INPUTS, TRANSFORMATIONS, AND TRANSPORT OF NITROGEN AND PHOSPHORUS IN CHESAPEAKE BAY AND SELECTED TRIBUTARIES

In this paper we assemble and analyze quantitative annual input-export budgets for total nitrogen (TN) and total phosphorus (TP) for Chesape ake Bay and three of its tributary estuaries (Potomac, Patuxent, and C hoptank rivers). The budgets include estimates of TN and TP sources (p oint, diffuse, and atmospheric), internal losses (burial in sediments, fisheries yields, and denitrification), storages in the water column and sediments, internal cycling rates (zooplankton excretion and net s ediment-water flux), and net downstream exchange. Annual terrestrial a nd atmospheric inputs (average of 1985 and 1986 data) of TN and TP ran ged from 4.3 g TN m(-2) yr(-1) to 29.3 g TN m(-2) yr(-1) and 0.32 g TP m(-2) yr(-1) to 2.42 g TP m-P yr(-1), respectively. These rates of TN and TP input represent 6-fold to and fold and 13-fold to 24-fold incr eases in loads to these systems since the precolonial period. A recent 11-yr record for the Susquehanna River indicates that annual loads of TN and TP have varied by about 2-fold and 4-fold, respectively. TN in puts increased and TP inputs decreased during the ll-yr period. The re lative importance of nutrient sources varied among these estuaries: po int sources of nutrients delivered about half the annual TN and TP loa d to the Patuxent and nearly 60% of TP inputs to the Choptank; diffuse sources contributed 60-70% of the TN and TP inputs to the mainstream Chesapeake and Potomac River. The direct deposition of atmospheric wet -fall to the surface waters of these estuaries represented 12% or less of annual TN and TP loads except in the Choptank River (37% of TN and 20% of TP). We found direct; although damped, relationships between a nnual rates of nutrient input, water-column and sediment nutrient stoc ks, and nutrient losses via burial in sediments and denitrification. O ur budgets indicate that the annual mass balance of TN and TP is maint ained by a net landward exchange of TP and, with one exception (Chopta nk River), a net seaward transport of TN. The budgets for all systems revealed that inorganic nutrients entering these estuaries from terres trial and atmospheric sources are rapidly converted to particulate and organic forms. Discrepancies between our budgets and others in the li terature were resolved by the inclusion of sediments derived from shor eline erosion. The greatest potential for errors in our budgets can be attributed to the absence of or uncertainties in estimates of atmosph eric dry-fall, contributions of nutrients via groundwater, and the sed imentation rates used to calculate nutrient burial rates.