MWRA developed a Contingency Plan to ensure that effluent discharge from the Massachusetts Bay outfall does not result in adverse impacts to water quality. The Plan identifies thresholds that can suggest that effluent quality or conditions in the Bay may be changing.
Ambient (Receiving Water) Thresholds
Nuisance Algae
Nuisance algal blooms are less predictable than the normal, beneficial algal blooms that produce oxygen and food for marine life; some nuisance blooms did occur during the baseline monitoring period. Because there was public concern that effluent nutrients could feed a red tide bloom in the vicinity of the Massachusetts Bay outfall, or otherwise increase the abundance of nuisance algae, the Contingency Plan includes thresholds for unusually high levels of certains species of nuisance algae.
Alexandrium fundyense
A species of nuisance algae that typically may bloom during April to June and can cause paralytic shellfish poisoning, known as PSP or red tide; it has been periodically found in Massachusetts since the 1970s. Toxicity is generally not found in shellfish until much higher cell counts are seen in the overlying waters. To calculate the threshold we determine the maximum number of total Alexandrium cells (A. fundyense plus unidentified Alexandrium spp.) seen in any nearfield sample.
Pseudo-nitzschia
Multiseries blooms can occur during November to March and produce domoic acid, which can cause a condition known as amnesic shellfish poisoning. To calculate the threshold we calculate the nearfield average count of algae in a group that includes the toxic species Pseudo-nitzschia multiseries, the closely related Pseudo-nitzschia pungens, and any unidentified Pseudo-nitzschia species.
Phaeocystis pouchetii
Blooms usually occur during February to May. As of 2017 seasonal Phaeocystis thresholds are no longer a part of the Contingency Plan. This species is not toxic, but individual cells can aggregate in gelatinous colonies that may be poor food for zooplankton and cause nuisance foam on the sea-surface. A series of threshold exceedances occurred between 2001 and 2016. In each case, MWRA and its monitoring team evaluated the exceedances and determined the blooms were natural occurrences with no evident connection to outfall discharge. MWRA's Science Advisory Panel, and state and federal regulators agreed with this conclusion.
Benthic Diversity
One way to track the status of a marine ecosystem is to measure the diversity of the organisms in the communities that comprise the ecosystem, such as the soft-sediment communities (benthic infauna) in the sediment. The benthic diversity thresholds are intended to indicate whether there is a change from baseline conditions (either toward more or less diversity) now that the outfall is discharging. Of the dozens of statistical measures of diversity that have been developed by researchers over the past few decades, four are tracked within the MWRA monitoring program to show possible changes in diversity.
Two of these indices, total number of species per sample and Fisher's log-series alpha, are measures of species richness (how many species are present). Both measures track species richness while total species per sample is easy to describe to a general audience, Fisher's Log-series alpha has a theoretical grounding favored by some researchers. The other two diversity indices tracked by MWRA's monitoring are among those most commonly used by ecologists in many environments. Pielou's J' is a measure of how evenly individuals are distributed among species in a community. Samples where most species have about the same number of individuals have high evenness, while samples where most of the individuals belong to one or a few species have low evenness. Finally, Shannon-Wiener H' is a diversity measure that is sensitive both to species richness and to species evenness in a community.
The extreme winter storms of December 1992 caused 24-foot seas in the vicinity of the outfall, moving sediments and burying some areas under inches of sand, mud, or gravel even though the ocean is about 100 feet deep in the area. This physical disturbance was at least partially the cause of the decline seen in the two richness indices between 1992 and 1993. The communities recovered rapidly, and by the late 1990s appeared to be showing a several-year cycle in species richness (data from farfield stations also show this apparent trend).
A series of diversity threshold exceedances occurred between 2010 and 2014, in each case triggered by high species diversity as measured by Pielou's J' and Shannon-Wiener H'. In each case, MWRA and its monitoring team evaluated the exceedances and determined the high species diversity represented natural fluctuations in sea-floor communities with no evident connection to outfall discharge (link). Regulators and OMSAP agreed with MWRA’s conclusion. In 2017 MWRA proposed deleting the upper range species diversity thresholds, and regulators agreed.
Benthic Opportunists
Pollution-tolerant or opportunistic species whose presence could signal pollution impacts on sediments in the vicinity of the outfall. These are species that can build up to high population levels in response to, for example, increased deposition of organic matter. In their selection of an outfall location in 1988, EPA modeled the deposition of organic matter and determined that with a secondary discharge, impacts would be minimal.
Based on a review of the species found in Boston Harbor, Massachusetts Bay, and Cape Cod Bay sediments during baseline sampling, several species have been identified as opportunists: Capitella spp. and Capitella capitata complex, Polydora cornuta, Streblospio benedicti, Ampelisca abdita, Ampelisca vadorum, Ampelisca macrocephala, and Mulinia lateralis. The Ampelisca species were included in the list because they are tolerant to moderate levels of organic enrichment, even though they cannot tolerate high levels. For example, the appearance of large populations of Ampelisca in Harbor sediments in the mid-1990s was one of the early signals of the Harbor's recovery.
The Contingency Plan thresholds for percent opportunists were set well below levels seen in Boston Harbor throughout the 1990s.
Chlorophyll
A measure of the amount of microscopic plants (phytoplankton or algae) in the water. In Massachusetts Bay, production of algae is the basis of the food web. However, excessive growth of algae can lead to undesirable consequences, such as oxygen depletion at depth due to decomposition of organic matter. Effluent from the outfall is rich in nutrients, and therefore could potentially cause excessive algal growth.
Annual and seasonal chlorophyll thresholds
Because the chlorophyll levels naturally vary over the year, there are separate thresholds for different seasons. The "nearfield" group of stations within about three miles from the outfall, which are most likely to be affected by nutrient-rich effluent, have seasonal thresholds. In most years, Massachusetts Bay experiences a "spring bloom" characterized by high chlorophyll levels as lengthening days provide enough sunlight for algae to grow quickly. Chlorophyll typically drops in summer, as the nutrients in well-lit surface waters are used up. When the weather cools, the surface and bottom waters mix, which usually gives rise to a "fall bloom" as nutrient-rich bottom waters are mixed up into the well-lit surface layers. As the days become short, chlorophyll levels drop again since there is not enough light for algae to grow.
Dissolved Oxygen (DO)
Dissolved oxygen in the water allows fish and other aquatic animals to breathe. Algae and other plants growing in the water produce oxygen, and atmospheric oxygen also dissolves in water at the surface. In polluted ecosystems DO can fall below levels necessary to sustain life.
The concentration of DO indicates the balance between production by algae and consumption by aquatic organisms and the decomposition of organic matter. Excessive organic matter can deplete oxygen, which may in turn harm the aquatic ecosystem. The amount of oxygen that the water can hold is related to water temperature, salinity, and pressure; thus, the percent saturation of DO is a measure that takes these factors into account. Monitoring locations for which there are DO thresholds include the "nearfield," the group of stations within about three miles from the outfall, and "Stellwagen Basin," a deep area nine miles east of the outfall. DO thresholds apply to the part of the year when the water column is stratified, i.e. from June - October.
Dissolved Oxygen Depletion Rate: The DO depletion rate is the speed at which dissolved oxygen is consumed by biological and chemical processes. Even if dissolved oxygen concentrations remain healthy, a rapid decrease could signal a future problem, such as excess nutrients. A low rate indicates a healthy system, with enough DO available for fish and other aerobic organisms to thrive. The threshold for the DO depletion rate is based on a change from the baseline; the caution threshold is a rate faster than 1.5 times the baseline mean rate, while the warning threshold is twice the baseline mean rate.