Nutrients and Eutrophication in Lakes

Aquatic systems are also classified according to their status with respect to nutrients or productivity (Table 15.10). Lakes that are relatively nutrient limited and low in productivity are called oligotrophic (''poorly fed''). Lakes with higher nutrient levels and higher productivity are called eutrophic (''well fed''). An intermediate designation is called mesotrophic.

Basically, an oligotrophic lake is one that would be perceived as "clean" vs. the relatively murky eutrophic lake. However, eutrophic does not necessarily mean polluted. Lakes proceed through a natural succession, starting as oligotrophic. Gradually, they accumulate sediments and nutrients. The shoreline grows more and more macrophytes as the lake becomes shallower. Eventually, they become marshes or wetlands, until they are finally transformed into terrestrial ecosystems.

Although it occurs naturally, addition of anthropogenic nutrients hastens this process. People contribute nutrients in the form of fertilizer runoff from lawns and agricultural lands, human waste from either treatment plant discharge or indirectly from septic tanks, and animal waste from pasture or feedlot runoff. In these cases the process is termed cultural eutrophication. Oligotrophic lakes are preferred for numerous human

TABLE 15.10 Comparison of Oligotrophic and Eutrophic Lakes

Criteria

Phosphorus conc. (winter) Organic matter Primary productivity

Depth Clarity

Hypolimnion dissolved oxygen in summer Bottom fauna

Bottom sediment Characteristic alga or genera

Examples

Oligotrophic

GPP < 150 g C/m2 ■ yr Chlorophyll a < 3 mg/L <15-25 m

Clear, blue, Secchi depth > 5 m >50% saturation

Diversified with deepwater fish

Low organic matter content Green algae Desmids (Staurastrum) Diatoms (Tabellaría, Cyclotella) Chrysophyceae (Dinobryon) Lake Superior Lake Geneva (Switzerland) Lake Baikal (Russia)

Eutrophic

GPP > 250 g C/m2 ■ yr Chlorophyll a > 6 mg/L <10-15 m

Dark, turbid, Secchi depth < 3 m <10% saturation (about 1 mg/L)

Low-DO-tolerant organisms;

no deepwater fish; carp Organic rich muck, high in N Cyanobacter (Anabaena,

Aphanizomenon, Microsystis) Diatoms (Molosira,

Stephanodiscus, Asterionella) Western Lake Erie Lake Lugano (Switzerland)

Source: Berner and Berner (1987); Conell and Miller (1984).

TABLE 15.11 Trophic Status of Several Lakes3

Lake

Status3

Avg Depth (m)

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