NUMBER TWENTY NINE - OCTOBER 1998

Phytoplankton communities can develop from a huge range of potential contributors (Fig 14), perhaps several hundreds of species in a given lake, compared with a handful of plant species. Most algae grow rapidly and thus one species or another can respond quickly to, for example, changes in weather affecting conditions in the lake through washout rates, inputs of nutrients, thermal stratification of the water column and light availability. Some phytoplankters start growth early and build up large spring populations, which may be grazed by Cladocera and replaced by grazer-resistant forms, with spines or larger size, which the grazers are unable to handle. Some species efficiently take up nutrients from very low concentrations and thrive in mid-summer when previous populations have depleted the water and inflows are too small to replenish the nutrients with supplies from the catchment. The phytoplankters are generally small and have lesser problems of obtaining dissolved substances from the water than do plants, for their diffusion pathways are short.

Fig 14. Algae come in a considerable variety of shapes, sizes and pigmentation. From Moss [108]

Their main problems, however, are in avoiding the consequences of their tendency to sink and in maintaining active or passive movement relative to the water to prevent shells of nutrient-depleted water forming over their surfaces. The greater density than water of most species means that they sink out of the illuminated surface layer and must be maintained in suspension by eddy currents generated by wind. They sink least rapidly if they are small, but then they are most vulnerable to zooplankton grazing. Survival in the phytoplankton thus means balancing the risks of grazing vulnerability, sinking, and failing to obtain sufficient nutrients and light.

In plant-dominated lakes, many of these pressures are exacerbated. The plant beds may damp eddy currents, zooplankton grazer populations can build up and the nutrient availability may be reduced. In a shallow lake free of plants, however, these pressures are greatly relaxed. In the open water, lacking any structures, or even deep, dark layers to provide refuges for the zooplankton, the fish quickly remove the large, most efficiently grazing Cladocera, so that grazing intensity is reduced. Grazing sometimes results in advantages for the larger phytoplankters, which can obtain nutrients that were previously more efficiently taken up by small phytoplankters; release from grazing pressure may lead to predominance of small forms, which, for the same total biomass are more efficient at intercepting light so that light penetration to the bottom is diminished. This will tend to suppress any plant growth from seeds or overwintering shoots, especially as the phytoplankters begin their growth, in temperate lakes, months earlier than the plants.

The growth of the phytoplankters will eventually be inhibited by scarcity of nutrients or, more probably, light, for various mechanisms contrive to make nutrients more available in a phytoplankton-dominated system than in a plant-dominated one. Phosphorus is as readily released from the sediments [142], whose surfaces become deoxygenated through decomposition of a rain of finely divided particles from the phytoplankton; nitrogen is more available because of lack of plant uptake. We do not know if denitrification is less important under phytoplankton-dominated conditions than under plant-dominated ones. The lack of plants also may lead to greater resuspension of sediment, which again worsens the light climate at the bottom. The phytoplankters are less affected by this because they move or are moved through the water column and can photosynthesise towards the surface. Not being bulky, they can also photosynthesise and grow at lower intensities than plants whose outer layers extinguish some light before it can reach the photosynthetic pigments.

The structureless open water habitat also favours build up of large populations of small zooplanktivorous fish because of a shortage of predators. Predators such as pike >(Esox lucius), require plant beds in which to lurk and thus are disadvantaged when plants are lost. Also the lack of large invertebrates otherwise found in the plant beds, results in poor survival of larger fish of most species, which require such prey on which to feed. Because these larger fish might otherwise compete for space with, or even cannibalise the small fish of their own species, survival of the smaller fish within their first year or so is increased and with it the pressure on the zooplankton.

The lack of refuges for the young zooplanktivorous fish against the piscivores, who will take the largest prey they can handle, also paradoxically displaces the size distribution of the fish community towards smaller animals. For a time a lesser proportion of the fish biomass will be found as small zooplanktivorous fish, decreasing the predation pressure on the zooplankton. Eventually as these cohorts of zooplanktivorous fish fail to survive to even moderately large size (for lack of food) to support the older and larger piscivores, piscivore reproduction and recruitment may be jeopardised. Later cohorts of young zooplanktivores may then be released from much predation. Such large populations of small zooplanktivorous fish may ensue that even a recovered population of the piscivores may be unable to exert much control.

The sediment laid down by phytoplankters is more fluid and amorphous than that laid down in plant beds. It may not provide a firm rooting medium for plants, and it may be more readily disturbed to give inorganic turbidity in the water and an unfavourable light climate for plant development. A summary of the buffer mechanisms which may stabilise the algal dominated state is given in Table 1.