Limnological observations of an intermittent tropical dry forest stream

This study examines aspects of the seasonal limnology of an intermittent stream in the dry forest region of northwest Costa Rica. It focuses on annual water level fluctuations and both seasonal and among pool variation in dissolved oxygen concentration and water temperature. Dry season pools differed in morphometry and the rate of decline in water levels subsequent to the seasonal floods. Rate of water level decline was related both to pool depth and to the exposure of the pool to the sun. Oxygen concentration was generally low in the dry season, but increased during the rainy season in association with rain events and seasonal flooding. A repeated-measures analysis of variance indicated that seasonal effects accounted for 40% of the variance in oxygen concentration for residual pools. Differences among pools were also significant. However, there was strong evidence for a significant interaction between seasonal and spatial influences on oxygen values in the system. Variation in water temperature was small, but differences among pools, sampling dates, and their interaction were all significant. Periphyton production increased significantly between the late wet season sample in November and the dry season sample in February. Incident light intensity explained 76% of the variation among pools in net periphyton production in the dry season.