Johan C. Varekamp¹, Ellen Thomas¹, Marilyn Buchholtz ten Brink², Marc Altabet³ , Sherri Cooper⁴, Francesca Sangiorgi⁵

• ¹Earth & Environmental Sciences, Wesleyan University, Middletown CT 06459 (jvarekamp@wesleyan.edu; ethomas@wesleyan.edu)
• ²Watershed Diagnostics Branch NHEERL, Atlantic Ecology Division, U.S. Environmental Protection Agency, 27 Tarzwell Drive, Narragansett, RI 02882 (Tenbrink.Marilyn@epamail.epa.gov)
• ³University of Massachusets at Dartmouth, MA (Maltabet@umass.edu)
• ⁴Bryn Athyn College, Bryn Athyn, PA (slcooper@newchurch.edu)
• ⁵Centro Interdipartimentale di Ricerca per le Scienze Ambientali, UniversitÃÝ di Bologna, Via S. Alberto, 163, 48100 Ravenna - Italy (franci@ambra.unibo.it)

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Goals of Research: The goal of our research is to search for climatic and local eutrophication signals in sediment cores that herald significant changes in the LIS ecosystem. We used sediment cores collected by the US Geological Survey in 1996/1997 (length 50-60 cm) and cores collected in 2001 (length 180-200 cm), accumulated at high sedimentation rates, and determined organic carbon, carbonate, foraminiferal faunal assemblages, diatom and dinoflagellates floral assemblages, the mass abundance of diatom valves (biogenic Si), Nitrogen isotopic composition, isotopic and trace element abundances of foraminiferal tests.

Results: The eutrophication of LIS started in most areas in the early 1800s AD, as indicated by the following observations:

• Increase in abundance of Clostridium perfringens (sewage input indicator, population density indicator)
• Increase in burial rate of organic carbon (productivity indicator)
• Increase in burial rate of biogenic silica (diatom productivity indicator)
• Increase in burial rate of sulfur (appears to correlate with increased rates of organic carbon burial)
• Increase in production rate of benthic foraminifera (productivity indicator)
• Increase in relative abundance of the foraminiferal species Elphidium excavatum (indicator of diatom availability, productivity)
• Increase in planktonic (centric) to benthic (pennate) diatom abundance ratio (water turbidity indicator)
• Increase in heterotrophic to autotrophic dinoflagellate ratio (eutrophication indicator)
• Increase in isotopic N signature of organic matter (d¹⁵N, sewage indicator)
• A shift in C/N values to more algal values (productivity indicator)
• Carbon isotopic composition of foram shells becomes lighter (d¹³C, indicator for amount of oxidized organic matter in bottom waters)

The following observations add more detail to this general pattern:

Changes in salinity (S) are small, but its variability increased during the 20^th century. This increased variability may be related to changes in land use patterns  which caused changes in precipitation rate to be transmitted more directly into discharge into the Sound (increased run off). Waters in westernmost LIS became less saline over the last 200 years. Discrete lows in salinity seem to correlate with periods of high precipitation, but the overall trend may be due to an increasing volume of fresh water influx from waste water treatment plants.

LIS has a general gradient from east to west: increased productivity, metal pollution and decreased salinity. Western LIS (the Narrows) may have had somewhat eutrophied conditions prior to 1800 AD as a result of sluggish circulation and a high ratio of water volume to land area discharging into this section of the Sound, as indicated by the relatively high levels of buried organic carbon and biogenic silica in the deeper sections of the core, as well as the high values of d¹⁵N in organic matter and generally light carbon isotopic values in foraminiferal shells.

Diatom, biogenic silica and benthic foraminiferal data indicate that in Western Long Island Sound and the Narrows a second wave of ecosystem changes started in the late 1960s and continued until today. These changes may have been caused by enhanced anthropogenic nitrogen inputs, which resulted in strongly increased N/Si values. The latter may have made diatoms silica-limited, and led to the emergence of cyanobacteria and/or dinoflagellates as important primary producers. This led to a decrease in overall foraminiferal abundances as well as changes in relative abundances, and may have reverberated through the higher trophic levels.

Levels of paleo-oxygenation in western LIS (estimated using d¹³C, T and S) suggest that most of LIS had average O₂ contents at about 70% saturation till about 1800 AD, when they dropped to about 30% for the next 200 years. Severe lows were usually associated with low salinity events, which may have been high nutrient inputs period.