Decadal acidification in a subtropical coastal area under chronic eutrophication
2022
Yao, Hongming | Wang, Jiujuan | Han, Yu | Jiang, Xiaoli | Chen, Jinsong
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 (ΔpHₘₑₐₙ) 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 (ΔpHₘₑₐₙ = −0.017 ± 0.009 unit·yr⁻¹) occurred in northwest, central south and central east HKC areas, much higher than the less acidified (ΔpHₘₑₐₙ = −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.
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