 the layer of water that lies below the thermocline, is noncirculating, and remains perpetually cold) were lower in 1990–1991 for a longer period than in 1960–1961, with values of less than 1 mg per litre (< 0.4 gr/cu ft) occurring in water as shallow as compared with a shallowest occurrence of greater than in 1961.The changes in oxygenation are considered consistent with measurements of higher algal biomass and productivity.These changes have arisen for multiple reasons