Decadal acidification in a subtropical coastal area under chronic eutrophication

Highlights

  • Acidification rate was −0.0085 ± 0.0069 pH·unit·yr−1 in Hong Kong Coast from 1986 to 2017.
  • The coastal acidification was dominated by water discharges particularly after 2001.
  • Increasing DIN:DIP was highlighted in this region during this acidification intensification.

Abstract

Acidification in coastal water usually becomes more intense due to the coupling of anthropogenic activities and climate change. In order 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.

Yao H., Wang J., Han Y., Jiang X. & Chen J., 2021. Decadal acidification in a subtropical coastal area under chronic eutrophication. Environmental Pollution: 118487. doi: 10.1016/j.envpol.2021.118487. Article (subscription required).


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