A new method of estimating carbon sequestration and its efficiency in coastal waters

Abstract

The biological pump (BP) in oceans refers to the fraction of phytoplankton organic matter sinking out of the euphotic zone (surface layer) into below the pycnocline layer (bottom layer) in the water column. Currently, sediment traps are commonly used to estimate organic settlement and carbon sequestration in open oceans, but the installation of the sediment traps in the ocean requires special efforts, let alone the temporal and spatial discordance of particle sinking trajectory from the surface to the bottom. Net community production is used only for the euphotic zone. Thus, there has been a lack of a simple method to estimate the export flux of organic carbon from the surface to bottom layer and to quantify BP efficiency in the coastal areas. In this study, we develop a conceptual model to illustrate carbon sequestration processes from the surface to the pycnocline layer and the bottom layer. The idea is to examine an increase (the release) in dissolved inorganic carbon (DIC) and organic carbon (DOC) in the bottom layer. Based on this model, a new method was developed to estimate carbon sequestration (CS) and carbon sequestration efficiency (CSE). Two cruises in May and August in 2016 were conducted to establish a three-end-member mixing model of θ-S which is used to estimate biologically mediated DIC (ΔDIC = DIC_in-situ-DIC_mixed) in relation to the conservative mixing of DIC. Based on the density gradient threshold of 0.03 kg m^-3m^-1, the water column is separated into the surface mixed layer, the pycnocline layer and bottom layer and integrated ΔDIC (IntΔDIC) in the three layers are estimated. The same approach is applied to dissolved organic carbon (DOC) data which are used to make the same calculation with the mixing model to obtain the sequestrated DOC mass in the bottom layer. Carbon uptake and carbon sequestration (CS) can be calculated as the integrated ΔDIC in the surface mixed layer and bottom layers, respectively. Carbon sequestration efficiency (CSE), which is defined as sum of bottom layer Int ΔDIC + Int ΔDOC divided by the whole water column integrated ΔDIC can also be calculated. The results showed that during algal blooms driven by abundant nutrients from the Pearl River Estuarine water in May, little sinking carbon was observed due to the absence of the bottom layer, resulting in low CSE. In contrast, in August, even no significant algal bloom occurred, the strengthened water stratification, lead to a substantial increase in the CS(449.49 ± 366.14 mmol C m^-2), leading to an increased CSE to a range of 0 ∼ 92.79 % (average 60.55 ± 25.07 %). The carbon sequestration rate was 55.61 ± 45.30 mg C m^-2 d^-1. The new method, based on vertical changes of DIC and DOC due to biological uptake or release in relation to the conservative mixing of water masses, provides an easy and direct tool to estimate carbon sequestration and carbon sequestration efficiency in the stratified water column in coastal waters.