Decadal acidification in a subtropical coastal area under chronic eutrophication

Coastal acidification is often much more intense than ocean acidification due to eutrophication. To better understand the relationship between long-term coastal acidification (CA) and coastal eutrophication (CE), in-situ monthly data over the past three decades (1986-2017) were analyzed from Hong Kong Coast (HKC). The coastwide annual mean pH change (ΔpH_mean) was estimated at -0.0085 ± 0.0069 unit·yr^-1 in last decades, which was over four times stronger than current estimation on open ocean acidification rate (∼-0.0019 unit·yr^-1). According to the CA spatial pattern, greater pH decline (ΔpH_mean = -0.017 ± 0.009 unit·yr^-1) occurred in northwest, central south and central east HKC areas, much higher than the less acidified (ΔpH_mean = -0.004 ± 0.002 unit·yr^-1) southwest and northeast HKC areas. The spatiotemporal CA variations were associated with water discharges, atmospheric CO₂ increase and respiration/production that was indicated by DIN:DIP structure changes. The annual mean DIN:DIP ratio increased progressively from initial ∼16 in 1986 to ∼37 in 2017, revealing excess nitrogen load from rapid urbanization in this region. Such discharge-induced acidification was estimated as the major contributor for the total CA in HKC over the last three decades. In addition, our simulation results indicated that a potential CA rate at ∼0.0035 unit·yr^-1 could be reached if reducing mean DIN:DIP from discharged water to ∼23 from HKC. This study revealed a previously not recognized relationship between coastal acidification and changing coastal nutrient stoichiometry, and proposed possible management approaches.