Origins of Particulate Organic Matter Determined from Nitrogen Isotopic Composition and C/N Ratio in the Highly Eutrophic Danshuei Estuary, Northern Taiwan

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 (

δ15N patterns in three subtropical estuaries show switch from nitrogen “reactors” to “pipes” with increasing degradation

“…This makes the BRE wet season 0‰ δ 15 N‐PN values that coincided with low PN concentrations and high (~ 15‰) δ 15 N‐DIN values clear outliers. Neither assimilation of NO 3 − fertilizers (δ 15 N ~ 0‰) or biological fixation of atmospheric N (δ 15 N ~ 0‰), previously used to explain such low aquatic δ 15 N‐PN (Schlarbaum et al ; Knapp et al ; Wu et al ), could reasonably explain the BRE values: high N loads make biological fixation energetically unfavorable and the relatively high δ 15 N‐NO 3 − + low δ 18 O‐NO 3 − values rule out NO 3 − fertilizer and atmospheric N inputs. This leaves a strong apparent fractionation of N assimilation, which single‐organism cultures show could reach −14‰ (Bauersachs et al ; Karsh et al ), as most likely explanation for the low BRE δ 15 N‐PN values.…”

A Tribute to Thomas M. Church: Exploring Chemical Oceanography in the Coastal Zone—The History and Future

“…This special volume includes six papers based on some of these AGU talks, and reading these papers you can clearly see many connections to topics worked on by Tom over his career. Atmospheric deposition of trace elements by wet and dry routes to coastal waters is discussed in the Gao et al (2016) paper, while estuarine processing of organic matter in the Danshuei Estuary, Taiwan, is discussed by Wu et al (2016) and mercury cycling in the Delaware estuary is covered in the Gosnell et al (2016) paper. It should be noted that the Wu et al (2016) paper was submitted just prior to its senior author's (K. K. Liu) passing, a friend and colleague whom we will greatly miss.…”