Estuary Trends

Scientists evaluate short- and long-term trends in nutrients, dissolved oxygen (DO), Secchi depth (a measure of clarity), and chlorophyll-a based on water quality samples taken 1-2 times per month since the 1980s at more than 130 stations located throughout the mainstem of the Chesapeake Bay and the tidal portions of numerous tributaries on the western and eastern shores using procedures described by Murphy et al. (2019).

Changes in observed conditions (i.e., the conditions experienced by the estuary’s living resources) are used to evaluate incremental progress towards improved habitats and attainment of water quality standards. Changes in flow-adjusted conditions account for year-to-year variations in streamflow or salinity and can be used for understanding the influence of watershed management actions on the estuary. The percent of stations improving, degrading, and showing no change using data collected through 2019 are summarized in the below table. Click here for further information about selected trends or click the “View Trends” button to plot customized maps of water quality concentrations and change.

Water Quality Variable

Observed Conditions

Flow-adjusted Conditions

Improving

No Change

Degrading

Improving

No Change

Degrading

Short-term Trend (2010-11 to 2018-19)

Dissolved Oxygen (summer, bottom layer)

22%

62%

16%

21%

56%

24%

Secchi Depth (annual, surface layer)

13%

58%

29%

27%

48%

25%

Chlorophyll-a (spring, surface layer)

26%

65%

9%

28%

63%

10%

Total Nitrogen (annual, surface layer)

16%

49%

35%

46%

40%

14%

Total Phosphorus (annual, surface layer)

29%

54%

17%

43%

49%

9%

Long-term Trend (Period of Record)

Dissolved Oxygen (Summer, bottom layer)

30%

47%

24%

24%

44%

32%

Secchi Depth (annual, surface layer)

12%

20%

68%

18%

25%

57%

Chlorophyll-a (spring, surface layer)

20%

38%

42%

27%

45%

28%

Total Nitrogen (annual, surface layer)

68%

18%

14%

88%

10%

1%

Total Phosphorus (annual, surface layer)

76%

13%

12%

81%

14%

5%

Murphy, R.R., E. Perry, J. Harcum, and J. Keisman. 2019. A generalized additive model approach to evaluating water quality: Chesapeake Bay case study. Environmental Modeling and Software 118(August 2019):1-13.