Establishing realistic restoration targets for nutrient-enriched shallow lakes: linking diatom ecology and palaeoecology at the Attenborough Ponds, U.K.

Abstract

Eutrophication is a major problem for shallow lakes in the U.K. lowlands. Over the last few decades issues relating to the management and restoration of these lakes have come to the fore, including the need to develop methods for establishing realistic restoration targets. One group of lakes for which restoration is a key concern are the Attenborough Ponds in the English Midlands, an interconnected series of shallow, nutrient-enriched gravel-pit lakes. In November 1972, the highly polluted River Erewash was diverted into the Attenborough Ponds. To determine the ecological effects of this event and to establish restoration goals for this lake system, two complimentary studies were undertaken here; (i) monitoring of the ecology and sedimentary representation of diatom communities in a disturbed lake connected to the R. Erewash (Tween Pond: high nutrient concentrations, no submerged macrophytes) and a relatively undisturbed lake isolated from the R. Erewash (Clifton Pond: lower nutrient concentrations, abundant submerged macrophytes), and; (ii) examination of the sedimentary diatom record in a short sediment core collected from the chosen disturbed lake (Tween Pond). The species composition, seasonality, relative productivity and sedimentary representation of planktonic and periphytic diatom communities were very different in Tween and Clifton Ponds. In Tween Pond two major phases of planktonic diatom production/sedimentation were observed during March–May and July–September. By contrast, planktonic diatoms were restricted to a single spring peak in Clifton Pond and after May there was a switch to the dominance of periphytic diatoms associated with the development of submerged macrophytes. The 1972 diversion of the River Erewash was clearly reflected in the sedimentary diatom record from Tween Pond, by; (i) the abrupt shift to planktonic diatom dominance, and; (ii) increasing percentages of late summer–autumn associated planktonic diatom species. These changes suggest both significant nutrient enrichment and the switch from submerged macrophyte to phytoplankton dominance. Numerical matching of pre-1972 diatom assemblages with surface sediment assemblages in the gravel-pits using Principal Components Analysis (PCA) and a squared chord distance measure revealed no close analogues. Nevertheless, similarities between; (i) percentages of different diatom habitat and seasonality groups, and; (ii) pre-1972 and contemporary macrophyte survey data suggested that Clifton Pond is probably a good pre-diversion analogue for habitat structure (e.g. macrophyte biomass, composition and architecture) and phenology (e.g. diatom and macrophyte seasonality) in the Ponds. The practical value of combining space-for-time substitution and palaeoecological approaches in restoration ecology studies of shallow lakes is discussed.