Kenly Hiller
Neponset Report
10/15/12
Status of nutrients and pollution in the Neponset river estuary
Introduction:
Over the past several centuries, the Neponset River Estuary has been
experiencing increased nutrient and pollutant input due to urbanization throughout
its watershed (Poppe and Moffett, 1993). Nitrogen is a key element in coastal
systems like the Neponset estuary because it regulates primary production
(including harmful or nuisance algal blooms) in these types of systems (Herbert,
1999). Understanding nitrogen cycling in estuaries is therefore key to assessing the
health of a system. In a healthy system, sources of N include nitrogen fixation and
recycling from decomposition. Microbes in the sediment play a key role in recycling
nutrients by oxidizing organic compounds. Ammonification, for example, releases
bioavailable ammonium to the water column, or it can be nitrified to nitrate,
providing fuel for denitrification (Herbert, 1999). Denitrification is important
because it’s the dominant process of nitrate reduction in most shallow marine
systems like Neponset (Herbert, 1984). It is one of very few ways to permanently
remove nitrogen from a system and can therefore be very important when
considering how to remove excess nitrogen added to a system from anthropogenic
sources.
Adding more of this limiting nutrient increases primary production, so
understanding the amount of nutrient-loading that’s taking place in Neponset is key
to understanding the health of the system. Nutrient-loading is defined here as
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human additions of limiting nutrients (in this case, N) that incredase primary
production from baseline levels sustained by nutrient recycling and N-fixation.
Humans have increased the transfer of nitrogen through rivers like the Neponset to
estuaries (Vitousek et al. 1997). Adding nitrogen causes blooms in phytoplankton
that subsequently die and sink to the bottom. They are then decomposed by
microbes that use up all the oxygen at the bottom, causing anoxia or hypoxia in
stratified waters (Vitousek et al. 1997). The increased turbidity due to these blooms
causes a loss of diversity in the primary producers that live in the estuary. Nuisance
algae become dominant and the diversity of phytoplankton drops (Cosper et al.
1987). This turbidity as well as an increase in epiphyte growth due to the nitrogen
surplus also causes eelgrass to die out (Cosper et al., 1987). The low oxygen and
high sulfide concentrations that occur during hypoxia events significantly reduces
the primary production rates of eelgrass (Holmer and Bondgaard, 2001).
All of these changes resulting from increase nitrogen loads, referred to as
eutrophication, have other harmful effects on the system as a whole. These changes
in the species composition of primary producers in estuaries can lead to a collapse
in fisheries (Vitousek et al. 1997). Fish will die if all of the oxygen is removed from
the water column by microbial decomposition. Removal of oxygen also changes
microbial populations and the processes like nitrification that they carry out
(Herbert, 1984). Nitrification is an aerobic process and is inhibited by a lack of
oxygen (Herbert, 1984). Also, the commercially relevant bay scallop population has
declined drastically due to loss of their eelgrass habitats (Cosper et al. 1987). Most
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importantly for Neponset, restoration of eelgrass will be nearly impossible if
nutrient concentrations are too high.
Another aspect of water quality is bacteria levels. Bacteria from leaky septic
or sewer systems can leach into estuaries. These pathogens lead to an increase in
waterborne gastroenteritis and therefore have hazardous effects on human health
(EPA 2008). Furthermore, bacteria can be harmful to the environment because
waste expelled into an estuary is broken down by microbes, which then deplete the
oxygen in the system just like the decomposition of dead phytoplankton that was
discussed earlier (EPA 2008).
Other types of pollution can be damaging to estuarine ecosystem health as
well. Polychlorinated biphenyls (PCBs) enter watersheds through waste produced
by factories that manufacture elecontronics, machining, painting, and other
industrial sources (Breault, 2011). They belong to a family of man-made organic
chemicals used in hydraulics, electrical, and heat transfer applications because of
their high boiling point and chemical stability (EPA.gov). They enter ecosystems
through illegal dumping or leakages and remain there for long periods of time due
to their chemical stability. They have negative effects on nearly every system in a
human body, including the nervous, immune, and endocrine systems (EPA.gov). it is
therefore vital to monitor the concentrations of PCBs in estuaries before any fish can
be eaten, because it can biomagnify up the food chain (Perry, 2009).
Heavy metal contamination can also be a problem. Mercury deposition has
increased worldwide (Fry and Chumchal, 2012), and this can lead to a buildup of Hg
in sediments. Depending on microbial community composition, this mercury can be
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methylated by sulfate reducing bacteria to methylmercury (Auer et al. 2009). This
form is much more toxic because it penetrates the blood-brain barrier quite easily
and can biomagnify to higher concentrations in predators (Hammerschmidt and
Fitzgerald, 2004). Cadmium and lead are also hazardous to human health.
Cadmium emissions from batteries can leach into estuaries and can cause kidney
damage if ingested (Jarup, 2003). Lead is related mainly to road transport (it used
to be in gasoline) and is widespread over many types of ecosystems (Jarrup, 2003).
These three metals are the most toxic and can all be harmful on human health when
eaten (Jarup, 2003). Mercury and cadmium are particularly toxic to eelgrass.
Concentrations of these metals were found to be up to 1,850 times higher than the
surrounding water concentrations and significantly inhibited eelgrass growth
(Lyngby and Brix, 1984). Monitoring heavy metal concentration is therefore
another key aspect of ecosystem health to monitor when considering how to restore
eelgrass to Neponset.
Current Data:
All sections of the Neponset River are on the water quality impaired list set
out by the Massachusetts Department of Environmental Protection for organics,
turbidity, low dissolved oxygen, pathogens, and metals. The actual data show that
dissolved oxygen (DO) actually is not too low in the river itself. Near
Foxboro/Canton the lowest DO is 6.6mg/L, and at Mother Brook only 15 of 397
samples taken had DO concentrations lower than 5mg/L (Boston Harbor
Watershed, 1999). The section with the lowest DO was closest to the estuary, in the
Milton to Dorchester Bay region. It went as low as 0.5mg/L at some sampling sites,
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but it was highly variable, with some sites showing percent saturations of over
124% (Boston Harbor Watershed, 1999). The report cautions that these data were
not taken pre-dawn (the time with the highest incidence of hypoxia), so they may
not be representative of the true extent of oxygen depletion in the system. However,
MWRA data complements this data. They found that DO ranged mostly from 59mg/L with only a few sites below 4ug/L and several above 10 (MWRA website, Fig.
1).
There are many sources of excess nutrients in the Neponet watersed.
Fertilizer, animal waste, runoff from impervious surfaces, direct input of grass
clippings from lawns and golf courses, leaky sewers, bad septic systems, and dams
all contribute excess nutrients to the system (Neponset.org). The Boston Water
Quality Assessment report data show that ammonia concentrations are below the
chronic water quality criteria for ammonia. Concentrations ranged from 0-0.295
mg/L, which is normal. The MWRA found that nitrate and nitrite (reported
combined) in the river mouth ranges from 1-12 uM mostly, with some spikes
reaching up to 30uM. Concentrations in the river itself were much higher, ranging
from 20-60 uM in some sites (Fig. 2). Chlorophyll concentrations do not seem to
predict nitrate concentrations, indicating that another factor (such as turbidity)
might be influencing chlorophyll concentrations. However, the Neponset River
Watershed Association (NRWA) stated that total nitrogen concentrations are
worrisome, with 39% of sampling sites exceeding accepted nitrogen concentrations
(Neponset.org). They also found that total nitrogen was most frequently excessive
along the river, while nitrate+nitrite was excessive at fewer sites. It wasn’t very
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specific information and the estuary has hardly been sampled at all. The MWRA and
the data from Neponset.org only had one sampling site in the estuary itself.
Phosphorus levels never exceeded 2uM, according to the MWRA website,
which is fine in terms of the estuary because P is not the limiting nutrient there.
However, P can be the limiting nutrient in the rivers and lakes in the Neponset
watershed. In fact, NRWA found that 89% of the samples they took had total
phosphorus concentrations exceeding acceptable water quality limits. They believe
part of this is due to the large, historic industrial P discharge from the Foxborough
Company, which is still causing eutrophic conditions in the Neponset Reservoir.
Colored dissolved organic matter (CDOM) is also important because it can reduce
the productivity of phytoplankton by blocking the light that is supposed to reach the
photic zone (Huang and Chen, 2009).
Bacteria seem to be the most pressing issue in Neponset. Reductions in bacteria
loads from 72-99% are called for to even meet total maximum daily loads. These stations
that don’t meet water quality standards have 200 or more organisms per 100ml sample, or
more than 10% of samples from a station have counts exceeding 400 organisms/100ml.
the areas of the estuary that are more developed have higher bacteria levels. This is
usually due to leaking or illicit sewer connections. These issues become worse in wet
weather as storm runoff flushes materials from pipe drainage systems. If the system is
full of impervious surfaces like pavement, there is no chance for the soil to filter the
bacteria out of the groundwater because the bacteria-laden water cannot percolate down
into the soil.
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Some segments of the Neponset River show contamination of heavy metals
from wastewater discharge from companies that dump waste into the river (Boston
Water Quality Report, 1999). In Foxboro, for example, Hg, Cd, Ni, Cu, Cr, and Pd are
all elevated, but the report states that there is no potential for biological harm from
current concentrations. Other sections of the river, such as Mother Brook, have been
contaminated with Hg and Cu.
High levels of polychlorinated biphenyls (PCBs) have also been found in this
system. The source is waste disposal from electronics research/equipment
manufacturing, machining, and painting, among other things (Brealt, 2011). There are
high levels in the estuary and in fish, particularly brown bullheads (dissertation).
Concentrations have been found to be as high as 26,000 mg/kg of sediment and 0.05-12
ug/L in groundwater (Brealt, 2011).
Future Directions
The Neponset Estuary is very neglected compared to its river. There is only one
sampling site in the estuary, so water quality data is virtually non-existent. Nitrogen is
most likely the limiting nutrient in this system, so we need to know how much nitrogen
and subsequent primary production is in the estuary. This will allow us to determine
whether nitrate concentrations in the watershed are contributing to eutrophication in the
estuary itself. Knowing DO levels will be helpful too, because it will tell us if
eutrophication-induced hypoxia or anoxia is occurring. Taking the same measurements
in the estuary that occur in the river would accomplish this goal. Stations could be set up
around the estuary perimeter as well as across the estuary itself along several gradients
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and depths to ensure that all variations are captured. There are plenty of boaters in
Neponset. If we can get volunteers to measure water quality along the river, we can
probably get some for the estuary as well. With these measurements, the health of the
estuary can be assessed, and total maximum daily loads for nutrients and pollutants can
be established for it. Otherwise, we have no idea how inputs from the watershed are
influencing the estuary.
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Fig. 2: DO concentrations from every sample taken in the Neponset. Concentrations
range from near zero to above 15mg/L
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Mouth
10
20
Chlorophyll a (ug/L)
20
0
0
10
Chlorophyll a (ug/L)
30
30
40
40
River
0
10
30
50
Nitrate+Nitrite (uM)
70
0
10
30
50
70
Nitrate+Nitrite (uM)
Fig. 2: ChlA concentrations don’t appear to be determined by nitrate+nitrite.
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References:
Breault, R.F., 2011, Concentrations, loads, and sources of polychlorinated biphenyls,
Neponset River and Neponset River Estuary, eastern Massachusetts: U.S. Geological
Survey Scientific Investigations Report 2011–5004, 143 p
Cosper, E.M. et al. 1987. Recurrent and persistent brown tide blooms perturb coastal
marine ecosystem. Estuaries and Coasts 10: 284-290.
Herbert, R.A. 1999. Nitrogen cycling in coastal marine ecosystems. FEMS
Microbiology Reviews 23: 563-590.
Huang, W, and R. Chen. 2009. Sources and transformations of CDOM in the Neponset
River watershed. Journal of Geophysical Research 114.
Perry, S.L. 2009. More Than One River: Local, Place-Based Knowledge and the Political
Ecology of Restoration and Remediation Along the Lower Neponset River,
Massachusetts. UMass Amherst Dissertations and Theses.
Vitousek, P.M. et al. 1997. Human alteration of the global nitrogen cycle: sources and
consequences. Ecological Applications 7: 737-750.
Total maximum daily loads of bacteria for Neponset river basin. Mass. Dept. of
Environmental Protection. http://www.mass.gov/dep/water/resources/neponset.pdf
Neponset river subwatershed river and estuary segment assessments. Boston Harbor
Watershed 1999 Water Quality Assessment Report
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