limitation. Rotifers counts were highest from mid-summer to early autumn whereas copepods were high in the spring and cladocerans were low in the summer. Removal of industrial and sewage point sources a decade ago resulted in a decrease in FRP. A phosphorus mass balance identified further reductions in external loading from the predominately agricultural catchment will decrease FRP levels further. The commercial fishery and lake hatchery also provides opportunities to control algal biomass through biomanipulation measures.     

Aspects of the thermal ecology of the rusty crayfish Orconectes rusticus (Girard)

Summary Orconectes rusticus currently is undergoing an explosive range expansion in the midwestern U.S.A., but information on the potentially important effects of water temperature on the species' biology is lacking. The thermal ecology of O. rusticus in southwestern Ohio, U.S.A., was examined by determining 1) the effects of four water temperatures (16, 20, 25, and 29°C) on survival and growth of juveniles, 2) the responses of juveniles and adults to a thermal gradient (7–27° C), and 3) the thermal tolerances (critical thermal maximum, CTMax, and critical thermal minimum, CTMin) of free-living, field-acclimatized juveniles and adults on a biweekly basis throughout the summer. Month-long growth experiments predicted maximum growth rates of juveniles at water temperatures between 26 and 28°C, but greatest survival between 20 and 22° C. Laboratory-acclimated (22° C) adults and field-acclimatized (2.5° C) juveniles both had an acute preferred temperature of 22° C. CTMaxs and CTMins of juveniles were 0.5–2.6° C higher than those of adults throughout the summer, suggesting that juveniles were exposed to water temperatures 1.5–6.8° C warmer than those of adults. Juvenile and adult O. rusticus prefer habitats where water temperatures favor maximum survival, but they usually are not found together in the same habitat; adults apparently displace the juveniles into warmer habitats. Warmer temperatures can decrease survival of juveniles but improve their growth rates, leading to enhanced fecundity and competitive ability. The past and future success of O. rusticus in expanding its range may depend, in part, on the species ability to adjust to new thermal environments occupied by other species of crayfish.     

Potential Effects of Climate Warming on Fish Habitats in Temperate Zone Lakes with Special Reference to Lake 239 of the Experimental Lakes Area (ELA), North-Western Ontario

We used simple statistics (e.g. mean temperature, degree days, cumulative volume days) to describe present thermal habitats for cool water (yellow perch, Perca flavescens) and cold water (lake trout, Salvelinus namaycush) fish of a small boreal lake. We then modelled changes in the vertical and temporal extent of these habitats under various scenarios of climatic change that included increases in air temperature of 2°C, 4°C, and 9°C, and positive and negative deviations from present levels of 10% in solar radiation and relative humidity, and 20% in wind speed and the lake water extinction coefficient. Model simulations indicated pronounced changes in the temporal and vertical availability of fish thermal niche space. These changes were mainly driven by the large increases in mean mixed layer temperatures that corresponded to 85% of the increases in air temperature, but, in particular, changes in light attenuation also resulted in some non-linear, unexpected effects in the distribution and seasonal availability of thermal niche space. As expected, classical lake trout thermal habitat (5–15°C) was progressively reduced and almost disappeared in littoral areas in spring and early summer. Perch thermal niche space expanded for air temperature increases of up to 4°C, but largely disappeared for the 9°C increase. We discuss changes in thermal habitat with regard to the life history of lake trout and yellow perch, and include other determinants of fish habitat to evaluate the potential of these species for long-term ecological success under climatic warming.     

The importance of avian-contributed nitrogen (N) and phosphorus (P) to Lake Grand-Lieu,France

The largest natural lake in France, Grand-Lieu, has suffered eutrophication. The objective of the study was to estimate the annual input of nutrients (N, P) resulting from avian excrement, deposited by birds feeding out of the lake and returning to its waters for breeding or roosting, as compared to the input by the rivers that enter in the lake. Two years are compared: 1981–82 and 1990–91. About 1600–2000 breeding herons and cormorants, 20 000–33 000 wintering ducks, gulls and cormorants and 1–2.4 million starlings deposited about 5800 kg total N in 1981–82 and 7640 kg in 1990–91. Respectively, 2000 and 2530 kg total P were deposited over the same time periods. These represent 0.7% and 0.4% of the total N input of the lake and 2.4 and 6.6% of the total P input in 1981–82 and 1990–91. Starlings account for 74% of the N and mallards most of the rest. P input by starlings (36% in 1981–82, 41% in 1990–91), and by mallards and herons (35% and 27% in 1981–82 and 22% and 24% in 1990–91 respectively) plays an appreciable role among birds. During the plant growing period (April–September), the contribution by birds can increase to 37% of total P input of the lake. Piscivorous bird colonies concentrate Phosphorus 42 times more within the colony than outside the colony. Overall, the role birds play in total N and P input is relatively small due to very high inputs from human sewage and agriculture run off. The monthly mean concentration of the water of the two rivers reaches currently 10 mg l^–1 of N (to 23 mg during peak floods) and 394 mg m^–3 of P (to 468 mg during peak floods). Earlier, for example in the 1960's, water in Brittany only contained 0.1 to 1.1 mg 1^–1 of N and 1 to 5 mg m^–3 of P during the maximum flow period. At this time, birds could probably have represented annually up to 37% of the N input and up to 95% of the P input to the lake.     

Thermal, mixing, and oxygen regimes of the Salton Sea, California, 1997–1999

The Salton Sea is a shallow (mean depth = 8 m; maximum depth = 15 m), saline (41–45 g l^–1), intermittently mixing, 57 km long, 980 km² lake located in the arid southwestern United States. The Sea is a wind driven system, with predominant winds paralleling the long axis of the lake, being strongest in spring and weakest in summer and fall. The Sea mixed daily or nearly daily between September and January. During this cooling period, moderate to high levels of dissolved oxygen (3–11 mg l^–1) were found throughout the water column. Mean water column temperature ranged from a minimum of 13–14 °C in early January to a maximum of 30–34 °C in July–September. During most of this warming period, the Sea was thermally stratified but subject to periodic wind driven mixing events. Winds were stronger in spring 1998 than in 1997 or 1999, causing more rapid heating of the lake that year and also delaying onset of anoxic conditions in bottom waters. During summer months, mid-lake surface waters were sometimes supersatured with oxygen, and bottom waters were hypoxic or anoxic with sulfide concentrations > 5 mg l^–1. Oxic conditions (> 1 mg O₂ l^–1) often extended a few meters deeper nearshore than they did well offshore as a consequence of greater mixing nearshore. Mixing events in late summer deoxygenated the entire water column for a period of days. Consumption of oxygen by sulfide oxidation likely was the principal mechanism for these deoxygenation events. Sulfide concentrations in surface waters were 0.5–1 mg l^–1 approximately 3 days after one mixing event in mid-August 1999. These mixing events were associated with population crashes of phytoplankters and zooplankters and with large fish kills. In the southern basin, freshwater inflows tended to move out over the surface of the Sea mixing with saline lake water as a function of wind conditions. Salinity gradients often contributed more to water column stability than did thermal gradients in the southeasternmost portion of the lake.     

Some characteristics of the community of autotrophs of Lake Sevan in connection with its eutrophication

Eutrophication of Lake Sevan caused by the artificial lowering of water level was accompanied by changes in the structure and dynamics of the planktonic communities. A dominance of diatoms up to 1983 was changed to that of green algae in the last years. Primary production of plankton rose and then decreased in the process of eutrophication. The annual average primary production in 1982–1986 — 250 g C m^–2 yr^–1 — is evidently close to the steady state production under the present morphometry of the lake. The activity coefficient of phytoplanktonic photosynthesis changed within relatively narrow limits, in spite of significant changes in the concentrations of major nutrients and in the structure and productivity of the phytoplankton.     

Caesium-137 and lead-210 dating of recent sediments from Mondsee (Austria)

Mean annual sedimentation rates over the last 20–30 years were determined in the pre-alpine Mondsee (Upper Austria) using Cs-137 and Pb(Po)-210 profiles for sediment core dating and two natural sediment markers. Lower sedimentation rates of about 2–3 mm yr^–1 were observed in the central part of the lake near the shore at 18–20 m and in the southern part at 30 m depth. Higher sedimentation rates of 4–7 mm yr^–1 were found in the central part of the lake at 47 and 65 m and in the northern bay at 18 and 41 m depth. At both these sites the Pb-210 profiles were strongly disturbed in the upper zone of the sediment cores, whereas the Cs-137 pattern remained intact. The higher annual sediment accumulation rates can be explained only partly by deposition of allochthonous material discharged by the streams, enhanced eutrophication in these parts of the lake, erosion and sediment focusing by turbidity currents being also probable.     

Thermal regimes of Florida lakes

Water column temperatures were determined monthly for 24 lakes and bimonthly for 5 lakes in peninsular Florida during 1979. Three geographical groups (north, central, south) were delineated from mean monthly water column temperatures for individual lakes. On a monthly basis, northern lakes were least similar to southern lakes, while central Florida lakes displayed greater affinity to the southern than to the northern lake group. Temperature differences between lake groups broke down during late summer. Subtropical lakes have been defined tentatively as those Florida lakes south of 28° latitude which possess warm monomictic circulation and a mean annual temperature of 24.2 ± 4.8 °C with minimum water column temperature rarely less than 14 °C and summer maxima rarely exceeding 31 °C. While all lakes in Florida are clearly warm monomictic annual nutrient cycling and productivity patterns may be influenced by inter-group differences in the timing and duration of water column circulation.     

Chemical composition of interstitial water and diffusive fluxes within the diatomaceous sediment in Lake Myvatn, Iceland

The sub-arctic Lake Myvatn is one of the most productive lakes in the Northern Hemisphere, despite an ice-cover of 190 days per year. This is due to relatively high solar radiation, nutrient rich inflow waters, N₂ fixation and internal nutrient loading. In order to define direction and magnitude of diffusive fluxes, soil water samplers were used to collect interstitial water from 25–150 cm depth, from within the diatomaceous sediment at the bottom of Lake Myvatn. Water depth at the sampling site was 225 cm. The pH of the interstitial water ranged from 7.16 to 7.30, while the pH of the lake water was 9.80–10.00. The concentrations of most solutes were similar 16 cm above the bottom of the lake at the sampling site and at the lake outlet. The concentrations of NO₃, S, F, O₂, Al, Cr, Mo, V, U, Sn and Sb were higher in the lake water than in the interstitial water. They will therefore diffuse from the lake water into the interstitial water. The concentrations of orthophosphates, PO₄, and total dissolved P were highest at 25 cm depth, but Co and NH₄ concentrations were highest at 50 to 100 cm depth. Thus they diffuse both up towards the lake bottom and down deeper into the sediments. The concentrations of Na, K, Ca, Mg, Sr, Mn, Li and alkalinity were greater within the sediments than in the lake water and increased continuously with depth. The Si concentration of the interstitial water was higher than in the lake water, it was highest at 25 cm depth and decreased slightly down into the sediments. The concentration gradient was greatest for bicarbonate, HCO₃ ^–, 1.5×10^–7 mol cm^–3 cm^–1, and then in declining order for the solutes with the highest gradient; NH₄, Si, Na, Ca, Mg, -S (diffusion into the sediments), K, PO₄, Cl, Fe and Mn. The estimated annual diffusive flux of PO₄ for Lake Myvatn was 0.1 g P m^–2 yr^–1, about 10% of the total PO₄ input to Lake Myvatn. The H₄SiO₄° flux was 1.3 g Si m^–2 yr^–1, <1% of both the input and the annual net Si fixation by diatoms within the lake and the diffusive flux of dissolved inorganic carbon was 1% of the annual net C fixation by diatoms. Annual diffusive flux of NH₄ ⁺ was 1.9 g N m^–2 yr^–1 similar to the input of fixed N to the lake and 24% of the net N fixation within Lake Myvatn. Thus it is important for the nitrogen budget of Lake Myvatn and the primary production in the lake since fixed nitrogen is the rate determining nutrient for primary production.     

The distribution,structure, and composition of freshwater ice deposits in Bolivian salt lakes

Freshwater ice deposits are described from seven, high elevation (4117–4730 m), shallow (mean depth <30 cm), saline (10–103 g l^-1) lakes in the southwestern corner of Bolivia. The ice deposits range to several hundred meters in length and to 7 m in height above the lake or playa surface. They are located near the lake or salar margins; some are completely surrounded by water, others by playa deposits or salt crusts. Upper surfaces and sides of the ice deposits usually are covered by 20–40 cm of white to light brown, dry sedimentary materials. Calcite is the dominant crystalline mineral in these, and amorphous materials such as diatom frustules and volcanic glass are also often abundant.Beneath the dry overburden the ice occurs primarily as horizontal lenses 1–1000 mm thick, irregularly alternating with strata of frozen sedimentary materials. Ice represents from 10 to 87% of the volume of the deposits and yields freshwater (TFR <3 g l^-1) when melted. Oxygen isotope ratios for ice are similar to those for regional precipitation and shoreline seeps but much lower than those for the lakewaters. Geothermal flux is high in the region as evidenced by numerous hot springs and deep (3.0–3.5 m) sediment temperatures of 5–10°C. This flux is one cause of the present gradual wasting away of these deposits. Mean annual air temperatures for the different lakes probably are all in the range of -2 to 4°C, and mean midwinter temperatures about 5°C lower. These deposits apparently formed during colder climatic conditions by the freezing of low salinity porewaters and the building up of segregation ice lenses.     

Phytoplankton primary production in a eutrophic cooling water pond

The seasonal variation of phytoplankton photosynthesis was measured with ¹⁴C-method in a warmed ice-free pond in central Finland. Simultaneously with in situ measurements the photosynthesis was also measured in an incubator with different water temperatures and constant light (ca. 16 W m^–2). The total annual photosynthesis was 57.2 C m^–2 a^–1. The portion of the winter and spring production of the annual photosynthesis was 18.4%, that of the autumn production ws 17.4%. Thus 64.3% of the total annual phytoplankton photosynthesis occurred in the three summer months. The range of the daily integrated photosynthesis per unit area was 1.9—563 mg C m^–2d^–1. The photosynthetic rate per unit chlorophyll a varied in situ from 0.94 to 33.1 mg C (mg chl. a)^–1 d^–1. The highest value was measured in the beginning of July and the lowest in mid-January. The photosynthetic rate increased in situ exponentially with increasing water temperature. In the incubator the highest photosynthetic rate values were also found in July and August (at+20 °C) when the phytoplankton population was increasing and the minimum values occurred after every diatom maximum both in spring and autumn. Light was a limiting factor for photosynthesis from September to Mid-January, low water temperature was a limiting factor from late January through May. The efficiency of the photosynthesis varied between 0.1 and 0.7% of P.A.R. According to the incubator experiments the Q₁₀ values for the photosynthesis were 2.45 and 2.44 for the winter population between 1 and 10° C and for the summer population between 5 and 15° C, respectively, but the Q₁₀ values decrease at the higher temperatures. The main effect of the warm effluents on the yearly photosynthesis was the increase of production in spring months due to the lack of ice cover. However, the increase of total annual phytoplankton photosynthesis was only ca. 10–15%, because the water temperature was during the spring months below 10° C.     

How does the cold stenothermal gadoid<Emphasis Type="Italic"> Lota lota</Emphasis> survive high water temperatures during summer?

The cold-stenothermal freshwater gadid Lota lota inhabiting the potamic regions of lowland rivers in central Europe, is exposed to summer temperatures up to 25 °C, which is far above the thermal preferendum of this species. Oxygen consumption rates, determined in field catches sampled at different times of the year, revealed that the basal metabolic rate is depressed during summer when water temperatures are high (152±16 mol O₂ 100 g^–1 h^–1at 22 °C in July compared to 250±33 mol O₂ 100 g^–1 h^–1 at 6 °C in November). This observation led us to investigate whether the observed depression of the metabolic rate is caused by oxygen limitation due to thermal impairment of the ventilatory system, as has been observed in other species. Determination of anaerobic end products (lactate and succinate) in the liver tissue of fish caught at different sampling dates did not show an accumulation of anaerobic end products during the summer, indicating no oxygen limitation. Measurements of enzyme activities in the white musculature and liver suggest that enzymes involved in aerobic metabolism were down-regulated during summer, which may have contributed to the observed reduction of metabolic rate.Abbreviations CS citric synthase - LDH lactate dehydrogenase - PK pyruvate kinase - TCA trichloroacetic acid Communicated by G. Heldmaier     

Oligochaeta of Lake Taimyr: a preliminary survey

Lake Taimyr in Siberia is northernmost among the world's large lakes: 73°40–75°20N, 99–106°E. The lake area is up to 4650 km² in summer, with a maximum depth of 26 m and a mean depth of only 2.8 m. The ice-free period lasts about three months. The water level sinks 5.5–6 m during winter, so that 85% of the bottom surface is frozen into ice for some time and subjected to negative temperatures, probably down to –20 °C. In artificially melted sediment samples, 75–92% of animals survived. The average summer biomass of zoobenthos is about 1 g m^–2 wet weight, a half of this being formed by Oligochaeta. Altogether 76 samples with 3742 oligochaete specimens collected by V. N. Grëze in 1943–1944 were studied. At least 14 taxa of Tubificidae, Lumbriculidae, and Enchytraeidae were found in the lake, and some more enchytraeids in an adjacent river. Many immature animals could not be identified to species. Naididae were completely lacking probably due to the absence of macrovegetation. The shallow freezing zone is inhabited mostly by Alexandrovia ringulata. The profundal fauna is dominated by Lamprodrilus isoporus, Stylodrilus sp., and Isochaetides sp.     

Perennially ice-covered Lake Hoare,Antarctica: physical environment,biology and sedimentation

Lake Hoare (77° 38 S, 162° 53 E) is a perennially ice-covered lake at the eastern end of Taylor Valley in southern Victoria Land, Antarctica. The environment of this lake is controlled by the relatively thick ice cover (3–5 m) which eliminates wind generated currents, restricts gas exchange and sediment deposition, and reduces light penetration. The ice cover is in turn largely controlled by the extreme seasonality of Antarctica and local climate. Lake Hoare and other dry valley lakes may be sensitive indicators of short term (< 100 yr) climatic and/or anthropogenic changes in the dry valleys since the onset of intensive exploration over 30 years ago. The time constants for turnover of the water column and lake ice are 50 and 10 years, respectively. The turnover time for atmospheric gases in the lake is 30–60 years. Therefore, the lake environment responds to changes on a 10–100 year timescale. Because the ice cover has a controlling influence on the lake (e.g. light penetration, gas content of water, and sediment deposition), it is probable that small changes in ice ablation, sediment loading on the ice cover, or glacial meltwater (or groundwater) inflow will affect ice cover dynamics and will have a major impact on the lake environment and biota.     

Precursors of carcinogenic N-nitroso compounds in Lake Peipsi

The concentration of precursors of carcinogenic N-nitroso compounds, nitrates and nitrites as well as ammonia, in the surface water of Lake Peipsi and its tributaries has been determined during the period 1985–1988. The nitrate and nitrite content was also analysed in bottom sediments and fish from the lake.The nitrate concentration in the water of Lake Peipsi varied from 0.01 to 2.33 mg NO₃&z.sbnd;N l^–1, the average value from 0.27 to 1.60 mg NO₃&z.sbnd;N l^–1, with the lowest concentrations in summer. The variations may be caused by different pollution loads, meteorological conditions, and assimilation of nitrates by plants and algae.The nitrate content in the water of rivers was on an average somewhat higher in comparison with its concentrations in the lake. The concentrations of nitrites were, as a rule, about an order of magnitude lower than those of nitrates. The amount of ammonia varied from 0.15 to 0.36 mg NH₄&z.sbnd;N l^–1.At present the concentrations of the studied nitrogen compounds are not essential and do not prevent from using the lake for recreation and drinking water supply.     

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.