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Formation and decline of a Heterosigma akashiwo layer in East Sound , Washington , USA 1

The formation and decline of a surface layer of the toxic raphidophyte Heterosigma akashiwo was observed in a shallow, coastal fjord in Washington state, USA. Physical and chemical water column properties and biological rates were simultaneously quantified to estimate the contribution of biotic factors (i.e. rates of cell accumulation and loss) to H.a. layer formation. The results suggest that reduced cell mortality, due to reduced protistan grazing pressure in addition to favorable environmental conditions may have contributed to the occurrence of the H.a. layer. Introduction The flagellate Heterosigma akashiwo (Hada) (H. a.) is a globally distributed raphidophyte that has been implicated in toxic events, leading to fish kills and aquaculture losses. The mode of toxicity for H.a. has not been identified, although several possible mechanisms exist (reviewed in Kempton et al. 2008). Blooms of H.a. have been observed for several decades around the Pacific (Honjo 1993, Horner et al. 1997) and more recently along the USA’s Atlantic coast (Kempton et al. 2008). Bloom formation for any alga can only occur if processes that lead to cell accumulation (e.g. growth, aggregation) exceed processes that lead to cell loss (e.g. dilution due to mixing, grazing). Although the factors driving H.a. bloom formation are unknown, there are suggestions that blooms coincide with changes in water properties due to run-off (Smayda 1998). Ecophysiological studies identify H.a. as particularly tolerant to a wide range of temperatures and salinities with potentially very high growth rates of multiple divisions per day (reviewed in Smayda 1998). Fewer studies have quantified H.a. mortality either in the lab or in-situ. Cell lysis due to viral infection has been implicated in rapid declines in H.a. abundance (reviewed by Tomary et al. 2008). However, grazing by heterotrophic protists typically accounts for the vast majority of phytoplankton mortality in the ocean. Laboratory results show that H.a. are either toxic to or not ingested by some protistan predators (Kamiyama & Arima, 2001, Clough & Strom 2005). This suggests that lack of predator-induced mortality may contribute to bloom formation in this, and by implication other, HAB species. Since HAB events are unpredictable, their study benefits from chance encounters. During a month-long field study a brief occurrence of dense H.a. was observed at the surface of a coastal fjord. This occurrence provided the opportunity to quantify the relative contribution of physical, chemical and biological factors to the formation, persistence and dissipation of the event. The results suggest that, in addition to environmental conditions, biological interactions may contribute to the formation of high-density layers of H. a. Methods Four stations spanning the longitudinal axis of East Sound, Washington, USA were sampled in July 2007. A site map and more methodological details are reported in Menden-Deuer (2008). A SeaBird 19+ CTD with mounted fluorometer recorded water column profiles. Water samples were collected from one to two depths, between 0.5 to 12 m, using a horizontally mounted Niskin bottle. For each sample, size-fractionated Chl a (GF/F, 5, 20μm), nutrient concentrations, phytoplankton growthand heterotrophic protist grazing rates were measured in triplicate (Strom & Frederickson 2008). Photosynthetic parameters were measured through incorporation of C at 14 light levels from 0 to 1400 μmol photons m s at 12oC. Statistical comparisons were made by 1-