Abstract

The health of the Chesapeake ecosystem is of vital importance to the United States for recreational, economical, and ecological reasons. An increase in nutrient inputs to the Chesapeake Bay result in an increase in available nutrients for spring algal blooms and a consequent increase in summer hypoxia. Recent studies have identified a nonlinear relationship between spring total nitrogen loading and summer hypoxic volume pre and post 1985. It is unclear whether this observation is a result of an artifact of data collection or a real change in the ecological processes that result in hypoxia formation in the Bay. This thesis examines the past fifty-five years of nitrogen loading to the Chesapeake Bay in order to examine a portion of this question.

Nitrogen loading has been monitored extensively at multiple sites in the Chesapeake Bay since 1984. Similar pre-1984 long term records do not exist. The previously available historical loading estimates were derived from estimated inputs at the Susquehanna River fall line. These indicate an increasing tread in nitrogen loading from 1950 to the mid 1980s followed by a decrease or leveling off of nitrogen loading to the present.

Many major nutrient inputs peaked in the Chesapeake Bay watershed during the mid 1980s. These included an increase in urban land use, and a maximum in corn acreage harvested and fertilizer sales within the basin. One significant finding in this thesis is that corn harvested and nitrogen fertilizer sales correlate well with the total nitrogen concentration at the Susquehanna River fall line, as well as at the Potomac and Patuxent River fall lines. Determining that multiple sources of loading independently reached a maximum in the mid-1980s supports the hypothesis that there has been a change in the relationship between total nitrogen loading and the extent of hypoxia.

The compiled loading information has been used to make an initial simulation of the fifty five year loading to the Bay using the HSPF model developed by the EPA Chesapeake Bay Program. The model simulates river flows well but needs to be updated to reflect the effects of changing fertilizer use in the basin.