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Water Quality Status and Trends

Water Quality Status and Trends Methodology

Maryland status and trends information is calculated from data collected as part of the Department of Natural Resources' Chesapeake Bay Water and Habitat Quality Monitoring Program. Water quality samples are collected once or twice a month (depending on season) from 55 tidal stations and 51 non-tidal stations in the Maryland tributaries. Status is a measure of current condition (most recent three years) at a station compared either to scientifically-based benchmark values or to a benchmark dataset. Based on this comparison, the station is given a ranking of "GOOD," "FAIR," or "POOR." Trends are a measure of how the system has been changing over time, either improving or degrading.

All of the data used in calculating the status and trends reported here are based on observed data (i.e., no flow-adjustment was made to the data).

Total nitrogen (N) levels used in the analysis are based on data from the surface-mixed layer (i.e., surface and above pycnocline samples, or surface only samples at shallow stations). Total nitrogen is calculated as:

(particulate N) + (total dissolved N)

Total phosphorus (P) levels used in the analysis are based on data from the surface-mixed layer (i.e., surface and above pycnocline samples, or surface only samples at shallow stations). Total phosphorus is calculated as:

(particulate P) + (total dissolved P)

Abundance of algae is estimated by measuring active chlorophyll a from surface mixed layer samples. Chlorophyll samples are taken but not analyzed for status.

Summer bottom dissolved oxygen is measured directly. The dissolved oxygen data used in this analysis are from samples taken during the months of June to September in bottom samples only. No dissolved oxygen measurements are taken in non-tidal or other shallow areas.

Total suspended solids (TSS) is measured directly from mixed-surface layer samples.

Water clarity is measured using Secchi disk depth.

For further information on parameters we measure please visit Our Monitoring Explained page.

Current Status

Status is a measure of current condition (most recent three years) at a station compared to a benchmark value or a benchmark dataset. Based on this comparison, the station is given a ranking of "GOOD," "FAIR," or "POOR."

Benchmark values based on concentrations needed by living resources are available for dissolved oxygen, but are not available for the other five parameters reported here (nitrogen, phosphorus, abundance of algae, water clarity, and total suspended solids). Because no scientifically accepted nutrient level goals are available for these five parameters, the most recent three years of data were compared to the benchmark dataset. The benchmark dataset includes the first six years of data (1985-1990) for that station and other stations throughout the entire bay that have similar salinity. Thus, status for these parameters is a relative measure, and "GOOD" levels are not necessarily levels needed to sustain healthy living resource populations. Specific details for status calculations follow.

Dissolved Oxygen

For dissolved oxygen, status is based on threshold levels needed by living resources and is set at the following concentration cutoff points:

GOOD: dissolved oxygen ≥ 5 mg/L
FAIR: 2 mg/L ≤ dissolved oxygen < 5 mg/L
POOR: dissolved oxygen < 2 mg/L

The Chesapeake Bay Program has established a target concentration for dissolved oxygen at 5 mg/L at all times throughout above-pycnocline waters (Jordan et al. 1992). Thus, summer bottom dissolved oxygen levels where the median for the most recent three-year period exceed 5 mg/L are rated as "GOOD." Hypoxia generally is defined as dissolved oxygen concentrations below 2 mg/L (Dauer et al. 1992). Thus, summer bottom dissolved oxygen levels less than 2 mg/L are rated as "POOR."

Total Nitrogen, Total Phosphorus, Chlorophyll A, Secchi Depth, and Total Suspended Solids

For total nitrogen, total phosphorus, chlorophyll a, and total suspended solids concentrations and Secchi depth, a Bay-wide scale was devised for each parameter by salinity zone (tidal fresh, oligohaline and mesohaline). This relative scale uses the distribution of all the data available Bay-wide (in cooperation with the Virginia Chesapeake Bay Monitoring Program) for the 1985-1990 period (the benchmark dataset) to establish a baseline against which to score water quality at each station.

Concentrations are measured in Maryland at up to four layers at deep stations with a pycnocline: surface, above pycnocline, below pycnocline, and bottom. A pycnocline is a layer in the water column where density changes rapidly between the warmer, fresher upper water, and the cooler, saltier, denser deep water. A strong pycnocline is common in deep areas of the Bay tributaries during the warmer months of the year. For this analysis, only surface and bottom measurements were used in determining the baseline dataset for total nitrogen, total phosphorus, and total suspended solids, because Virginia's data does not include above pycnocline and below pycnocline data. For chlorophyll a concentrations and secchi depth, only surface data were used in both the baseline dataset and the current status dataset.

A cumulative logistic function on the monthly medians from the benchmark data was used to establish the cut points that divide the benchmark dataset into three sections. Monthly medians from the current dataset (i.e., data for the last three years) at each station are then compared to these cutoff points, and rated as "GOOD," "FAIR," or "POOR." It is important to note that all of these ratings are relative to similar stations elsewhere in the Bay during the benchmark period (1985-1990), and that "GOOD" levels are not necessarily at levels needed to sustain healthy living resource populations.

Long-term trends

Trends are a measure of how the system has been changing over time, either improving or degrading. If the trend is not statistically significant at α = 0.05, “no trend” is reported. If the trend is significant, an arrow indicates the direction of the trend (increasing or decreasing concentration). The color of the arrow indicates whether it is a good or bad trend; green arrows indicate an improving trend, and red arrows indicate a degrading trend. The magnitude of the trend is expressed as the percent change since the beginning of the study period. Trends are measured over the most recent 10-years of data.

The Maryland Department of Natural Resources (DNR) analyzes long-term monitoring program data to address water quality monitoring objectives of the Chesapeake Bay Water Quality Monitoring Program. Longterm water quality datasets are analyzed to determine if trends are present; trends, either increasing or decreasing, are used to evaluate effectiveness and track progress of management actions to reduce nutrient and sediment pollution. Tidal water quality information is linked with other monitoring information (shallow-water monitoring, benthic monitoring, SAV monitoring, non-tidal monitoring) and examined in the context of the whole to gain a more comprehensive understanding of water quality processes and the relationship between water quality and living resources.

Trend: General Additive Models (GAMS) for linear and nonlinear trends:

Trend tests are conducted using an R statistical package developed by the Chesapeake Bay Program and partners. Starting in 2017, Seasonal Kendall trend analyses were no longer determined; trend tests are now completed using a Generalized Additive Model (GAMs) approach. The methods used in are described in Murphy and Perry 2017, available here.

Previous trends analysis testing determined that the GAM2 model is the chosen model, based on Akaike information criterion (AIC) score, in almost all cases for all parameters when interventions are not required.

GAM2: water quality = linear trend + nonlinear shape + seasonal cycle + interaction of seasonal cycle and date

wq = linear(cyear) + s(cyear) + s(doy) + ti(cyear,doy)

For more details on the field sampling, laboratory analysis, or statistical methods, visit the Monitoring News and Notes page to view our Quality Assurance Project Plan (QAPP).

Find out more details on the statistical methods used in analyzing status and trends, and the history of those methods over time.


Alden, R.W, III and E. Perry. 1997. Presenting Measurements of Status. Chesapeake Bay Program Data Analysis Work Group. USEPA, Chesapeake Bay Program. Annapolis, Maryland. 16 p.

Dauer, D.M., A.J. Rodi Jr., and J.A. Ranasinghe. 1992. Effects of low dissolved oxygen events on the macrobenthos of the lower Chesapeake Bay. Estuaries 15(3): 384-391.

Gilbert, R.O. 1987. Statistical Methods for Environmental Pollution Monitoring. Van Nostrand Reinhold: New York. 253 p.

Hirsch, R.M., J.R. Slack, and R.A. Smith. 1982. Techniques of Trend Analysis for monthly water quality data. Water Resources Research 18(1): 107-121.

Kendall, M.G. 1975. Rank Correlation Methods. Charles Griffin: London.

Mann, H.B. 1945. Non-parametric tests against trend. Econometrica 13:245-259.

Sen, P.K. 1968. Estimates of the regression coefficient based on Kendall's tau. Journal of the American Statistical Association 63:1379-1389.

Van Belle, G. and J.P. Hughes. 1984. Nonparametric tests for trend in water quality. Water Resources Research 20(1):127-136.