AND BIOTA
IN LONG ISLAND SOUND:
THE LAST 40 YEARS
E. Thomas, J. C. Varekamp (Earth and Environmental Sciences, Wesleyan University) and E. L. Mecray, M. R. Buchholtz ten Brink (Center for Coastal and Marine Geology, U.S. Geological Survey, Woods Hole, MA)
With help from Wesleyan students: Irina Abramson, Taras Gapotchenko, Ryan Huggins, Polina Rabinovich.
And financial support from Connecticut Sea Grant College Program (grant nr.DSG443 R/ER-2), from Wesleyan University, and from the U. S. Geological Survey Coastal and Marine Geology Program.
Presented at the Long Island Sound Symposium, fall 2000.
Facts on Long
Island Sound (LIS):
First severe anoxia: 1971
LIS
hypoxia/anoxia:
1. influx of ‘fertilizer’
2. stratification of water
Hypoxia: oxygen concenrtation less thann 5 ml/L; Anoxia: no oxygen.
Both the influx of nutrients and the climatic conditions that
influence stratification play a role in the development of
hypoxia/anoxia.
Benthic Foraminifera as
Monitors of Long Island Sound Environments:
We have data on the faunal composition in the past, before the 1971 episode of anoxia, so we can see whether faunas changed over the last few decades.
See
map below for sample location of these three
studies.
Data on benthic foraminifera for the 1960s:
The most common species are Elphidium excavatum, Buccella frigida, Eggerella advena; Ammonia beccarii is rare
Things had
changed in 1996/1997, and keep changing in
1999/2000:
In western LIS we see the
most
severe faunal change. But
many things differ in western LIS (as compared to central and eastern
basins):
WHICH of these linked to faunal change?
We look
at the Ammonia -
Elphidium
index,
which was defined in the Gulf of Mexico
(Barun K. Sen Gupta).
This index expresses the relative abundance of Ammonia as compared to
that of Ammnoia+Elphidium. In the Gulf of Mexico the
A-E
index is
related to oxygenation:
high
A-E values,
low
oxygenation. Note
that values of A-E remain high in 1999, sampled after the summer
hypoxia period. Values for 2000 appear lower, but hypoxia was not
very severe that summer according to data collected by the
Connecticut Department of Environmental Protection.
In Long Island Sound, the
Department of Environmental Protection shared some samples with us,
from locations where they measured the dissolved oxygen in July 2000.
Although these are few samples, there does appear to be a negative
correlation between disslved oxugen levels and the A-E
index.
We also compared the A-E index
with the abundance of the bacterial spore Clostridium
perfringens, which lives in mammalian (including human) guts. The
bacterium forms the spore when it leaves the body of its host, and is
preserved in effluent from waste water treatment plants. Its
abundance thus can be used as a tracer for such effluent.
Note that we see high values for the A-E index ( low oxygenation) in western LIS with high abundance of Clostridium perfringens spores, indicators for sewage (even treated sewage).
Data on the A-E index in a core taken in western LIS (age model tentative) show a strong increase in A-E index since the early 1970s, and the first severe hypoxia was recorded in 1971.
We
therefore suspect wastewater influx (nutrients) as the main cause of
the benthic faunal change and increased abundance of A. beccarii, and
we think that increased
abundance of Ammonia
beccarii
in
western LIS may be linked to
effluent
from Waste Water Treatment Plants
But what exactly caused this increase in abundance? Several possibilities must be investigated:
There is another possibility as well: in the laboratory it has been shown that A. beccarii needs temperatures at bottom >17oC for more than 30 days in order to reprodce successfully. E. excavatum is a cold-water species. Maybe recent warming has resulted in making it possible for A. beccarii to survive well in the present of abundant food.
Long Island Sound is at about the boundary between colder and warmer benthic invertebrate ecosystems along the US eastern seaboard; the American lobster, for instance, occurs in LIS and just along it southern shore in deeper waters, but not further south. It is a possibility that boundaries between these faunal zones are moving northward as a result of 'global warming'. We need to look at temperature records for Long Island Sound to see whether such a temperature change indeed occurred, or we need to look at proxies for temperatures. Comparison of surface water temperatures at Norwalk Harbor between 1978 and 1998 suggests that the period of temperatures high enough for efficient reproduction for A. beccarii indeed increased over that period (see red horizontal line).
Isotopic
studies: Oxygen
and carbon isotope values on the most abundant species,
Elphidium
excavatum
Note that both oxygen isotopes values for oxygen in ea water are about 0, for river water about -9.5 per mille. For dissolved inorganic carbon in rivers and the oceans the values are about the same (see table).
S, ‰ Ocean River
So if we mix river and sea water we expect lower isotopic values for both parameters. BUT concentrations on water and dissolved inorganic carbon are not the same in btoh these sources, we we need a mixing model in order to figure out what carbon isotopic value we expect to co-occur with what oxygen isotopc value, IF all isotopic change results from mixing of river and sea water. We calculate d18O values as function of salinity, while assuming bottom water temperatures of 7.0 to 10.6oC (late spring through summer ).
Oxygen isotope data (preliminary):
Note from the above figure that we expect a larger range in values for oxygen isotopes than for carbon isotopes!
Now look at the figure below, which gives the data: the range in oxygen isotope values is smaller than that in the carbon isotope values (contrary to expectation, IF all the isotope data could be explained by mixing of river and sea water).
Carbon isotope data:
thus, some other process (other than mixing) MUST have played a role. That process most likely is:
Can we say how much
oxidation of organic matter occurred, and where in LIS?
Yes, we can estimate d13C variability not related to mixing of sea and river water (d13C*)
NOTE: the "salinity-corrected" carbon isotope values in western LIS
were much lighter in 1996 than in 1961, and the slope of the line
through these values when plotted against longitude was steeper in
1996. This suggests that more organic matter was oxidized in western
LIS in 1996 than in 1961.
Carbon isotope data suggest increased oxidation of organic matter between the 1960s and 1990s. Since oxidation uses oxygen, this suggests that hypoxia also increased over that time. What we do not know, is whether that organic matter was locally produced (i.e., consists of algae produced during blooms). or was organic matter added in the effluent from waste water treatment plants. Studies of nitrogen isotopes of C/N ratios might help find this out.
Conclusions
Speculation:
There might be linkages between global to regional climate variability (particularly precipitation patterns) and the devlopment of hypoxia in LIS: there are few data points available, but there seems to be a possible correlation between duration of hypoxia and El Nino events (as well as between the severity of hypoxia and the index of North Atlantic Oscillation).
