Fe20 el20

Surface sediment samples from the PRE region (21.64° to 23.07° N, and 113.71° to 114.36° E) along the salinity gradient were collected during two boat cruises in July 2015 (samples ZJA, ZJB, ZJC, ZJD, and ZJE) and July 2017 (samples A01, A09, A12, A14, F101, F307, F408, and F804) (Figure 1). The sediments were sealed in 50 ml tubes (Falcon) immediately after sampling of the water bed, stored in liquid nitrogen on board, and then transferred to a −40°C refrigerator in the laboratory for further analysis. Water samples from wet sediment samples were collected by centrifugation, and the pH and salinity were determined by using with a pH meter (FE20/EL20, Mettler Toledo) and a salinity meter (MASTER-S/MillM, Atago) on board. Sediment samples (5 g each) were air-dried to calculate the dry weight (d.w.) and to measure physicochemical parameters in the laboratory. The concentrations of ^{NH_4^+}, ^{NO_2^–}, and ^{NO_3^–} were measured using AutoAnalyzer 3 HR (Seal, United States) (Zhou et al., 2017 (link)). The TOC, inorganic carbon (IC), total carbon (TC) and total nitrogen (TN) content, were analyzed using TOC-L_CPH/CPN (Shimadzu, Japan) as reported previously (Pan et al., 2019 (link)).

Ten grams of the samples was weighted and homogenized with a mixer (FastPrep-24, MP Biomedicals, Solon, OH, USA) for 2 min with 20 mL of deionized water to make a thick slurry. The pH of this slurry was determined using a digital pH meter (Mettler Toledo FE20/EL20, Schwerzenbach, Switzerland). The moisture of each sample (1–3 g) was measured using the oven-dry method at 105.0 ± 1.0 °C for drying, followed by the determination of the sample weight until a constant weight was achieved. Water activity was determined by an Aqualab 4TE (Decagon Devices, Pullman, WA, USA) at 25 °C. The salt (NaCl) content was determined using Mohr’s titration method [16 ].

The determination of muscle pH was determined using the method described by Fan et al. (2014) and modified slightly. A sample (2.00 g) of salting fillets was dispersed in 18 ml of distilled water and centrifuged at 10,000 g for 10 min, and then filtered. The pH value of the filtrate was determined by a digital pH meter (Mettler Toledo FE20/EL20) and performed in triplicate.

The pH of fillets was detected using a digital pH meter (Mettler Toledo FE20/EL20, Shanghai, China) according to the method of Gui et al. [1 (link)]. The TVB-N was extracted and determined according to semi-micro steam distillation method in the China National Food Safety Standard methods (Determination of Total Volatile Basic Nitrogen in Food; GB/T 5009.228–2016). The homogenized samples (10 g) were measured by distillation using a Kjeldahl Apparatus (KDY-9820, Beijing, China). The value was expressed as mg of TVB-N per 100 g of flesh.