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Buzzards Bay National Estuary Program

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Buzzards Bay Eutrophication Index Information

Eutrophication Index Information

The Buzzards Bay Eutrophication Index was created by Dr. Joe Costa in 1992 as a tool to summarize water quality monitoring data collected in the Citizen's monitoring program. Its first public use was in 1992, when the Buzzards Bay NEP issued the first Baywatchers report (read the 1992 Baywatchers Report (850 kb pdf). (Read also the second 1994 Baywatchers Report 1.3 MB pdf file), the third report issued in 1996, (1996 water quality report page). For subsequent reports and the latest information on the Coalition's water quality monitoring program, go to The Buzzards Bay Coalition website.).

The the Buzzards Bay nitrogen monitoring assessment approach was formulated in 1990 and 1991, and was articulated in this 1992 publication (the eutrophication index was not developed until after the manuscript was written):

Costa, J. E., B. L. Howes, A. Giblin, and I. Valiela. 1992. Monitoring Nitrogen and indicators of nitrogen to support management action in Buzzards Bay, p. 497-529. In McKenzie et al.(eds) Ecological Indicators, Elsevier, London.

The Buzzards Bay Eutrophication Index was roughly modeled after a water quality index adopted by Hillsborough County in Florida to evaluate changes in Tampa Bay water quality. This water quality index approach was based on defining, for each water quality parameter used, a "poor" water quality value (0 points), and an "excellent" water quality value (100 points). The adoption of the 0 and 100 point values was made after reviewing data from Buzzards Bay and other Southern New England embayments, and after consultation with Dr. Brian Howes, who had set up the monitoring program with Dr. Costa.

The citizen's program measure dissolved oxygen concentrations with Hach KitsTM, secchi depth, salinity, and temperature) approximately 15 times between June 1 and September 30. The citizens also collect 2-4 water samples during summer, which we analyze for dissolved, and particulate organic nitrogen, nitrate + nitrite, ammonia, orthophosphate, and chlorophyll. The samples are analyzed by Howes' laboratory, and the methodologies are described by Howes and Goehringer (1994). Data for four years (1992 to 1996) are used in this report.

Generally 2 to 4 sites within each embayment were monitored in each embayment. In some smaller embayments only one site was monitored; in larger embayments 5 or more sites were sampled. Samples for nutrient analyses were taken on outgoing tides, while oxygen and secchi data included both incoming and outgoing tides because the oxygen measurements were needed in the early morning hours, generally taken between 6-9 AM, as indicated by Taylor and Howes, (1994).

Mean summertime values of dissolved oxygen percent saturation, secchi disk depth, chlorophyll a, total organic nitrogen (TON), and dissolved organic (DIN) were combined in a Eutrophication Index modeled after a similar index developed by Hillsborough County, Florida, U. S. A. (Johansson, 1991). The Hillsborough County Index included 7 water quality parameters(% dissolved O2 saturation, Chl a, total coliform, light penetration, total phosphorus, TKN, and BOD). Because the focus of the Buzzards Bay Eutrophication Index was on the effects of nitrogen loading, BOD and coliforms were omitted from our index, and DIN and DON were included instead of TKN because of problems associated with measuring low levels of nitrogen in seawater using the TKN methodology (D'Elia et al., 1987).

As noted by Harkins (1974), it is acceptable for parameters used in a water quality index to show interaction or interdependence, but there must not be any redundancy in the parameters. Thus, it acceptable to have DIN and DON included in a water quality index as we have done, but inappropriate to include both TKN and DIN because the parameters have redundancy in that the both include NH4 concentrations.

Like the Hillsborough County Index, the Buzzards Bay Eutrophication Index evaluated water quality parameters against a scoring curve like the one we used for mean secchi depth shown in Figure 5. As shown, if the summertime mean secchi depth was less than 0.5 m, then a score of 0 was received for that parameter. Conversely, a secchi depth greater than 3.0 m received a score of 100. If Secchi depth was between 0.5 and 3.0 m, the score was calculated using the following equation:

Score=(ln(value)-ln(0 pt. value))/(ln (100 pt. value)-ln(0 pt. value))

The 100 and 0 point values for each parameter is shown in the table below. All summertime means of the five parameters were applied to the equation above. These end points shown were chosen for Buzzards Bay based on the authors' knowledge of conditions typically found in a range of southern New England embayments. During the course of the water Citizen Monitoring Program, 100 and 0 point values changed somewhat, and the table reflects the current values employed. Most notably, at inception, mean summertime oxygen concentration were used in the index, then the lowest 20% of values. This was latter changed to the mean of the lowest 1/3 of all summertime values with a concurrent change in the 0 point and 100 point values.

The Eutrophication Index equaled the mean of the scores for the five parameters (i.e., all parameters were equally weighted). The Buzzards Bay NEP also experimented with Eutrophication Index scores without oxygen scores included. When several sites were monitored in an embayment, we averaged only those data from sites in the upper half of the estuary because of the steep gradient in water quality near the mouth to Buzzards Bay conditions. Secchi depth remains a problematic measurement in many embayments because many stations are not deep enough to document extinction of the disk.

Parameter Scale endpoints currently employed by the Buzzards Bay NEP for the Buzzards Bay Eutrophication Index. End Point values have been modified somewhat since inception.

0 point 100 point
Parameter value value
Oxygen saturation
(mn of lowest 20%)
40 % 90 %
Transparency 0.6 m 3 m
Chlorophyll 10 ug/l 3 ug/l
DIN 10 uM 1 uM
Total Organic N 0.6 ppm 0.28 ppm

Oxygen Saturation Table

Water quality monitors measure oxygen concentration, but also report oxygen percent saturation. Because salinity and temperature are important factors affecting how much oxygen can dissolve in water, these two factors must be corrected for, using the conversion table below.

Pdf file: oxygen-saturation-table.pdf     Word file: oxygen-saturation-table.doc     Excel file: oxygen-saturation-table.xls    

This online calculator is useful for deriving percent saturation for any given salinity and temperature.

References Cited

Harkins, R.D. 1974. An objective water quality index. J. Water Pollut. Contr. Fedr., 46, 589.

D'Elia, C. F., R. E. Magnien, C. F. Zimmerman, P. A. Vaas, N. L. Kaumeyer, C. W. Keefe, D. V. Shaw and K. V. Wood. 1987. Nitrogen and phosphorus determinations in estuarine waters: A comparison of methods used in Chesapeake Bay monitoring. Chesapeake Bay Liaison Office, UMCEES 97-19 CBL.

Howes, B. L. and C. D. Taylor. 1989. Nutrient regime of New Bedford Outer Harbor: Infaunal community structure and the significance of sediment nutrient regeneration to water column nutrient loading. Final Report, New Bedford Sewage Treatment Facilities Plan. Volume IV, Appendix R.

Howes, B.L. and D.D. Goehringer. 1994. Water quality monitoring of Falmouth's coastal ponds: results from the 1993 season. Technical Report for the WHOI Sea Grant Program and the Town of Falmouth, Mass. 72 pp.

Johansson, J.O.R. 1991. Long-term trends of nitrogen loading, water quality and biological indicators in Hillsborough Bay, Florida. Pp. 157-176 in S.F. Treat and P.A. Clark (eds.), Proceedings, Tampa Bay Area Scientific Information Symposium 2. 1991 Feb. 27-March 1; Tampa, FL. Text, Tampa, Fla.