Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

Species composition and phytoplankton biomass in a tropical African lake (Lake Awassa,Ethiopia)

The species composition and phytoplankton biomass of Lake Awassa, Ethiopia were studied from September 1985 to July 1986 in relation to some limnological features of the lake. During the study period, three phases of thermal stratification were recognized: a period of unstable stratification and near-complete mixing was followed by a stable stratification period and another period of complete mixing. Complete mixing was associated with cooling of air temperature with an influx of cool rain and high rainfall. The underwater light penetration showed a similar pattern over the whole period with the highest in the red, and the lowest in the blue spectral region. Euphotic depth varied between 1.6 and 3.0 meters with the highest measurements corresponding to the stable stratification period. PO₄-P concentrations ranged between 23 and 45 µg l^–1 and NO₃-N concentrations varied between 7 and 14 µg l^–1 during the study period. Both nutrients showed increasing values associated with mixing periods and/or the rainy season.A total of 100 phytoplankton species were identified with 48% of the taxa represented by green algae, 30% by blue-green algae, 11% by diatoms, and the rest by chrysophytes, dinoflagellates, cryptomonads and euglenoids. The dominant phytoplankton species were Lyngbya nyassae, Botryococcus braunii and Microcystis species. Seasonal biomass variation was pronounced in the first two species but not in Mycrocystis. Phytoplankton biomass increased following the mixing period in December, and thermal destratification during May to July which was also a period with high rainfall and relatively high nutrient concentration. While the seasonal variation of the total phytoplankton community in Lake Awassa was relatively low (coefficient of variation < 20%), it was higher in some of the individual component species.     

Sediment oxygen demand in man-made Lake Ton-Ton (Uruguay)

A study on sediment metabolism was carried out during 1986 in Lake Ton-Ton, Uruguay. Sediment oxygen demand (SOD) from chemical and biological origin was measured in undisturbed sediment cores taken from the deepest part of the lake. Mean SOD rate for the study period (51.56 mgO₂ m^–2 h^–1) corresponded well with the eutrophic state of the lake. During stratification, SOD from chemical origin accounted for 69–87% of total SOD, while SOD from biological origin was dominating for the rest of the year, except in July. Biological respiration was principally of microbial origin. Hypolimnetic temperature was the main factor controlling SOD rates (r = 0.771,p < 0.001). Nevertheless, freshly sedimented phytodetritus from anAnabaena bloom, together with a renewed input of oxygen to bottom water were responsible for the maximum SOD values, recorded at the beginning of a mixing period in April (72.51 mgO₂ m^–2 H^–1).     

Nutrient cycling and productivity in a desert saline lake: observations from a dry,low-productivity year

Seasonal variability of nutrients and productivity were examined in Pyramid Lake, a hyposaline, N-deficient, terminal desert lake, during a dry period. River inflow and N-fixation during 1990 were minimal allowing internal nutrient cycling to be more closely studied. Nutrient cycling was strongly affected by seasonal thermal stratification that was typical for a warm monomictic lake. Concentrations of nitrate, phosphate, and silicate in surface waters were highest during winter mixing and decreased rapidly in the spring due to a diatom bloom. Maximum average chlorophyll concentration in surface waters was 2.7 ± 1.2 µg 1^–1 and occurred in April while surface nutrients were being depleted. In contrast to chlorophyll, maximum particulate carbon in surface waters occurred in July–August when areal productivity was highest (367–398 mg C m^–2 day^–1). Concurrent with spring nutrient depletion in surface waters was increasing N-deficiency in the plankton. After the spring bloom dissipated in May, particulate matter (POM) became increasingly N-deficient reaching maximum elemental C : N of > 18 during summer-fall. Profiles of the C : N ratio of POM were nearly constant with depth for individual sampling dates suggesting that the residence time of POM in the water column was short (< 1 month). While surface waters were nutrient depleted during summer stratification, nutrient concentrations of bottom waters progressively increased, presumably through the oxidation of POM sinking to the bottom (103 m). Converting the rate of oxygen depletion in bottom waters to carbon equivalents of POM suggests that 42 % of mean annual phytoplankton production in overlying waters during 1990 was mineralized in bottom waters.     

Physical and Chemical Limnology of a Subsaline Athalassic Lake in West Greenland

Physical and chemical profiles of a shallow (c. 12-m-deep) subsaline (total dissolved solids 2.3-2.8 g l^–1) closed-basin lake in the continental area of southwestern Greenland are described for the first time. Watercolumn data for every 5th consecutive day between April 20 and October 6, 2001, and continuous recordings of lake water level and meteorological conditions are used to infer controls on contemporary lake functioning, sediment formation and climate-lake interactions. Limnological observations demonstrate the importance of lake-ice formation and its role in haline convection and the development of meromixis. Observed lake cycling suggest that the lake at present is in a state of near-meromixis where stagnant bottom waters de-stratify through deep penetration of weak haline convective cells by the end of June. From this study, the primary reasons the shallow Greenlandic low salinity lakes develop meromixis are:(i) lack of an outflow (ii) meltwater dilution and chemical stratification of surface waters, (iii) insubstantial wind mixing, (iv) a weak winter thermohaline convective cell forced by cryoconcentration, and (v) biogeochemically enhanced solute concentrations near the sediment bed. Throughout the open water period the hydrological balance is dominated by evaporative losses. Lake surface water conductivities change from 2110 to 2890 S cm^–1 due to the combined effects of open water evaporation, meltwater dilution, diffusive exchanges over the seasonal pycnocline, and boundary mixing. Freeze-out of salts and resulting deep haline convection increase overall water column salinity during winter. Owing to deep convective mixing, plant nutrients are relatively high in the upper watercolumn with a dominant internal source of phosphorous. Extreme productivity pulses of phytoplankton are observed as soon as sub-ice radiation levels increase and directly after ice-out when sufficient wind mixing can support an intense monospecific diatom bloom of Diatoma spp. leading to the rapid depletion of dissolved silica.     

Nitrogen limitation and particulate elemental ratios of seston in hypersaline Mono Lake, California, U.S.A.

Particulate elemental ratios (C:N, N:P and C:Chl a) of seston in hypersaline (70–90 g kg^–1) Mono Lake, California, were examined over an 11-year period (1990–2000) which included the onset and persistence of a 5-year period of persistent chemical stratification. Following the onset of meromixis in mid-1995, phytoplankton and dissolved inorganic nitrogen were substantially reduced with the absence of a winter period of holomixis. C:N, N:P and C:Chl a ratios ranged from 5 to 18 mol mol^–1, 2 to 19 mol mol^–1 and 25 to 150 g g^–1, respectively, and had regular seasonal patterns. Deviations from those expected of nutrient-replete phytoplankton indicated strong nutrient limitation in the summer and roughly balanced growth during the winter prior to the onset of meromixis. Following the onset of meromixis, winter ratios were also indicative of modest nutrient limitation. A 3-year trend in C:N and N:P ratios toward more balanced growth beginning in 1998 suggest the impacts of meromixis weakened due to increased upward fluxes of ammonium associated with weakening stratification and entrainment of ammonium-rich monimolimnetic water. A series of nutrient enrichment experiments with natural assemblages of Mono Lake phytoplankton conducted during the onset of a previous episode of meromixis (1982–1986) confirm the nitrogen will limit phytoplankton before phosphorus or other micronutrients. Particulate ratios of a summer natural assemblage of phytoplankton collected under nitrogen-depleted conditions measured initially, following enrichment, and then after return to a nitrogen-depleted condition followed those expected based on Redfield ratios and laboratory studies.     

Polymixis of a shallow lake (Groβer Müggelsee,Berlin) and its influence on seasonal phytoplankton dynamics

The polymictic properties of Lake Müggelsee, a eutrophic shallow lake in Berlin, are described by the water column stability (N ²) and gradients in saturation of oxygen at the deepest site of the lake (7.5 m). Mixing and stratification changed irregularly up to 7 times during the vegetation season (April to September), as was indicated by all of the stratification parameters. Thermally stable conditions generally lasted 1–2 weeks. A maximum of 5 weeks stratification was observed in 1982.In order to investigate the response of algal development, the internal rates of change of the dominant algal species in the lake during the vegetation period were estimated from weekly measurements of phytoplankton biomass from 1980 to 1990. The necessity taking a mixed sample in a shallow lake is discussed. The polymictic properties favoured the development of specific blue-green algal species; there dominance was also favoured by the trophic conditions. Among the dominant blue-greens the growth of Limnothrix redekei was independent of polymixis whereas stratification supported the starting conditions for the summer blue-greens Aphanizomenon flos-aquae and Planktothrix agardhii. After these algae reached a distinct level of biomass, they grew under mixing as well as under stratified conditions.For the development of solitary centric diatoms during summer regulation by growth restriction through nutrient limitation, esp. dissolved silicon was more important. However, Melosira sp. developed well under stratified conditions but collapsed due to increased sinking losses when the water column became too stable.An attempt is made to apply Reynolds' possibility matrix of the most likely phytoplankton assemblages as a function of nutrients and mixing in the shallow Lake Müggelsee.     

Biogeochemical response to physical forcing in the water column of a warm monomictic lake

Based on microstructure measurements of temperature and horizontalcurrent velocity the physical structure in the water column of Lake Kinneretwascharacterized as a five layer system consisting of a surface mixed layer, lowerepilimnion, metalimnion, upper hypolimnion and benthic boundary layer. Usingoxygen and hydrogen sulfide as natural chemical tracers, the time scale ofchemical change was identified in relation to advection, mixing and biologicalprocesses. Rapid changes due to advection that took place on an hourly timescale were removed by referring the data back to the temperature of the water.Biological activity dominated the hydrochemical changes observed in the meta-and upper hypolimnion. These were expressed by DO depletion rates of 2.0 and0.4g m^–2 d^–1, respectively.Verticaland horizontal mixing were shown to occur on a seasonal time scale. Once thechemical stratification process was completed the slow mixing through thebenthic boundary layer became the limiting factor for subsequent reactions inthe water column.     

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.     

Population dynamics of Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) in different areas of Lake Sevan

Potamothrix alatus paravanicus Poddubnaya & Pataridze (Tubificidae) is the most abundant species of benthic invertebrate in Lake Sevan and the only species in the deep profundal. Differences in annual temperature, length of the stratification period (low oxygen content) and bottom sediment characteristics influence the population of P. a. paravanicus in different depth zones of Lake Sevan. From shallow to deep water a regular increase of the following indices was observed in 1984–1987: annual biomass from 1.2–2.0 to 8.2–17.0 g m^–2 wet weight; mean individual wet weight of clitellate specimens from 2.0–3.8 to 8.3–16.6 mg; total life span from one and a half to more than 20 years; the length of breeding season from 2–3 to 12 months.An important ecological distinction between P. a. paravanicus and allied Potamothrix hammoniensis (Michaelsen) is the much lower fecundity of the former species.     

Convection in bottom sediments and its role in material exchange between water and sediments

In two Finnish lakes, in winter and summer, a constant temperature in the sediment was not reached until a depth of 1.5–2 m. The thermal stratification pattern in the sediment was similar to that in water. However, the lack of turbulent mixing in the sediment resulted in a thin episediment. This stratification was caused by convection. Convection currents in the sediment were most prevalent during spring and autumn overturn when the density of overlying water was greater than that of the interstitial water. Convection was also possible in winter. The duration and magnitude of convection was dependent on warming and cooling rates. Such convection currents may be important in material exchange between sediment and water since material concentrations in interstitial water are 5–100 times greater than in overlying water.     

A Short-Term Study of Vertical and Horizontal Distribution of Zooplankton During Thermal Stratification in Lake Kinneret,Israel

This study documents for the first time both vertical and horizontal distribution patterns of the zooplankton community in Lake Kinneret during the period of thermal stratification. The zooplankton distribution patterns were explored in relation to abiotic (temperature, oxygen) and biotic (picocyanobacteria, ciliates, flagellates, phytoplankton, fish) environmental gradients. Sampling was carried out on 6–7 July 1992 at five stations and six depths from nearshore to offshore. Zooplankton abundance and biomass varied from 5 to 267 ind. l^–1(mean: 95 ind. l^–1), and from 0.1 to 65 d.w. mg m^–3(mean: 24 d.w. mg m^–3). Zooplankton taxonomic groups (Rotifera, Cladocera, Cyclopoida, Calanoida) and size classes (micro-, meso- and macrozooplankton) showed peaks of maximal density and biomass in the epilimnetic and metalimnetic strata (5 and 14 m). Depth, accounting for 31–39% of total spatial variation, reflected the vertical distribution of zooplankton in relation to temperature and oxygen declines, and the higher concentration of food resources (protists and phytoplankton) in the epilimnion and metalimnion. Onshore–offshore distance, accounting for 17–22% of the total spatial variance, reflected different distribution patterns shown among zooplankton groups and size classes. The macrozooplankton (Copepoda, Cladocera) was more abundant offshore, whereas microzooplankton (Rotifera and nauplii) predominated nearshore. These horizontal distribution patterns were related to small increases in temperature and phytoplankton biomass, and higher concentrations of fish in the littoral zone. Although limited to a short temporal scale, our study indicated that zooplankton spatial distribution in Lake Kinneret during the period of thermal stratification was related to physicochemical, food and predation factors, manifested differently along the vertical and nearshore–offshore gradients.     

Sedimentation Rate of Seston during the Formation of Temperature Stratification after Ice Break-up in the Partly Meromictic Lake Verevi

The small strongly stratified hard-water hypertrophic lake Verevi (max. depth 11.0 m, surface area 12.6 ha, mean depth 3.6 m) was investigated in 2000 and in 2001. The lake is sheltered from winds, and the role of waves in mixing the water column is minimal. Eutrophication favours the strengthening of stratification. Early warm springs cause a fast stagnation of the water column forming partly meromictic conditions. Seston content of water and in sediment traps in 3 layers was measured several times during the formation of stratification. Besides measuring particulate matter, in 2001, the nutrient content of the trapped sediment was analysed. During the first 7 days of the investigation, 30% of the total particle sedimentation took place. The sedimentation rate of particulate matter was 0.4–6.3 g m^–2 d⁻¹ dry weight in different layers of the water column. Daily average sedimentation loss rate was 27% of the total amount of seston of the epilimnion, whilst from the meta- and hypolimnion the settling was much slower (9.6 and 7.3%, respectively). In our experiments with twin sediment traps, to one of which formaldehyde was added, the PO₄³⁻-P concentration was 19% smaller in the trap without formaldehyde, probably due to planktonic uptake. The relationship between primary and export production is loop-like. The shape was irregular, indicating a high grazing rate of zooplankton.     

Picoplankton photosynthesis and diurnal variations in photosynthesis-irradiance relationship in a eutrophic and meso-oligotrophic lake

The abundance and relative importance of autotrophic picoplankton were investigated in two lakes of different trophic status. In the eutrophic lake, measurements of primary production were performed on water samples in situ and in a light incubator three times during the day whereas for the oligotrophic lake, only one measurement of primary production was performed on water samples in the incubator. Dark-carbon losses of phytoplankton from Lake Loosdrecht were investigated in time series. Cell numbers of autotrophic picoplankton in eutrophic Lake Loosdrecht (3.2 × 10⁴ cells ml^–1) were lower than in meso-oligotrophic Lake Maarsseveen (9.8 and 11.4 × 10⁴ cells ml^–1 at the surface and bottom respectively). In the phytoplankton of both lakes the ratio of picoplankton production increased with decreasing light intensity. In Lake Loosdrecht depth-integrated contribution of picoplankton to total photosynthesis was less than 4%. The P-I-relationship showed diurnal variations in light saturated photosynthesis, while light limited carbon uptake remained constant during the day. Dark carbon losses from short-term labelled phytoplankton during the first 12 hours of the night period accounted for 10–25% of material fixed during the preceeding light period.     

Sulfte-reduction process in sediments of Lake Kinneret,Israel

Monomictic Lake Kinneret is stratified during summer and autumn, resulting in a hypolimnion rich in H₂S (3–7 mg 1^–1). In winter and spring every year a bloom of dinoflagallate Peridinium gatunense produces an average biomass of 150000 ton wet weight. Part of this biomass sinks to the hypolimnion and sediments where it is decomposed and mineralized, with some of the mineralization due to the activity of sulfate-reducing bacteria (SRB). The sulfate-reduction potential of the upper sediment layer at the deepest part of the lake (42 m) was measured. The activity of the enzyme arylsulfatase was also monitored. Rates of sulfate-reduction ranged from a minimum of 12 nmoles SO_f4 ^p2–-reduced cm^–3 day^–1 in December before lake overturn to a maximum of 1673 nmoles SO_f4 ^p2– reduced cm^–3 day^–1 in July during stratification. These rates are considerably higher than those recorded from other freshwater lakes in the world and are probably limited more by the availability of organic matter than by sulfate concentrations.     

Dissolved organic carbon fluctuations in Dal Lake waters

Dissolved organic carbon (DOC) fluctuations were studied in the surface water of Dal Lake from May 1984 to April 1985, at monthly intervals. Concentration of DOC varied from 0.140 mg l^–1 to 0.725 mg l^–1. A minimum was recorded in May and September, while an increase was observed during December and January. The fluctuations during the observation period showed a relationship with temperature and pH.     

The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe,Ontario, Canada

Observations of the interactions of large amplitude internal seiches with the sloping boundary of Lake Simcoe, Canada show a pronounced asymmetry between up- and downwelling. Data were obtained during a 42-day period in late summer with an ADCP and an array of four thermistor chains located in a 5 km line at the depths where the thermocline intersects the shallow slope of the lakebed. The thermocline is located at depths of 12–14 m during the strongly stratified period of late summer. During periods of strong westerly winds the thermocline is deflected as much as 8 m vertically and interacts directly with the lakebed at depth between 14–18 m. When the thermocline was rising at the boundary, the stratification resembles a turbulent bore that propagates up the sloping lakebed with a speed of 0.05–0.15 m s⁻¹ and a Froude number close to unity. There were strong temperature overturns associated with the abrupt changes in temperature across the bore. Based on the size of overturns in the near bed stratification, we show that the inferred turbulent diffusivity varies by up to two orders of magnitude between up- and downwellings. When the thermocline was rising, estimates of turbulent diffusivity were high with K_Z ∼10⁻⁴ m²s⁻¹, whereas during downwelling events the near-bed stratification was greatly increased and the turbulence was reduced. This asymmetry is consistent with previous field observations and underlines the importance of shear-induced convection in benthic bottom boundary layers of stratified lakes.     

International study on Artemia LXIII. Field study of the Artemia urmiana (Günther, 1890) population in Lake Urmiah, Iran

Lake Urmiah is a large (total surface 4750–6100 km² in recent times) thalassohaline hypersaline lake (150–180 g l^–1 in the period 1994–1996), located in northwestern Iran. It is the habitat of the endemic Artemia urmiana. Over the period July 1994–January 1996 a sampling campaign was organized: 36 fixed sampling stations, distributed over the entire lake's area, were sampled weekly to determine water temperature, salinity and transparency. At each occasion a filter net was dragged over a distance of 400 m in the superficial water layer to assess the density and composition of the Artemia population. A more limited sampling campaign focused on the annual fluctuations in chlorophyll concentration and on the reproductive behaviour of the brine shrimp population. Several stages of brine shrimp survived during winter months (water temperature 3°C) at low densities. Compared to available data for the Great Salt Lake, USA, Lake Urmiah shows a low algal biomass and overall low Artemia density. The increasing grazing pressure of the developing brine shrimp population in spring seems to prevent the phytoplankton from reaching high blooming concentrations, and oviparity is the dominant reproductive mode throughout the reproductive season.     

Production and decomposition processes in a saline meromictic lake

Bacterial and phytoplankton cell number and productivity were measured in the mixolimnion and chemocline of saline meromictic Mahoney Lake during the spring (Apr.–May) and fall (Oct.) between 1982 and 1987. High levels of bacterial productivity (methyl ³H-thymidine incorporation), cell numbers, and heterotrophic assimilation of ¹⁴C-glucose and ¹⁴C-acetate in the mixolimnion shifted from near surface (1.5 m), at a secondary chemocline, to deeper water (4–7 m) as this zone of microstratification gradually weakened during a several year drying trend in the watershed. In the mixolimnion, bacterial carbon (13–261 µgC 1^–1) was often similar to phytoplankton carbon (44–300 µgC 1^–1) and represented between 14–57% of the total microbial (phytoplankton + bacteria) carbon depending on the depth interval. Phototrophic purple sulphur bacteria were stratified at the permanent primary chemocline (7.5–8.3 m) in a dense layer (POC 250 mg 1^–1, bacteriochlorophyll a 1500–70001µ 1^–1), where H₂S changed from 0.1 to 2.5 mM over a 0.2 m depth interval. This phototrophic bacterial layer contributed between 17–66% of the total primary production (115–476 mgC m^–2 d^–1) in the vertical water column. Microorganisms in the phototrophic bacterial layer showed a higher uptake rate for acetate (0.5–3.7 µC 1^–1 h^–1) than for glucose (0.3–1.4 µgC 1^–1 h^–1) and this heterotrophic activity as well as bacterial productivity were 1 to 2 orders of magnitude higher in the dense plate than in the mixolimnetic waters above. Primary phytoplanktonic production in the mixolimnion was limited by phosphorus while light penetration appeared to regulate phototrophic productivity of the purple sulphur bacteria.     

Long-term observations of deepwater renewal in Crater Lake,Oregon

We examine observations of key limnological properties (primarily temperature, salinity, and dissolved oxygen), measured over a 14-year period in Crater Lake, Oregon, and discuss variability in the hypolimnion on time scales of days to a decade. During some years (e.g., 1994–1995), higher-than-average wintertime deep convection and ventilation led to the removal of significant amounts of heat and salt from the hypolimnion, while dissolved oxygen concentrations increase. In other years, such as the winter of 1996–1997, heat and salt concentrations increase throughout the year and dissolved oxygen levels drop, indicating conditions were dominated by the background geothermal inputs and dissolved oxygen consumption by bacteria (i.e., minimal deep convection). Over the entire 14 year period, no statistically significant trend was observed in the annual hypolimnetic heat and salt content. Measurements from several thermistors moored in the hypolimnion provide new insight into the time and space scales of the deep convection events. For some events, cool water intrusions are observed sequentially, from shallower depths to deeper depths, suggesting vertical mixing or advection from above. For other events, the cooling is observed first at the deepest sensors, suggesting a thin, cold water pulse that flows along the bottom and mixes more slowly upwards into the basin. In both cases, the source waters must originate from the epilimnion. Conditions during a strong ventilation year (1994–1995) and a weak ventilation year (1996–1997) were compared. The results suggest the major difference between these 2 years was the evolution of the stratification in the epilimnion during the first few weeks of reverse stratification such that thermobaric instabilities were easier to form during 1995 than␣1997. Thus, the details of surface cooling and wind-driven mixing during the early stages of␣reverse stratification may determine the net␣amount of ventilation possible during a particular year.