A sulfur hexafluoride-based Lagrangian study on initiation and accumulation of the red tide Cochlodinium polykrikoides in southern coastal waters of Korea

We report continuous in situ measurements of a population of the harmful algal bloom species Cochlodinium polykrikoides in a fixed volume of inshore waters near the island of Naro-do (34.478N and 127.558W), off the southern coast of Korea and where the earliest bloom of C. polykrikoides occurs regularly. This Lagrangian exper-iment was carried out by injecting the inert chemical tracer sulfur hexafluoride (SF6) into a patch of seawater carrying C. polykrikoides and tracking the SF6-labeled water mass for 4 d. Our results suggest that in situ growth of C. polykrikoides within the SF6-labeled water accounts for only a fraction of the total cell increase. A probable mechanism we invoke here is that bloom initiation and much of the cell accumulation in inshore waters near Naro-do are due to the input of C. polykrikoides cells via lateral mixing of inshore waters with the alongshore current containing high C. polykrikoides cell density. During the last two decades, massive accumulations of autotrophic algae and some heterotrophic protists, collec-tively referred to as harmful algal blooms (HABs), have in-creased considerably in frequency, size, and cell density across the globe (Anderson 1989; Pelley 1998; Sellner et al. 2003). Several lines of evidence suggest that this trend is due to increasing coastal eutrophication; however, strong ev-idence for this relation is not yet available, largely because of the ephemeral and complex nature of HABs (Smayd