Importance of selenium and humic substances from land runoff in the development of Gymnodinium catenatum toxic dinoflagellate blooms

Toxic dinoflagellate blooms of Gymnodinium catenatum Graham (causative organism of paralytic shellfish poisoning) can have major impacts on aquaculture, human health and the aquatic environment. In south-east Tasmanian estuaries, these blooms are preceeded by rainfall events and the associated input of freshwater and humic substances (from soil leachates). This study examined the potential nutritive role of terrestrially-derived humic substances / micronutrients and the importance of vertical water column stratification in the development of G. catenatum blooms. Laboratory experiments showed that humic additives (standard aquatic humic acid purchased from the International Humic Substances Society, and dissolved organic matter isolated from the Huon Estuary by ultrafiltration) stimulated G. catenatum growth and biomass production. Under these conditions biomass was limited by the macro-nutrients nitrate and phosphate, but in seawater medium with no humic additives the micro-nutrient selenium was limiting. This suggests that humic substances change overall nutrient availability and / or uptake by G. catenatum, either by adding nutrients or by interaction with other nutrients. Bioassays indicated that 1 - 100 nM selenium (IV) stimulated G. catenatum growth and biomass production. However not all strains of this dinoflagellate species tested (including isolates from Tasmania, Japan and Spain) had an obligate selenium requirement. Another PSP dinoflagellate Alexandrium minutum showed a selenium requirement similar to G. catenatum, but the bloom forming diatom, Chaetoceros cf. tenuissimus showed no reduction in growth or biomass production under seleniumdeficient conditions. Inorganic selenium (selenite and selenate) concentrations in the Huon Estuary were commonly <0.01 nM (below detection) and thus are potentially limiting for G. catenatum. Since selenium enters estuarine systems primarily through river run-off, rainfall may be a crucial trigger for dinoflagellate blooms in these waters by increasing selenium levels. Water column stability and stratification, also often associated with rainfall, occurs in south-east Tasmanian coastal waters during summer, and this is also critical in the development of G. catenatum blooms. Laboratory experiments in stratified laboratory water columns (0.1 x 1m) examined the migration behaviour of G. catenatum and showed that nutrient-deficient cells migrate downwards to access nutrients from bottom layers at night, while nutrient-replete cells tend to remain at the surface. This migration pattern was similar when using humic and non-humic surface waters, indicating that humic substances do not enhance cellular nutrient concentrations to such a degree as to preclude the need for deep nutrient uptake. Diel vertical migration to facilitate dark nutrient assimilation provides an ecological advantage for G. catenatum over other nonmigratory species, particularly during late summer in Tasmanian estuaries when surface nutrients are depleted. This study indicates that selenium and humic substances contained in river run-off are stimulatory to G. catenatum growth and biomass production, providing essential micro-nutrients during bloom initiation. Furthermore, water column stability and vertical stratification caused by river run-off are of paramount importance in maintaining G. catenatum blooms. During this latter stage, vertical migration by nutrient-deficient cells forms an integral part of the successful ecological strategy utilised by this red tide dinoflagellate.