Lake Production, Succession, and Eutrophication - Hydro-Environmental Analysis: Freshwater Environments - page 460

All of these metrics may be related to biomass or productivity as a useful metric for eutrophi-
cation. For example, Vollenweider (1968) demonstrated the importance of phosphorus loading as
an indicator of lake production, and these and similar relationships have been widely used in lake
management. Similarly, while phosphorus is commonly thought of as the limiting nutrient for fresh-
water lakes, nitrogen loadings are important for some lakes, so they are related to production and
biomass. Chlorophyll can be related to biomass and, since light is essential for growth, so can the
Secchi depth. Algal biomass and productivity may also be measured directly, but that measurement
is commonly dificult, particularly in computing variations in annual averages. Each of these has
advantages and disadvantages as discussed by Carlson and Simpson (1996). The single metric that
will be discussed here is related to oxygen consumption. Carlson and Simpson suggested that some
limnologists would use oxygen consumption as the sole indicator of a trophic state, but that it would
be better to represent a result of the lake's trophic state than an indicator of that state.
A potential impact of eutrophication is a reduction of the oxygen concentration in the hypolim-
nion. That is, material from the surface falls to the hypolimnion and is decomposed, which reduces
oxygen concentrations. The greater the productivity is (e.g., for a eutrophic system), the greater the
resulting hypolimnetic oxygen demands will be. The larger the hypolimnion is, the greater will be
its capacity to absorb these losses related to oxygen depletion.
Hypolimnetic oxygen depletion has been used to estimate the trophic status of lakes, usually
based on the hypolimnetic oxygen deicit, or the difference between the oxygen concentrations and
the saturation concentrations (Thienemann 1926, 1928). The rate of hypolimnetic oxygen consump-
tion (increase in deicit) is impacted by, among other factors, the productivity in surface waters and
the morphometry of the hypolimnion. To eliminate the impact of morphometry, Strom (1931) and
Hutchinson (1938) proposed that the demand be expressed as an areal rate, the areal hypolimnetic
oxygen deicit (AHOD g O 2 m −2 day −1 ). Other factors that inluence the rate include the temperature
and the presence of dissolved organic compounds. Dissolved organic compounds also contribute to
the depletion of oxygen, even in lakes of low productivity, and thus have become one of the deining
characteristics of dystrophy (Thienemann 1921; Naumann 1932).
The AHOD is normally calculated as the slope of the linear plot of the product of mean depth
and mean hypolimnetic DO against time. Mortimer (1941) proposed limits of 0.25 g m −2 day −1 for
the upper limit of oligotrophy and 0.55 g m −2 day −1 for the lower limit of eutrophy.
Since this is only applicable to stratiied lakes, other metrics may be used, such as net dissolved
oxygen (Porcella et al. 1980). Another oxygen-related index is the anoxic factor (AF) represented
in Equation 16.2:

∑ tA
A

as

AF = (16.2)

o

where
t a is the days of detectable anoxic conditions
A s is the sediment area
A o is the surface area (Nurnberg 1995)

So, the AF describes the relationship of the lake bottom area exposed to anoxia, which is often
more useful in assessments of benthic organisms than the AHOD.

16.4.2 f orSberG and r ydInG ' S c rIterIa
Forsberg and Ryding (1980) developed criteria for classifying lakes into trophic states based on
four water chemistry parameters (total chlorophyll, total phosphorus [TP], total nitrogen [TN], and
water clarity). These criteria were based on studies of a series of Swedish lakes; however, the criteria