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

Eutrophication is a worldwide phenomenon affecting the ecology of water bodies, yet little is known of the timing, magnitude and characteristics of the resulting change in highly calcareous (marl) lakes. This project aimed to fill this knowledge gap through the combination of multi- proxy palaeolimnological analyses (last few hundred years) and comprehensive limnological monitoring (2009, 2010) of three English marl lakes, Cunswick Tarn, Hawes Water, and Mal- ham Tarn. The specific objectives were to characterise the centennial-scale eutrophication response of macrophyte, microalgal and invertebrate communities, and to assess ecological status and reference conditions for the sites. Eutrophication-driven declines in macrophyte colonisation depth, diversity, and evenness have occurred in all sites. While such changes were expected in the most eutrophic site, Cunswick Tarn, relatively marked changes also in Mal- ham Tarn and Hawes Water evidence high ecological sensitivity. Detailed palaeolimnological analyses of Cunswick Tarn revealed not only gradual, but also rapid and dramatic change and importantly synchroneity in response across biological groups. Shifts in community composi- tion closely followed changes in marl deposition with (i) substantial increases over a decade in the early 1900s characterised by increases in charophytes, molluscs and plant-associated cladocerans, indicating a persistence of benthic pathways and high water transparency (ii) decreases around the 1920s, accompanied and followed by progressive increases in floating- leaved macrophytes, cyanobacteria and pelagic cladocerans. Stable isotope analysis of fossil resting eggs of Daphnia in Cunswick Tarn sediment appeared to indicate key changes in the balance between benthic and pelagic production through changes in both carbon uptake and patterns of Daphnia abundance and seasonality. Although marl lakes may initially buffer eutrophication through increased benthic production, key ecological changes identified early in the eutrophication process suggest marl lakes are as susceptible to eutrophication as other lake types. Near-complete collapse of their characteristic ecology is possible with moderate levels of enrichment