Oxygen concentrations, Lake

Areas experiencing hypoxia. Counts are accumuiated by decade. Less than 6% of areas in which hypoxia has occurred have iater experienced increases in dissoived oxygen concentrations. 2007 data are based on press reiease reports from UNEP. Other data from Diaz, R. J. (2006). NOAA Great Lakes Seminar Series 2006 Past Seminars. ftp //ftp.gieri.noaa.gov/webcast/2006/diaz/ 20061019.pdf. 

We will try our hand at applying the diffusion equation to a couple of mass transport problems. The first is the diffusive transport of oxygen into lake sediments and the use of oxygen by the bacteria to result in a steady-state oxygen concentration profile. The second is an unsteady solution of a spill into the groundwater table. 

EXAMPLE 2.1 Steady oxygen concentration profile in lake sediments (steady-state solution with a first-order sink)... 

Figure E2.1.2. Solution (equation (E2.1.5)) to Example 2.1. for oxygen concentration in lake sediments with first-order sink.

EXAMPLE 7.2 Effect of fall turnover on oxygen concentration in lake sediments (unsteady, one-dimensional transport with step boundary conditions and a first-order sink, solved using explicit, central differences)... 

Before fall turnover, there is zero oxygen concentration in the sediments and in the water above the sediments of Lake Harriet. At fall turnover, stratification of the lake is broken down and the water overlying the sediments abruptly reaches approximately Co. You are interested in how fast the sediments will respond to the higher oxygen concentration. To determine, this you must answer two questions (1) What is the oxygen profile in the sediments over time (2) What is the flux rate across the sediment-water interface over time ... 

We can use Henry s law to verify that the concentration of oxygen in lake water is normally adequate to sustain aquatic life, which requires a concentration of at least 1.3 X 10-4 mol-L 1. The partial pressure of oxygen is 0.21 atm at sea level, so the molar solubility of oxygen is... 

Regression analysis in time series analysis is a very useful technique if an explanatory variable is available. Explanatory variables may be any variables with a deterministic relationship to the time series. VAN STRATEN and KOUWENHOVEN [1991] describe the dependence of dissolved oxygen on solar radiation, photosynthesis, and the respiration rate of a lake and make predictions about the oxygen concentration. STOCK [1981] uses the temperature, biological oxygen demand, and the ammonia concentration to describe the oxygen content in the river Rhine. A trend analysis of ozone data was demonstrated by TIAO et al. [1986]. 

Minerals such as phosphorous and oxygen are also required for life to flourish. Phosphorous levels can vary over a range of parts per billion (ppb). Most fish require an oxygen concentration of at least 5 parts per million (ppm). Oxygen concentration is primarily due to photosynthesis in lake plants and surface wind agitation. Some oxygen can also come from tributary streams. 

Figure 7. Correlation between concentration of nitrate, phosphate, and oxygen in Lake Zurich (a and b) and Lake Norrviken (c)...

Hickling et al, 2005), and threats to cold stenothermal taxa (e.g. Griffiths, 2007). In addition, recent resnlts from the EU CLIME project have shown how the extremely hot summer of 2003 cansed significant changes in the temperatnre, stratification and hypolimnetic oxygen concentration of deep lakes in Switzerland (Jankowski et al., 2006), and hot snmmers are likely to be more frequent in fntnre, particularly over central Europe (IPCC, 2007). 

Increased algal growth, a decrease in piscivorous fish and a temporary increase in benthic and forage fish to the point where oxygen depletion results in overall decline and a decline in hypolimnetic oxygen concentrations in eutrophic lakes. 

Examples of physical and chemical equilibria abound in nature. Many environmental systems depend on fragile equilibria. The amount of oxygen dissolved in a certain volume of lake water (the oxygen concentration) is governed by the principles of equilibrium. The lives of plants and animals within this system are critically related to the levels of dissolved oxygen in the water. 

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