A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemicalcycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhorn Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (> 560 Mu M O2: > 250% saturation) during sunny days to suboxic conditions (< 5 Mu M) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O2 concentrations ranged from 346 Mu M to sustained non-detectable levels in the night hours. In concert withthe DO fluctuations, diet phosphate cycling was on the order of 4 MuM in response to tidal flushing events and biological assimilation and regeneration. The I03- /I- redox couple quickly responded to suboxic conditions in the water column bya marked increase in I- concentrations and corresponding depletion of I03-. The extreme fluctuations of the pe in the water column resulted in diel dissolved Mn2+variations of nearly 5 Mu M, with observed dissolved Mn removal rates on the order of 1 Mu M h-1. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments. [ABSTRACT FROM AUTHOR]