ANTHROPOGENIC EUTROPHICATION AS RECORDED BY VARVED SEDIMENTS IN THE PETTAQUAMSCUTT RIVER ESTUARY, RHODE ISLAND, USA

Eutrophication in coastal water bodies has seen much attention recently due to its negative ecological and aesthetic impacts. This study takes advantage of a varved sediment record from an estuary in southern Rhode Island to document historical eutrophication and its ecological impact on the system as recorded in the geologic record. The high-resolution stratigraphy sheds light on the ecosystem structure and productivity locally over a time period long enough to document the natural variability as well as anthropogenic effects. The Pettaquamscutt River Estuary is a north-south trending coastal environment averaging approximately 3 meters depth and whose watershed occupies roughly 3600 hectares. In the upper reaches there are two former kettle depressions, which form density-stratified water columns due to their depths (20 meters). The stagnant and anoxic bottom waters in these holes are ideal for varve formation since bioturbation is non-existent and there is a seasonal biological-erosional sedimentation cycle. The watershed of the estuary experienced a developmental boom in the middle part of the twentieth-century, leading to increased nutrient inputs from septic systems and fertilizer use. We have examined two freeze-cores taken from the Lower Basin (1999, 2001) in order to document the ecological consequences of this increased nutrient flux. The higher nutrient loads are evident in the stable nitrogen isotope shift that is seen in the late 1950’s. Concurrent with this shift, the ecology and yield of the aquatic producers shifts as observed in both increased summer (biogenic) varve laminae thickness and increases in fossil pigment (chl-a, chl-b-, bchl) concentrations. The levels for all of these parameters are higher over the past 50 years than they have been at any other time during our 250-year record. It is therefore concluded that the anthropogenic nutrient influx over the last half century has resulted in eutrophication to a greater degree than would be expected from natural variability.