Winter Anoxia Research Articles

Under‐Ice Oxygen Depletion and Greenhouse Gas Supersaturation in North Temperate Urban Ponds

AbstractStormwater ponds are common features in urbanized landscapes because they enhance flood reduction and nutrient retention. With shallow depths and high inputs of organic matter, these systems can be highly productive with rapid oxygen depletion when thermally stratified or ice‐covered. However, most of our understanding of the biogeochemistry of stormwater ponds comes from the open water period. We explored under‐ice oxygen dynamics in 20 stormwater ponds in Madison, WI (USA) that were ice covered from late December to early March to investigate the drivers of bottom water oxygen saturation and the impact on the accumulation of carbon dioxide (CO2) and methane (CH4). Winter anoxia was driven by ice transmissivity, winter nutrient concentrations, and precedent summer productivity. Oxygen depletion led to overall higher concentrations of greenhouse gases in pond surface waters. This research enhances our understanding of winter pond biogeochemistry and its links to summer productivity.

Predator recognition learning by northern redbelly dace Chrosomus eos from a small kettle lake with a dynamic predator community

Predation, and the risk of predation, shape the ecology, behavior, and evolution of many species. Small fishes navigate a perilous landscape of risk in the shallow waters of the littoral zone. Moreover, in small lakes, the predator community can be dynamic due to stochastic colonization and extirpation events. These conditions select for the ability by small fish to acquire recognition of novel predators through associative learning. Chemical cues associated with predation events, such as damage-released chemical cues from conspecifics, and the odor of predators, inform prey of the presence of risk, and facilitate acquired recognition of novel predator odor. Deming Lake, MN, is a small meromictic lake with intermittent connections to neighboring lakes in the watershed. Annual sampling of the littoral fish community between the years 2000 and 2023 reveals a history of colonization and extirpation by relatively large-bodied species such as yellow perch Perca flavescens and pumpkinseed sunfish Lepomis gibbosus. Ice cover data, combined with limnological depth profiles of dissolved oxygen, confirm that dissolved oxygen is limiting during the winter and consistent with the hypothesis that extirpation of large-bodied species in Deming Lake is likely due to winter anoxia. These data set the stage for an experimental demonstration of acquired recognition of the odor of allopatric rock bass Ambloplites rupestris by bass-naïve northern redbelly dace Chrosomus eos from Deming Lake. Rock bass have been absent from Deming Lake since at least the year 2000. Predator-recognition learning allows redbelly dace, and many other small-bodied fishes that face variable predator species over ontogenetic, spatial, and temporal scales, a mechanism to adapt quickly to indicators of predation risk.

Assessment of the hypolimnetic withdrawal system at Pine Lake, Alberta

Sosiak A. 2021. Assessment of the hypolimnetic withdrawal system at Pine Lake, Alberta. Lake Reserv Manage. 38:47–66. In 1998, the Pine Lake Restoration Society (PLRS) installed a gravity-fed hypolimnetic withdrawal (HW) system in 4.13 km2 Pine Lake, Alberta, which had severe cyanobacterial blooms. Under HW, median euphotic total phosphorus (TP) declined by 27–32%, and chlorophyll a by 37–41%, typical of HW projects elsewhere. Furthermore, cyanobacterial blooms were less severe during HW. Decreasing hypolimnetic TP and internal loads suggest sediment P release declined in at least the middle basin under HW. There was no evidence of increased hypolimnetic temperatures during HW, which could have enhanced sediment P release. Summer hypolimnetic anoxia declined in the middle and south basins, while in winter anoxia declined in the north and middle basins. No change in TP occurred in 9 Alberta reference lakes, and external P loadings were not reduced by management efforts at Pine Lake, which suggests improvements were due to HW. After N declined during HW in Pine Lake, cyanobacteria shifted from a diverse community with species that fix atmospheric N, and others without that capacity, to one dominated by Aphanizomenon flos-aquae. Correlation analysis found Pine Lake blooms were associated with euphotic TP, total nitrogen (TN), earlier spring ice breakup, and higher air temperatures. Due to increased rainfall, the PLRS was able to operate the system more often than predicted. Pine Lake remains productive and the goal of mesotrophic productivity was not achieved. However, the anoxia analysis suggests lake productivity should decline appreciably by 2032 with continued HW operation. HW provides an affordable option for stratified lakes with high internal P loading. Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2021.2001609 .

Indirect evidence that anoxia exposure and cold acclimation alter transarcolemmal Ca2+ flux in the cardiac pacemaker, right atrium and ventricle of the red-eared slider turtle (Trachemys scripta)

We indirectly assessed if altered transarcolemmal Ca2+ flux accompanies the decreased cardiac activity displayed by Trachemys scripta with anoxia exposure and cold acclimation. Turtles were first acclimated to 21 °C or 5 °C and held under normoxic (21N; 5N) or anoxic conditions (21A; 5A). We then compared the response of intrinsic heart rate (fH) and maximal developed force of spontaneously contracting right atria (Fmax,RA), and maximal developed force of isometrically-contracting ventricular strips (Fmax,V), to Ni2+ (0.1–10 mM), which respectively blocks T-type Ca2+ channels, L-type Ca2+ channels and the Na+-Ca2+-exchanger at the low, intermediate and high concentrations employed. Dose-response curves were established in simulated in vivo normoxic (Sim Norm) or simulated in vivo anoxic extracellular conditions (Sim Anx; 21A and 5A preparations). Ni2+ decreased intrinsic fH, Fmax,RA and Fmax,V of 21N tissues in a concentration-dependent manner, but the responses were blunted in 21A tissues in Sim Norm. Similarly, dose-response curves for Fmax,RA and Fmax,V of 5N tissues were right-shifted, whereas anoxia exposure at 5 °C did not further alter the responses. The influence of Sim Anx was acclimation temperature-, cardiac chamber- and contractile parameter-dependent. Combined, the findings suggest that: (1) reduced transarcolemmal Ca2+ flux in the cardiac pacemaker is a potential mechanism underlying the slowed intrinsic fH of anoxic turtles at 21 °C, but not 5 °C, (2) a downregulation of transarcolemmal Ca2+ flux may aid cardiac anoxia survival at 21 °C and prime the turtle myocardium for winter anoxia and (3) confirm that altered extracellular conditions with anoxia exposure can modify turtle cardiac transarcolemmal Ca2+ flux.

Migration, homing and spatial ecology of common carp in interconnected lakes

AbstractThe common carp,Cyprinus carpio, is a large, long‐lived, fecund and mobile cyprinid, which evolved in complex inter‐braided Ponto‐Caspian rivers that experience both springtime flooding and freezing winters. Studies suggest adults often move to productive, shallow lakes and floodplains to spawn because they often lack egg predators and then return to deeper normoxic waters to overwinter. Whether these movements involve individuals consistently selecting, or homing to, the same spawning and refuge lakes as part of a strategy benefiting their reproductive success is unknown. To address this question, we examined the movements of 67 radio‐tagged adult carp for 3 years in a watershed with 11 interconnected lakes. Carp were tagged and released into a centrally located, normoxic deep lake in spring and fall. Each spring over 95% of its adults left via a single stream and swam into one of 5 shallow lakes, with most individuals (84%) selecting the same lake(s) in which to spawn each year (median Bhattacharyya affinity coefficient of similarity of 0.82). Young were later found in those lakes without egg predators, which cannot survive winter anoxia. After spawning, carp spent summers moving between productive lakes in an individualistic fashion, presumably foraging, with most (89%) eventually returning to the same deep lake to overwinter (median Bhattacharyya affinity of 1.0). These movements appear to reveal a life‐history tactic involving seasonal homing migration, first to a spawning location and later to winter refuges, that is well adapted to productive but highly heterogeneous and interconnected freshwater environments.

H2S-producing enzymes in anoxia-tolerant vertebrates: Effects of cold acclimation, anoxia exposure and reoxygenation on gene and protein expression.

To lend insight into the potential role of the gasotransmitter hydrogen sulfide (H2S) in facilitating anoxia survival of anoxia-tolerant vertebrates, we quantified the gene expression of the primary H2S-synthesizing enzymes, 3-mercaptopyruvate sulfurtransferase (3MST), cystathionine γ-lyase (CSE) and cystathionine β-synthase (CBS), in ventricle and brain of normoxic, anoxic and reoxygenated 21°C- and 5°C-acclimated freshwater turtles (Trachemys scripta) and 10°C-acclimated crucian carp (Carassius carassius). Semi-quantitative Western blotting analysis was also conducted to assess 3MST and CBS protein abundance in ventricle and brain of 5°C turtles and 10°C crucian carp subjected to normoxia, anoxia and reoxygenation. We hypothesized that if H2S was advantageous for anoxia survival, expression levels would remain unchanged or be upregulated with anoxia and/or reoxygenation. Indeed, for both species, gene and protein expression were largely maintained with anoxia exposure (24h, 21°C; 5 d, 10°C; 14 d, 5°C). With reoxygenation, 3MST expression was increased in turtle and crucian carp brain at the protein and gene level, respectively. Additionally, the effect of cold acclimation on gene expression was assessed in several tissues of the turtle. Expression levels were maintained in most tissues, but decreased in others. The maintenance of gene and protein expression of the H2S-producing enzymes with anoxia exposure and the up-regulation of 3MST with reoxygenation suggests that H2S may facilitate anoxic survival of the two champions of vertebrate anoxia survival. The differential effects of cold acclimation on H2S enzyme expression may influence blood flow to different tissues during winter anoxia.

The expression of genes involved in excitatory and inhibitory neurotransmission in turtle (Trachemys scripta) brain during anoxic submergence at 21 °C and 5 °C reveals the importance of cold as a preparatory cue for anoxia survival

We investigated if transcriptional responses are consistent with the arrest of synaptic activity in the anoxic turtle (Trachemys scripta) brain. Thirty-nine genes of key receptors, transporters, enzymes and regulatory proteins of inhibitory and excitatory neurotransmission were partially cloned and their expression in telencephalon of 21 °C- and 5 °C-acclimated normoxic, anoxic (24 h at 21 °C; 1 and 14 days at 5 °C) and reoxygenated (24 h at 21 °C; 13 days at 5 °C) turtles quantified by real-time RT-PCR. Gene expression was largely sustained with anoxia at 21 °C and 5 °C. However, the changes in gene expression that did occur were congruous with the decline in glutamatergic activity and the increase in GABAergic activity observed at cellular and whole organism levels. Moreover, at 21 °C, the alterations in gene expression with anoxia induced a distinct gene expression pattern compared to normoxia and reoxygenation. Strikingly, acclimation from 21 °C to 5 °C in normoxia effectuated substantial transcriptional responses. Most prominently, 56% of the excitatory neurotransmission genes were down-regulated, including most of the ones encoding the subunits composing excitatory N-methyl-d-aspartate (NMDA) and 3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) glutamate receptors. By contrast, only 26% of the inhibitory neurotransmission genes were down-regulated. Consequently, the gene expression pattern of 5 °C normoxic turtles was statistically distinct compared to that of 21 °C normoxic turtles. Overall, this study highlights that key transcriptional responses are consonant with the synaptic arrest that occurs in the anoxic turtle brain. In addition, the findings reveal that transcriptional remodelling induced by decreased temperature may serve to precondition the turtle brain for winter anoxia.

Modelling Dissolved Oxygen/Sediment Oxygen Demand under Ice in a Shallow Eutrophic Prairie Reservoir

Dissolved oxygen is an influential factor of aquatic ecosystem health. Future predictions of oxygen deficits are paramount for maintaining water quality. Oxygen demands depend greatly on a waterbody’s attributes. A large sediment–water interface relative to volume means sediment oxygen demand has greater influence in shallow systems. In shallow, ice-covered waterbodies the potential for winter anoxia is high. Water quality models offer two options for modelling sediment oxygen demand: a zero-order constant rate, or a sediment diagenesis model. The constant rate is unrepresentative of a real system, yet a diagenesis model is difficult to parameterise and calibrate without data. We use the water quality model CE-QUAL-W2 to increase the complexity of a zero-order sediment compartment with limited data. We model summer and winter conditions individually to capture decay rates under-ice. Using a semi-automated calibration method, we find an annual pattern in sediment oxygen demand that follows the trend of chlorophyll-a concentrations in a shallow, eutrophic Prairie reservoir. We use chlorophyll-a as a proxy for estimation of summer oxygen demand and winter decay. We show that winter sediment oxygen demand is dependent on the previous summer’s maximum chlorophyll-a concentrations.

Lowering Temperature is the Trigger for Glycogen Build-Up and Winter Fasting in Crucian Carp (Carassius carassius).

Seasonal changes in physiology of vertebrate animals are triggered by environmental cues including temperature, day-length and oxygen availability. Crucian carp (Carassius carassius) tolerate prolonged anoxia in winter by using several physiological adaptations that are seasonally activated. This study examines which environmental cues are required to trigger physiological adjustments for winter dormancy in crucian carp. To this end, crucian carp were exposed to changing environmental factors under laboratory conditions: effects of declining water temperature, shortening day-length and reduced oxygen availability, separately and in different combinations, were examined on glycogen content and enzyme activities involved in feeding (alkaline phosphatase, AP) and glycogen metabolism (glycogen synthase, GyS; glycogen phosphorylase, GP). Lowering temperature induced a fall in activity of AP and a rise in glycogen content and rate of glycogen synthesis. Relative mass of the liver, and glycogen concentration of liver, muscle and brain increased with lowering temperature. Similarly activity of GyS in muscle and expression of GyS transcripts in brain were up-regulated by lowering temperature. Shortened day-length and oxygen availability had practically no effects on measured variables. We conclude that lowering temperature is the main trigger in preparation for winter anoxia in crucian carp.

Effects of prolonged anoxia on electrical activity of the heart in crucian carp (Carassius carassius).

The effects of sustained anoxia on cardiac electrical excitability were examined in the anoxia-tolerant crucian carp (Carassius carassius). The electrocardiogram (ECG) and expression of excitation-contraction coupling genes were studied in fish acclimatised to normoxia in summer (+18°C) or winter (+2°C), and in winter fish after 1, 3 and 6 weeks of anoxia. Anoxia induced a sustained bradycardia from a heart rate of 10.3±0.77beatsmin-1 to 4.1±0.29beatsmin-1 (P<0.05) after 5 weeks, and heart rate slowly recovered to control levels when oxygen was restored. Heart rate variability greatly increased under anoxia, and completely recovered under re-oxygenation. The RT interval increased from 2.8±0.34 s in normoxia to 5.8±0.44 s under anoxia (P<0.05), which reflects a doubling of the ventricular action potential (AP) duration. Acclimatisation to winter induced extensive changes in gene expression relative to summer-acclimatised fish, including depression in those genes coding for the sarcoplasmic reticulum calcium pump (Serca2a_q2) and ATP-sensitive K+ channels (Kir6.2) (P<0.05). Genes of delayed rectifier K+ (kcnh6) and Ca2+ channels (cacna1c) were up-regulated in winter fish (P<0.05). In contrast, the additional challenge of anoxia caused only minor changes in gene expression, e.g. depressed expression of Kir2.2b K+ channel gene (kcnj12b), whereas expression of Ca2+ (cacna1a, cacna1c and cacna1g) and Na+ channel genes (scn4a and scn5a) was not affected. These data suggest that low temperature pre-conditions the crucian carp heart for winter anoxia, whereas sustained anoxic bradycardia and prolongation of AP duration are directly induced by oxygen shortage without major changes in gene expression.

Quantification of heat shock protein mRNA expression in warm and cold anoxic turtles ( Trachemys scripta) using an external RNA control for normalization

The mRNA expression of heat-shock protein 90 (HSP90) and heat-shock cognate 70 (HSC70) was examined in cardiac chambers and telencephalon of warm- (21 °C) and cold-acclimated (5 °C) turtles ( Trachemys scripta) exposed to normoxia, prolonged anoxia or anoxia followed by reoxygenation. Additionally, the suitability of total RNA as well as mRNA from β-actin, glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and cyclophilin A (PPIA) for normalizing gene expression data was assessed, as compared to the use of an external RNA control. Measurements of HSP90 and HSC70 mRNA expression revealed that anoxia and reoxygenation have tissue- and gene-specific effects. By and large, the alterations support previous investigations on HSP protein abundance in the anoxic turtle heart and brain, as well as the hypothesized roles of HSP90 and HSC70 during stress and non-stress conditions. However, more prominent was a substantially increased HSP90 and HSC70 mRNA expression in the cardiac chambers with cold acclimation. The finding provides support for the notion that cold temperature induces a number of adaptations in tissues of anoxia-tolerant vertebrates that precondition them for winter anoxia. β-actin, GAPDH and PPIA mRNA expression and total RNA also varied with oxygenation state and acclimation temperature in a tissue- and gene-specific manner, as well as among tissue types, thus disqualifying them as suitable for real-time RT-PCR normalization. Thus, the present data highlights the advantages of normalizing real-time RT-PCR data to an external RNA control, an approach that also allows inter-tissue and potentially inter-species comparisons of target gene expression.

Seasonality of glycogen phosphorylase activity in crucian carp (Carassius carassius L.)

Seasonal changes in the activity of glycogen phosphorylase (GP), a rate-limiting enzyme of glycogen degradation, were examined in an anoxia-tolerant fish species, the crucian carp (Carassius carassius L.). In muscle and brain, the activity of GP remained constant throughout the year when tested at 25°C. In contrast, the activities of liver and heart GP displayed striking increases in summer. When seasonal temperature changes are taken into account, the activity of GP during the anoxic mid-winter is only 4-6% of its summer time activity in the muscle, heart and liver, and 13% in brain. In winter-acclimatized fish, experimental anoxia (1-6weeks) caused sustained depression of the GP activity in heart and gills. In liver and muscle, a transient depression of GP activity occurred during the first week of anoxia but later GP activity recovered back to the normoxic level. GP of the brain was completely resistant to anoxia. In all studied tissues, the constitutive activity of GP is more than sufficient to degrade glycogen deposits during winter anoxia without anoxia-induced activation of GP. The seemingly paradoxical summer-time increase in the activity of liver and heart GP could be related to active life-style of the summer-acclimatized fish (growth, reproduction), the increased demand of energy and molecular precursors of anabolic metabolism being satisfied by preferential degradation of glycogen. The high glycogen content of winter-acclimatized crucian carp is not associated with the elevated GP activity or anoxic activation of GP.

The impact of raised water level on a winter anoxia lake on the population of narrow-clawed crayfish Astacus leptodactylus (Esch.)

We’ve built this digital research platform to provide academics everywhere with fast, stable and secure digital access to our library of over 110,000 scholarly books and 800,000 journal articles. We’ve tested the platform in close cooperation with leading academic institutions. If you have any questions or you notice something doesn’t quite work as it should, please visit our <a href="/publishing/faq?lang=en" class="text-primary">Help/FAQ</a> page and let us know.

Effect of temperature and prolonged anoxia exposure on electrophysiological properties of the turtle (Trachemys scripta) heart

Cardiac activity of the turtle (Trachemys scripta) is greatly depressed with cold acclimation and anoxia. We examined what electrophysiological modifications accompany and perhaps facilitate this depression of cardiac activity. Turtles were first acclimated to 21 degrees C or 5 degrees C and held under either normoxic or anoxic (6 h at 21 degrees C; 14 days at 5 degrees C) conditions. We then measured cardiac action potentials (APs) using spontaneously contracting whole heart preparations and whole cell current densities of sarcolemmal ion channels using isolated ventricular myocytes under appropriate normoxic and anoxic conditions. Compared with 21 degrees C-acclimated turtles, 5 degrees C-acclimated turtles exhibited a less negative resting membrane potential (by 18-29 mV), a 4.7- to 6.8-fold slower AP upstroke rate, and a 4.2- to 4.9-fold greater AP duration. Correspondingly, peak densities of ventricular voltage-gated Na(+) (I(Na)) and L-type Ca(2+) currents and inward slope conductances of inward rectifier K(+) (I(K1)) channel current were approximately 1/7th (Q(10) = 3.4), 1/13th (Q(10) = 5.0), and one-half (Q(10) = 1.4) of those of 21 degrees C-acclimated ventricular myocytes, respectively. With anoxia at 21 degrees C, peak I(Na) density doubled and ventricular AP duration increased by 47%, a change proportional to the reported approximately 30% reduction of intrinsic heart rate. In contrast, with anoxia at 5 degrees C, ventricular AP characteristics were unaffected; of the ion currents investigated, only the inward conductance via I(K1) changed significantly (reduced by 46%). The present findings indicate that cold temperature, more so than prolonged anoxia, results in substantial modifications of cardiac APs and reduction of ventricular ion current densities. These changes likely prepare cardiac muscle for winter anoxia conditions.

Interglacial History of a Palaeo-lake and Regional Environment: A Multi-proxy Study of a Permafrost Deposit from Bol’shoy Lyakhovsky Island, Arctic Siberia

Chironomid, pollen, and rhizopod records from a permafrost sequence at Bol’shoy Lyakhovsky Island (New Siberian Archipelago) document the development of a thermokarst palaeo-lake and environmental conditions in the region during the last Interglacial (MIS 5e). Open Poaceae and Artemisia associations dominated vegetation at the beginning of the interglacial period. Rare shrub thickets (Salix, Betula nana, Alnus fruticosa) grew in more protected and wetter places as well. Saalian ice wedges started to melt during this time, resulting in the formation of an initial thermokarst water body. The high percentage of semi-aquatic chironomids suggests that a peatland-pool initially existed at the site. A distinct decrease in semi-aquatic chironomid taxa and an increase in lacustrine ones point to a gradual pooling of water in the basin, which could in turn induce thermokarst and create a permanent pond during the subsequent period. The highest relative abundance of Chironomus and Procladius reflects unfrozen water remaining under the ice throughout the ice-covered period during the later stage of palaeo-lake development. The chironomid record points to three successive stages during the history of the lake: (1) a peatland pool; (2) a pond (i.e., shallower than the maximum ice-cover thickness); and (3) a shallow lake (i.e., deeper than the maximum ice-cover thickness). The trend of palaeo-lake development indicates that intensive thermokarst processes occurred in the region during the last Interglacial. Shrub tundra communities with Alnus fruticosa and Betula nana dominated the vegetation during the interglacial optimum. The climate was moister and warmer than present. The results of this study suggest that quantitative chironomid-based temperature reconstructions from Arctic thermokarst ponds/lakes may be problematic due to other key environmental factors, such as prolonged periods of winter anoxia and local hydrological/geomorphological processes, controlling the chironomid assemblages.

Associations between water chemistry and fish community composition: a comparison between isolated and connected lakes in northern Sweden

Summary1. The correlation between water chemistry, physical variables and fish community composition was examined in 40 small (≤30 ha) coastal lakes in northern Sweden. Twenty of the 40 lakes were isolated from other water bodies and 20 were connected to the Baltic Sea. Lakes were fished in summer, using three different methods. Water chemistry was sampled in late winter prior to ice‐out and pH was measured additionally in summer.2. Our central question was whether water chemistry plays a greater role in the composition of fish communities in isolated lakes than in connected lakes, as isolated lakes cannot be recolonised once a species has become extinct.3. Results indicate that winter anoxia affects community composition only in isolated lakes, whereas acidity is of importance in both connected and isolated lakes. Methane (indicating anoxia), was significantly correlated with variation in fish community composition in isolated lakes, and a group of variables that indicate anoxia (CH4, pCO2, inorganic carbon and dissolved oxygen) explained 24–34% of the variation. pH alone explained 12% of the variation in community composition for connected lakes and a group of variables indicating acidity (summer and winter pH and ANC) explained 10–20% of the variation in isolated lakes. Lake area was the most important physical variable, being significantly correlated with the variation in fish community composition in connected lakes.4. In isolated lakes, the presence of pike (Esox lucius), perch (Perca fluviatilis) and roach (Rutilus rutilus) was associated with low CH4. The occurrence of crucian carp (Carassius carassius) and roach was positively correlated with pH, and the crucian carp was apparently also affected by predation by pike and perch. In connected lakes the effect of anoxia was low, probably due to the possibility of recolonisation and pockets of oxygenated water, allowing pike and perch to persist and thereby limiting the distribution of crucian carp.

Simulated climate change effects on dissolved oxygen characteristics in ice-covered lakes

A deterministic, one-dimensional model is presented which simulates daily dissolved oxygen (DO) profiles and associated water temperatures, ice covers and snow covers for dimictic and polymictic lakes of the temperate zone. The lake parameters required as model input are surface area ( A S), maximum depth ( H MAX), and Secchi depth as a measure of light attenuation and trophic state. The model is driven by daily weather data and operates year-round over multiple years. The model has been validated with extensive data (5976 points). Standard error between simulated and measured dissolved oxygen is 1.9 mg/l. The model is applied to simulate effects of climate change on dissolved oxygen characteristics of 27 lake classes in Minnesota. The projected climate changes due to a doubling of atmospheric CO 2 are obtained from the output of the Canadian Climate Center Global Circulation Model (CCC GCM). Climate change delays the ice formation and shortens the ice cover period. Winter anoxia, even in shallow lakes, therefore disappears under a projected 2 × CO 2 climate condition. This eliminates winterkill in these lakes. Herein, the simulated DO characteristics have been plotted and interpolated graphically in a coordinate system with a lake geometry ratio ( A 0.25 S H MAX ) on one axis and Secchi depth on the other. The lake geometry ratio expresses a lake's susceptibility to stratification. To illustrate the effect of projected climate change on DO characteristics, separate graphs are presented for values simulated with inputs of past climate conditions (1961–1979) and with a projected 2 × CO 2 climate scenario.

MECHANISMS CREATING COMMUNITY STRUCTURE ACROSS A FRESHWATER HABITAT GRADIENT

▪ Abstract Lentic freshwater habitats in temperate regions exist along a gradient from small ephemeral ponds to large permanent lakes. This environmental continuum is a useful axis for understanding how attributes of individuals ultimately generate structure at the level of the community. Community structure across the gradient is determined by both (a) physical factors, such as pond drying and winter anoxia, that limit the potential breadth of species distributions, and (b) biotic effects mediated by ecological interactions, principally predation, that determine the realized success of species. Fitness tradeoffs associated with a few critical traits of individuals often form the basis for species turnover along the gradient. Among species that inhabit temporary ponds, distributions are often constrained because traits that enhance developmental rate and competitive ability also increase susceptibility to predators. In permanent ponds, changes in the composition of major predators over the gradient limit distributions of prey species because traits that reduce mortality risk in one region of the gradient cause increased risk in other regions of the gradient. Integrated across the gradient, these patterns in species success generate distinct patterns in community structure. Additionally, spatial heterogeneity among habitats along the gradient and the fitness tradeoffs created by this heterogeneity may hold important evolutionary implications for habitat specialization and lineage diversification in aquatic taxa.

Importance of Runoff and Winter Anoxia to the P and N Dynamics of a Beaver Pond

Annual retentions of total phosphorus (TP) (−11%) and total nitrogen (TN) (−5%) of a headwater beaver pond in central Ontario were low during 1987–88. Annually, inputs exceeded outputs of total reactive P (71%) and NO3–N (35%), and outputs exceeded inputs of total unreactive P (−33%) and total organic N (−26%), and inputs approximated outputs of NH4–N (−8%). Seasonal trends in P and N retention were inversely correlated with runoff. Monthly retention was weakly positively related to average water temperature and redox potential (ORP). Positive monthly retention coincided with low runoff and high biotic assimilation during the growing season. Winter ice cover was associated with undetectable dissolved oxygen and low ORP and increased levels of P and N, particularly NH4–N. High levels of P and N in the surface water during winter were coupled with increased runoff and potentially low biotic assimilation resulting in a net release of TP and TN. Large flow-through of waterborne inputs and flushing of regenerated P and N occurred during peak snowmelt resulting in low annual retention. Initial accumulation of flooded forest material and input of organic matter by beaver may be very important to P and N dynamics, representing a long-term source of nutrients to the pond water and outflow.

BOUNDARIES OF ECOLOGICAL SUSTAINABILITY OF PRAIRIE LAKES AND RESERVOIRS

The dynamics of algal growth and algal bloom development in eutrophic prairie lakes and reservoirs offers an opportunity to assess the negative consequences of exponential growth and its sustainable and unsustainable phase. Extreme situations of ecological perturbation and instability, where the structure and function of an aquatic ecosystem breaks down, are demonstrated on summary results from lakes undergoing collapse (crashes) of algal blooms or winter anoxia as a result of unsustainable productivity. Algal blooms provide warning signals during their development to indicate various levels of ecological sustainability. Phytoplankton biomass was found to be a measure of ecological sustainability with chlorophyll a levels less than 50 μg/L being sustainable and over 100 μg/L unsustainable.Due to high natural input of nutrients from prairie soils and advanced eutrophication, the carrying capacity of many prairie lakes and reservoirs is often fully utilized and leaves little room for further loads from popu...