Historical trends in Chesapeake Bay dissolved oxygen based on benthic foraminifera from sediment cores

Environmentally sensitive benthic foraminifera (protists) from Chesapeake B ay 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 by Pb-210, Cs-131, C-14, and pollen stratigraphy were analyzed from the ti dal portions of the Patuxent, Potomac, and Choptank Rivers and the main cha nnel of the Chesapeake Bay. Ammonia parkinsoniana, a facultative anaerobe t olerant of periodic anoxic conditions, comprises an average of 74% of moder n Chesapeake foraminiferal assemblages (DO = 0.47 and 1.72 ml l(-1)) compar ed to 0% to 15% of assemblages collected in the 1960s. Paleoecological anal yses show that A. parkinsoniana was absent prior to the late 17th century i ncreased to 10-25% relative frequency between approximately 1670-1720 and 1 810-1900, and became the dominant (60-90%) benthic foraminiferal species in channel environments beginning in the early 1970s. Since the 1970s, deform ed tests of A. parkinsoniana occur in all cores (10-20% of Ammonia), sugges ting 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 occur red, as dysoxic (DO = 0.1-1.0 mi l(-1)), perhaps short-term anoxic (DO < 0. 1 mi l(-1)) conditions developed. The most extensive (spatially and tempora lly) anoxic conditions were reached during the 1970s. Over decadal timescal es, DO variability scents to be Linked closely to climatological: factors i nfluencing river discharge; the unprecedented anoxia since the early 1970s is attributed mainly to high freshwater now and to an increase in nutrient concentrations From the watershed.