Plus Island Ecosystem LTER site: Parker River Plum Island Sound Direct Measurement of Gas Exchange Rates in the Parker River Estuary

The Plum Island Ecosystem (PIE) research site is part of the
National Science Foundation's (NSF) Long Term Ecological Research
(LTER) Network. PIE-LTER research is conducted by scientists from the
Ecosystems Center at the Marine Biological Laboratory, the Univeristy
of South Carolina, the Massachusetts Audubon Society, Wells National
Estuarine Research Reserve, and the University of New Hampshire.

The Plum Island Ecosystem was originally called The Plum Island Sound
Comparative Ecosystems Research Program and was part of the The Land
Margin Ecosystems Research Program funded by the National Science
Foundation (NSF) from 1992 to 1996.

The Plum Island Ecosystem Research site investigates the importance to
estuarine ecosystems of organic carbon and organic nitrogen inputs
from watersheds with various land covers and uses. It also asks
whether the interaction of inorganic nutrients with the quantity and
quality of organic carbon and organic nitrogen play an important role
in determining the trophic structure, production and trophic
efficiency in estuaries. To answer these questions, the study:
Measures the quantity of dissolved and particulate organic carbon and
organic nitrogen entering coastal waters from lands; Conducts
experiments to determine the effects of various nutrient and organic
matter inputs and interactions on the flow and recycling of C and N
through pelagic and benthic food webs including higher trophic levels;
Models food chain transformations and the effects of changes in land
use and land cover.

Description of this data set:
Ecosystem processes such as respiration,
denitrification, and methanogenesis may be
quantified by measuring the flux of gases such as
oxygen, carbon dioxide, nitrous oxide, and
methane. For instance, we have used changes in O2
or CO2 concentrations in the water column to yield
estimates of whole system metabolism in the Parker
River Estuary. These fluxes must be corrected for
the non-biological or diffusive flux of the gas of
interest between the water and the atmosphere.
Corrections are typically made by using published
relationships between wind speed and a factor
known as the gas transfer velocity, k. The mass
flux (F) of a gas across the air-water interface
is equal to the gas transfer velocity multiplied
by the difference between the surface (Csfc) and
equilibrium concentrations (Ceq) of the gas [F= k
(Csfc-Ceq ) ] For large rivers and estuaries, k
has been empirically derived using either the
"dome" technique, which measures gas fluxes into a
floating dome, or by injecting inert gases as
tracers into the water column and directly
measuring loss to the atmosphere. Ideally, these
measurements are made over a variety of wind
conditions. Regression relationships between the
gas transfer velocity and wind velocity are then
developed and used for predicting k during gas
flux measurements. Part of LMER Plum Island Sound
Comparative Ecosystems Study Published
relationships between gas exchange rates and wind
speed derived from studies done in deep, wide
water bodies with long fetches may not be
applicable to tidal systems such as the upper
Parker River which are narrow, shallow, and
serpentine. The wind acting on such a system does
not have a uniform effect: a) the serpentine
nature of the Parker results in short reaches of
river oriented in all directions, a morphometry
that produces a generally short fetch; b) a three
meter tide range in this narrow channel results in
two vastly different situations at high and low
tides, such that at high tide the river surface is
somewhere near the same level as the marsh surface
and exposed to the wind, and at low tide, the
river surface is shielded from the wind by the
high marsh banks. Another consideration in shallow
systems is that currents and bottom topography may
contribute significantly to surface turbulence
through bottom shear, an effect not accounted for
by wind speed. To avoid the surface turbulence
effects associated with the dome technique, we
chose to directly measure gas exchange across the
air-water interface using sulfur hexafluoride gas
(SF6) as a tracer. SF6 occurs in nature at
extremely low concentrations (<10-15 M), is
chemically and biologically nonreactive, and is
easily detectable at these very low levels. SF6
has been used successfully in open ocean, lake,
and river applications.

Geographic Description of this data set:
Parker River Plum Island Sound Estuary, Massachusetts

The information for this metadata was taken in
part from the Long-Term Ecological Research Program site
See "http://ecosystems.mbl.edu/PIE/".