Harmful Algal Blooms in Chinese Coastal Waters Will Persist Due to Perturbed Nutrient Ratios

The three large marine
ecosystems (LMEs) bordering China (Yellow
Sea/Bohai Sea, East China Sea, and South China Sea) have received
excess nitrogen (N) and phosphorus (P) in the past decades with detrimental
consequences for ecosystem functioning, such as increased productivity,
loss of biodiversity, and proliferation of harmful algal blooms (HABs).
N loading increased much faster than that of P. Here, we show that
HABs in the three LMEs started to proliferate after the N:P molar
ratio exceeded the threshold value of 25 in the 1980s. The mismatch
of N and P inputs is not only related to differences in loads but
also inherent to the differences in their biogeochemical cycles which
more efficiently filter P than N in land- and waterscapes. Future
Shared Socioeconomic Pathways show that high N:P ratios will persist
for decades to come, even worsening in a future oriented toward sustainability,
and indicate that HABs may be a persisting problem in China’s
coastal waters. While efforts in agricultural systems are governed
by the agronomic crop requirements and are not easy to manage with
respect to N:P ratios, the separate collection of urine in urban and
rural areas could contribute to decreases in both total nutrient loads
and N:P ratios.