Winter enhancement of bacterial carbon demand extends from the euphotic zone into the mesopelagic ocean

Two basin-scale cruises were conducted aboard the R/V Tan Kah Kee in the South China Sea during winter (November–December 2022) and summer (August–September 2023). Eight stations distributed between 111.5–119°E and 12–22°N were occupied during the two cruises (Figure 1). A total of 165 seawater samples were collected from depths of 5–1000 m, including samples from the surface, the deep chlorophyll maximum, intermediate depths, and the mesopelagic zone. Seawater for microbial and biogeochemical analyses was collected using 12-L Niskin bottles. The data includes the bottom depth, depth, temperature, salinity, nutrient concentrations (nitrate plus nitrite: NOx, phosphate: PO₄³⁻, silicate: Si(OH)₄), chlorophyll a (Chl-a), bacterial abundance, bacterial production (BP), bacterial respiration (BR), bacterial carbon demand (BCD), net primary production (NPP), total organic carbon (TOC), particulate organic carbon (POC), particulate nitrogen (PN), chromophoric dissolved organic matter (CDOM) and fluorescent dissolved organic matter (FDOM). Vertical profiles of temperature, salinity, pressure, and density were obtained with a Sea-Bird 911 conductivity-temperature-depth system mounted on a rosette sampler. Nutrients were analyzed using an AA3 autoanalyzer (Du et al. 2013), Chl-a fluorometrically after GF/F filtration (Xiao et al. 2019), and bacterial abundance by flow cytometry after SYBR Green staining (Jiao et al. 2010). POM were measured using an elemental analyzer, while TOC was analyzed with a Shimadzu TOC-V analyzer following previous studies (Dai et al. 2009; Cai et al. 2015). CDOM absorption spectra and FDOM excitation-emission matrices were measured following Wang et al. (2017). Weekly net primary production (NPP) was estimated from satellite-based productivity products (Behrenfeld and Falkowski 1997; Behrenfeld et al. 2005; Silsbe et al. 2016).