Does reduced sediment load contribute to increased outbreaks of harmful algal
blooms off the Changjiang Estuary?
Baodong Wang and Ming Xin
The First Institute of Oceanography, State Oceanic Administration, Qingdao, PR China
E-mail: wangbd@fio.org.cn
Harmful algal blooms are increasingly frequent in coastal waters around the world
over the last several decades. The accelerated coastal eutrophication, which resulted
from the increasing anthropogenic loadings of nutrients, is commonly assumed to be
the primary cause of this increase. However, although important, the accelerated
coastal eutrophication may be not the only explanation for the increasing blooms or
toxic outbreaks in estuarine waters. Changes in riverine material fluxes other than
nutrients, such as sediment load, may significantly affect biological activities as well
as HAB incidence in estuarine and coastal waters. A case study off the Changjiang
Estuary indicated that, with the increasing riverine loadings of nutrients, sediment
load from the Changjiang River has been reduced by 70% in the past four decades. A
comparison of long-term data revealed that the phytoplankton biomass maximum has
expanded to a much lower salinity region due to the drastic reduction in riverine
sediment load and the subsequent improvement in light penetration in the Changjiang
River plume. Furthermore, the number of HAB incidences off the Changjiang Estuary
are positively related with the sediment load from the Changjiang River over the past
four decades. Therefore, it is argued that the drastic decline in sediment load from the
Changjiang River reduced turbidity in the Changjiang Estuary and thus contributed to
increased frequency of HABs in buoyant discharge plumes.