The concentrations of the inorganic nutrients, chlorophyll a, phaeopigment, dissolved organic nitrogen and phosphorus, and Gelbstoff were studied in Loch Creran throughout the year of 1971. The surface concentrations (1 m) of phosphate, nitrate, silicate, ammonium, and nitrite during winter and autumn, ranged between 0.25–0.58 μg at. PO 4-P l , 2.6–7.2 μg at. NO 3-N l , 7.9–12.6 μg at. SiO 4-Si l , 0.14–0.53 μg at. NH 4-N l , and 0.01–0.22 μg at. NO 2-N l , respectively. In the summer months, the values of phosphate, nitrate and silicate were between < 0.03–0.24 μg at. PO 4-P l , < 0.05–0.50 μg at. NO 3-N l , and 0.43–6.80 μg at. SiO 4-Si l . Ammonium and nitrite at the surface (1 m) were usually below the limit of detection. The shortage of nitrogen compounds might have limited the size of the phytoplankton crop during the summer months. At the bottom layers (40–45 m, and 15–20 m) the levels of phosphate, nitrate, and silicate during winter and autumn were between 0.38–0.76 μg at PO 4-P l , 3.80–7.70 μg at. NO 3-N l , and 7.00–13.40 μg at. SiO 4-Si l , respectively. In the summer months, the bottom values (40–45 m, and 15–20 m) of phosphate, nitrate, and silicate fluctuated between 0.10–0.34 μg at. PO 4-P l , 0.14–2.70 μg at. NO 3-N l , and 1.20–4.60 μg at. SiO 4-Si l , respectively. The levels of ammonium and nitrite at 40–45 m, and 15–20 m during the year ranged between 0.14–2.0 μg at. NH 4-N l , and 0.06–0.60 μg at. NO 2-N l . The lowest values of both nutrients were found in winter and the highest in autumn. The ratios of nitrate/ phosphate, silicate/phosphate, and silicate/nitrate at the surface (1 m) were influenced mainly by biological utilization and river run-off, and at the bottom (40–45 m. and 15–20 m) by the release of nutrients from the sediment, and water circulation. The concentrations of chlorophyll a and phaeopigment at 1, 5, and 10 m during the year, ranged between 0.07–10.80 μg/l, and 0.06–7.30 μg/l, respectively. The lowest and the highest values were found in the winter and spring bloom, respectively. During the spring, increase of phytoplankton growth, the percentage of phaeopigment rose from a maximum of 18% in the early stages of the bloom to a maximum of 60% in the decline. The high percentage of phaeopigment and ammonium, associated with low levels of chlorophyll a, suggest that zooplankton grazing contributed largely to the decline of the spring bloom of 1971. The surface concentrations (1 m) of dissolved organic nitrogen and phosphorus throughout the year, ranged between 3.5–8.6 μg at. NO 3-N l , and 0.12–0.50 μg at. PO 4-P l , respectively. At the bottom of Station 3 (40–45 m) and Station 5 (15–20 m) the values were between 3.31–7.0 μg at. NO 3-N l for the dissolved organic nitrogen, and 0.08–0.22 μg at. PO 4-P l for the dissolved organic phosphorus. The highest values of both entities were recorded in the summer months when bottom temperatures (40–45 m, and 15–20 m) were between 10.0–13.8°C. The surface levels of Gelbstoff (1 m) during the year were markedly affected by the volume of run-off. The surface optical density of the water (1 m) fluctuated between 0.320–0.176 at 320 nm, and 0.004–0.050 at 400 nm. In the bottom layers (40–45 m, and 15–20 m) the values were between 0.035–0.094 at 320 nm, and < 0.004–0.022 at 400 nm. During winter and autumn, the optical density of the bottom water (40–45 m, and 15–20 m) at 320 nm followed the same pattern of phosphate. In the summer, the low ratios of the optical density at 320 nm/400 nm were accompanied with high levels of ammonium and phosphate. The preceding findings may be of ecological importance in the loch, with regards to phytoplankton nutrition.