Academic literature on the topic 'Hypolimnetic anoxia'

The ability to infer long-term changes in hypolimnetic oxygen levels is important for ecological studies of eutrophication and the impacts of climatic change on freshwater lakes. We examined the distributions of fossil midge (Diptera: Chironomidae) assemblages in the surface sediments of 54 south-central Ontario lakes and, using canonical correspondence analysis (CCA), determined if fossil chironomid assemblages could be used to reconstruct levels of hypolimnetic anoxia in thermally stratifying Laurentian Shield lakes. Anoxia was expressed as the anoxic factor (AF), which represents the days per season that a sediment area equal to a lake's surface area is overlain by anoxic water. Forward selection in CCA showed that AF, maximum depth, [SO4], [Na], and watershed area all explained significant portions of species variation. A weighted-averaging regression and calibration model of the chironomid-anoxia relationship was developed which suggests that it is possible to infer AF from fossil chironomid assemblages ( (r2appar ) = 0.70, bootstrapped RMSE = 6.5 days/summer). Quantitative reconstruction of past changes in anoxia using fossil Chironomidae should provide a new and useful tool for paleoecological assessments of lake ecosystems.

Abstract Anoxia is a common phenomenon at the bottom of large reservoirs during thermal stratification. In an anoxic environment, an increasing amount of reducing substances and nutrients are released and settle at the hypolimnion of the reservoir, leading to water quality deterioration and eutrophication. This work presents a case study on Daheiting Reservoir, a part of the Water Diversion Project from the Luanhe River to Tianjin city. With the monitored data of the water temperature and dissolved oxygen content in the reservoir, and based on the mechanism of redox reactions, the water quality response to the hypolimnetic anoxia in Daheiting Reservoir was systematically analyzed. It was found that the release of total phosphorus from the sediments in Daheiting Reservoir was a joint effect of the biological and chemical processes, and the redox reaction in the anoxic zone boosted release of phosphorus. Anoxia in the reservoir caused the ammonia nitrogen released from sediments in the reservoir to accumulate at the hypolimnion, which increased the concentration of ammonia nitrogen in the water. Anoxia in the reservoir led to an increase in the concentration of iron and manganese, which accounts for the major driving factor of release of iron and manganese.

Abstract Many lakes experience oxygen depletion in their hypolimnia during summer or winter stratification. This study investigates the remedial actions that are available to combat the harmful effects of hypolimnetic anoxia in three types of Central Ontario lakes with different trophic states. In more eutrophic lakes, e.g., Lake Wilcox, southern Ontario, much phosphorus accumulates in the hypolimnion during anoxia and represents ca. 60% of the annual P budget. To diminish the detrimental effects of such a high internal phosphorus load, withdrawal of the hypolimnetic waters, together with damming of the surface water outflow, has been suggested. In oligo- to mesotrophic Chesley Lake, on the Niagara Escarpment, phosphorus accumulation in the hypolimnion is only slight and metals with a binding capacity for phosphorus are available. Here a hypolimnetic oxygenation has been suggested to alleviate the stress on fish and also to prevent further P release from the sediments. Lakes in the District of Muskoka are softwater lakes because of their location on the Canadian Shield. Although the geochemistry of the catchment typically renders these lakes nutrient poor and oligotrophic, long-term development on their shores has led to phosphorus accumulation in the sediments that is released under anoxic conditions in some lakes. To prevent the deterioration of the water quality in these lakes, water quality models to manage and control future development in its watersheds, as those of the district municipality, should include anoxia and internal P recycling in a quantitative manner.

Previous work suggests that a high rate of internal ferrous iron (Fe2+) loading from anoxic sediments into overlying waters favours cyanobacteria dominance (>50% of the phytoplankton biomass) over eukaryotic algae. This Cyanobacteria–Ferrous conceptual model was assessed along the Georgian Bay coastline of Lake Huron, Ontario, in one meso-eutrophic and three oligotrophic embayments that experience natural hypolimnetic anoxia. Cyanobacteria dominated all embayments in the relatively warmer summer of 2012 but not in the much cooler summer of 2014, although hypolimnetic anoxia and internal Fe2+ loading were observed in both summers in all embayments. A cyanobacteria bloom large enough to turn the lake visibly green was observed only in warmer 2012 in the meso-eutrophic embayment. Results show that warm summer temperatures and internal Fe2+ loading are necessary preconditions for cyanobacteria dominance, while high nutrient levels are needed to form large blooms. There were no consistent patterns between dominance and total and dissolved phosphorus (P), total nitrogen, ammonium, and nitrate. Internal P loading was not a necessary precondition for dominance. While P removal programs will decrease phytoplankton biomass in eutrophic waters, oxidized surficial sediments must be maintained throughout an aquatic system to prevent cyanobacteria dominance.

Abstract. Hypolimnetic anoxia in eutrophic lakes can delay lake recovery to lower trophic states via the release of sediment phosphorus (P) to surface waters on short timescales in shallow lakes. However, the long-term effects of hypolimnetic redox conditions and trophic state on sedimentary P fraction retention in deep lakes are not clear yet. Hypolimnetic withdrawal of P-rich water is predicted to diminish sedimentary P and seasonal P recycling from the lake hypolimnion. Nevertheless, there is a lack of evidence from well-dated sediment cores, in particular from deep lakes, about the long-term impact of hypolimnetic withdrawal on sedimentary P retention. In this study, long-term sedimentary P fraction data since the early 1900s from Lake Burgäschi provide information on benthic P retention under the influence of increasing lake primary productivity (sedimentary green-pigment proxy), variable hypolimnion oxygenation regimes (Fe∕Mn ratio proxy), and hypolimnetic withdrawal since 1977. Results show that before hypolimnetic withdrawal (during the early 1900s to 1977), the redox-sensitive Fe∕Mn-P fraction comprised ∼50 % of total P (TP) in the sediment profile. Meanwhile, long-term retention of total P and labile P fractions in sediments was predominantly affected by past hypolimnetic redox conditions, and P retention increased in sedimentary Fe- and Mn-enriched layers when the sediment-overlaying water was seasonally oxic. However, from 1977 to 2017, due to eutrophication-induced persistent anoxic conditions in the hypolimnion and to hypolimnetic water withdrawal increasing the P export out of the lake, net burial rates of total and labile P fractions decreased considerably in surface sediments. By contrast, refractory Ca–P fraction retention was primarily related to lake primary production. Due to lake restoration since 1977, the Ca–P fraction became the primary P fraction in sediments (representing ∼39 % of total P), indicating a lower P bioavailability of surface sediments. Our study implies that in seasonally stratified eutrophic deep lakes (like Lake Burgäschi), hypolimnetic withdrawal can effectively reduce P retention in sediments and potential for sediment P release (internal P loads). However, after more than 40 years of hypolimnetic syphoning, the lake trophic state has not improved nor has lake productivity decreased. Furthermore, this restoration has not enhanced water column mixing and oxygenation in hypolimnetic waters. The findings of this study are relevant regarding the management of deep eutrophic lakes with mixing regimes typical for temperate zones.

Amisk Lake was artificially oxygenated year-round by bubbling oxygen through a diffuser installed at the deepest point of its north basin. The hypolimnion of both basins became anoxic in summer before operation of the system whereas anoxia was never observed while it was fully operational (July 1990 until October 1993). The system increased the average hypolimnetic oxygen content of the south basin by an average of 83 tonnes (t) more than under natural conditions between 2 September and 18 May. This considerable increase is approximately accounted for by the transport of dissolved oxygen (DO) through the connecting channel by currents driven by the bubble plume ( approx 52 t) and by internal seiche pumping ( approx 17 t): both mechanisms are contingent on the thermocline depth being less than that of the connecting channel. Internal seiche pumping was possible because the connecting channel is shorter than the maximum distance across which hypolimnetic fluid moved between flow reversals. Other factors that may have been important include changes in the replenishment of DO during turnover, oxygen transfer during winter through the ice-free patch created by the bubble plume, and an increase in DO consumption.

Abstract. The concentration of oxygen is fundamental to lake water quality and ecosystem functioning through its control over habitat availability for organisms, redox reactions, and recycling of organic material. In many eutrophic lakes, oxygen depletion in the bottom layer (hypolimnion) occurs annually during summer stratification. The temporal and spatial extent of summer hypolimnetic anoxia is determined by interactions between the lake and its external drivers (e.g., catchment characteristics, nutrient loads, meteorology) as well as internal feedback mechanisms (e.g., organic matter recycling, phytoplankton blooms). How these drivers interact to control the evolution of lake anoxia over decadal timescales will determine, in part, the future lake water quality. In this study, we used a vertical one-dimensional hydrodynamic–ecological model (GLM-AED2) coupled with a calibrated hydrological catchment model (PIHM-Lake) to simulate the thermal and water quality dynamics of the eutrophic Lake Mendota (USA) over a 37 year period. The calibration and validation of the lake model consisted of a global sensitivity evaluation as well as the application of an optimization algorithm to improve the fit between observed and simulated data. We calculated stability indices (Schmidt stability, Birgean work, stored internal heat), identified spring mixing and summer stratification periods, and quantified the energy required for stratification and mixing. To qualify which external and internal factors were most important in driving the interannual variation in summer anoxia, we applied a random-forest classifier and multiple linear regressions to modeled ecosystem variables (e.g., stratification onset and offset, ice duration, gross primary production). Lake Mendota exhibited prolonged hypolimnetic anoxia each summer, lasting between 50–60 d. The summer heat budget, the timing of thermal stratification, and the gross primary production in the epilimnion prior to summer stratification were the most important predictors of the spatial and temporal extent of summer anoxia periods in Lake Mendota. Interannual variability in anoxia was largely driven by physical factors: earlier onset of thermal stratification in combination with a higher vertical stability strongly affected the duration and spatial extent of summer anoxia. A measured step change upward in summer anoxia in 2010 was unexplained by the GLM-AED2 model. Although the cause remains unknown, possible factors include invasion by the predacious zooplankton Bythotrephes longimanus. As the heat budget depended primarily on external meteorological conditions, the spatial and temporal extent of summer anoxia in Lake Mendota is likely to increase in the near future as a result of projected climate change in the region.

Abstract Many water bodies in western Canada experience high internal phosphorus recycling rates and excessive primary production, with associated water column anoxia and restricted fisheries habitat. Lime (Ca(OH)2 and CaCO3) application and hypolimnetic oxygenation were investigated in Alberta as nontoxic, inexpensive management alternatives. In hardwater lakes, multiple Ca(OH)2 applications at dosages <100 mg L-1 reduced chl a and TP concentrations by up to ~30 and 50%, respectively, often for several years. In contrast, high flushing rates in stormwater retention basins made repeated Ca(OH)2 treatment necessary. Hypolimnetic oxygenation in Amisk Lake from 1988-93 increased hypolimnetic dissolved oxygen concentrations from 0.9 to 4.6 mgL-1 in the treated basin while maintaining thermal stratification. Epilimnetic whole-lake chl a and TP concentrations were reduced by 55 and 13%, respectively. Deep-water habitat was improved for fish, zooplankton and macroinvertebrates. Our research shows that water quality can be improved in lakes that are naturally eutrophic (i.e., due to internal P cycling) and has implications for lake management projects worldwide.

Hypolimnetic O2 consumption, methanogenesis, and denitrification were estimated from concentration changes in O2, CH4, and nitrogen oxides (NO3−, NO2−, N2O), respectively, in the eastern basin of Lake St. George, Ontario, during thermal stratification from 1980 to 1984. NH4+ accumulated in the hypolimnion throughout the period of stratification and provided an overall measure of organic matter mineralization. Anaerobic metabolism, particularly methanogenesis, predominated because anoxia was total within less than 2 mo of spring water column turnover and nitrogen oxides were always completely depleted before fall turnover. The time of onset and rate of NO3−, depletion varied considerably between years and were not always directly correlated with O2 concentration. It was estimated that, on average, O2 consumption, denitrification, and methanogenesïs accounted for approximately 80% of hypolimnetic organic C mineralization in the periods surveyed.

Between May and December 1996, monthly samples of surface sediment (0–1 cm), settling matter, and water were taken at a shallow site and a deep site in each of two consecutive Hg-polluted riverine lakes. In the upper lake, the sediment was polluted also with cellulose fiber. Both hypolimnia turned anoxic, but sulfide was detected only in the upper lake. When sulfide appeared, hypolimnetic methyl mercury (MeHg) increased and reached 47 pM (9.4 ng·L–1), whereas MeHg in the sediment below decreased. The increase in hypolimnetic inorganic Hg (IHg = total Hg – MeHg), which reached a peak of 40 pM (8.0 ng·L–1), was slower, possibly because mobilized IHg was methylated. In the lower lake, hypolimnetic MeHg and IHg increased less dramatically during summer stratification, reaching only 5 and 24 pM (1.0 and 4.8 ng·L–1), respectively. There was no detectable concomitant decrease in sediment MeHg. In both lakes, MeHg appeared to increase simultaneously with total Fe and Mn in the hypolimnion, as did IHg in the lower lake. Our observations suggest that the presence of hydrous ferric and manganese oxides decreased the mobility of Hg in both lakes but increased MeHg production in the upper lake.

The seasonal periodicity of hydrology, physical and chemical water quality parameters and phytoplanktonic assemblages was studied at two sites in a large tropical Australian riverine impoundment. This study, the first in the lower Fitzroy River at Rockhampton, occurred between August 1990 and November 1993. It covered extremes in riverine flow conditions including major flooding and drought.

The annual flow regime was characterized by major flows in the "wet" season (summer and autumn) and greatly reduced or no flow in the "dry" season of winter, spring and sometimes early summer. Consequently, the thermal regime at both of the study sites was divided into two phases. The first was a phase of water column heating in the late winter to early summer. Features of this heating phase were long term stratification with progressive epilimnetic deepening, high pH, regular occurrence of epilimnetic oxygen supersaturation and decreased or undetectable levels of oxidized nitrogen in the surface layer. Hypolimnetic anoxia was recorded late in this phase. The second, between substantial wet season inflows and late winter was characterized by nutrient rich inflows and water column cooling and mixing.

Distinct interannual differences occurred in the volume, source and timing of inflows and subsequent water chemistry. In 1991, conductivity, water clarity, filterable reactive phosphorus (FRP) and pH increased markedly following major flooding from northern tributaries, while oxidized nitrogen decreased. This was in marked contrast to the drier years of 1992 and 1993 where turbidity and oxidized nitrogen were higher during the initial post-flood period and conductivity and FRP were lower. Extremes of mostly abiogenic turbidity (range 1.6 to 159 NTU) were a feature of the light climate. Ratios of euphotic depth/mixing depth below 0.3 occurred in early 1992 and 1993.

Steep gradients in the physical and chemical environment were paralleled by variations in the phytoplankton. Algal biomass (as chlorophyll a) at Site 1, midstream opposite the water intake for the city of Rockhampton, ranged from 1.5 to 56.6 ug L-1. The vertical water column distribution of chlorophyll was variable with assemblages normally dominated by phytoflagellates and various species of cyanoprokaryotes. There was also higher relative abundance of chlorophyll a (reflecting increasing dominance of cyanoprokaryotes) in the latter half of the year and at the lower end of light availability. The specific vertical water column positioning with respect to light and temperature is shown for assemblages dominated by the genera Anabaena, Aphanizomenon and Cylindrospermopsis.

The most striking aspect of the phytoplankton was the long term dominance of cyanoprokaryotes and the species richness (particularly that of cyanoprokaryotes) when compared with the dearth of information to date on other tropical rivers. Seasonal successions were varied. Regularly occurring assemblages were cyanoprokaryotes (Oscillatoriales), euglenophytes or non-flagellated chlorophytes during flows followed by flagellated chlorophytes and then cyanoprokaryotes (Nostocales) during the dry season. Genera present indicated highly eutrophic conditions. Hierarchical agglomerative clustering of phytoplankton data and comparison with a principal components analysis of corresponding environmental data were used to demonstrate the linkage between steep environmental gradients and variation in the phytoplankton assemblage. The specific environmental conditions associated with the success of various species were also analysed and presented. Using the above information, a two-part model was proposed which predicts the most likely genera of phytoplankton with respect to multidimensional environmental gradients. This model covers a wide gamut of conditions varying from highly variable lotic to lentic environments.

As Cylindrospermopsis raciborskii was considered a most important species in relation to the quality of the water supply for Rockhampton, the physical, chemical and biotic conditions prior to and during a bloom of this species are described. A number of possible grazers of C. raciborskii were identified with a view to future biomanipulation. One of these, the large ciliate, Paramecium cf. caudatum was found to be an effective grazer of toxic straight C. raciborskii in the laboratory.

This study is unique in that it analyses the impact of episodic events (eg. major flooding) on the subsequent phytoplankton in the lower Fitzroy River. The model relating phytoplankton to multidimensional environmental gradients provides great information for use in management, particularly in relation to the prediction of toxic algal blooms.