Coastal Services Center

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Monitoring Phytoplankton Levels

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Monitoring chlorophyll in the water column is difficult because it varies so greatly in time and space. In order to determine whether discharges of treated sewage are significantly increasing the phytoplankton chlorophyll in Massachusetts Bay, coastal managers are supplementing ship and mooring measurements with satellite imagery of chlorophyll near the surface from SeaWiFS (Sea-viewing Wide-Field-of-View Sensor). High chlorophyll levels at the surface of the water column indicate high phytoplankton concentrations.

The Project: Monitoring Phytoplankton Levels in Boston Harbor and Massachusetts Bay

In compliance with a federal court order, the Massachusetts Water Resources Authority (MWRA) completed construction of the world's largest marine sewage outfall, and in September 2000 began discharging metropolitan Boston's secondary-treated sewage 9.5 miles offshore into Massachusetts Bay.

Environmental concerns about the outfall led the regulatory agencies to require an extensive monitoring program as part of the discharge permit. Ninety-seven specific thresholds were established to represent expectations for environmental quality. When a threshold is exceeded, MWRA must promptly notify regulators, the public, and a science advisory panel. The panel may decide that the exceedance was due to natural causes, or may recommend further study or other response.

Threshold testing includes measurements of effluent, the water column, the seafloor, and fish and shellfish. The threshold for chlorophyll concentration is based on seasonal and annual aggregation of shipboard measurements taken about every month at a few dozen stations in the bay. Because there are substantial data gaps between stations and surveys, MWRA needed a way to collect and summarize regional data more frequently.

Deriving Phytoplankton Data from SeaWiFS Imagery

To monitor the water column in Massachusetts Bay, MWRA needed a sensor capable of detecting subtle changes in ocean color. Changes in ocean color can be caused by varying amounts of chlorophyll, the photosynthetic pigment that is found in all plants, including phytoplankton. SeaWiFS measures the light intensity reflected off the ocean's surface at several wavelengths. Algorithms are applied to the data to discriminate between changes in color caused by chlorophyll, sediments, and other substances in the water.

SeaWiFS data show chlorophyll levels with false color to emphasize the concentration ranges. Violet represents the lowest concentrations, green and yellow are intermediate, and red is high. The data show an increasing trend in chlorophyll levels.

The Result

MWRA has found the SeaWiFS images to be indispensable in decision-making. The images showed that a large algal bloom coincident with the start-up of the bay outfall was regional and thus not caused by the outfall. The images also help with interpreting the shipboard data, with comparisons of model output, and with interpretation of input to the bay from the Gulf of Maine, which can include blooms of toxic algae. Local resource managers are using the daily SeaWiFS data to complement the monthly or quarterly boat surveys that make a more comprehensive set of measurements in the water column. SeaWiFS is helping to answer basic questions:

• Is there a phytoplankton bloom out there?
• Where is the bloom?
• When did it happen?

The answers to these questions can then be used to address the following questions:

• Why did it occur?
• What are its consequences?
• How can it be handled?

For More Information

• Massachusetts Water Resources Authority
• Outfall Monitoring Science Advisory Panel

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