Field nutrient data obtained in the Bohai Sea in June and November 2011, and January 2016.

The field surveys were conducted during 24–28 June 2011 (summer), 25–30 November 2011 (early winter), and 28–31January 2016 (mid-winter). At 13–23 grid stations, depth water samples were collected for nutrient analyses. Depth profiles of temperature and salinity (practical salinity scale of 1978) were determined with a calibrated conductivity-temperature-depth/pressure (CTD) recorder (SBE911+, Sea-Bird Co., USA) onboard R/V Dongfanghong 2. Water samples were collected at two or three depths using a rosette sampler fitted with 8 L or 12 L Niskin bottles, which were mounted with the CTD unit. The sea surface water samples were collected from a depth of 2–3 m, and the bottom water samples were collected from a depth of 2–3 m above the seabed. Water samples were filtered on board using clean 0.45 μm cellulose acetate membranes. All of the filtered samples were divided into three subsamples. One of the subsamples was frozen at –20°C and then analyzed for nitrate, nitrite and DIP in the laboratory. And trichloromethane (0.1% v/v) was added into other two of the subsamples which were stored at 4°C and then analyzed for ammonium and DSi in the laboratory. Nitrate and nitrite were measured by reducing nitrate to nitrite with a Cd column, and then determining nitrite using the pink azo dye spectrophotometric method. DIP, DSi and ammonium were measured based on standard phosphor-molybdenum blue (for DIP), silicon molybdenum blue (for DSi), and indophenol blue (for ammonium) spectrophotometric procedures. Nitrate, nitrite, DIP and DSi were determined using an AA3 Auto-Analyzer (Bran+Luebbe Co., Germany), while ammonium was measured using a self-made Tri-223 continuous Auto-Analyzer. Detection limits were 0.1 μmol L⁻¹ for nitrate, 0.04 μmol L⁻¹ for nitrite, 0.08 μmol L⁻¹ for DIP, 0.08 μmol L⁻¹ for DSi, and 0.5 μmol L⁻¹ for ammonium. Samples with high nutrient concentrations were properly diluted before determination. For data quality assurance, Reference Materials for Nutrient in Seawater from The General Environmental Technos Co., Ltd (http://www.kanso.co.jp/) were used during analyses, achieving a precision level of ±0.1 μmol L⁻¹ for most nutrient species other than ammonium.