Da. Hodell et Cl. Schelske, PRODUCTION, SEDIMENTATION, AND ISOTOPIC COMPOSITION OF ORGANIC-MATTERIN LAKE-ONTARIO, Limnology and oceanography, 43(2), 1998, pp. 200-214
Organic matter and its carbon and nitrogen isotopic composition were m
easured in sequential sediment trap and core samples from the Rocheste
r Basin of Lake Ontario to evaluate their usefulness in reconstructing
historic changes in lake productivity. The greatest flux of organic m
atter from the epilimnion occurred during late summer and coincided wi
th whiting events, indicating that calcite precipitation is an effecti
ve mechanism for sedimenting organic matter. Carbon isotopes of organi
c matter were low prior to the onset of stratification, increased to m
aximum values in late summer, and then decreased following fall overtu
rn. This pattern is controlled mainly by the timing of stratification
and primary productivity, which preferentially removes (CO2)-C-12 from
the epilimnion. The physiological effect of decreased carbon isotopic
fractionation with decreasing supplies of [CO2](aq) may have also con
tributed to increased delta(13)C(orgC). Nitrogen isotopes showed a sea
sonal pattern opposite to that of carbon, whereby delta(15)N values we
re low during the summer stratified period and high for the remainder
of the year. Seasonal variability in delta(15)N(orgN) probably reflect
s changes in the source of sedimented organic particles, which is domi
nated by isotopically depleted phytodetritus during the stratified per
iod and isotopically enriched organic matter from heterotrophic or det
rital sources during the mixed period. A comparison of organic carbon
accumulation rates and delta(13)C(orgC) between sediment cores collect
ed in 1987 and 1993-1994 confirms earlier predictions that diagenetic
processes reduce the mass accumulation of organic carbon in the zone o
f oxic pore waters, but will not change the delta(13)C(orgC) values. A
ll cores analyzed for delta(13)C(orgC) display the reproducible patter
n of a progressive increase in the 1900s, peaking in the early to mid-
1970s, and then decreasing to the present. This pattern matches the hi
storical trends of phosphorus loading to the basin, suggesting that de
lta(13)C of organic carbon is a reliable proxy for paleoproductivity a
nd responds to spring phosphorus supplies in the water column. The del
ta(15)N of sedimentary organic matter increased linearly from 1840 to
1960 at a rate of 0.3 parts per thousand per decade, and remained rela
tively constant thereafter except for an increase in the upper few cen
timeters of sediment. The increase in delta(15)N(orgN) reflects a comb
ination of factors, including early forest clearance by Europeans, inc
reased sewering by municipalities after 1940, and increased nitrate ut
ilization as productivity increased in the lower Great Lakes. Increase
d rates of denitrification in the central basin of upstream Lake Erie
from the 1930s to the early 1970s may have also contributed to the ris
e in delta(15)N(orgN) values.