From 1963 through 1969, the U.S. Geological Survey operated a water-quality monitor system in the upper-middle Patuxent estuary, near Benedict, Maryland. Over the period of record, a pronounced change occurred in the diel (24 hr.) oxygen measurement, particularly during the months of July, August, and September. Annual variations of dissolved oxygen ranged from 3.6 to 15.0 mg liter⁻¹ in 1964 and from 2.3 to 16.5 mg liter⁻¹ in 1969 with percentage saturation varying from 60 to 130 percent in 1964 and from 30 to 184 percent in 1969. The magnitude of diel summer oxygen variation changed from about 3 to 7 mg liter⁻¹ day⁻¹ over the same period.

From hourly values of temperature, oxygen, and conductivity, gross primary production (GP) and respiration (R) were estimated. Data representing greatest daily change for spring, summer, and autumn of each year and “average” values for each month of years 1964 and 1969 were analyzed for GP and R. From the seasonal data, GP ranged from about 3 to 25 g O₂ m⁻² day⁻¹, and R ranged from about 4 to 32 g O₂m⁻² day⁻¹. From the “average” days, GP ranged from 2 to 10 g O₂m⁻² day⁻¹ in 1964 and from 1 to 16 in 1969. In 1964, daily average GP was 5.2 g O₂m⁻² day⁻¹ as compared to 6.1 in 1969, and respiration was 6.1 g O₂m⁻² day⁻¹ in 1964 versus 10.2 g O₂m⁻² day⁻¹ in 1969, increases of 20 and 16 percent, respectively. Ratios of GP/R averaged 0.84 in 1964 versus 0.60 in 1969, reflecting a larger respiration in 1969 and suggesting a trend towards community instability.

Productivity estimates by other investigators showed a two-to threefold increase in production over the same period of time. The increases in production were attributed to upstream domestic waste loading. If the present trend increases, metabolic imbalance and anaerobic conditions may develop.