1) Earth & Environmental Sciences, Wesleyan University, Middletown CT 06459 (jvarekamp@wesleyan.edu),

2) U.S. Geological Survey, Coastal Geology, Woods Hole MA (Mtenbrink@usgs.gov)

3) School for Marine Science and Technology, University of Massachusetts at Dartmouth, MA (Maltabet@umass.edu)

4) Biology Department, Bryn Athyn College, Bryn Athyn, PA (slcooper@newchurch.edu)

• For more data on geochemistry: see abstract for Goldschmidt Conference (2002).
• For more data on foraminifera: see abstract for annual meeting GSA (2002)

The goal of our research is to document recent environmental changes in Long Island Sound (LIS) and their effects on the ecosystem as shown in microbiota which leave fossil evidence (photosynthesizing diatoms and dinoflagellates, and heterotrophic foraminifera and dinoflagellates), and put these into the historical perspective of the last 500-1000 years. We emphasize changes that occurred over the last 150 years, with the main anthropogenic imprint since the middle part of the 19^th century, the changes that occurred over the last 30-40 years since anoxia/hypoxia in western LIS became common after the late1960s-early 1970s, and changes in the late 1990s when the lobster die-off occurred. In order to document these environmental and biotic changes we have studied samples from sediment cores to record changes in dinoflagellate and diatom floras and benthic foraminiferal faunas, to obtain evidence for changes in water temperature and salinity (using d¹⁸O and Mg/Ca in carbonate foraminiferal shells), to document pollutant burdens, degree of bottom water oxygenation (using d¹³C in the calcite of foraminiferal shells), to provide evidence for changes in the magnitude of sewage input (using abundances of the bacterial spore C. perfringens), changes in diatom productivity (using analyses of sediment-stored biogenic Silica), and changes in the storage and origin of C_org, and N_org , using C and N abundances and isotopic compositions (d¹⁵N and d¹³C) and changes in sulfur abundances. We have also put effort into calibrating these indicators used in the core samples in the modern LIS environment through water sampling, surface sediment sampling and measurements of water column parameters (temperature, dissolved oxygen, salinity). We are working on providing age models for the sediment cores to obtain temporal records of the environmental and biotic changes.

Nutrient fluxes have increased in the Sound with increased population density and changes in land use patterns. The Narrows and Western LIS have the largest inputs of effluents from Waste Water Treatment Plants (WWTP) and there we find the most pronounced increased organic productivity as well as the resulting bottom water hypoxia/anoxia as a result of this eutrophication process.

Records of C_org concentrations in sediment cores show an increase from ~ 1850 on, in central LIS going from 1.2 to 2.5 % and in western LIS up to 4.5 %. A core in western LIS also shows an increase in N concentration from ~0.1 at the bottom to ~0.2 % at the top. Recalculation of C_orgdata as C_org accumulation rates shows that these rates have increased by a factor of 5 between 1850 and 2000. Accumulation rates of Biogenic Silica have increased as well, by a factor of 4-5 over the last 150 years. This finding confirms and quantifies the long-held suspicion that the primary productivity and resulting flux of organic carbon to the LIS bottom waters has increased strongly as a result of eutrophication. We can, however, not conclude without further consideration that this enhanced carbon flux is the direct cause of the bottom water hypoxia/anoxia, because the carbon is covered more rapidly by sediment as the result of the higher sediment accretion rates of the last 150 years.

The records of paleo-salinity and -temperature (mean bottom water temperature over several years) show strong variations over the last millennium, with a positive correlation between temperature and salinity (warm and dry versus cool and wet). The highest temperatures occurred about 1000 years ago (Medieval Warm period) whereas the lowest water temperatures were reached about 200 years ago (end of Little Ice Age). The water temperatures have increased over the last 100 years. The paleo salinity shows a narrow window between 26 and 31 ^o/_oo, with more extreme events during the 20^th century. We assume that these short, low-salinity events are the result of wet periods, which, with changes in land use, resulted in more direct pulses of fresh water input into the Sound than before. The d¹³C values of carbonates in LIS sediment become substantially lighter over the last 200 years (Figures 2 and 3), the result of the oxidation of organic carbon in the bottom waters. The Nitrogen isotope signal becomes heavier by almost 2 per mille over that same period, evidence for the influx of anthropogenic nitrogen into the LIS system (see figures 2 and 3, below).

Diatom floras from a core in western LIS show a major decrease in diversity and species richness and an increase in the centric/pennate ratio (C:P; an indicator of eutrophication and increasing water turbidity) starting in the middle 19^th century, when C_org data indicate increasing organic storage in the sediments (figure 3). During the last few decades, the number of diatom valves declined as did diatom diversity, while the ratio of centric to pennate diatoms increased even more. Preliminary data on dinoflagellates show a strong east-west gradient in surface samples, with heterotrophic dinoflagellates (indicators of eutrophication) more abundant in western LIS.

Benthic foraminiferal faunas show the most severe changes with time in western LIS. In most cores, the total abundance of foraminifera (nr/gr bulk sediment) increased from the middle of the 19^th century, but decreased again during the last ~30 years, and most prominently in the last few years. The mid 19^th century increase in number of benthic foraminifera was caused by an increase in absolute and relative abundance of the diatom-consuming species Elphidium excavatum. The decrease in total foraminiferal numbers in western LIS and the Narrows was caused by the decrease in numbers of E. excavatum during the last few decades (figures 2 and 3). The decrease in relative abundance of this species was caused by an increase in abundance of the omnivorous Ammonia beccarii, a cosmopolitan omnivorous species which was very rare in LIS before the mid-1960s.

We speculate that the major changes in the benthic foraminiferal faunas were largely caused by the increased diatom productivity in the middle 19^th century, followed by the decrease in diatom abundance in the last few decades. The increase in C:P in diatoms may be explained by increasing eutrophication and resulting increase in turbidity of the water column. The recent decrease in abundance of diatom valves and decrease in diversity could be explained by silica limitation during the spring phytoplankton bloom. The strong increase in relative abundance of the benthic foraminifer A. beccarii in western LIS at a time of decreasing abundance of the diatom-consuming species could have been caused by blooming of non-diatom phytoplankton (Figure 1). In the last few decades, the main primary producers in LIS may thus have changed from diatoms to organic-walled phytoplankton such as dinoflagellates, which has potentially a major impact on all LIS biota.

Conclusions

The paleo environmental records (examples shown below for core WLIS 75 in the Narrows, figure 1; cores A1C1 and A4C1 in West LIS, figures 2 and 3) show clear evidence for eutrophication of the Sound since the mid 19^th century, as evidenced by enhanced storage of Organic Carbon, Biogenic Silica, and Nitrogen, heavier nitrogen isotopes, lighter carbon isotopes, and changes in benthic foraminiferal faunas and diatom floras. A decrease in Biogenic Silica and change in fauna in the last 20-30 years in the extreme western Sound (Figure 1) may signal the onset of new changes in the LIS ecosystem. Salinity and water temperature have not moved dramatically outside their long-term range, but over the last century stronger variations in salinity seem to occur and waters have been warming. The warming with the occurrence of more extreme events in low-salinity together with the enhanced production of organic carbon is most likely the cause for the hypoxia/anoxia in western LIS. The documented changes in the LIS microfloral and faunal ecosystem may have propagated throughout the LIS ecosystem.

---

Figure 3: Records for Core A4C1 in western LIS, water depth 25 m, latitude 41.033047 'N, longitude 73.287978 'W (decimal degrees). Top figure: percentage of E. excavatum), a diatom-consuming foraminiferal species (red), absolute abundance of foraminifera (blue), and centric/pennate ratio of diatoms (green, eutrophication indicator). Middle figure: weight percent organic carbon in bulk sediment (black), excess carbon isotope value in foraminiferal carbonate (light blue). Lowest figure: abundance of the sewage-indicating bacterial spore Clostridium perfringens (number of spores per gram dry sediment (black) and mercury concentration (ppb, green).

 

---

return to home page