Variations of dissolved inorganic nutrients and their influences on harmful algal blooms in Bohai Sea over the past thirteen years

The Bohai Sea (BHS) has been subjected to harmful algal blooms (HABs). In this study, we compiled and analyzed datasets of HABs and related environmental parameters in BHS from 2006 to 2018 including dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP). However, quarterly monitoring cannot give direct in-situ observations when HABs happened. Therefore, we reconstructed spatially discrete seasonal observations to a spatial resolution of 0.25° × 0.25° continuous monthly data from August 2006 to October 2019 using Chebyshev Polynomial Fitting (CPF) and seasonal decomposition methods. It was concluded that a coherent increasing trend of the DIN and DIN/DIP ratio transitioned to a decreasing trend from 2013, whereas the DIP concentration exhibited a consistent falling trend from 2012 to 2019. the occurrence of HABs was successfully controlled by the reduction of DIN and DIP, but the reduction may promote the occurrence or intensity of HABs events in some particular algal species.