Temporal variation and stoichiometric ratios of organic matter remineralization in bottom waters of the northern Gulf of Mexico during late spring and summer

Abstract

An improved extended optimum multiparameter (eOMP) analysis was applied to hydrographic (temperature and salinity), and water chemistry data, including dissolved oxygen (O2), nutrients (nitrate plus nitrite, phosphate, and silicate), dissolved inorganic carbon (DIC), and total alkalinity (TAlk) data collected during late spring and summer from 2006 to 2012 in bottom waters off the Louisiana coast, to explore the dynamics and stoichiometry of DIC production during the development and maintenance of summer hypoxia. Our analysis demonstrated that DIC in bottom water was relatively low from April to June, but increased significantly in July, peaked in August, and dropped slightly in September. Furthermore, DIC production resulted from both aerobic organic carbon (OC) respiration and denitrification, as well as substantial loss due to vertical mixing with surface water. The average summer gross OC respiration rate was estimated to be 0.19 g C m−2 d−1, with the highest values occurring in late summer when hypoxic conditions dominated. We also found that Corg/N/P/‐O2 remineralization ratios for aerobic respiration were generally consistent with the classic Redfield ratio (106/16/1/138) except individual C/N and C/P ratios were slightly lower, indicating that marine OC was the major source of the DIC production in the bottom water. This study quantified the role of temporal bottom‐water microbial respiration to seasonal DIC dynamics and provided a means for studying the stoichiometry of biogeochemical processes in coastal waters.

Physical and Biogeochemical Controls on pH Dynamics in the Northern Gulf of Mexico During Summer Hypoxia

“…In hypoxic bottom waters, the DIC increase resulting from denitrification was estimated to account for 1.6–20.9% (average 9.5%) of the total DIC release. Our result is similar to the finding by Xue et al (), which suggested that the bottom DIC production in the nGOM during summer resulted from the combination of aerobic organic carbon respiration (contributing ~80%) and denitrification.…”

“…The deficit in DIN release, relative to the release of DIC and SRP from the aerobic decomposition of organic carbon (Figures e and f), also suggested the removal of DIN especially under low oxygen conditions (Nicolas Gruber, ). In the nGOM, the decomposition of organic matter generally follows the classic Redfield ratio (Huang et al, ; Xue et al, ). We thus calculated the denitrification‐induced DIN deficit (ΔN denit ) and DIC release (ΔDIC denit ) as ΔN denit = ΔDIC biol /106 × 16 − ΔDIN biol and ΔDIC denit = ΔN denit /104 × 106.…”

Spring net community production and its coupling with the CO<sub>2</sub> dynamics in the surface water of the northern Gulf of Mexico

“…In our study region, the riverine input of NO 3 - (Table 1) is the main nitrogen source for biological uptake and we considered the average Redfield ratio of C:O 2 =106:138 to be appropriate. Although the stoichiometry of C:N may differ from the Redfield ratio (Geider and La Roche 2002;Sambrotto et al 1993), the applicability of the Redfield C:N ratio in our study region has been proved (Huang et al 2012;Xue et al 2015; also the result of this study).…”

Hypoxic Bottom Waters as a Carbon Source to Atmosphere During a Typhoon Passage Over the East China Sea

“…In shelf seas, CO 2 efflux during a typhoon is usually much higher than that of normal weather due to combined effect of mixing or upwelling of subsurface water with high dissolved inorganic carbon (DIC; Mathis et al, 2012;Ye et al, 2017) and extreme wind speed (Huang & Imberger, 2010;Nemoto et al, 2009). Such influence can be amplified when bottom shelf waters are hypoxic and enriched with CO 2 (Cai et al, 2011;Rabalais et al, 2014;Xue et al, 2015;Yu et al, 2014). Surface pCO 2 in the hypoxic inner East China Sea shelf is predicted to increase by 312 μatm after vertical mixing based on model simulation (Chou et al, 2009), which is much higher than that observed in the South China Sea shelf (increase by 20 μatm; Ye et al, 2017).…”