Zinc (Zn) is an essential component of more than 300 enzymes, and an important cofactor for the unique functions of relative enzymes. The rapid detection of Zn concentration and its bioavailability in the seawater can help us to know the impact of Zn restriction on the sinking rates of diatoms under acidification. The effects of trace Zn²⁺ concentration (0-72 pmol L^-1) on the phytoplankton fluorescence and enzyme activity of Thalassiosira weissflogii were investigated experimentally to establish the methods of Zn availability (Zn limitation) detection. It was found that the correlation between Zn²⁺ concentration, chlorophyll a concentration, growth rate, Fv/Fm, and alkaline phosphatase activity per unit algae was relatively low, and it was not suitable for Zn restriction detection. The activity of soluble total alkaline phosphatase increased with the increase of Zn²⁺ concentration and varied with the addition of organic materials species and concentration, which influenced the bioavailability of Zn，showing that the bioassay method provides an indication of the bioavailable Ni content in the seawater samples. The urease bioassay was able to detect as little as 0.02 pmol L^-1 of bioavailable Ni in seawater. Nickel bioavailability can be decreased with an increase of humic acid and EDTA, but increased with cysteine added. This method was mainly affected by Zn²⁺ and PO₄^3-concentration. This method was suitable for the offshore with low inorganic phosphorus content, and was not suitable for the detection of Zn limitation in seawater with high inorganic phosphorus concentration. Our bioassay method is accurate, sensitive, simple, and can be used in most laboratories.
 

Assessment； bioavailable； nickel；Thalassiosira weissflogii