EFFECTS OF TIDAL STAGE AND SUN ANGLES ON INTERTIDAL BENTHIC MICROALGAL PRODUCTIVITY

Motile benthic diatoms exhibit rhythmic vertical migrations that are influenced by tidal and light cycles. As a consequence of these periodic migrations, corresponding periodicities in benthic microalgal production should occur. Using oxygen microelectrodes, hourly measurements of microalgal production were obtained from subaerially exposed cores collected from low-intertidal muddy sediments in North Inlet estuary, South Carolina, USA. Microalgal productivity at low tide was twice that at high tide (mean difference 52%) and was significantly correlated with diurnal and tidal periodicities (r2 = 0.41; p < 0.0001). Production values ranged from 28.0 to 460.5-mu-mol O2 mg chl a-1 h-1 and maximum rates were achieved during mid-afternoon low tides. A curvilinear regression equation was constructed to simulate daily and monthly benthic microalgal production based on tidal and light cycles. Comparisons between predictions of the curvilinear equation and published data sets showed a reasonable agreement (r2 = 0.77), suggesting similar phenomena in other estuaries. Current benthic microalgal production models do not account for hourly variability in productivity, leading to potentially large errors when measurements are extrapolated over monthly and annual time scales. Although other physiological and abiotic factors also influence benthic microalgal productivity, much of the short-term variability in production rates may be simply attributed to migratory rhythms within estuarine sediments.