This study investigated the ecology of subtidal epipelic diatoms in a sea-water loch. A methodology for the sampling and measurement of subtidal epipelic algae was established. The spatial heterogeneity and seasonal distribution of epipelon was investigated and related to physicochemical (salinity, temperature, light) and biological parameters (grazing and disturbance). The relationships between the epipelon and invertebrates were examined by survey and field experiments. The epipelic flora of Loch Goil consisted almost entirely of diatoms (163 spp, 39 genera) and was characterised by a few numerically dominant species (Navicula cf. tripunctata, Naviculasp. H, Navicular forcipata var. densistriata; Amphora proteus). The proportion of these species remained relatively constant with depth and season. The abundance of epipelic diatoms was found to be related to microtopography with relatively high densities concentrated on thesides and bases of sedimentary mounds formed by the polychaetes Arenicola marina and Eupolymnia nebulosa. Analysis of the pattern using a 'paired-Quadrat Variance' method showed clumping at intervals of 4cm, 2m and 3m. The bathymetric (6-40m) distribution of epipelic diatoms was investigated and found to be divided into shallow (6m, 12m, 15m) and deep (20m and 30m) water zones, with differences in the density of algae and composition of the rarer species along the depth gradient. The seasonal variability of the epipelon was determined from 1988 to 1990 at 9m and 11m depth. Maximum epipelic growth was recorded from May to September followed by an over-winter period (October to April) of minimum growth. The seasonal distribution of epipelic abundance and chlorophyll-a was found to be correlated with annual cycles of water temperature, irradiance and water clarity.