Carbon and nitrogen cycling in the Zhubi coral reef lagoon of the South China Sea as revealed by 210Po and 210Pb

Abstract

The radionuclides 210Po and 210Pb were examined to trace the cycling of particulate organic carbon (POC) and particulate organic nitrogen (PON) in the Zhubi coral reef lagoon. The net export flux of POC to the open sea is 14mgCm−2d−1. However, the net exchange of PON has not yet been observed. On average, the vertical export fluxes in the lagoon of POC and PON, as derived from 210Po/210Pb disequilibria, are 43mgCm−2d−1 and 13.8mgNm−2d−1, respectively. The deficit of 210Po relative to 210Pb in particulate matter provides evidence for the degradation of particulate organic matter. According to the mass balance budgets, 310mgCm−2d−1 and 121mgNm−2d−1 were recycled into dissolved fractions. Based on a first-order kinetics model, the degradation rate constants of POC and PON are 0.28 and 0.30m−1, respectively. Thus, 210Po and 210Pb can quantify the cycling of carbon and nitrogen in this coral lagoon.