Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores |
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| Authors: | Alexander W Karlsen Thomas M Cronin Scott E Ishman Debra A Willard Randy Kerhin Charles W Holmes Marci Marot |
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| Institution: | (1) State Key Laboratory of Tropical Oceanography, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, China;(2) Graduate University of Chinese Academy of Sciences, Beijing, China; |
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| Abstract: | Environmentally sensitive benthic foraminifera (protists) from Chesapeake Bay were used as bioindicators to estimate the timing
and degree of changes in dissolved oxygen (DO) over the past five centuries. Living foraminifers from 19 surface samples and
fossil assemblages from 11 sediment cores dated by210Pb,137Cs,14C, and pollen stratigraphy were analyzed from the tidal portions of the Patuxent, Potomac, and Choptank Rivers and the main
channel of the Chesapeake Bay.Ammonia parkinsoniana, a facultative anaerobe tolerant of periodic anoxic conditions, comprises an average of 74% of modern Chesapeake foraminiferal
assemblages (DO-0.47 and 1.72 ml l−1) compared to 0% to 15% of assemblages collected in the 1960s. Paleoecological analyses show thatA. parkinsoniana was absent prior to the late 17th century, increased to 10–25% relative frequency between approximately 1670–1720 and 1810–1900,
and became the dominant (60–90%) benthic formaniferal species in channel environments beginning in the early 1970s. Since
the 1970s, deformed tests ofA. parkinsoniana occur in all cores (10–20% ofAmmonia), suggesting unprecedented stressful benthic conditions. These cores indicate that prior to the late 17th century, there
was limited oxygen depletion. During the past 200 years, decadal scale variability in oxygen depletion has occurred, as dysoxic
(DO=0.1–1.0 ml l−1), perhaps short-term anoxic (DO<0.1 ml l−1) conditions developed. The most extensive (spatially and temporally) anoxic conditions were reached during the 1970s. Over
decadal timescales, DO variability seems to be linked closely to climatological factors influencing river discharge; the unprecedented
anoxia since the early 1970s is attributed mainly to high freshwater flow and to an increase in nutrient concentrations from
the watershed. |
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