Mesozooplankton selective feeding in subtropical coastal waters as revealed by HPLC pigment analysis

Phytoplankton community composition and mesozooplankton selective feeding on different phytoplankton taxa were studied by HPLC pigment analysis at 2 sites with contrasting hydrography in the coastal areas of Hong Kong. The site (WE) on the western part of Hong Kong is located in the Pearl River estuary, whereas the site (EO) on the eastern part of Hong Kong is influenced by oceanic waters. Despite the strong difference in hydrographic conditions, diatoms were the dominant phytoplankton at both sites throughout most months of the year, contributing on average more than 40% of the total phytoplankton in terms of chlorophyll a (chl a). Cryptophytes were the second most abundant phytoplankton group at both sites, whereas contributions from haptophytes, green algae, cyanobacteria and dinoflagellates were small. Diatoms and cryptophytes, the 2 most dominant phytoplankton groups, accounted for about 80 and 75% of the diets of mesozooplankton at EO and WE, respectively. One consistent pattern observed throughout the study period was that mesozooplankton assemblages strongly preferred dinoflagellates over other types of phytoplankton, especially in estuarine waters, even though dinoflagellates only contributed to a small fraction of total chl a. On the other hand, phytoplankton groups with relative small cell sizes, such as green algae and cyanobacteria, were not efficiently ingested. Overall, mesozooplankton feeding selectivity in subtropical eutrophic coastal waters, where planktonic food is sufficient, appears to be influenced by a combination of different factors, including the compositions of predators and the size and quality of the prey.