Role of biological activity in mediating acidification in a coastal upwelling zone at the east coast of Hainan Island

Abstract Coastal upwelling that brings to the surface both CO2- and nutrient-rich deep seawater is related to acidification and intense biological productivities in surface waters. However, the impact of biological activities on upwelling-induced acidification from nearshore to offshore areas in an upwelling zone remains insufficiently known. In this paper, we present daily records of high-resolution profiles of carbonate chemistry and hydrographic parameters from the nearshore to the offshore upwelling zone off the east coast of Hainan Island (ECH) in July 2014 (summer season). A three end-member mixing model was adopted to discriminate biological processes from physical mixing and to further semiquantitatively diagnose biological contribution to the upwelling-induced acidification. The results show divergent responses in pH, aragonite saturation state (ΩAr) and biomediated nonconservative dissolved inorganic carbon (ΔDIC) at depths of 10 m–30 m between nearshore and offshore regimes under the on-going coastal upwelling, which were attributed to the distinct roles of biological activities in mediating acidification in the upper 30 m across the ECH. Specifically, in the offshore regime, enhanced photosynthesis reduced the upwelling-induced acidification by 40% (i.e., type C: relief role), whereas aerobic respiration increased acidification in the subsurface waters of the nearshore regime by 15% (i.e., type B: aggravation role). This led to the pH and ΩAr minimum in upwelled waters along the transect from the nearshore to the offshore. In contrast, the biological contribution was almost negligible over surface waters in the nearshore regime because of the balance between net community productivity and net community calcification (i.e., type A: limited or minor role).