Mid-Channel Water Clarity

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Chesapeake Bay Program Indicator Framework 

Reporting Level Indicators

Indicator and Data Survey

A. Category/Name/Source/Contact 

(1) Category of Indicator 

___ Factors Impacting Bay and Watershed Health 

___ Restoration and Protection Efforts 

___ Watershed Health 

__X__ Bay Health 

(2) Name of Indicator: Mid-Channel Water Clarity

(3) Description of Dataset used to calculate percent of goal achieved: For what purpose(s) were the data collected? (e.g., tracking, research, or long-term monitoring.)   All of the above.

Which parameters were measured directly? Which were obtained by calculation? 

Secchi depths were measured directly at fixed stations during Bay transect cruises.Percent goal is then calculated based on this data.

(4) Source(s) of Data: 

Is the complete data set accessible, including metadata, data-dictionaries and embedded definitions? Secchi depth is measured by the MD Department of Natural Resources (MD mainstem and tributary data), the VA Department of Environmental Quality (VA tributary data), and Old Dominion University (VA mainstem data); these data are submitted to the Bay Program’s database manager and subsequently posted on the CIMS data hub. 

If yes, please indicate where complete dataset can be obtained.

Data included in standard monitoring programs are located on the CIMS data hub. These data can be downloaded from the Chesapeake Bay Program website (http://www.chesapeakebay.net/data_waterquality.aspx).

(5) Custodian of Source Data (and Indicator, if different):

Jeni Keisman, UMCES/CBPO

(6) CBPO Contact: 

Jeni Keisman, UMCES/CBPO

B.  Communication Questions

(complete either part 1, 2, or 3) 

2. Bay Health or Watershed Health indicators only 

(7b) What is the long-term trend? (since start of data collection, 1985-2009); Water clarity goal achievement decreased from 37.5 percent to 26 percent. During this time, goal achievement has averaged 23.3 percent and has ranged from 7.1 percent to 41.1 percent. 

(8b) What is the short-term trend? (10-yr trend) Between 2000 and 2009, the water clarity score increased from 14.28 percent to 26 percent of goal achieved.

(9b) What is the current status in relation to a goal? The goal is for 100 percent of the Chesapeake Bay to meet guidelines for water clarity. A device called a Secchi disk is used to measure water clarity and the depth to which light penetrates the water column during the growing season for underwater bay grasses.  In 2009, 26 percent of tidal waters met or exceeded thresholds for water clarity. This was an increase from 2008, when about 14 percent met guidelines. 

(10b) What is the key story told by this indicator? 

Secchi depth is a measure of water clarity and the depth to which light penetrates the water column. All plants need light to grow, and clear water allows light to reach underwater plants.  Phytoplankton and aquatic grasses are the predominant underwater plants in Chesapeake Bay.  Phytoplankton,or algae, are microscopic and usually live suspended in open waters.  They are the base of food chains that support most living resources in the bay, including oysters and fish.  Underwater grasses, also known as submerged aquatic vegetation (SAV), are rooted in shallow areas fringing the bay.  They provide vital habitat for a number of living resources.  Greater water clarity typically leads to healthier communities of both plant types.

As a measure of water clarity, Secchi depth is directly influenced by the amount of suspended matter in water, including plankton (microscopic organisms), sediment and detritus. Phytoplankton affect water clarity and their concentrations are highly dependent on the amount of available nutrients. 

River flow also influences Secchi depth. Freshwater inflow is a major factor in the delivery of sediment and nutrients to the Bay and its tributaries. Increased runoff from high flows, such as in 1993, 1994, 1996, the first half of 1998 and in 2003, contributed to widespread degradation in water clarity. Drier conditions, as in 1999, 2000, 2001 and 2002 tended to improve water clarity.

The Secchi depth indicator is currently judged by the frequency that Secchi depth exceeds (i.e., is deeper than) the following thresholds, weighted by Bay segment area:

Salinity zoneMonthsdepth (m)

Tidal FreshApr-Oct≥0.85




These thresholds are based on conditions during the SAV growing season (April through October for all salinity regimes except the most saline (polyhaline); in the polyhaline region, the season is March through November).  Note that Secchi depth measurements for this indicator are from fixed stations located in open water areas of the Bay and do not necessarily reflect Secchi depth in shallow water areas where SAV is most abundant.    

(11b) Why is it important to report this information?  Secchi depth is a measure of water clarity, and good water clarity is an important indicator of healthy phytoplankton communities in the open waters of Chesapeake Bay.  Phytoplankton are the base of food chains supporting important living resources such as oysters and fish; therefore it is essential to maintain good water clarity in order to support healthy open water phytoplankton communities.  Good water clarity is also vital to the survival of submerged aquatic vegetation (SAV); however it is important to note that this indicator reports mid-channel water clarity, whereas SAV tend to grow in the shallow waters closer to shore.

(12b) What detail and/or diagnostic indicators are related to this reporting level indicator?  Spatial interpolation maps and a current conditions indicator.  

C. Temporal Considerations 

(13) Data Collection Date(s):  1985 through 2008

(14) Planned Update Frequency (e.g. - annual, bi-annual): 

(a) Source Data:  Annual

(b) Indicator: Annual 

(15) For annual reporting, month spatial data is available for reporting:  January of the following year.

D. Spatial Considerations 

(16) Type of Geography of Source Data (point, line polygon, other):  Point

(17) Acceptable Level of Spatial Aggregation (e.g. - county, state, major basin, tributary basin, HUC):  Fixed-station data are aggregated to 78 tidal water segments for the Chesapeake Bay (2003 revised Chesapeake Bay Program (CBP) segmentation and zonation scheme) and then aggregated to the bay-wide scale.

(18) Are there geographic areas with missing data? If so, where?  Yes.  For 2007, there are no secchi depth data included for segments CHOTF, CHSTF, ANATF, HNGMH, NANOH, LYNPH, or POCOH.   

(19) The spatial extent of this indicator best described as:  

(a) Chesapeake Bay (estuary) 

(b) Chesapeake Bay Watershed 

(c) Other (please describe): ____

Please submit any appropriate examples of how this information has been mapped or otherwise portrayed geographically in the past. 

(20) Can appropriate diagnostic indicators be represented geographically?  Yes

E. Data Analysis and Interpretation

(Please provide appropriate references and location of documentation if hard to find.) 

(21) Is the conceptual model used to transform these measurements into an indicator widely accepted as a scientifically sound representation of the phenomenon it indicates? (i.e., how well do the data represent the phenomenon?).  Yes. This indicator has undergone technical and peer review by state, Federal and nongovernment organization partner members of the Tidal Monitoring and Analysis Workgroup (TMAW) and the Monitoring and Assessment Subcommittee (MASC). Data collection, data analysis and QA/QC are conducted by the principal investigators/scientists. The data are peer-reviewed by scientists on the workgroup. Data selection and interpretation, the presentation of the indicator, along with all supporting information and conclusions, are arrived at via consensus by the scientists in collaboration with the resource manager members of the workgroup. The workgroup presents the indicator to the subcommittee where extensive peer review by Bay Program managers occurs.  

(22) What is the process by which the raw data is summarized for development and presentation of the indicator?   Secchi data for the growing season are compared to the thresholds in each salinity zone to determine the frequency that a particular station passes the thresholds indicated above (i.e., has better water clarity).  The mean of the percentages from all stations within a Bay segment is then weighted by the respective segment area to obtain a bay-wide value.  

(23) Are any tools required to generate the indicator data (e.g. - Interpolator, watershed model).  Yes, there is a Fortran program used to generate the results.

(24) Are the computations widely accepted as a scientifically sound?  Yes The methods for mid channel water clarity are the same as those for the Eco-check Report Card Report Card:


Specifically the document (Development of an Integrated and Spatially Explicit Index of Chesapeake Bay Health (Bay Habitat Health Index - BHHI))

(25) Have appropriate statistical methods been used to generalize or portray data beyond the time or spatial locations where measurements were made (e.g., statistical survey inference, no generalization is possible)?  Not applicable

(26) Are there established reference points, thresholds or ranges of values for this indicator that unambiguously reflect the desired state of the environment? (health/stressors only)  Yes.  There are thresholds developed for Secchi depth that have been applied to the entire Bay for this indicator (see above).  

F. Data Quality

(Please provide appropriate references and location of documentation if hard to find.) 

(27) Were the data collected according to an EPA-approved Quality Assurance Plan?  Yes.  Methods are described in the Quality Assurance Project Plan (QAPP) on file for the EPA grant.  Documentation is available at: http://www.chesapeakebay.net/qualityassurance.aspx.

If no, complete questions 28a – 28d

(28a) Are the sampling design, monitoring plan and/or tracking system used to collect the data over time and space based on sound scientific principles? 

(28b) What documentation clearly and completely describes the underlying sampling and analytical procedures used?

(28c) Are the sampling and analytical procedures widely accepted as scientifically and technically valid?

(28d) To what extent are the procedures for quality assurance and quality control of the data documented and accessible?

(29) Are the descriptions of the study or survey design clear, complete and sufficient to enable the study or survey to be reproduced?  Yes.  Methods are described in the Quality Assurance Project Plan (QAPP) on file for the EPA grant.  Documentation is available at:http://www.chesapeakebay.net/qualityassurance.aspx.

(30) Were the sampling and analysis methods performed consistently throughout the data record?  Yes.  Methods are described in the Quality Assurance Project Plan (QAPP) on file for the EPA grant.  Documentation is available at:http://www.chesapeakebay.net/qualityassurance.aspx.

(31) If datasets from two or more agencies are merged, are their sampling designs and methods comparable?  Yes.  Methods are described in the Quality Assurance Project Plan (QAPP) on file for the EPA grant.  Documentation is available at:http://www.chesapeakebay.net/qualityassurance.aspx.

(32) Are uncertainty measurements or estimates available for the indicator and/or the underlying data set?  No

(33) Do the uncertainty and variability impact the conclusions that can be inferred from the data and the utility of the indicator?  Yes.  Secchi depth is highly variable both spatially and temporally.  Moreover, Secchi depths from the fixed-station data used to create these indicators are not representative of the water clarity in shallow water areas that support submerged aquatic vegetation (SAV).  Therefore, there are limitations to how the data can be applied and interpreted both spatially and temporally. 

(34) Are there noteworthy limitations or gaps in the data record?   No   Please explain.  

G. Additional Information


(35) Please provide any other information about this indicator you believe is necessary to aid communication and any prevent potential miss-representation. 

Criteria Documentation, available at:

• Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries, April 2003, http://www.chesapeakebay.net/content.../cbp_13142.pdf

• Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries – 2004 Addendum, October 2004, http://www.chesapeakebay.net/content.../cbp_13268.pdf

• Ambient Water Quality Criteria for Dissolved Oxygen, Water Clarity and Chlorophyll a for the Chesapeake Bay and Its Tidal Tributaries – 2007 Chlorophyll Criteria Addendum, http://www.chesapeakebay.net/content.../cbp_20138.pdf

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