Bennion, H; Goldsmith, B; Yang, H; Goodrich, S; (2016) A palaeolimnological study of Loch Shin, Scotland. Final report to SEPA and the Forestry Commission. (ECRC Research Report 171 ). UCL Environmental Change Research Centre: London, UK.

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Abstract

This project reports on analysis of a sediment core collected from the lower basin of Loch Shin in Sutherland, Scotland on 24 August 2015, spanning a total sediment depth of 31.5 cm. Palaeoecological techniques, principally diatom analysis of a dated core, were employed to assess environmental change at the loch since 1850 AD. The project aims to determine the reference conditions of the loch and to establish the onset, rate and magnitude of any changes in the diatom flora. The Scottish Environment Protection Agency are particularly interested in any changes in the loch that may have occurred in association with fish farming and forestry practices and whether the hydropower scheme has had any impact. Radiometric methods were applied to the core in order to date the recent sediments, revealing that the 31.5 cm core dates back to ~1850 AD. There is an increase in sedimentation rates before the 1920s, followed by small changes around a mean value of 0.026 g cm-2 yr-1 until the 2000s, after which the sedimentation rates increase slightly again in recent years to 0.038 g cm-2 yr-1. Diatom analysis was carried out on 21 samples distributed throughout the core and a diatom-total phosphorus (TP) transfer function was applied to the diatom data to reconstruct trophic status. There were marked changes in the diatom assemblages during the 160 year period represented by the core and cluster analysis revealed four major zones with the most significant split, reflecting floristic change, at 7.75 cm which corresponds to 1994. In the period from 1860 to 1930 (Zone 1), the sedimentary diatom assemblages of Loch Shin were very stable with a diatom flora typical of nutrient-poor, deep lakes and the diatom-inferred TP concentrations were low and stable at 5 μg L−1 indicating oligotrophic conditions. The diatom reference conditions of Loch Shin can thus be described as a community of oligotrophic, acidophilous-circumneutral taxa, particularly Cyclotella kuetzingiana, Achnanthidium minutissimum and Brachysira vitrea. The diatom assemblages began to change from ~1930 with the arrival of taxa more typically associated with mesotrophic waters, namely Aulacoseira subarctica and Asterionella formosa, marking an initial enrichment phase (Zone 2). The progressive and gradual nature of the shifts suggests a response to the cumulative effect of increasing pressures in the catchment during the mid to late twentieth century including dam construction and the consequent water level rise in the 1950s, as well as forestry plantation and fertilisation which took place from the 1960s. A more pronounced enrichment phase was evident from the mid-1990s with the expansion of Aulacoseira subarctica, the arrival of Fragilaria crotonensis and the decline, and in some cases the loss, of numerous taxa seen in the early part of the record (Zone 3). The diatom-inferred TP concentrations increased to 16 μg L−1. While the exact causes of these shifts cannot be established, the most marked changes in the diatoms are coincident with the arrival of the fish farms on the loch in 1994-1995 suggesting that aquaculture may have played a role. Since 2009 Fragilaria crotonensis has disappeared although the reasons for this are not clear (Zone 4). A. subarctica remains the dominant species and the planktonic component of the assemblages remains high at 50-60%. The diatom-inferred TP concentrations decreased to 11 μg L-1 at the core surface, indicating that there may be a recent phase of reduction in trophic status. However, given that there are no reported changes in land use or management since 2009 and that diatoms respond to a host of factors in addition to nutrient concentrations, it would be unwise to suggest that the disappearance of F. crotonensis from the recent sediment record is reflective of improving water quality. Clearly a more detailed study of recent management practices in the catchment and fish farms is required to establish whether there have been any real reductions in nutrient loads to the loch.

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