Timing is Everything:
The Importance of Salinity to Estuaries
by Tracey Crago, WHOI Sea Grant
Cape Cod draws many to its shores
come spring, but the melting snow beckons Woods Hole scientist Anne
Giblin north, to the marshes of the Parker RiverPlum Island Sound
region on Massachusetts' north shore.
co-investigator Chuck Hopkinson, Jr., takes water samples in
the Parker River.
courtesy of Debbie Scanlon, MBL Ecosystems Center
Giblin, an associate
scientist at the Marine Biological Laboratory's Ecosystems Center,
is co-investigator in a Woods Hole Sea Grant-supported project examining
the effects of freshwater discharge on the nitrogen dynamics of
estuarine systems. Along with Chuck Hopkinson, Jr., a senior scientist
at MBL's Ecosystems Center, Jane Tucker, a senior research assistant
at MBL, professor Gary Banta and two of his graduate students from
Roskilde University in Denmark, Giblin is hoping to add key pieces
to the complex puzzle of estuarine productivity. Previous Sea Grant
support for the research team has contributed to a greater understanding
of the effects of salinity on nitrogen cycling in the subtidal portions
of the oligohaline zone, or low salinity portion, of the estuary.
The oligohaline zone in many estuaries is a region of high biodiversity
and productivity. It is also the area where high inputs of riverine
nutrients can cause nuisance algal blooms. Surprisingly, says Giblin,
it is not as well studied as the saline regions of estuaries.
In a previous Sea Grant
study, the team sampled the upper reaches of Plum Island Sound in
the Parker River, an area that experiences significant temporal
salinity changes: a gradual increase in average salinity from late
spring to late summer as river discharge decreases; a monthly oscillation
in the maximum salinity that corresponds to neap and spring tidal
cycles; and twice-daily oscillations in the salinity caused by the
semi-diurnal tidal cycle.
"Salinity changes in
the overlying water can have a profound effect on both the timing
and magnitude of benthic nitrogen release," says Giblin. The researchers'
work has shown that an increase in salinity causes a decrease in
ammonium adsorption (adherence to sediments or organic matter) and
a decrease in the rate of denitrification.
While this is important
in subtidal areas, it may be even more significant in the surrounding
fresh and brackish tidal marshes. In the Plum Island study site,
for example, intertidal marshes occupy more than 10 times the area
of subtidal sediments, and they experience greater changes in salinity.
Could intertidal marshes—considered
a net sink for nitrogen due to denitrification and burial—also act
as a source of nitrogen in the summer, when salinity is higher?
Giblin and her colleagues are midway through their project, with
and her colleagues collect salinity and exchangeable ammonium
samples at several subtidal and intertidal stations within the
Plum Island Sound estuary.
courtesy of Anne Giblin, MBL Ecosystems Center
Sampling the intertidal
region is "not nearly as easy" as measuring in subtidal areas, says
Giblin. For starters, she says, measuring denitrification is a little
more problematic because of the high nitrogen concentrations in
air. And then there are the smaller-scale interfaces that characterize
marsh systems. "There is a whole family of places where the interaction
between the marsh and open water represent varying degrees of potential
exchange, making quantifying such interactions difficult," says
and hard-working graduate students to study these interactions helps,
says Giblin. One, Ketil Koop-Jakobsen, focused much of his master's
thesis project on exchange processes in the marsh, focusing on the
importance of adsorbed ammonium in the tidal flushing of ammonium
from intertidal salt marsh sediments.
"In a freshwater environment,
ammonium tends to stickbetter to sediment and organic matter—up
to 100 times more than you would see in a saline environment," says
Giblin. But in summer, when nitrogen inputs are at their lowest
and the freshwater input decreases dramatically, investigators see
an ammonium pulse. "The high nitrogen fluxes we see in the summer
suggest a physiochemical effect. How important is that? Why are
there plankton blooms in the summer when there are very few external
inputs of nitrogen to the system?"
Early results, including
Koop-Jakobsen's work, have provided some clues. In summer, the salinity
causes adsorbed ammonium to be released from the sediment and, through
diffusion and convection, it becomes available in the water column
where it could stimulate a phytoplankton bloom.
that the amount of adsorbed ammonium released from the sediment
was substantial during inundation at high tides.
While these results
contribute to the broader goal of modeling the nitrogen cycle of
the estuary—a goal Giblin shares with many others working in the
Plum Island Sound region—the project also speaks to the importance
of hydrological manipulation within estuarine systems.
"A dammed system, like
the Parker River, has a greatly accentuated hydrographic flow,"
says Giblin. "By controlling the timing and/or flow of freshwater
inputs, the nitrogen cycling and residence time in an estuary could
A model of the nitrogen
cycle could, for example, be used to assess the impact of water
withdrawal or addition on the watershed and to look at storm and
drought events. What's more, says Giblin, the hydrologic conditions
of an estuary could play a role in enhancing or retarding eutrophication.
"This model could be a powerful management tool for estuaries like
the Parker River, where there is some control over the freshwater
inputs on a seasonal basis, to help better manage estuaries to minimize