The size structure and pigment composition of phytoplankton in a semi-enclosed bay, Hong Kong, after a 12-year nutrient reduction

Eutrophication is one of the major factors contributing to the globally increasing occurrence of harmful algal blooms, which may consequently do harms to aquatic lives in the regional ecosystems. Tolo Harbor, a semi-enclosed bay located in the north-eastern part of Hong Kong, has a long history of eutrophication and frequent algal blooms occurrence. This paper presents a 11-month (September 2010-July 2011) examination of the spatial and seasonal variations in the size structure and pigment composition of phytoplankton after 12 years of nutrient reduction. Concentration of chlorophyll a (Chl a) increased landward and the phytoplankton was dominated by algae in the 2-20 and 20-200 μm size fractions. Algal blooms were recorded in the inner part of Tolo Harbor in May (dinoflagellate bloom) and June (diatom bloom) 2011. High performance liquid chromatography (HPLC) analysis revealed that the composition of phytoplankton pigments varied seasonally, but did not differ between the inner and outer parts of the harbor. The prevalence of fucoxanthin in all size fractions indicated that diatoms were the most dominant phytoplankton taxa. Peridinin, an unambiguous marker of dinoflagellates, appeared sporadically. The presence of cryptophytes, green algae, prymnesiophytes, and cyanobacteria was denoted by the pigments alloxanthin, chlorophyll b, 19′-hexanoyloxyfucoxanthin, and zeaxanthin, respectively. Nutrient concentrations correlated poorly with Chl a concentrations from micro- (20-200 μm) and nano-sized (2-20 μm) algal cells while temperature affected significantly the pigment concentrations of pico-sized (<2 μm) and taxonomically minor algal groups.