Contribution of sediment focussing to heterogeneity of organic carbon and phosphorus burial in small lakes

Summary 1. Sediment distributions within lakes are typically heterogeneous. Much of our understanding comes from the study of large and/or shallow lakes, where wave mixing is the dominant sediment focussing mechanism. 2. We elucidated the heterogeneity of surface sediment distribution in a small lake, Esthwaite Water (U.K.). We considered multiple focussing mechanisms (downslope gravitational transport, waves, and wind-driven currents) and their effect on multiple sediment variables (water content, organic content, total phosphorus and benthic diatom taxon richness). In particular, we investigated the implications of different focussing processes for calculations of burial rates (BR) of organic carbon and total phosphorus. We used a new, high-resolution bathymetric survey of the lake in our calculations and compared the results to those from an earlier low-resolution bathymetric survey. 3. Wave-driven focussing and downslope gravitational transport were not significant in Esthwaite Water. However, calculated wind-induced current speeds were sufficient to mobilise small particles at all depths of the lake and therefore could potentially be an important resuspension process in small lakes. 4. We calculated that approximately half of the phosphorus entering the lake is retained in the sediments. This has important implications for the ability of the lake to recover from eutrophication because of the prolonged internal phosphorus loading capability. 5. Differences in calculated BRs because of sediment heterogeneity were much larger than those because of the difference in bathymetric resolution. Ignoring sediment heterogeneity when calculating flux-to-lake bed rates for organic carbon and total phosphorus can lead to large inaccuracies, with implications for BR and budget studies