No abstract

The Danshuei River flows through the heavily populated metropolitan area of Taipei and New Taipei cities, which causes remarkable additions of nutrient elements. In spite of the rather short residence time of water, the Danshuei estuary is distinctive for the very high ammonium concentration and extensive hypoxia in its lower reach. Because particulate organic matter (POM) is potentially the culprit of hypoxia, we investigate the isotopic characteristics of POM collected in February and July 2009 at a fixed station over four semidiurnal tidal cycles. By using nitrogen isotopic composition and C/N ratio of POM, we derive the relative contributions of POM from different sources. One potential source that combines dead and living phytoplankton, phytodetritus, has d 15 N values that can be predicted by the d 15 N of ammonium and the isotope effect during ammonium uptake; however, the isotope effect is concentration dependent. We employ a three-endmember mixing model based on d 15 N and C/N ratio to calculate the fractional contributions from three major POM sources, i.e., phytodetritus, soil, and sediment. Sensitivity test was conducted for the derivations from both carbon and nitrogen basis. For February 2009 we found the three fractions (in terms of contribution to the particulate organic carbon) to be 45 ± 19, 10 ± 11 and 45 ± 13 %, respectively; for July 2009, 71 ± 18, 11 ± 10 and 18 ± 13 %, respectively. The results imply that phytodetritus is probably the major culprit for the hypoxic conditions in the estuary, especially, in summer. K. K. Liu: Deceased on 12 August 2015. Electronic supplementary material The online version of this article (