By
ALAN DESBONNET/special to ecoRI.org News
The
waters off the coast of Rhode Island, specifically Rhode Island Sound and Block
Island Sound, while traversed routinely for maritime trade and recreational
cruising, have largely been scientific enigmas regarding the details of their
physical and chemical properties. This is even more so regarding the waters
outside their backdoor—the waters extending out to the edge of the Continental
Shelf.
Those
drafting the Rhode Island Ocean Special Area Management Plan (Ocean SAMP),
however, needed a better understanding of those waters. The oceanographic
characteristics of an area have a major influence on the ecology, and you can’t
adequately avoid or mitigate ecological impacts if you don’t understand the
ecology, which in this case means you need to understand the physical
oceanography.
While
Rhode Island and Block Island Sounds are connected and indeed share many
characteristics, Dan Codiga and Dave Ullman, both physical oceanographers at
the University of Rhode Island Graduate School of Oceanography who explored the
two sounds to gain a better understanding of their oceanography to help develop
the ongoing Ocean SAMP, found them to be far from identical twins.
For
starters, Block Island Sound was found to be a much more dynamic environment.
Stronger currents and greater turbulence turned out to be a hallmark of Block
Island Sound, and Codiga and Ullman found this to be largely due to the
influence of the tides pulsing in and out of Long Island Sound. Even more
interesting is that they found the Connecticut River to be a major influence on
the salinity of Block Island Sound.
“During
extreme events such as the flood of 2010,” Ullman said, “the influence of
Connecticut River fresh water can be seen as lower salinities in surface waters
as far to the east as the middle of Rhode Island Sound. That’s an amazingly
broad influence on our coastal waters.”
The Race
The
Connecticut River drains a watershed that extends to the northern edges of
Vermont and New Hampshire, and all the water and snowmelt in that vast
watershed pours into Long Island Sound. Long Island Sound water is then pumped
into Block Island Sound via a narrow gap called The Race, which gets its name
from the way the currents race through the opening.
When
all that fresh water meets higher salinity ocean waters, it rides up over it
— fresh water is “lighter” than salt water; it doesn’t have the weight of
the salt — and tries to create two distinct layers that oceanographers call a
stratified water column. This layering effect can hamper mixing of the water
column, which in extreme cases can reduce oxygen levels in the water to a point
that is stressed (hypoxia).
“Fortunately,
the turbulence created by flows through The Race keeps the water column in
Block Island Sound generally stirred and well mixed, reducing the chance of
severe stratification,” Ullman said.
Conversely,
Rhode Island Sound doesn’t have any significant source of fresh water — there
are no major rivers pouring directly into it, and Narragansett Bay isn’t a
significant source of fresh water either. In this case, and because it’s more
tranquil, particularly in the summer, a two-layer system (stratification) often
sets up in Rhode Island Sound. Whether this stratification results in the onset
of stressful, hypoxic, conditions isn’t known.
“The
area that has the greatest potential for hypoxia to occur,” Ullman said, “is
the area in Rhode Island Sound where it meets the entrance of Narragansett
Bay.”
Stressful place
Dissolved
oxygen measures of 4 milligrams per liter, which are just on the borderline of
being considered stressful, have been observed in the area noted by Ullman, but
not enough measurements have been taken to indicate if things get worse or not.
Given increasing temperatures and reduced summer winds due to changing climate,
this is an area where further research would be appropriate, according to the
researchers.
Although
the two sounds behave somewhat differently, they do interact with one another.
Codiga and Ullman have been able to piece together an overall working model of
the physical oceanography of these offshore waters. They describe surface water
flowing into eastern Rhode Island Sound from the southeast (Nantucket/Vineyard
region), then moving west into central Rhode Island Sound, where it then turns
southwest along the east side of Block Island.
Water
moving out of Long Island Sound moves to the east and southeast, where it joins
up with water moving out of Rhode Island Sound, and all the water then moves
southwest around the tip of Montauk Point on Long Island, continuing south and west
into the Mid-Atlantic Bight.
While
collecting the information needed to pull together a picture of the general
circulation of these offshore waters, the researchers documented a unique
occurrence — the intrusion of a deep-water tongue of saltier, warmer water into
Rhode Island Sound.
“What
we observed was an extreme event,” Codiga said. “The deep water that flowed
into Rhode Island Sound was saltier and warmer than any that have been measured
in that area.”
According
to Ullman, “The water that we saw move into Rhode Island Sound had all the
characteristics of water that is typically found on the Shelf Break — the area
where the Continental Shelf drops off into the depths of the Atlantic.”
Both
Codiga and Ullman said they recorded a rare event, and both said that such
events could be significant to the biology and ecology by opening an avenue for
plants and animals from the tropics to gain access to northern temperate
inshore waters.
Ullman
sifted through archived data and found no previous documentation of such an
event. The research duo, in collaboration with Sea Grant-funded researchers
Chris Kincaid and Anna Pfeiffer-Herbert, placed in-water monitoring arrays
offshore during the winter of 2010–11, but found no evidence of a recurrence of
the deep-water intrusion.
Alan Desbonnet is the interim director Rhode Island Sea Grant.
This article originally was published in the Fall 2012
41˚ N.
