Article: Tucker, Jane, Anne E. Giblin, Charles S. Hopkinson, Samuel W. Kelsey, and Brian L. Howes. “Response of benthic metabolism and nutrient cycling to reductions in wastewater loading to Boston Harbor, USA.” Estuarine, Coastal and Shelf Science 151 (2014): 54-68. DOI:10.1016/j.ecss.2014.09.018
Boston Harbor: A decade of clean-up
Estuaries, or that special coastal place where rivers meet the ocean, are some of the most beautiful and biologically productive places on Earth. Estuaries make great beaches and are home to many of the delicious seafoods that help boost our local economies. Unfortunately, these ecosystems are extremely susceptible to human activities which cause eutrophication. Eutrophication is when an aquatic environment receives too many nutrients. For example, “extra” nitrogen can get dumped into the ocean when we apply fertilizers to our gardens or even when we use the rest room. Eutrophication is very detrimental to estuaries, sometimes causing hypoxia (low oxygen) and nuisance algal blooms, all of which hinder the wonderful services estuaries provide us.
Sediments are a great place to look for eutrophication since they can store the organic matter generated by biological processes that take place above. In places such as Boston Harbor, sediments are so rich in organic matter that environmental recoveries are often slower in the sediments, making the sea bottom a highly competitive environment for its inhabitants (Figure 1). Boston Harbor is also a special case, since unlike other highly disturbed estuaries, strong tidal flushing often prevents hypoxia from occurring. So, from the outside, Boston Harbor’s eutrophication problems may not seem so bad compared to a place like Chesapeake Bay, but these sediments still receive massive amounts of organic matter and nutrients. Thus these ecosystems are vulnerable.
Boston Harbor’s major pollution problem stemmed from its waste water. Prior to 1991, waste water treatment facilities were not able to remove much of the total suspended solids (TSS, which includes the organic rich sediments) and nitrogen caused by the Boston-area’s large population, resulting in the transport of this material into the harbor. In 1991, the Massachusetts Water Resources Authority (MWRA) began a multi-year project to reduce the pollution problem (Table 1). In this year, the MWRA implemented a better primary treatment of waste water, reducing the TSS by 60%. A second phase from 1997 to 1998 improved the secondary treatment protocols, which now removed up to 90% of TSS from the Boston Harbor. While this was excellent progress, the dissolved inorganic nitrogen (DIN) was still being dumped into the ocean at alarming concentrations! In the final phase in 2000, effluent was re-routed offshore, reducing the total nitrogen by 82% and the DIN by 50%!
The decade-long clean-up of Boston Harbor created an interesting “field study” to observe how sediments reacted to these clean-up efforts. Tucker et al., investigated this unique situation!
The approach: Let’s play in the mud!
Four sampling stations where set up in Boston Harbor, all with different ranges of exposure to the waste water effluent that was predicted using a model of the pollution plume (Figure 1).
Sample collection began in 1992 for the northern stations and 1995 for the southern stations. Sediments were collected using SCUBA divers to collected cores of sediment (how fun does that sound!). The retrieved sediments were measured for a suite of different items including the pH (how acidic was the sediments?), Eh (a measure of how reducing an environment is, or how reactive the oxygen is), chlorophyll-a, dissolved oxygen, and nutrients (Table 2). This data was used to calculate important biogeochemical cycles such as the sediment oxygen demand (or the “breathing” needs of the sediment critters) and nitrogen fluxes.
Amphipods and other large fauna were counted and identified in all collected sediments and this measurement was supplemented by pre-existing studies in the area that measured benthic fauna.
What did they find?
The massive and decade-long clean-up of the waste water entering Boston Harbor had significant positive impacts to the sediments. For example, the sediment oxygen demand decreased from the start to finish of this study. At the start of the investigation in 1992, only one year into the improvements, the sediment oxygen demand of Boston Harbor sediments were incredibly high (in fact the highest these authors could uncover compared to all previous studies). The high sediment oxygen demand makes oxygen harder to come by in these sediments and creates truly harsh living conditions. By the end of the study, this sediment oxygen was markedly improved; in fact, the new oxygen demand in Boston Harbor was similar to clean sediments. In an abstract way, the amphipods living in Boston Harbor went from living in a landfill to a peaceful meadow.
A similar story for oxygen levels unfolded for the dissolved organic nitrogen. Fluxes of nitrogen to the sediments, for example, decreased by 44% between Phase II and IV of the clean-up (Table 1). Similarly, sediment organic matter also decreased in three of the four study sites. Organic matter is the “food” for these sediment-dwelling critters. Too much food is bad (think about how you feel after an all-you-can-eat buffet!). Temporally (between years), there was some variability in these trends which was influenced by who was living in the sediments. So the reduction in organic matter will help improve the high sediment oxygen demand (in fact, it was estimated to contribute up the a third of the original sediment oxygen demand). Notably, this organic matte reduction was correlated to the sewage organic carbon, meaning that these improvements were likely responsible for the improved sediment health!
The oxygen and nutrient demand responded quickly to the reduction in polluted waste water. The surprisingly fast improvement was likely due to the quick tidal flushing associated with Boston Harbor. (In other words, Boston harbor was a great place for this type of improvement). Because the above water column and sediments rarely experienced hypoxia, the local sediment fauna was always present (so the critters were not replaced by more hypoxic-tolerant organisms).
One item to note was that in the beginning of the study, there were a few oxygen excursions, or periods when the demand for oxygen went off the charts. Upon first glance, someone could think “this clean-up is making things worse!” However, the magnitude of these few and short lived excursions decreased over time and the overall net effect was a reduction in this need for oxygen and nutrients, making the sediments cleaner and less prone to hypoxia.
This 10-year clean-up project in Boston Harbor was a success. The improvement of waste water treatment improved the quality of living for the sediment critters (such as amphipods) by lowering the amount of organic matter, demand for oxygen, and need for nutrients. Different sediments in different ecosystems will react differently to nutrient loading and eutrophication, but in the case of Boston Harbor, this project improved the living conditions of the benthic organisms! For you at home, this means that Boston Harbor will likely experience even less hypoxic events and nuisance algal blooms.
Would a long-term clean-up effort like Boston Harbor be worth the money in a water body near you? Leave a comment!
I received a Ph.D. in oceanography in 2014 from the Graduate School of Oceanography (URI) and am finishing up a post-doc at the University of Maryland Center for Environmental Science (Horn Point Laboratory). I am now the Research Coordinator for the Delaware National Estuarine Research Reserve.
Carbon is my favorite element and my past times include cooking new vegetarian foods, running, and dressing up my cat!