Inhibition of marine coastal bloom-forming phytoplankton by commercially cultivated Gracilaria lemaneiformis (Rhodophyta)

While harmful algal blooms (HABs) have become a threat to fisheries, environmental and public health, and economies worldwide, most of the approaches under investigations for prevention, control, and mitigation (PCM) of HABs are limited by high cost and possible secondary pollution. This study reports our laboratory and field experiments demonstrating effective growth-inhibiting effects of the widely cultivated seaweed Gracilaria lemaneiformis on phytoplankton possibly forming HABs. Laboratory experiments demonstrated significant growth-inhibiting effects of fresh G. lemaneiformis thalli in a dose-response manner on four of six test microalgae commonly observed in the seaweed cultivation area (Scrippsiella trochoidea, Prorocentrum micans, Skeletonema costatum, Dunaliella salina) and on the natural phytoplankton assemblage in seawater. Significant inhibiting effects were also observed from in situ bottle (1.0 L) and mesocosm (50 and 1000 L) experiments on the harmful alga Phaeocystis globosa during its blooms. Based on a large-scale field survey, the phytoplankton abundance in the areas with Gracilaria cultivation was demonstrated to be significantly lower than that in the areas without Gracilaria. Collectively, our results showed that large-scale cultivation of G. lemaneiformis is an effective approach to prevent HABs in coastal waters, as an added value to the economically viable industry of Gracilaria cultivation.