A filamentous green algae-dominated temperate shallow lake: Variations on the theme of clear-water stable states?

Abstract

Shallow lakes can have two alternative stable states over wide range of nutrient concentrations: a macrophyte-dominated clear-water state and a phytoplankton-dominated turbid-water state. Little Budworth Pool, Cheshire, UK is a small (surface area 3 ha), shallow (max. depth < 1m), clear-water lake with very high nitrate concentrations (annual mean 9.2 mg NO 3 -N 1 -1 ), relatively low phytoplankton growth (annual mean 13.5 μg chlorophyll-a 1 -1 ) and high metaphytic filamentous green algal abundance (FGA, mainly Spirogyra), but without any submerged plant-beds. To understand the mechanism limiting phytoplankton biomass in summer, the lake was sampled every two weeks for two years (November 2001-October 2003) for water chemistry, phytoplankton, zooplankton, benthic macroinvertebrates, FGA and benthic diatoms. Phytoplankton growth was mainly dominated by small species like Cyclotella spp., Chroomonas acuta and Cryptomonas spp., whereas among the zooplankton Cladocera showed high abundance of both small and large species (Bosmina longirostris and Daphnia longispina) especially in summer and autumn. FGA attained highest biomass in the warmer months and showed negative correlations with nitrate-N and total nitrogen concentrations suggesting nutrient removal ability of these plants, but also possible increased denitrification in summer. While phytoplankton growth was negatively correlated with FGA biomass, cladoceran densities were positively related to the latter. A comparison among macrophyte-dominated lakes and FGA-dominated lakes showed similarities among them regarding water clarity, nutrient concentrations, phytoplankton growth and species composition, zooplankton assemblage and abundance, but not for abundance of benthic macroinvertebrates. We suggest that in a lake lacking submerged plants, abundance of FGA may act as a buffer against a forward switch to phytoplankton dominance, thus maintaining a clear-water state. The control of phytoplankton growth was primarily not through FGA removing nutrients from the water because sufficient available nutrients were recorded in water during summer. Rather it was by intense grazing by cladocerans ostensibly supported in refuges provided by FGA.