Is inorganic nutrient enrichment a driving force for the formation of red tides? A case study of the dinoflagellate Scrippsiella trochoidea in an embayment Kedong Yin a,b,c, *, Xiu-Xian Song d , Sheng Liu a , Jinjun Kan e , Pei-Yuan Qian b a Key Laboratory of Tropical Marine Environmental Dynamics, South China Sea Institute of Oceanology, CAS, Guangzhou 510301, China b Atmospheric, Marine and Coastal Environment (AMCE) Program, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong SAR, China c Australian Rivers Institute, Griffith University (Nathan Campus), Brisbane, QLD4111, Australia d Key Laboratory of Marine Ecology and Environmental Sciences, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China e Department of Earth Sciences, University of Southern California, Los Angeles, CA90089-0740, USA 1. Introduction There has been an increasing tendency of HABs in the coastal waters around the world (GEOHAB, 2006). In general, enrichment of nutrients is often considered to be responsible for the increased number of HABs (Paerl, 1997; Anderson et al., 2002; Glibert et al., 2005). Many HABs can be directly linked to nutrient loading. However, the unexpected occurrence and disappearance of red tide blooms in Hong Kong waters and other areas do not appear to support the notion that an increase in inorganic nutrients or a depletion of nutrients is a driving mechanism, as in lakes. For example, Port Shelter, a semi-enclosed bay in the northeast of Hong Kong waters, is one of the hot spots for red tides in Hong Kong waters, occurring mostly in March and April and least prevalent in summer months (Yin, 2003). The red tides here are defined as high biomass blooms and usually are visible as colored patches in Hong Kong waters. However, the monthly average chl a concentrations during 10 years was <4 mgl À1 (Yin, 2003), much lower than red tide biomass. This raises an important question: are inorganic nutrients a direct driving force for the formation of these red tides? That is, when nutrients are added to those waters, can a red tide be Harmful Algae 8 (2008) 54–59 ARTICLE INFO Article history: Received 30 March 2007 Received in revised form 14 March 2008 Accepted 1 August 2008 Keywords: HAB Hong Kong water Nutrient enrichment Scrippsiella trochoidea ABSTRACT Red tides (high biomass phytoplankton blooms) have frequently occurred in Hong Kong waters, but most red tides occurred in waters which are not very eutrophic. For example, Port Shelter, a semi-enclosed bay in the northeast of Hong Kong, is one of hot spots for red tides. Concentrations of ambient inorganic nutrients (e.g. N, P), are not high enough to form the high biomass of chlorophyll a (chl a) in a red tide when chl a is converted to its particulate organic nutrient (N) (which should equal the inorganic nutrient, N). When a red tide of the dinoflagellate Scrippsiella trochoidea occurred in the bay, we found that the red tide patch along the shore had a high cell density of 15,000 cells ml À1 , and high chl a (56 mgl À1 ), and pH reached 8.6 at the surface (8.2 at the bottom), indicating active photosynthesis in situ. Ambient inorganic nutrients (NO 3 , PO 4 , SiO 4 , and NH 4 ) were all low in the waters and deep waters surrounding the red tide patch, suggesting that the nutrients were not high enough to support the high chl a >50 mgl À1 in the red tide. Nutrient addition experiments showed that the addition of all of the inorganic nutrients to a non- red-tide water sample containing low concentrations of Scrippsiella trochoidea did not produce cell density of Scrippsiella trochoidea as high as in the red tide patch, suggesting that nutrients were not an initializing factor for this red tide. During the incubation of the red tide water sample without any nutrient addition, the phytoplankton biomass decreased gradually over 9 days. However, with a N addition, the phytoplankton biomass increased steadily until day 7, which suggested that nitrogen addition was able to sustain the high biomass of the red tide for a week with and without nutrients. In contrast, the red tide in the bay disappeared on the sampling day when the wind direction changed. These results indicated that initiation, maintenance and disappearance of the dinoflagellate Scrippsiella trochoidea red tide in the bay were not directly driven by changes in nutrients. Therefore, how nutrients are linked to the formation of red tides in coastal waters need to be further examined, particularly in relation to dissolved organic nutrients. ß 2008 Elsevier B.V. All rights reserved. * Corresponding author at: Australian Rivers Institute, Griffith University (Nathan Campus), Brisbane, QLD 4111, Australia. E-mail address: k.yin@griffith.edu.au (K. Yin). Contents lists available at ScienceDirect Harmful Algae journal homepage: www.elsevier.com/locate/hal 1568-9883/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.hal.2008.08.004