Variations in triple isotope composition of dissolved oxygen and primary production in a subtropical reservoir

Tania Guha,Hana Jurikova,Mao-Chang Liang,Fuh-Kwo Shiah,Osamu Abe,Chung-Ho Wang

doi:10.5194/bg-13-6683-2016

Abstract

Abstract. Lakes and reservoirs play an important role in the carbon cycle, and therefore monitoring their metabolic rates is essential. The triple oxygen-isotope anomaly of dissolved O2 [17Δ = ln(1+δ17O) − 0.518 × ln(1 + δ18O)] offers a new, in situ, perspective on primary production, yet little is known about 17Δ from freshwater systems. We investigated the 17Δ together with the oxygen : argon ratio [Δ(O2 ∕ Ar)] in the subtropical Feitsui Reservoir in Taiwan from June 2014 to July 2015. Here, we present the seasonal variations in 17Δ, GP (gross production), NP (net production) and the NP ∕ GP (net to gross ratio) in association with environmental parameters. The 17Δ varied with depth and season, with values ranging between 26 and 205 per meg. The GP rates were observed to be higher (702 ± 107 mg C m−2 d−1) in winter than those (303 ± 66 mg C m−2 d−1) recorded during the summer. The overall averaged GP was 220 g C m−2 yr−1 and NP was −3 g C m−2 yr−1, implying the reservoir was net heterotrophic on an annual basis. This is due to negative NP rates from October to February (−198 ± 78 mg C m−2 d−1). Comparisons between GP rates obtained from the isotope mass balance approach and 14C bottle incubation method (14C–GP) showed consistent values on the same order of magnitude with a GP ∕ 14C–GP ratio of 1.2 ± 1.1. Finally we noted that, although typhoon occurrences were scarce, higher than average 17Δ values and GP rates were recorded after typhoon events.

Highlights

It is well established that marine photosynthesis plays a critical role in the global biogeochemical cycling of carbon and oxygen, which sustain the great majority of ecosystems on our planet
The main drawback is the in vitro methodology, which involves the removal of plankton communities from the natural environment and confining them in a small volume of water, with variability in primary production (PP) observed under laboratory conditions
In an effort to contribute to the understanding of production rates measured in situ using the 17 method and the in vitro estimates from the 14C bottle incubation approach, and to expand this to freshwater systems, we provide comparisons between the respective rates

Summary

Introduction

It is well established that marine photosynthesis plays a critical role in the global biogeochemical cycling of carbon and oxygen, which sustain the great majority of ecosystems on our planet. Assessing primary production (PP) and providing accurate estimates of ecosystem metabolic rates are the key to understanding each system’s fluxes and variability in biogeochemical cycling. PP has been evaluated by in vitro 14C bottle incubation method introduced by Steeman-Nielsen (1952). These measurements are associated with a number of biases and the interpretation of the PP estimates is problematic. The main drawback is the in vitro methodology, which involves the removal of plankton communities from the natural environment and confining them in a small volume of water, with variability in PP observed under laboratory conditions. The PP rates observed in vitro cannot be fully representative of natural PP rates (e.g. Harrison and Harris, 1986; Marra, 2002)

Methods

Results

Discussion

Conclusion