The recovery of a very shallow eutrophic lake, 20 years after the control of effluent derived phosphorus

Abstract

1. Monitoring at fortnightly to monthly intervals of a very shallow, lowland lake over 24 years has enabled the time course of recovery from nutrient enrichment to be investigated after high external P loading of the lake (>10 g P m )2 year )1 ) was reduced between 1977 and 1980. 2. The lake showed a relatively rapid response during the spring and early summer, with a reduction in phytoplankton biomass occurring after 5 years when soluble reactive phosphorus concentration was <10 lg L )1 . 3. However, during the later summer the response was delayed for 15 years because of sustained remobilisation of phosphorus from the sediment. The greater water clarity in spring and a gradual shift from planktonic to benthic algal growth may be related to the reduction in internal loading after 15 years. 4. Changes in the phytoplankton community composition were also observed. Centric diatoms became less dominant in the spring, and the summer cyanobacteria populations originally dominated by non-heterocystous species (Limnothrix/Planktothrix spp.) almost disappeared. Heterocystous species (Anabaena spp. and Aphanizomenon flos-aquae) were slower to decline, but after 20 years the phytoplankton community was no longer dominated by cyanobacteria. 5. There were no substantial changes in food web structure following re-oligotrophication. Total zooplankton biomass decreased but body size of Daphnia hyalina, the largest zooplankton species in the lake, remained unchanged, suggesting that the fish population remained dominated by planktivorous species. 6. Macrophyte growth was still largely absent after 20 years, although during the spring water clarity may have become sufficient for macrophytes to re-establish.

Restoring macrophyte diversity in shallow temperate lakes: biotic versus abiotic constraints

“…These examples show that reducing external nutrient loading does result in a return of macrophytes and an increase in species richness 20-40 years after peak nutrient loading, when macrophytes had almost disappeared (Table 1). Macrophyte return is slow and there can be a delay in recovery, where macrophytes do not yet colonise even though transparency has increased in response to reduced nutrient loading, a phenomenon also observed in other studies of nutrient reduction (Jeppesen et al, 2005a, b;Phillips et al, 2005). Furthermore, whereas macrophytes returned and species richness improved after reduction of nutrient loading, a longer term comparison shows that the species richness and macrophyte community is different from the records about a century ago, from the early 1900s, which was the start of human-induced large-scale eutrophication (Table 1).…”

A model‐based assessment of non‐compliance of phosphorus standards for lakes in England and Wales

“…Of the limited ecological studies available, most have focused on responses of chlorophyll concentrations to nutrient load reductions. These case-studies (Carvalho and Kirika, 2003;Ferguson et al, 2008;Jeppesen et al, 2005;Moss et al, 2005;Phillips et al, 2005) all indicate strong ecological resilience to reduced in-lake phosphorus concentrations, with limited improvements observed in chlorophyll concentrations or algal blooms even following large reductions in both external loads and in-lake phosphorus concentrations. One of the reasons for ecological resilience is that even following reductions of in-lake phosphorus concentrations, phosphorus may still be in excess of plant and algal requirements and production may, therefore, be limited by other factors, such as nitrogen or light availability (Moss et al, 2005).…”

Long-Term Trends and Temporal Synchrony in Plankton Richness, Diversity and Biomass Driven by Re-Oligotrophication and Climate across 17 Danish Lakes

“…However, the response of plankton communities to nutrient reduction has sometimes been found to be weak or inconsistent in phytoplankton [27,32,33] and zooplankton [13,34,35]. Therefore, a joint effect of changes in nutrient concentrations and climate is more likely [16,36].…”