Estuarine oxygen and regional meteorological conditions were studied for 24 Massachusetts estuaries to determine regional influences of light on oxygen dynamics. During July and August 2006, near-bottom dissolved oxygen (DO), temperature, salinity, chlorophyll-a, tide stage, and photosynthetically active radiation (PAR) were measured. Relative low or high DO extrema occurred simultaneously across multiple estuaries despite differences in total oxygen levels. July and August had a total of 19 days with low DO in more than 25 % of sites, including 8 days with low DO in more than 50 % of sites. PAR averaged across nine monitoring locations accounted for a significant fraction of regional DO variance. Correlations between regional DO and regional light were greatest for a 1-day lag in light. Low DO occurred more frequently during low light conditions (PAR < 400 μE m −2 s −1 ), and high DO during high light conditions (PAR > 400 μE m −2 s −1 ). Precipitation did not appear to be a direct driver of DO during this timeframe. Comparison of chlorophyll-a with DO showed a negative correlation, indicating that respiration outpaced photosynthesis. These results suggest that local factors create the potential for hypoxia, but that regional light levels provide a key trigger.