Nearshore Spatial Distribution of Fishes in Gill Net Samples,Cayuga Lake,New York

Cayuga Lake fishes were collected monthly with experimental gill nets at depths of 2 to 31.5 m. Horizontally-placed nets were arranged to allow determinations of vertical and inshore-offshore variation in fish distribution and occupied temperatures. Warmwater species concentrated in inshore and surface waters at 19-23° C during the summer. Coldwater species concentrated in hypolimnetic waters at 9-15° C during the summer. Seasonal inshore-offshore movements were evident with all species except those which were captured only close to shore (northern pike, carp, smallmouth bass, white sucker). Depth ranges in which greatest numbers of common species occurred in the summer (July-September) were: alewife, inshore and surface to 15.3 m; lake trout, 15.3 to 30.5 m or more; northern pike, inshore and surface to 15.3 m; rainbow smelt, 7.6 to 30.5 m or more; spottail shiner, inshore and 7.6 m; trout-perch, inshore and 7.6 to 15.3 m; yellow perch, inshore and 7.6 to 15.3 m. Mean occupied temperatures (° C) of common species during the summer were: alewife 18.4 to 20.9; lake trout, 12.5 to 13.6; northern pike, 19.4; rainbow smelt, 10.4-14.4; spottail shiner, 19.6 to 21.5; trout-perch, 18.7-21.3; yellow perch, 19.4 to 21.0. Depth distribution was obviously influenced by temperature preferences and also by availability of food in certain species. Catch size was directly related to water temperatures in several species.     

Combined Effects of Cadmium and Zinc on a Lake Mighican Zooplankton Community

Two 3-week experiments with small enclosures were conducted in situ in northeastern Green Bay, Lake Michigan, to compare the effects of separate and combined additions of cadmium and zinc on the zooplankton community. The radioisotopes ¹⁰⁹Cd and ⁶⁵Zn were used as tracers to determine the sorption of cadmium and zinc by different particle size fractions. Separate additions of 2 μg Cd/L or 100 μg Zn/L, as well as combined additions of 2 μg Cd/L + 100 μg Zn/Lor 1 μg Cd/L + 50 μg Zn/L, caused significant reductions of total crustacean density, species diversity, two community similarity indices, and final dissolved oxygen concentration. The effects of separate additions of 100 μg Zn/L were generally more pronounced than those of 2 μg Cd/L, but were similar to those of ～5 μg Cd/L. The effects of combined additions of 2 μg Cd/L + 100 μg Zn/L were not significantly different from those of 100 μg Zn/Lalone and were primarily due to zinc because it reduced cadmium uptake by the plankton (10–85 μm and >85 μm fractions). The effects of combined additions of l μg Cd/L + 50 μg Zn/L probably were also mainly due to zinc. The overall results of this study suggest that less than a ten-fold increase in the concentration of zinc in Lake Michigan (presently ～5 μg/L) could have pronounced effects on the plankton community.     

Organochlorine Residues in Suspended Solids Collected from the Mouths of Canadian Streams Flowing into the Great Lakes 1974–1977

Suspended solids were collected from 105 streams on the Canadian side of the Great Lakes during snow melt and spring flow between 1974 and 1977. Analyses were performed for organochlorine and organophosphorus insecticides and three industrial chemicals. All streams sampled contained suspended solids that were contaminated with DDT, its metabolites, and polychlorinated biphenyl (PCB). The highest mean concentrations of ΣDDT (356 μg/kg) occurred in 1974 from streams draining the Niagara Peninsula and entering Lake Ontario; these were correlated to areas of high use on fruit in the past. The highest loadings by lake section, however, were along the north shore of Lake Erie involving a very much larger delivery of suspended solids. The highest concentrations of PCB were on suspended solids entering Lake Ontario in 1974 along the northern (191 μg/kg) and southwestern (172 μg/kg) shore and were associated with the high urban-industrial activity in the megalopolis of Hamilton-Burlington-Oakville-Mississauga-Toronto. However, the highest loadings occurred along Lake St. Clair where two large rivers, the Thames and Sydenham, discharge a much larger volume of suspended solids. The highest individual concentrations of HEOD (dieldrin) were found on suspended solids in two streams entering along the southwestern shoreline of Lake Ontario. However, the highest mean residues on solids for all streams was in waters entering Lake Erie (3 μg/kg) and the highest loadings were into Lake St. Clair. Endosulfan, chlordane, and heptachlor epoxide were detected in some streams entering all five lakes. No aldrin, endrin, mirex, heptachlor, or organophosphorus insecticides were detected.     

Bacterial Enrichments in Surface Films of Freshwater Lakes

Numerous freshwater lakes were examined for enrichment of bacteria in their surface films as compared to their subsurface waters. Surface films were sampled with a glass plate (60-100 μm) surface samples), a Teflon plate (～7 μm samples), or a hydrophilic Nuclepore filter (～40 μm samples). Subsurface samples (～0.1 m) were obtained with a sterile flask. Total bacterial counts were determined by epifluorescent microscopy. Enumerations of benzoic acid-degrading microorganisms were performed using a¹⁴C-most-probable-number technique, with ¹⁴C-substrate added to lakewater media at μg/L concentrations. Most lakes examined did not show significant enrichments of either total bacteria or benzoic acid-degrading bacteria in their surface films (～ 10–100 μm samples). Certain highly transparent lakes did show small enrichments of total bacteria in their surface films. A survey of lakes of various transparencies revealed a direct relationship (r = 0.94) between increasing transparency and increasing surface film enrichment of bacteria.     

Growth,Reproduction, Mortality,Distribution, and Biomass of Freshwater Drum in Lake Erie

Predominant age-groups in the Lake Erie freshwater drum Aplodinotus grunniens population were 3, 4, and 5 as determined from gill net, trap net, bottom trawl, and midwater trawl samples. Age and growth calculations indicated that females grew faster than males. However, the length-weight relation did not differ between sexes and was described by the equation: log W = ?5.4383 + 3.1987 log L. Some males became sexually mature at age 2 and all were mature by age 6. Females matured 1 year later than males. Three sizes of eggs were present in ovaries; the average total number was 127,000 per female for 20 females over a length range of 270 to 478 mm. Seasonal analysis of the ovary-body weight ratio indicated that spawning extended from June to August. A total annual mortality rate of 49% for drum aged 4 through 11 was derived from catch-curve analysis. Freshwater drum were widely distributed throughout Lake Erie in 1977–1979, the greatest concentration being in the western basin. They moved into warm, shallow water (less than 10 m deep) during summer, and returned to deeper water in late fall. Summer biomass estimates for the western basin, based on systematic surveys with bottom trawls, were 9,545 t in 1977 and 2,333 t in 1978.     

Carbonate Equilibria and the Distribution of Inorganic Carbon in Saginaw Bay

The spatial and temporal distribution of selected inorganic carbon equilibrium species is presented for eleven locations, representing the near surface waters of Saginaw Bay in Lake Huron during 1974. Carbon dioxide and calcite equilibrium conditions are determined through solution of temperature and ionic strength adjusted equilibria and evaluated with respect to suitability as indicators of physical conditions and biological activity in the waters of Saginaw Bay. The participation of inorganic carbon species in several chemical and biochemical reactions leads to the utility of these species in reflecting documented spatial and temporal trends in phytoplankton biomass, temperature, ionic strength, and ionic composition. Substantial seasonal variations in carbon dioxide and calcite equilibrium conditions were observed at several locations throughout the bay. The magnitude of variation was greatest in the more productive areas of the bay, where extensive calcite supersaturation and carbon dioxide undersaturation occurred. Maximum calcite saturation and minimum carbon dioxide saturation were calculated for the warm, productive summer months. Transformations in chemical equilibria were mediated by both physical and biochemical factors,as reflected by seasonal changes in temperature and pH. The significance of photosynthetic activity was most pronounced in the inner bay and in shore zones, while temperature became of greater relative importance where bay waters mixed with the open waters of Lake Huron. Strong correlations between time averaged data for chlorophyll a and carbon dioxide (R = 0.97) and chlorophyll a and calcite saturation (R = 0.95) indicate the importance of photosynthetic activity in establishing the distribution of equilibrium conditions of inorganic carbon species.     

Downflux of Sediment,Organic Matter,and Phosphorus in the Niagara River Area of Lake Ontario

Vertical arrays of sediment traps were installed near the mouth of the Niagara River and at nearshore and offshore sites. Surveys of temperature, turbidity, and conductivity were used to establish that the traps nearest the river mouth were under the river outflow plume. A single offshore array in 1980 indicated downfluxes of 1-2 g · m^?2 · d^?1 of dry matter in the mid-water column. These fluxes were similar to those of small lakes but much higher fluxes were found near the bottom of Lake Ontario. More extensive work in 1981 confirmed the presence of a near-bottom nepheloid layer and the effect it has on increasing downflux extimates. Sediment traps near shore and at the river mouth caught substantially more material at all depths than did traps at the offshore stations. Downfluxes of phosphorus and sediment were consistent with independent estimates of loading and retention.     

PCBs in Lake Superior, 1978–1980

Polychlorinated biphenyls (PCBs) were measured in the water column of Lake Superior during the summers of 1978 to 1980. The total PCB water concentrations were relatively uniform areally and vertically within any one year: 1978, 1.3 ± 1.3 ng/L; 1979, 3.8 ±1.9 ng/L; 1980, 0.9 ±0.4 ng/L. The volume-weighted abiotic PCB burden in the water column fluctuated with climatic conditions between ～10,000 and 40,000 kg, but would be in the range of 10,000 to 20,000 kg most of the time. Analysis of 24 large volume, filtered samples from 1980 showed that 27±12% of the PCBs were in the paniculate phase. The distribution coefficient for PCBs, defined by use of a glass-fiber filter, ranged from 10⁴ to 10⁷ L/kg, exhibiting a strong, inverse log relationship to the suspended solids concentration. This behavior is partially attributed to the differences in the organic carbon content of the sampled particles in different regions of the lake and to artifacts of the filtration process. The elevated total PCB concentrations in the summer of 1979 in conjunction with the previous winter's severe weather conditions are evidence of seasonal fluctuations of PCBs in the water column resulting mostly from sediment resuspension.     

Sediment geochemistry and contributions to carbon and nutrient cycling in a deep meromictic tropical lake: Lake Malawi (East Africa)

The biogeochemical functioning of large tropical lakes differs substantially from temperate lakes, yet remains poorly understood. We characterized the carbon, nitrogen, and phosphorus cycling in the water column and sediments of a deep meromictic tropical Lake Malawi (East Africa) by measuring geochemical distributions and compiling whole-lake geochemical budgets. Four locations (100 to 650 m water depth) were characterized. The results reveal that sediments contribute significantly to lake-wide biogeochemical budgets. Sedimentation rates have significantly increased in recent decades. While the export efficiency of organic matter from photic zone to deep sediments is low (14%), organic carbon is buried in the anoxic sediments with high efficiency (27–46%). Area-specific rates of carbon mineralization (4.1 mmol m^? 2 d^? 1) are similar to those in temperate well-oxygenated large lakes and marine sediments in similar water depths. Ammonium effluxes from sediments (0.44 mmol m^? 2 d^? 1) contribute 29% to the total nitrogen inputs into the water column, while sediment denitrification (0.035 mmol m^? 2 d^? 1) and burial of organic nitrogen (0.27 mmol m^? 2 d^? 1) remove 28% of total inputs in the lake. The recycling efficiency of phosphorus in anoxic sediments is high (73%). P effluxes average 0.037 mmol m^? 2 d^? 1, suggesting a large and previously unquantified contribution (42%) to water column P inputs. The results underscore the importance of sediments in the geochemical budgets of even large lakes and suggest trends in lacustrine carbon cycling that hold across a wide range of environments.     

Microzooplankton distribution,dynamics, and trophic interactions relative to phytoplankton and quagga mussels in Saginaw Bay,Lake Huron

Invasive quagga mussels have recently replaced zebra mussels as the dominant filter-feeding bivalves in the Great Lakes. This study examined microzooplankton (i.e., grazers < 200 μm) and their trophic interactions with phytoplankton, bacteria, and bivalve mussels in Saginaw Bay, Lake Huron, following the zebra to quagga mussel shift. Microzooplankton distribution displayed strong spatial and temporal variability (1.73–28.5 μg C/L) relative to phytoplankton distribution. Ciliates were the dominant component, especially in the spring and early summer. Rotifers and dinoflagellates increased toward late summer/fall in the inner and outer parts of the bay, respectively. Microzooplankton grazing matched bacterial growth rates and removed ca. 30% of the phytoplankton standing stock in the < 100 μm size fraction per day. The greatest herbivory occurred at the site dominated by colonial cyanobacteria. Microzooplankton, which comprised < 4% of the quagga mussels prey field (i.e. available prey), contributed 77% and 34% to the quagga carbon-based diet during Microcystis and diatom blooms, respectively. Feeding on microzooplankton could buffer mussels during lean periods, or supplement other consumed resources, particularly during noxious cyanobacterial blooms. The results of this study demonstrate that microzooplankton are a resilient and critical component of the Saginaw Bay ecosystem.     

Mapping cyanobacterial blooms in Lake Champlain's Missisquoi Bay using QuickBird and MERIS satellite data

C-phycocyanin (C-PC) and chlorophyll-a (Chl-a) concentrations for the eutrophic waters of Missisquoi Bay, Lake Champlain (VT–QC) were retrieved from Envisat's MERIS radiance data (300 m spatial resolution) and validated against coincident georeferenced transect observations. Pigment concentrations were also predicted from empirically calibrated QuickBird data (2.4 m spatial resolution) using selected band ratios and principal components analysis. The QuickBird NIR/Red band ratio accounted for approximately 80% of the variability in observed Chl-a concentration, allowing for detailed mapping of phytoplankton spatial distributions. C-PC concentrations, in contrast, were somewhat poorly modeled (R² = 0.68). Use of these data for monitoring purposes, however, is also limited by the need for coincident field observations. Chl-a concentrations were also accurately retrieved from the MERIS data (Mean Relative Error = -0.6%) despite high concentrations of suspended particles and dissolved organic matter in the bay waters. C-PC concentrations were underestimated on average by 2.1%, but by 10–20% at high C-PC concentrations (≥ 80 μg/L) and as the proportion of cyanobacteria in the phytoplankton community decreased. The relatively high overall accuracies observed, however, attest to the robustness of the MERIS semi-analytical retrieval algorithms used to quantify potentially toxic cyanobacteria cell densities without the need for coincident field data. Our analyses over a 17 day period captured the peak and collapse of a late summer cyanobacterial bloom, illustrating the value of remote sensing to provide synoptic and timely information on the abundance and distribution of cyanobacterial populations that, in turn, can facilitate public health risk assessment.     

A mesocosm investigation of the effects of quagga mussels (Dreissena rostriformis bugensis) on Lake Michigan zooplankton assemblages

Dreissenid mussels are known to disrupt the base of the food web by filter feeding on phytoplankton; however, they may also directly ingest zooplankton thereby complicating their effects on plankton communities. The objective of this study was to quantify the effects of quagga mussel feeding on the composition and size structure of Lake Michigan zooplankton assemblages. Two mesocosm (six 946 L tanks) experiments were conducted in summer 2013, using quagga mussels and zooplankton collected near Beaver Island, MI, to examine the response of zooplankton communities to the presence and absence of mussels (experiment 1) and varying mussel density (experiment 2). Mesocosms were sampled daily and zooplankton taxa were enumerated and sized using microscopy and FlowCAM® imaging. In experiment 1, the presence of quagga mussels had a rapid negative effect on veliger and copepod nauplii abundance, and a delayed negative effect on rotifer abundance. In experiment 2, mussel density had a negative effect on veliger, nauplii, and copepodite abundance within 24 h. Multivariate analyses revealed a change in zooplankton community composition with increasing mussel density. Ten zooplankton taxa decreased in abundance and frequency as quagga mussel density increased: except for the rotifer Trichocerca sp., treatments with higher mussel densities (i.e., 1327, 3585, and 5389 mussels/m²) had the greatest negative effect on small-bodied zooplankton (≤ 128 μm). This study confirms results from small-scale (≤ 1 L) experiments and demonstrates that quagga mussels can alter zooplankton communities at mesoscales (~ 1000 L), possibly through a combination of direct consumption and resource depletion.     

Quantifying a shift in benthic dominance from zebra (Dreissena polymorpha) to quagga (Dreissena rostriformis bugensis) mussels in a large,inland lake

Dreissenid mussels are aggressive invasive species that are continuing to spread across North America and co-occur in the same waterbodies with increasing frequency, yet the outcome and implications of this competition are poorly resolved. In 2009 and 2015, detailed (700 + sample sites) surveys were undertaken to assess the impacts of invasive dreissenid mussels in Lake Simcoe (Ontario, Canada). In 2009, zebra mussels were dominant, accounting for 84.3% of invasive mussel biomass recorded. In 2015, quagga mussels dominated (88.5% of invasive mussel biomass) and had expanded into profundal (> 20 m water depth) sites and onto soft (mud/silt) substrates with a mean profundal density of 887 mussels/m² (2015) compared to ~ 39 mussels/m² in 2009. Based on our annual benthos monitoring, at a subset of ~ 30 sites, this shift from zebra to quagga mussels occurred ~ 2010 and is likely related to a population decline of zebra mussels in waterbodies where both species are present, as recorded elsewhere in the Great Lakes Region. As the initial invasion of dreissenid mussels caused widespread ecological changes in Lake Simcoe, we are currently investigating the effects this change in species dominance, and their expansion into the profundal zone, will have on the lake; and our environmental management strategies. Areas of future study will include: changes in the composition of benthos, fish, or phytoplankton communities; increased water clarity and reduction of the spring phytoplankton bloom; energy/nutrient cycling; and fouling of anthropogenic in-lake infrastructures (e.g. water treatment intakes) built at depths > 25 m to avoid previous zebra mussel colonization.     

Preference for Nannoplankton Size Fractions in Lake Ontario Zooplankton Grazing

In in situ zooplankton grazing experiments in Burlington Canal, smaller nannoplankton (1-20 μm) were greatly preferred over the larger (20 - 64 μm)fraction. The majority (94.4%) of the cells ingested by the rotifer Kellicottia longispina were from the smaller fraction. Similarly, the 1 - 20 μm fraction accounted for 91.9% of the cells ingested by the cladoceran Bosmina longirostris and 94.8% of those ingested by mixed copepod nauplii. The rotifer feeding rates were slower than those of the crustacean zooplankton studied. The results imply that contaminant transfer pathway studies should focus on smaller algae as being crucial in phytoplankton-zooplankton links in trophic models.     

Variability of light absorption properties in optically complex inland waters of Lake Chaohu,China

Absorption coefficients of phytoplankton, colored detrital matter (CDM), non-algal particles (NAP), colored dissolved organic matter (CDOM), and their relative contributions to total non-water absorption (a_t ? w) are essential variables for bio-optical and radiative transfer models. Light absorption properties showed large range and variability sampled at 194 stations throughout Lake Chaohu between May 2013 and April 2015. The a_t ? w was dominated by phytoplankton absorption (a_ph) and NAP absorption (a_d). The contribution of CDOM absorption to a_t ? w was lower than 30%. Phytoplankton and NAP were the primary sources of spatial and vertical variability in absorption properties. Light absorption by CDOM, though significant in magnitude, was relatively constant. CDM absorption (a_dg) was dominated by NAP. The spatial variation of the absorption coefficients from each of the optically active constituents were driven by several main inflow rivers in the western and middle part of Lake Chaohu. Algal blooms and bottom resuspension contributed to vertical variability as observed by phytoplankton and NAP profiles. Specific absorption of phytoplankton had significant spatial and seasonal variations without vertical variation. The spectral slope of absorption showed no significant spatial variability (p > 0.05). Variations of absorption affected different ranges of remote sensing reflectance (R_rs) spectrum, thereby increasing the difficulty of applying the remote sensing algorithm in optically complex waters. Parameters and relationships presented in this study provide useful information for bio-optical models and remote sensing of lakes similar to Lake Chaohu in terms of optical properties.     

Contaminants Associated with Suspended Sediments in Lakes Erie and Ontario, 1997–2000

Sediment traps were installed at individual index stations in the western basin of Lake Erie and the Mississauga (central) basin of Lake Ontario, and refurbished seasonally during the period 1997–2000. In Lake Ontario, sediment down flux rates and corresponding contaminant down flux rates were highest in winter and increased with depth due to the influence of resuspended bottom sediments. Sediment down flux rates in western Lake Erie (22 to 160 g m⁻² d⁻¹) were far greater than in Lake Ontario (0.19–3.0 g m⁻² d⁻¹). Suspended material in western Lake Erie was characterized as predominately resuspended bottom sediments; down flux rates were roughly 5- to 10-fold higher in spring and fall, compared to summer. Suspended sediment concentrations of PCBs and other organochlorine contaminants, represented by both annual means and individual seasonal values, were higher in Lake Ontario throughout the duration of the study, compared to Lake Erie. The mean annual concentration of PCBs in suspended sediments over the period 1997–2000 was 330 ng/g in western Lake Erie and 530 ng/g in Lake Ontario. Based on a comparison with historical data from Lake Ontario, mean contaminant concentrations over the period 1997–2000 for PCBs, hexachlorobenzene, and mirex corresponded to decreases of 38%, 74%, and 40%, respectively, since the mid-1980s. Corresponding down flux rates for PCBs, hexachlorobenzene, and mirex decreased by approximately 70%, 90%, and 80%, respectively, since the 1980s.     

Factors in the Determination of Selected Trace Elements in Near-Shore U.S. Waters of Lakes Superior and Huron

Determinations of total As, Cr, Cu, Fe, Mn, and Pb in acidified unfiltered water samples from 48 stations representing the near-shore U.S. waters of Lake Superior, Lake Huron, and selected incoming streams gave overall mean concentrations and standard deviations (μg/L) of 1.0 ± 0.7, 0.9 ± 1.8, 2.0 ± 1.8, 236 ± 819, 14 ± 34, and 0.7 ± 0.8, respectively, using flameless atomic absorption analytical methods. Total concentrations of Ni and Hg were usually less than the instrumental detection limits (μg/L) of 0.8 and 0.02, respectively. Small losses of Hg, As, and Cr from solutions are possible in addition to a molecular spectral interference in the determination of As using the graphite furnace method. River waters usually were only slightly higher in total element concentrations than lake waters, except for Fe andMn. Nearshore waters of both lakes were very similar in elemental composition.     

Survival,growth, and production of Hexagenia bilineata mayflies in fluidized sediment from lower Green Bay,Lake Michigan

Organic pollution in lower Green Bay, Lake Michigan over the past century was accompanied by the local extirpation of Hexagenia (primarily H. limbata) mayflies. Recoveries were made in other systems where population crashes had occurred (e.g., western Lake Erie); however, an active recovery does not appear to be taking place in Green Bay. Excessive primary production has caused substantial benthic organic matter accumulation resulting in a fluidized “sludge-like” substrate as the majority of the sub-littoral habitat. Fluidized substrate potentially hinders Hexagenia nymphs' abilities to construct and maintain burrows critical to their life cycles. In this study, Hexagenia bilineata nymphs were collected from an Upper Mississippi River backwater where their presence at high densities in relatively fluid sediment had been observed, and reared in oxygenated aquaria containing lower Green Bay or Upper Mississippi River sediment. Their survival, growth, secondary production, and biomass turnover were calculated for a 166 day period. Seventy-five percent of nymphs survived or metamorphosed into winged sub-imagos in lower Green Bay substrates compared to 40.6% in Upper Mississippi River substrates. Nymph dry weight more than tripled in Green Bay substrates and more than doubled on Upper Mississippi River substrates. Production was notably higher in lower Green Bay substrates. Differences in survival and production between the two treatments were statistically significant (P < 0.05), while differences in growth and biomass turnover were not (P > 0.05). Based on these results, the high fluidity of lower Green Bay substrates did not appear to hinder burrow construction or maintenance.     

Polychlorinated Biphenyl Contamination in Surficial Sediments of Northeastern Lake Michigan

The results of this study indicate that the polychlorinated biphenyl contamination in northeastern Lake Michigan surficial sediments is relatively low. Concentration ranges from 1.6 to 6.7 μg/L PCB (Aroclor 1254) were found. The concentration of PCBs in these sediment samples was also found to be highly correlated with % organic carbon (r=0.93) and the % silt-clay fraction (r=0.98). The % organic carbon and % silt-clay fraction were also highly correlated (r=0.93).     

Spatial and temporal variations in water temperature in a high-altitude deep dimictic mountain lake (Nam Co), central Tibetan Plateau

Water temperature and the related thermal structure and stratification of a lake are very important to lake ecosystems because of their significant effects on the vertical exchanges of dissolved and particulate matter. In this study, we present high resolution, seasonal variations in water temperature at different depths of a large deep lake on the central Tibetan Plateau. The results show that Nam Co is a typical dimictic lake whose thermal stratification begins and ends in early June and early November, respectively. Increases in the water temperature during spring and the establishment of thermal stratification in the eastern small sub-basin occur approximately one month prior to the main basin which is likely caused by the different morphometry, different water transparency during spring, and the possible presence of a spring thermal bar. The Schmidt stability of the water column is directly controlled by surface water temperature. During the ice-covered period, the homogeneous water temperature exhibits a continuous increasing trend from approximately 0.5 °C to 3.5 °C. The daily mean surface water temperature of the main open lake area is highly correlated to the air temperature but shows a hysteresis effect of approximately 38 days, which shows the significant heat storage in such a large deep lake. Nam Co is a typical lake in this area in terms of its altitude, water depth and climatic conditions, so our results have broader significance for limnological and paleolimnological studies of similar lakes on the Tibetan Plateau.