Succession in Lakes

The epilimnion has plenty of oxygen since it is in contact with the atmosphere. It is also fairly productive since it has more sunlight available. However, biomass production gradually reduces nutrient availability during the growing season. Sedimentation of dead organisms transports the nutrients to the hypolimnion or to the sediments. Degradation reduces oxygen and releases nutrients in the hypolimnion. However, twice a year in the temperate zone, in spring and fall, these differences disappear as temperature changes destroy the stratification. The resulting mixing is called turnover (not to be confused with the idea of turnover time in nutrient cycling). The turnover returns nutrients to the surface and oxygen to the depths.

Spring turnover brings nutrients and light together with a warming environment. This initiates successional changes in populations. A sequence of species goes through boom-and-bust cycles. Each flourishes temporarily as it enjoys a competitive advantage over the others. Eventually, it depletes some necessary resource or comes under attack by zooplankton predators or fungal parasites, and the population crashes. This leaves the field for another opportunist, which can best compete in the resulting environment.

The largest total algal populations occur shortly after turnover. Large, sudden increases in phytoplankton populations are called algal blooms. During an algal bloom, cell counts on the order of millions per liter can color the lake green. The diatom Asterionella is often the first algae to bloom as longer days in spring increase the sunlight available. It is faster growing than its competitors Fragilaria, TabellarĂ­a, and the cyanobacter, and it performs luxury uptake of phosphorus, up to 100 times its immediate requirement. However, it is susceptible to fungal attacks, which can give Fragillaria and Tabellaria a temporary advantage. If the filamentous cyanobacter Oscillatoria gains a foothold, it can shade Asterionella, outcompeting it for sunlight. Ultimately, silica limits Asterionella blooms (Figure 15.14). Unless some other nutrient becomes limiting, Asterionella stops growing when the silica concentration falls below 0.5 mg/L.

Because they can float on the surface, cyanobacter may gain an advantage when turbidity limits light to the green algae. A cyanobacter bloom may continue into the summer as nitrate and ammonia are depleted, since they can fix atmospheric nitrogen. Eventually, nutrients become depleted even for the cyanobacter. Iron is often one of the limiting

Spring turnover

Fall turnover

Spring turnover

150 200 250 300 350 Day of year

Figure 15.14 Typical algal succession. (Based on Home and Goldman, 1994.)

150 200 250 300 350 Day of year

Figure 15.14 Typical algal succession. (Based on Home and Goldman, 1994.)

nutrients. It is needed for the nitrogenase enzyme for nitrification. Anabaena and Microcystis often form late summer-autumn blooms as nutrients are recycled from sediments or the hypolimnion.

When the fall turnover comes, large numbers of the diatom Melosira can be resus-pended from the sediments, becoming the dominant alga over the winter period. During the summer they settle back to the sediment before nutrient limitation begins to affect them. This, plus their tolerance of anoxic conditions, enables them to survive in the sediment.

Phytoplankton growth responds most strongly to light availability. But zooplankton feed at a rate strongly related to the temperature. Thus, they often peak later than the phy-toplankton from the spring bloom, bringing it to an end. The elimination of phytoplankton brings about the clearwater phase, during which the lake is most transparent. This initial peak in zooplankton is usually dominated by cladocerans such as Daphnia or Bosmina. Other animals present in this peak include predatory rotifers, carnivorous and herbivorous copepods, and protozoans. Herbivorous rotifers follow the clearwater phase as they can eat larger and harder-to-digest algae, as well as toxic or repulsive cyanobacter rejected by the cladocerans.

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