Spring-summer cycling of DOC, DON and inorganic N in a highly seasonal system encompassing the Northeast Water Polynya, 1993

Dissolved organic nitrogen (DON), dissolved organic carbon (DOC) and inorga nic nutrient concentrations were determined in samples from an area encompa ssing the Northeast Water Polynya from June to August 1993. In June, still ice-covered polynya area surface waters (PySW) had significantly higher (p < 0.05) DOC concentrations (110 muM, n = 68) than surface water outside the polynya area (96 muM, n = 6). Melting ice and ice algae are suggested as D OC sources. DOC concentrations found in this study are consistent with othe r studies showing higher DOC concentrations in the Arctic than in other oce an areas. As the productive season progressed, DOC concentrations in Polyny a surface water (PySW) decreased (p < 0.05) from 110 to 105 muM, while DON concentrations increased (p < 0.05) from 5.6 to 6.1 muM, causing a signific ant decrease (p < 0.05) in the C: N ratios of DOM from spring (C: N ratio 2 0) to summer (C: N ratio 17). We found a significant (p < 0.05) decrease in the DOM C: N ratio in all water masses within the polynya area as the prod uctive season progressed. DON was the largest fraction of total dissolved n itrogen (TDN) in PySW and surface waters outside the polynya area. TDN was calculated as the sum of DON, nitrate, nitrite and ammonium concentrations. DON increased (p < 0.05) from 62% to 73% of TDN in PySW from spring to sum mer, a result of increasing DON concentrations and decreasing inorganic nit rogen concentrations over the productive season. The seasonal accumulation of DON and the corresponding decrease in nitrate concentrations in waters w ith primary production indicate that it is important to take the DON pool i nto account when estimating export production from nitrate concentration de creases in surface waters. PySW TDN concentrations decreased (p < 0.05) fro m 9.1 (n = 61) to 8.6 muM (n = 60) from spring (May 25 through June 19) to summer (July I through July 27). The seasonal decrease in surface water TDN concentrations corresponded to increases in TDN concentrations in deeper w ater masses within the Polynya. Most of the TDN increase in deep water was in the form of DON. A possible explanation is that PON was dissolved (parti ally remineralized) in the water column at mid depths, causing increases in the DON concentration. Transfer of N from PySW (with a short residence tim e in the polynya area) to Polynya Intermediate Water and deep waters of the Norske and Westwind Trough with multi-year residence times keeps N from le aving the polynya area. In spring, nutrients from degradation of OM in PyIW could support primary production. The role of PyIW as an OM trap could be important in supporting primary production in the polynya area. (C) 2001 El sevier Science Ltd. All rights reserved.