Abstract
Hypoxia and anoxia occur frequently in freshwater systems and have biological and chemical implications. Anoxia can be expressed and quantified as the anoxic factor; hypoxia, for a specific level of oxygen depletion, can be expressed as the hypoxic factor in lakes, reservoirs, and river sections. These methods summarize information of individual dissolved oxygen profiles as annual values or factors that facilitate comparison between and within lakes. Therefore, these factors are useful in the formulation and testing of hypotheses related to the dissolved oxygen status in water bodies. Methods of calculating different factors for different oxygen levels and water layers, including those applying separately to the epilimnion and hypolimnion, are presented in detail. Proven and potential applicability include: (1) the quantification of relationships with lake water quality variables and lake classification (trophic state), (2) the evaluation of restoration techniques with respect to their effects on hypolimnetic oxygen depletion, (3) the determination of internal phosphorus loading in stratified and polymictic lakes, (4) the exploration of habitat constraints due to hypoxia (e.g., fish species richness and winterkill), (5) forecasting potential effects of climatic change on oxygen content and internal phosphorus loading, and (6) the establishment and examination of criteria and guidelines with respect to hypoxia by custom-made definitions.
Highlights
Hypoxia and anoxia occur frequently in freshwater systems[1], and increases in organic and nutrient loading and changes in water flow have increased oxygen depletion in lakes[2] and reservoirs[3]
Ways to quantify oxygen depletion in lakes were introduced before including oxygen depletion rates[7], the probability of anoxia[8], anoxic factor (AF)[1], and hypoxic factor (HF)[9]
Epilimnetic factors were smaller than factors for the whole water column, as expected (Fig. 3). Severe anoxia in this reservoir is apparent since the long-term average lacustrine AFepi was 10 d yr–1, indicating that overall, an area in the lacustrine epilimnion equal to the lacustrine surface area is overlain by water below 2 mg L–1 dissolved oxygen (DO) for 10 d summer–1
Summary
Hypoxia and anoxia occur frequently in freshwater systems[1], and increases in organic and nutrient loading and changes in water flow have increased oxygen depletion in lakes[2] and reservoirs[3]. The anoxic and hypoxic factors quantify the extent and duration of anoxia and hypoxia They are based on a series of measured oxygen profiles and morphometric (hypsographic) data and can be computed for any water body (lake, reservoir, river, estuary, or marine area). To consider the mixed surface layer separately, epilimnetic factors (AFepi, HFepi) can be computed These factors are determined by subtracting, for each period i, terms corresponding to anoxia or hypoxia in the seasonal stratified area below the thermocline (athermo_i) from those for the whole water column before summation, according to Eq 2. Such spatially distinct factors are especially useful in determining certain habitat requirements and for the support of guidelines specific to the mixed zone. Where: fall turnover date, Julian day of the year; T, average July–August temperature at ca. 1 m above the bottom at the deepest location of the lake (Co); z, mean depth, i.e., lake volume/lake surface area (m)
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