Tidal Water Quality Change

Scientists evaluate short- and long-term changes, or trends, in nutrients, dissolved oxygen (DO), Secchi depth (a measure of clarity), and chlorophyll-a using a Generalized Additive Modeling (GAM) approach. The approach includes selecting a GAM structure to describe nonlinear seasonally varying changes over time, incorporation of hydrologic variability via either river flow or salinity, the use of an intervention to accommodate method or laboratory changes suspected to impact data values, and representation of left- and interval-censored data (Murphy et al, 2019, 2021).

 

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 2020 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 2019-20)

Dissolved Oxygen (summer, bottom layer)

13%

65%

22%

21%

50%

38%

Secchi Depth (annual, surface layer)

18%

58%

23%

27%

57%

25%

Chlorophyll-a (spring, surface layer)

34%

58%

9%

28%

61%

7%

Total Nitrogen (annual, surface layer)

18%

62%

20%

46%

37%

14%

Total Phosphorus (annual, surface layer)

25%

55%

20%

43%

60%

14%

Long-term Trend (Period of Record)

Dissolved Oxygen (Summer, bottom layer)

21%

51%

27%

24%

44%

38%

Secchi Depth (annual, surface layer)

10%

22%

68%

18%

25%

59%

Chlorophyll-a (spring, surface layer)

22%

34%

45%

27%

45%

31%

Total Nitrogen (annual, surface layer)

73%

21%

6%

88%

12%

2%

Total Phosphorus (annual, surface layer)

71%

15%

14%

81%

22%

7%

†Note that two or three months of data at each station were missing in 2020 due to sampling restrictions, but an analysis of the potential impact indicates that these results were not greatly impacted.

Murphy, R, E. Perry, J. Keisman, J. Harcum, and E. Leppo. 2021. baytrends: Long Term Water Quality Trend Analysis. R package version 2.0.5.https://CRAN.R-project.org/package=baytrends.

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.