Chemical investigations of Loch Etive,Scotland. II. Dissolved organic compounds

Variations in the levels of dissolved organic compounds in Loch Etive were studied during 1971. The amount of dissolved organic nitrogen, dissolved organic phosphorus, and Gelbstoff were markedly affected by the freshwater run-off. The surface (1 m) concentrations of the dissolved organic nitrogen and dissolved organic phosphorus throughout the year ranged from 3.0–9.0 μg-at. NO₃-N/l and 0.02-0.45 μg-at. PO₄-P/1, respectively. The surface levels of Gelbstoff during the year as measured by the optical density of the water at 320 nm and 400 nm fluctuated between 0.08-0.490 and 0.020-0.140, respectively. In the bottom layers of the Lower Basin (40–50 m) the optical density of the water at 320 nm and 400 nm throughout the year ranged between 0.060-0.150 and 0.010-0.047, in the Deep Basin (115–140 m) the values were between 0.086-0.130 and 0.018-0.037 and at the Head of the Loch, (25–35 m) they fluctuated between 0.080-0.229 and 0.017-0.052, respectively. The vertical concentrations of the dissolved organic nitrogen, dissolved organic phosphorus and Gelbstoff decreased irregularly from the surface downward. The concentrations of the dissolved organic nitrogen at the surface of the Lower Basin during the year were similar to those found at 40–50 m. In the Deep Basin and at the Head of the Loch the surface values were higher than those observed at 115–140 m and 25–35 m, respectively. The concentrations of the dissolved organic phosphorus at the surface of the Lower Basin were close to those found at the bottom (40–50 m). In the Deep Basin the surface values were higher than those at 115–140 m and at the Head of the Loch the surface concentrations were variably higher or lower than the bottom layers (25–35 m). The ratio of the optical density at 320 nm and 400 nm in the photic zone (1 m, 4–5 m, and 10 m) during the year ranged between 3.2–4.0 with the lowest ratios in winter and autumn. A similar pattern was observed in the water column of the Lower Basin and at the Head of the Loch. In the deeper strata of the Deep Basin, ratios were usually between 4.0–5.3 for most of the year. A comparison between the optical densities of the surface water during winter and autumn and that at 115–140 m (Deep Basin), suggests that the fraction more liable to precipitate is found in the visible region of the spectrum (400 nm).