A Field and Laboratory Study of Oxygen Uptake by Planktonic Blue-Green Algae

Lough Neagh, Northern Ireland (540 36' N, 6? 30' W) is a shallow eutrophic lake whose phytoplankton is dominated by blue-green algae and, in the spring, by diatoms (Gibson et al. 1971). Since the largest axis of the lake coincides with the prevailing wind and since the mean wind speed across the lake is 6 18 m s-' (12 knots) (Millway 1970), the lake is well mixed for most of the year. As a consequence, there is no lasting hypolimnion which might de-oxygenate. Nevertheless, under calm conditions oxygen and temperature gradients do occur and the oxygen demand of the water is such that even short periods of stagnation can produce considerable de-oxygenation in the lower part of the water column. A study was made of the oxygen demand of the lake water to elucidate the conditions bringing about de-oxygenation of the water column (C. E. Gibson, in preparation), since this event is likely to be of great significance in the ecology of the lake. This paper reports on the annual cycle of oxygen demand in Lough Neagh with particular reference to its distribution through the water column. It shows that oxygen demand varies not only with the time of the year, but also with the light history of the phytoplankton. Laboratory experiments with a blue-green alga common in the plankton further elucidate the factors affecting the oxygen demand. Descriptive details of Lough Neagh are given by Gibson et al. (1971), Wood & Gibson (1973), and Gibson & Stewart (1973).