Coastal Eutrophication – The impact of Agriculture in Chesapeake Bay

Over the past 300 years, Chesapeake estuary in mid-Atlantic USA has been converted from natural forests and wetlands into agricultural fields and urban development. Brush (2009) took sediment cores from the estuary, dating back 14,000 years ago, to look at the effects of these changes.

Sediment, nitrogen, pollen, diatom, and seed profiles from sediment cores suggest that prior to disturbance, the nitrogen cycle of this area consisted of a balance between biological nitrogen fixation and denitrification (Brush, 2009). This balance was maintained while small agricultural settlements were developed. However as land use changed to incorporate more intensive farming to support a growing population, this balance was disturbed. As agriculture grew and became more intensive, wetlands were drained, land was deforested and streams were channelized to reduce flooding of agricultural land and chemically produced nitrogen fertilizers were used to farm less fertile, marginal land. Changes to the landscape vegetation, hydrology and geochemistry resulted in a reduced denitrifying capacity of the area. This resulted in increased nitrogen loadings into the estuary, which lead to coastal eutrophication.

This diagram was produced to highlight the changes in the area over time. It is evident that many changes occurred as the population increased and with the onset of intensive agriculture (Brush, 2009).

The diagram shows how pollen types reflect the changes in land use. Sedimentation rates also increase as land was cleared for agriculture. These changes led to a shift from a benthic dominated system to planktonic dominated system, due to lack of light in the water column, which was further reduced by continued Planktonic algal growth. The increase in fertilizer purchases was mirrored by nitrogen fluxes in the sediment column. Through the process of eutrophication, this eventually led to the deep waters becoming anoxic and productivity declined (Brush, 2009).

Brush (2009) highlights how coastal eutrophication in Chesapeake Bay has significantly reduced coastal shellfish and fishery resources (especially oyster farming as shown in the diagram), which are important food sources for humans. Therefore in an attempt to feed the growing population of Chesapeake Bay through intensive agriculture, other valuable natural food resources were destroyed, and so far, all efforts to return Chesapeake Bay to its natural state have failed.