An evaluation of methods for sampling macrophyte maximum colonisation depth in Loch Leven, Scotland

The maximum colonisation depth (MCD) of macrophytes in lakes is considered a robust ecological indicator of system health. The accuracy of five boat-based survey methods (double-headed rake, Ekman grab, bathyscope, hydroacoustics, and underwater photography) was evaluated along depth-transects within a shallow eutrophic loch (Loch Leven, Scotland, UK) in 2006. Data from an earlier field campaign conducted in 1993 (33 transects) were used to assess sample frequency and error associated with the use of the double-headed rake method. The double-headed rake consistently sampled more species than the Ekman grab or bathyscope, probably as a result of its ability to sample relatively larger areas of the lake bed in comparison to the other methods. The bathyscope method underestimated the MCD in comparison to all other methods. This appeared to be because the turbidity of the water-column restricted the depth of view when using the bathyscope. The double-headed rake and underwater photography methods were in good agreement, with maximum observed MCD estimates of 5.0 m and 4.5 m water depth, respectively (2006). Using the 1993 data (MCD observations taken from 33 selected transects), we estimated that over 5 transects were required to ensure a MCD estimate greater than the 75th percentile from the distribution of all 33 colonisation depths, more than 15 replicate transects were required to ensure an estimate greater than the 90th percentile, and more than 20 replicate transects were required to ensure an estimate greater than the 95th percentile.