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Abstract

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.

Item Type:	Thesis - PhD
Authors/Creators:	Doblin, Martina A
Keywords:	Algal blooms, Toxic marine algae, Dinoflagellate blooms, Runoff
Copyright Holders:	The Author
Copyright Information:	Copyright 1998 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (Ph.D.)--University of Tasmania, 1998. Includes bibliographical references
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