Inference of nitrogen cycling in three watersheds of northern Florida, USA, by multivariate statistical analysis

Abstract

Nitrogen in fresh waters of three rivers in northern Florida-the Apalachicola-Chattahoochee-Flint (ACF) River system, Ochlockonee (Och), and Sopchoppy (Sop)- is inferred to be derived mostly from atmospheric deposition. Because the N:P mole ratios in the rivers are nearly three times higher than the Redfield ratio for aquatic photosynthesis, N is saturate in the ecosystems, not a limiting nutrient, although it may be chemically transformed. Absolute principal component analysis (APCA), a receptor model, was applied to many years of monitoring data for Apalachicola River water and rainfall over its basin in order to better understand aquatic chemistry of nitrogen in the watershed. The APCA model aged rain and groundwater. In the fresh rain component, the ratio of atmospheric nitrate to sulfate is close to that in rainwater, as if some samples had been collected following very recent rainfall. The aged rain component of the river water is distinguished by a low NO[sup [minus][sub 3]]/SO[sup 2[minus][sub 4]] ratio, signifying an atmospheric source but with most of its nitrate having been lost or transformed. The groundwater component, inferred from its concentration to contribute on average about one fourth of the river water, contains abundant Ca[sup 2+] but no detectable nitrogen. Results similar tomore » ACF were obtained for Sop and Och, though Och exhibits some association of NO[sup [minus][sub 3]] with the Ca[sup 2+]-rich component. Similar APCA of wet precipitation resolves mainly components that represent acid rain, with NO[sup [minus]][sub 3], SO[sup 2[minus]][sub 4], and NH[sup +][sub 4], and sea salt, with Na[sup +], Cl[sup [minus]], and Mg[sup 2+]. Inland, the acid rain component is relatively more prominent and Cl[sup [minus]] is depleted, while at atmospheric monitoring sites nearer the coastal region sea salt tends to be more prominent.« less