Changes in the nutrient ratios and phytoplankton community after declines in nutrient concentrations in a semi-enclosed bay in Hong Kong

Tolo Harbour is a landlocked bay with poor tidal flushings in the northeastern part of Hong Kong. During the 1980s, excessive nutrient loading led to dramatic increase in nutrient concentrations, accompanied by lower N:P ratios, higher algal biomass and shifts in the phytoplankton community. We studied the effects of nutrient loading reduction measures on nutrient concentrations, nutrient ratios and phytoplankton dynamics in Tolo Harbour by comparing data collected before the full implementation of nutrient loading reduction measures (1986–1997) to those after the implementation (1998–2008). Such measures led to declines in nutrient concentrations, changes in N:P and N:Si ratios, lower chlorophyll-a concentrations and fewer algal blooms. Diatoms were the most abundant phytoplankton group in Tolo Harbour both before and after declines in nutrient concentrations. The density of dinoflagellates did not change, but substantial increase in other algal group abundance was recorded.

Research highlights► We investigated the long-term changes in physico-chemical parameters and phytoplankton in a semi-enclosed bay in Hong Kong. ► Implementation of nutrient loading reduction measures had lead to decreases in nutrient concentrations, as well as changes in the nutrient ratios. ► While changes in the nutrient concentrations did not lead to changes in the density of dinoflagellates, densities of diatoms and other algal groups increased significantly. ► The changes in Tolo Harbour was compared to the recovery of other freshwater and marine ecosystems.