Understanding the ecological response of marl lakes to enrichment: a combined limnological and palaeolimnological approach

Marl lakes have been designated a priority habitat within the EU Habitats Directive for their nutrient-poor waters with benthic vegetation of charophytes (stoneworts). However, it is thought that the ecological status of several English marl lakes, including Semerwater and Malham Tarn, has been adversely affected by eutrophication including an increase in algae and a decline in the diversity of the submerged plant community. In the absence of long-term records the nature of these changes are poorly understood and hence the information needed to successfully conserve and manage these unique waters is lacking. This project seeks to combine contemporary limnological and biological investigation with palaeoecological analysis to elucidate the nature, timescales and magnitude of changes in ecological and limnological processes involved in the response of marl lakes to nutrient enrichment. The study will address three main hypotheses: 1. In the absence of enrichment, marl lakes exist in a stable, clear water state, dominated by a species-rich community of charophytes 2. In response to eutrophication, marl lakes exhibit a gradual (>100 years) transition from macrophyte to phytoplankton dominance. A decline in charophyte species richness leads to an encroachment of elodeid macrophytes, characterised by sub-decadal oscillations between charophyte and elodeid dominance. These changes occur in conjunction with a reduction in marl precipitation. Finally, submerged macrophytes are displaced by phytoplankton populations concomitant with the total cessation of carbonate precipitation. 3. On a shorter timescale, reductions in plant species richness result in a progressive decrease in the seasonal duration of plant cover with associated alterations in zooplankton, invertebrate and fish populations. It is expected that the main findings of the work will be incorporated into management plans for the three study lakes, specifically assisting Natural England in setting conservation objectives for marl lakes as required by the EU Habitats Directive, and providing information to the Environment Agency on reference conditions and ecological data as required by the EU Water Framework Directive.