Loch Lomond is divisible into two natural regions, a northern, deep, steep-sided trough and a southern, relatively shallow expanse. These are separated by what is, in many respects, a transitional middle region. This topographical diversity has engendered widely different environments for the benthic fauna, especially for the profundal fauna.
A survey of selected sites in each region has shown a distribution of species in accordance with the individuality of these major divisions of the lake.
Chironomid midge larvae and the bivalve mollusc, Pisidium, each have dominant species in different regions. Of the former, Sergentia coracinus characterizes the northern and species of Tanytarsus the southern region; both being absent from the transitional region where Chironomidæ of any species are few in number.
Pisidium conventus predominates in the deep cold waters of the northern and middle regions and is replaced by other species, chiefly P. lilljeborgi, in the southern.
Regional differentiation of oligochaete worms appears to be less clearly defined but with Limnodrilus sp. avoiding the deep water and Stylodrilus heringianus favouring it.
Despite marked differences in proportions of the component species, tentative estimates of biomass of the profundal benthic animals show a tendency towards equality in all regions.