A Paleoecological Record of Changes on the Land and Estuarine Degradation
Speaker: Grace Brush, John Hopkins University
Series: CEE Departmental Seminars
Location: Bowen Hall Auditorium
Date/Time: Monday, March 28, 2011, 4:30 p.m. - 5:30 p.m.
The Chesapeake Bay, in mid-Atlantic USA, like many coastal regions of the world, has become highly eutrophic and in some areas anoxic over the last century. This ecological degradation has led to a collapse of the fish and shellfish economy. Many of the nation’s degraded waterways are now being required to develop plans to restore these ecosystems by reducing pollutants to an allowable load that it is hoped will result in restoration of the fishery and economy. Among the practices proposed to reduce nitrogen loads in particular is the use of anerobic systems such as restored and constructed wetlands.
Since wetlands were a large part of the pre-colonial landscape in the Chesapeake watershed, I decided to revisit sediment cores we have collected in the wetlands and the estuary itself, in order to determine when and why the estuarine system changed. The paleoecological record might also allow us to evaluate the feasibility of using wetlands to bring back the estuarine resource, and determine additional or alternative landscape conditions that could also aid in restoration goals.
Using sediment cores collected throughout the estuary, we have reconstructed the history of land use and fluxes of materials that flowed into the estuary over a few hundred to several thousand years. Based on the fossil pollen and seed record, the landscape prior to European colonization was almost entirely forested, consisting of a mosaic of different forest species on different geologic/soils substrates. Wetlands, particularly those created by beavers, were extensive. The ratio of arboreal to non-arboreal (excluding ragweed) to ragweed pollen in sediment cores records changes in the percentage of land in forest and in agriculture. Following colonization, 300 years ago, the land mosaic changed from one of different forest types to forested and non-forested patches. As most of the land was converted to agriculture, the original landscape pattern became obscured. In the 1600s and into the early 1700s less than 20% of the land was farmed. During this time, sediment and nutrient influxes into the estuary were slightly higher than in pre-colonial time, but still remained low. During the late 1700s and into the 1800s, about 40 to 50% of the land was in agriculture and beavers were more or less eliminated. Coincident with these changes on the land, sedimentation increased from two to tenfold, as did influxes of metals and nutrients from fertilizers. In the late 1800s and into the early 1900s, 80% of the land was agricultural. Sediment and nutrient influxes increased accordingly. Based on nutrient influxes and the ratio of benthic to planktonic diatoms preserved in the sediment, eutrophication and anoxia of the Chesapeake Bay began at approximately the time 40% of the land was under cultivation and when small farms separated by forest patches were being replaced by large commercial operations. Imported guano, phosphates and other sources of nutrients were used to increase crop production. During this time, wetlands were further diminished, streams were channelized, runoff greatly increased and the groundwater table lowered. As the landscape changed from wet to dry, and nitrogen loads including synthetic nitrogen increased, the estuarine ecosystem changed from a benthic system to a pelagic-microbial system and the fish and shellfish economy was greatly diminished.