In shallow lakes the contemporary view is that two alternative stable states can exist over a range of nutrient concentrations: a clear water state dominated by submerged macrophytes; and a turbid phytoplankton-dominated state (Blindow et al., 1993; Scheffer et al., 1993; Timms and Moss, 1984; Hosper, 1989; Irvine et al., 1989). At either end of the nutrient continuum one of these alternatives will exist but for a wide range of nutrient concentrations (<50 to several thousand µgl^-1 Total Phosphorus) either of these two states can exist, with each state stabilised by a number of buffer mechanisms (Moss and Leah, 1982; Timms and Moss, 1984; Moss, 1990; Scheffer et al., 1993; Scheffer, 1990).

Macrophyte buffering mechanisms that help maintain a clear water state are numerous. Macrophytes act as habitats and refugia for macro-invertebrates and cladocerans that reduce the epiphyton and phytoplankton communities by grazing (Carpenter and Lodge, 1986; Timms and Moss, 1984). Davis (1975) suggested that the plant bed environment, often de-oxygenated, favours grazers by discouraging their predators. Macrophytes may also release allelopathic chemicals (Wium-Andersen et al., 1982), partake in the "luxury uptake" of nutrients, removing available nutrients for phytoplankton (Phillips et al., 1978; Van Donk et al., 1989; Ozimek et al., 1993) and reduce available nitrate by anaerobic decomposition processes such as de-nitrification (Takii and Fukui, 1996). Moreover certain species of macrophytes can oxidise the sediment and reduce the release of phosphorus; with less available phosphorus in the water column phytoplankton populations are reduced (Carpenter et al., 1983; Van Donk et al., 1990). In addition stands of macrophytes reduce water movement within them; in consequence suspended sediment re-settles and turbidity falls aiding the growth of macrophytes (Schiemer and Prosser, 1976). Moss (1990) suggested that even if light penetration is falling, submerged macrophytes may switch from low growing forms to taller species and that plants heavily encrusted with epiphyton may be able to shed their leaves followed by replacement of new growth. All these processes competitively disadvantage phytoplankton and may buffer the macrophyte-dominated state.

Once established, on the other hand, phytoplankton can buffer the switch back to a macrophyte state by growing much earlier in the season than macrophytes in temperate regions. In doing so they can curtail the development of turions, rhizomes or seeds by shading in spring or reduce the formation of propagules by shading and competition for CO₂ in late summer (Moss, 1990). Submerged plants require carbon for growth but algae have shorter CO₂ and HCO₃ diffusion pathways owing to their small size thus removing the carbon available to the bulkier macrophytes (Simpson and Eaton, 1986). Van Vierssen and Prins, (1985) suggested that certain blue-green algae may even release chemicals toxic to macrophytes. In addition, phytoplankton-dominated open water has few refugia for grazing Cladocera, thus any Cladocera venturing forth are typically removed by zooplanktivorous fish. This may be compounded by Timms' and Moss' (1984) suggestion that, with reduced macrophytes, few large macro-invertebrates would be available to large fish, favouring smaller-sized fish feeding largely on zooplankton. Finally, in poorly flushed systems, phytoplankton species can consist of large filamentous or colonial blue-green algal species that are inedible to zooplankton. This and the above mechanisms all help to maintain phytoplankton-dominated water states over a wide range of nutrient concentrations.

It is clear that once macrophytes are removed and a turbid phytoplankton-dominated state is achieved, the shift back to a clear water macrophyte state is difficult to accomplish. At present English Nature (EN), together with the Environment Agency (EA), give permission to landowners and occupiers to stock SSSI freshwaters with fish. Unfortunately, without a full understanding of the effect that these stocking regimes and practices have on the lake ecosystem English Nature have had to make many decisions on the basis of limited information. It is imperative to understand these complex interactions and this experiment was designed to understand how macrophyte decline may be affected by:

bottom-up effects of fish in disturbing sediment that shades out plants (Meijer et al.,1990) and nutrient release from disturbed sediment (Miller et al., 1961) or via fish excretion (Andersson et al., 1988) that aids the growth of phytoplankton (Bruekelaar et al., 1994) and epiphytes (Phillips et al., 1978).

top-down effects of different species and densities of fish in removing zooplankton (Timms and Moss, 1984; Irvine et al., 1989) and macro-invertebrate grazers (Bronmark and Weisner, 1992).

Eventually it is hoped that a complete understanding of fish species, size and density interactions can be developed to provide individual management plans for stocking of SSSIs. Any plan must mitigate between maximum usage and profit and detrimental effects to the aquatic ecosystem.

For the most up to date results and data please look at my published papers found on my CV page.