NUMBER TWENTY NINE - OCTOBER 1998

A major component, the buffering mechanisms, of the alternative states model in Figure 9 has now been described. But there are two other key components to be discussed before the use of the model in shallow lake restoration can be made clear. These are the roles of nutrients and of switches.

Implicit in an alternative states model is that the states can each be maintained under a wide range of imposed nutrient loads. It should thus be possible to demonstrate that increases or decreases in loading will not by themselves cause a switch in states and that other mechanisms must cause the switches between states. These conditions are generally satisfied but there are still uncertainties and complications.

First the alternative states exist over only part of the potential nutrient range found in natural and polluted waters. There certainly are uniquely plant-dominated clear-water states at the lower end of the nutrient range and it appears that severe phosphorus limitation then occurs. The mechanisms that change such systems from charophyte or isoetid dominance to pondweed dominance are still uncertain. The increased nutrient loading may lead first to an increase in periphyton [118], which may be less vulnerable to the mechanisms which plant beds are able to use to suppress phytoplankton growth. Experiments show a later response of phytoplankton compared with periphyton to nutrient increase and there is evidence from sediment cores, taken from eutrophicated shallow lakes, of an increase in periphyton diatoms prior to increases in phytoplankton (Fig 15) [105].

Fig 15 Diatoms, with their silica walls are readily preserved in sediments and can be used to deduce the history of lakes. In Strumpshaw Broad, increases in nutrient loading from the developing Norwich human population led at first to increases in periphyton. The plants disappeared and with them their associated snails, and phytoplankton came to dominate. From Moss [105]

The response of the plant community is to replace the low-growing isoetids and charophytes with taller-growing plants, sometimes including the more vigorous species of charophytes, but usually pondweed species which can cope with the increasing periphyton burden. These may produce leaves closer to the surface, where a denser periphyton burden can be sustained in the greater light intensities. The plants may also readily shed periphyton-burdened leaves and replace them with new ones. They take advantage, through leaf uptake, of the increased nutrient availability to build up their biomass and hence to establish the various buffer mechanisms associated with the plant beds. Further nutrient increase may then be easily sustained.

We do not know, however, whether it can be sustained indefinitely or whether there is a state of unique phytoplankton dominance at very high nutrient loads. The nature of the phytoplankton community is affected by the nutrient status. Algal communities associated with high loadings may be less vulnerable to the devices employed by plant beds to suppress algae.

The highest loads are often thought to support blue-green algal populations, though extensive surveys in Denmark have shown this to be untrue (Fig 16), and that green algae then dominate. Blue-green algae, nonetheless, thrive at these upper limits and may be more resistant to grazing than other algae [120]. Blue-green algae are often large (though they can be very tiny) and seem to be poorly nutritious food for grazers [3,42,79]. Their well-known production of toxins [36,94,99,115] may also be a grazer-deterrent.

As a group [133] they are favoured by microaerophilic conditions such as are found overnight and close to the sediment in highly enriched conditions. (They evolved during the anaerobic Precambrian period, over two billion years ago and appear to retain many characteristics which would have been adaptive then). They are also associated with low N to P ratios (favoured by sediment release of phosphates in shallow lakes), availability of ammonium rather than nitrate), low CO₂ concentrations (often established in conditions of high algal production by uptake by the algae themselves), heavy grazing (which removes other, smaller, algal competitors), low washout rates and high temperatures. Most of these conditions are established by late summer at high nutrient loadings.

Blue-green algae are not totally immune from grazing, however, and at least the coccoid (spherical) forms, as opposed to the elongate, filamentous species, may be grazed alongside other more palatable algae. Blue-green growths, once established, are largely invulnerable to grazing [59,113] but their establishment may be prevented if grazing begins early enough on a small inoculum, which is thus prevented from developing.

Fig 16 Changes in algal communities in relation to total phosphorus concentrations. Blue green algae do not necessarily dominate at the highest nutrient concentrations; green algae often do. From Jensen et al [85]

If there is a unique phytoplankton-dominated state there should be some effects on it of nutrient reduction . However, these will generally be changes in the nature of the algal community and will move the system back to the range in which the alternative states can occur. They will not per se re-establish clear water and conditions for plant growth unless they are so stringent that they return conditions to those which uniquely support a plant-dominated state. This will mean re-establishing total phosphorus concentrations of perhaps less than 25µg l-1 and will usually be impossible to achieve, at least in catchments with much agriculture.

Despite these arguments, and the existence, admittedly under fairly extreme conditions, of plant communities in little-diluted sewage effluent at Little Mere (see Box 3) [10,34], nutrient concentrations may be important even within the range in which the alternative states can occur. The argument is that although switch mechanisms are required to move the system between states, the ease of switching is affected by nutrient loading. The higher the load, the more readily the plant-dominated system will switch to an algal-dominated one and vice versa.

Evidence for this is largely from case studies of restoration, where attempts to reverse a loss of plants have been unstable unless nutrients have been significantly reduced, for example at L. Sobygaard in Denmark [86,88,90], and from observations that dramatic loss of plants is much more likely (though not inevitable) at high nutrient loads. Nutrient-independent switches are involved but in some cases, e.g. in the stocking of common carp, (see below) the switch mechanisms themselves can influence the distribution of nutrients by mobilizing supplies from the sediment and hence the outcome of switch operation. Nutrient control is thus likely to be necessary for restoration of reasonably stable plant-dominated states but it is not yet certain whether this should be control of phosphorus, nitrogen or both.