NUMBER TWENTY NINE - OCTOBER 1998

The current version of the alternative states model is shown in Fig 9. It hypothesises that over a wide range of nutrient concentrations, both plant-dominated and algal-dominated states can exist as alternatives. At very low nutrient concentrations (less than about 25 µg l-1 total P, with usually a high N:P ratio, much greater than about 10:1, there is only one possible state, of dominance by plants, frequently with communities of stoneworts (charophytes) in calcareous waters, and isoetids, Lobelia and Littorella in more acid, base-poor waters. These are low-growing plants which are very vulnerable to shading. Water lilies (nymphaeids) are also likely to be present.

The plants derive phosphorus from the sediments and the water, but phosphorus will in general be limiting to algal growth [22,137] so that large populations of phytoplankton cannot easily develop. At higher phosphorus and nitrogen concentrations, this plant community may be replaced by one of pondweeds and other relatively larger water plants, including species of Potamogeton, Myriophyllum, Stratiotes etc and greater numbers of lilies.

These communities are very productive and mostly rooted in richer sediment from which they will derive most of their phosphorus [7,44]. Low oxygen conditions created in the depths of their beds at the sediment surfaces will favour release of phosphorus from the sediment and will promote denitrification. Nitrogen is thus likely to be limiting to both plant and algal growth and will generally be absent in available form during the summer growth period [116]. Increases in nitrogen loading will tend to increase the plant growth and decrease the diversity of the community so that at the highest nutrient levels (milligrams per litre of N in winter from external loading from the catchment and in summer almost as much of P, due to sediment release) a very few tall plant species will dominate the community, particularly Ceratophyllum, Potamogeton pectinatus, Myriophyllum spicatum and nymphaeids. These plant communities have mechanisms (buffers) which reduce the chances of damage to themselves and stabilise their own existence.

Over the range of nutrient concentrations in which the pondweed-dominated communities can exist, there can also be a parallel range of phytoplankton algal communities. These will often be dominated by diatoms in spring, a mixture of green algae, dinoflagellates, other flagellates and some blue-green algae in summer, and blue green algae, sometimes with diatoms in late summer and autumn. There is a very large number of possible combinations, for there are hundreds of algal species, but as the nutrient concentrations increase, the proportion of blue-green algae may increase, whilst at the very highest concentrations, green algae may predominate [85]. Shortage of nitrogen will often favour the development of nitrogen-fixing blue green algae. At the very highest nutrient loadings and concentrations, phytoplankton algal - dominance may normally be the only possibility, just as isoetid or charophyte communities appear to be at the very lowest, but this is uncertain. The algal communities are also stabilised by particular buffer mechanisms, which make them difficult to dislodge, once they are established.

With the existence of such buffer mechanisms, which are described in detail below, a switch mechanism is necessary to cause a change from one state (plant or algal dominance) to the other. This switch may operate over the entire range of nutrient concentrations but a 'flip' from plants to algae tends to occur more easily at high nutrient concentrations and vice versa. 'Forward' switches convert plant-dominance to algal-dominance and 'reverse' switches, the converse. Forward switches may include mechanisms that directly destroy the plants or those that interfere with the buffer mechanisms. Reverse switching usually involves restructuring of the fish community in a process called biomanipulation, designed to reinstate the plant buffer mechanisms and destroy those that stabilise the algal communities.