Summer/Fall 2001 Table
The Proof is in the Plants
By Tracey Crago, WHOI Sea Grant
In science, it is more often the norm
than the exception that what works in one location does not always
work in another. But for Marci Cole, a graduate student in the Boston
University Marine Program (BUMP), the science gods must be smiling.
Coles WHOI Sea Grant-supported research tracks land-derived
wastewater nitrogen in groundwater and aquatic plants in Waquoit
Bay, a saltwater embayment on Cape Cod. Cole has been delighted
to find that the model also works in three estuaries (Great and
Green Ponds in Falmouth, and Mashpee River), two freshwater ponds
(Ashumet and Coonamessett Ponds in Falmouth), and one salt pond
(Miacomet Pond on Nantucket). And whats more: it works from
season to season.
"This information-and the
method- should be useful for managers," says Cole. "Its
easy, relatively cheap, and transferable."
Coles work is based on the so-called
nitrogen loading model, or NLM, developed by her advisor, Ivan Valiela,
a BUMP professor of biology, and Jim McClelland, a 1998 graduate
of BUMP now a postdoctoral researcher at the Marine Biological Laboratory,
where BUMP is based. The NLM, funded in part by WHOI Sea Grant,
estimates the percentage of wastewater nitrogen in an estuary that
comes from groundwater. It can also estimate the nitrogen load from
atmospheric deposition and fertilizer. Those calculations are based
in part on calculations of land use data, gathered via aerial photography
(for example, houses, open space, wooded areas, farms, cranberry
bogs, golf courses, and so on), and GIS data. Of the three sources
of nitrogen, the signature from wastewater nitrogen-d15N-is
the highest in groundwater. That makes it easy to spot, explains
Thats where Coles work
comes in. Hard work. She has spent countless hours in all kinds
of weather collecting groundwater with an instrument called a piezometer,
driving it into the ground every 50 meters around each of her six
sites for each of three seasons. Coles reward for hard physical
labor: samples that provide actual nitrogen concentrations for groundwater
(measured in micromolar, or ÁM), as well as the d15N signature.
While in the field, Cole collects samples
of particulate organic matter (POM) and producers: algae at marine
sites and macrophytes at freshwater sites, and marsh vegetation
at all sites. These samples are collected near the shore or from
a boat at 10 locations within each estuary. "We collect a handful
of whatever is most abundant from each site. We try for the same
thing in each estuary, for example Spartina in the saltwater sites
and cattails in the freshwater sites," says Cole.
The samples are then taken back to
the lab where they are rinsed, dried, ground up, and sent off for
analysis. That analysis provides in a stable isotopic signature
(d15N) for each producer. Each producer is then assigned a single
value per season, per estuary. Coles results indicate that
macrophytes (Spartina, Typha, submergent plants) are the best indicators
of wastewater nitrogen, though macroalgae (Enteromorpha, Gracilaria)
are "not bad." POM, says Cole, "was not very reliable."
Based on those results, and the fact
that no seasonal pattern emerged, Cole believes that collecting
producers-perhaps only macrophytes-in summer would suffice.
Recalling her days spent trudging along shorelines in boots and
sampling from a small boat, Cole thinks that is a real plus. "That
is good for people, say coastal managers, who may be interested
in using this method: you dont have to go out in a boat or
even get your feet wet."
collected samples of estuarine producers (particulate organic
matter or POM, macroalgae, and macrophytes) at each site to
obtain a stable isotopic signature (d15N) for each producer.
She compared these results with wastewater derived nitrogen
values for the same estuary. Her results suggest that macrophytes
are the best indicator of wastewater nitrogen in an estuary.
With the signatures in hand, Cole then
compared her results to modeled wastewater nitrogen load for each
estuary (from the NLM). Coles results show that a relationship
exists between the isotopic signal of groundwater and producers,
and the wastewater nitrogen load value derived from the model (see
graph). This relationship can be used to predict what percentage
of nitrogen-of all the nitrogen coming into an estuary-is
coming from wastewater via groundwater," explains Cole. "A
value of 020 percent would be considered a good scenario,
and 6080 percent would represent the high end." To get
a sense of the range involved, Sage Lot Pond (a salt pond located
just east of Waquoit Bay) is 8 percent; Miacomet Pond on Nantucket
is around 30 percent; Great and Green Ponds (salt ponds west of
Waquoit Bay) and Quashnet and Childs Rivers (freshwater inputs
to Waquoit Bay) are between 40 and 65 percent; and Ashumet Pond
(a fresh pond northwest of Waquoit Bay) is between 8182 percent.
In addition to the analyses conducted
on the producers, the groundwater samples are analyzed for nitrate,
ammonium, total dissolved nitrogen, and stable isotopes for nitrate
and ammonium. For freshwater sites, phosphate is measured as well.
For someone who came to graduate school
hoping to work on estuarine sediment cores, Cole has not spent much
time looking at sediment, other than the occasional groundwater
sample containing fine grains ("it looked like a chocolate
milkshake and took three hours to filter," she says). But her
results have made her happy with few regrets. "I like the idea
that my work has management implications," she says. "It
makes me feel good about what I do;knowing that its useful
in some way. But I wont miss the briars, poison ivy, and ticks."
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