Fig. 1.

Depth profiles of physical-chemical parameters and NH₄⁺ consumption rates in the South China Sea in the summertime of 2014. a, d Station D1; b, e Station H3; c, f Station X2. Panels a−c show NO₃⁻ concentrations (gray solid dots); NO₃⁻ uptake rates (pink open squares); potential density anomalies (blue line) and chlorophyll fluorescence values (green line). Panels d−f show NH₄⁺ concentrations (gray solid dots); percent of surface PAR (pink curve); NH₄⁺ uptake rates (green open squares), and nitrification rates (blue open squares). Gray bars mark the depth range with the steepest NO₃⁻ gradient and the black dashed lines within them show the nitracline depth; pink triangles on the right Y-axis mark the euphotic zone depth (0.1% of surface PAR). Error bars of NH₄⁺ uptake rates, NH₄⁺ oxidation rates, and NO₃⁻ uptake rates are represented by the standard deviation of duplicates, and are smaller than the symbols where not visible