Existence of a macrophyte-dominated clear water state over a very wide range of nutrient concentrations in a small shallow lake

Abstract

Little Mere, a small shallow lake, has been monitored for four years, since its main source of nutrients (sewage effluent) was diverted . The lake has provided strong evidence for the persistence of a clear water state over a wide range of nutrient concentrations. It had clear water at extremely high nutrient concentrations prior to effluent diversion, associated with high densities of the large body-sized grazer, Daphnia magna, associated with low fish densities and fish predation . Following sewage effluent diversion in 1991, the nutrient concentrations significantly declined, the oxygen concentrations rose, and fish predation increased . The dominance of large body-sized grazers shifted to one of relatively smaller body-sized animals but the clear water state has been maintained . This is probably due to provision of refuges for grazers by large nymphaeid stands (also found prior to diversion) . There has been a continued decrease in nutrient concentrations and expansion of the total macrophyte coverage, largely by submerged plants, following effluent diversion . The grazer community of Little Mere has also responded to this latter change with a decline in daphnids and increase in densities of weed-associated grazers . The presence of large densities of such open water grazers was the apparent main buffer mechanisms of the clear water state until 1994 . The lake has, so far, maintained its clear water in the absence of such grazers . Thus, new buffer mechanisms appear to operate to stabilize the ecosystem . Little Mere appears to have shifted from previous top-down controlled clear water state to a bottom-up controlled clear water state .

Frequent regime shifts in trophic states in shallow lakes on the Boreal Plain: Alternative "unstable" states?

“…Thus, the concentrations of TP associated with a shift in alternative state in many shallow lakes (50-150 mg TP L 21 ; Scheffer 1998) are appropriate only for lakes with little or lower amounts of SAV. Once a substantial SAV cover has become established, lakes can tolerate much higher concentrations of TP without shifting to a turbid state, as has been noted by others (Beklioglu and Moss 1996;Bachmann et al 2002).…”

Ecological regime shifts in salinised wetland systems. II. Factors affecting the dominance of benthic microbial communities

“…The existence of positive feedbacks for macrophyte and phytoplankton dominance in permanent waters have been well-documented (e.g. Scheffer, 1990;Beklioglu & Moss, 1996), and a role for benthic microbial communities as the homeostatic regulators of permanent wetlands has also been proposed (Burke & Knott, 1997). The role of water permanence in ecosystem dynamics requires further investigation.…”

Swimming restricted foraging behavior of two zooplanktivorous fishes Pseudorasbora parva and Rasbora daniconius (Cyprinidae) in a simulated structured environment

“…In littoral zones of aquatic systems, vegetation distribution patterns produce marked structural variation (Dionne and Folt 1991), and can provide refuge for prey communities by hindering predator-foraging activity (Beklioglu and Moss 1996). Aquatic macrophytes can contribute to an increase in fish abundance (Heck Jr and Thoman 1981), as well as in invertebrate abundance (Savino et al 1992).…”