The benthic ecology of Loch Linnhe and Loch Eil, a sea-loch system on the west coast of Scotland. III. The effect on the benthic fauna of the introduction of pulp mill effluent

Abstract

A five year survey of the macrobenthic fauna of Lochs Linnhe and Eil on the west coast of Scotland was undertaken to assess the effects on the fauna of the discharge of effluent from a pulp and paper mill into the narrows between the two lochs. Effluent discharge began early in 1966, and the survey covered the two years prior to this date and the subsequent years. The characteristics of the effluent are described. Variations in the total organic carbon content of the bottom sediments show little change over the period in the carbon levels at the deep-water stations, but show rises in the amounts at the shallowwater stations adjacent to the effluent outfall. The total biomass of the fauna at six regular sampling stations showed increases of between 13 and 62% in the three-year post-effluent period when compared with the pre-effluent period; however, the largest increases were recorded 1–2 years after introduction of effluent and the later samples showed a return to the biomass levels existing during the pre-effluent period. Variations in the numbers of the major molluscan and echinoderm species are analysed. A general increase in the numbers of the molluscs Corbula gibba (Olivi), Nucula sp. and Myrtea spinifera (Montagu) and the echinoderm Amphiura chiajei Forbes took place in the period September 1966 to July 1968, and were followed by a decline in the later months of 1968. Corbula gibba showed the rise earliest at the station at the head of Loch Eil where it had its highest resident populations, followed successively by rises at the station in mid-loch Eil and mid-loch Linnhe, and in 1968 it extended its range to the station furthest down Loch Linnhe. The feeding ecology of these species is discussed and the possible effect of the effluent on larval and adult stages are considered. It is concluded that, although following introduction of the effluent the successful larval settlement of the species considered may have been enhanced, the fluctuations observed are within the known range of benthic population changes, so that it seems unlikely that the introduction of effluent into the system has yet had any marked positive or negative effect on the benthic populations in the survey area.