Influence of land use and hydrology on exports of carbon and nitrogen in a Maine River Basin

The influences of land use patterns and hydrology on element transport were examined in the Aroostook River Basin of northern Maine. The drainage area contains 80% forest, 13% agriculture, 5% wetland cover, 2% surface water, and <0.5% urban development. Surface water chemistry in the Aroostook River and 18 tributary streams varied with geologic conditions, land use, and seasonal hydrology. Concentrations of Ca(2+), Mg(2+), NO(3)(-), Cl(-), SO(4)(2-), and suspended solids were up to an order of magnitude higher in streams draining agricultural watersheds, compared to tributaries with forest and wetland cover. Mean concentrations of dissolved organic carbon (DOC) were 50% higher in tributaries dominated by forest and wetland cover, as compared to those with extensive agriculture. Dissolved organic carbon in streams influenced by agriculture was proportionately enriched in lower molecular weight solutes (<1000 Da) and depleted in higher molecular weight fractions (>10 000 Da). Daily export of DOC by the Aroostook River varied over nearly three orders of magnitude from maximum values of 890 000 kg C d(-1) in November to minimum values of 1600 kg C d(-1) in September. Peak fluvial exports of suspended solids in the Aroostook River amounted to 870 000 kp d(-1). Annual runoff exports of inorganic N ranged 10-fold from 0.7 kg N ha(-1) yr(-1) in forested watersheds to 7.2 kg N ha(-1) yr(-1) in agricultural watersheds, and amounted to approximately 1 x 10(6) kg N yr(-1) for the entire river basin.