Records of nutrient-enhanced coastal ocean productivity in sediments from the Louisiana continental shelf

Brian J. Eadie, Brent A. McKee, Maragret B. Lansing, John A. Robbins, Simone Metz, John H. Trefry

ABSTRACT

Shelf sediments from near the mouth of the Mississippi river were collected and analyzed to examine whether records of the consequences of anthropogenic nutrient loading are preserved. Cores representing approximately 100 yr. of accumulation have increasing concentrations of organic matter over this period, indicating increased accumulation of organic carbon, rapid early diagenesis, or a combination of these processes. Stable carbon isotopes and organic tracers show that virtually all of this increase is of marine origin. evidence from two cores near the river mouth, one within the region of chronic seasonal hypoxia and one nearby but outside the hypoxic region, indicate that changes consistent with increased productivity began by approximately the mid-1950s when the inorganic carbon in benthic formas rapidly became isotopically lighter at both station. Beginning in the mid-1960s, the accumulation of organic matter, organic 13dC and 15dN all show large changes in a direction consistent with increased productivity. This last period coincides with a doubling of the load of nutrients from the Mississippi River, which leveled off in the mid-1980s. These data support the hypothesis that anthropogenic nutrient loading has had a significant impact on the Louisiana shelf.