Horner Rosser, S. School of Environmental Biology, Curtin University, jane.rosser@marine.csiro.au

An intensive study of aspects of a dinoflagellate bloom in the upper Swan River estuary, southern Western Australia, was undertaken. Physical (temperature, light, oxygen, salinity, nutrient) and physiological (chlorophyll profiles, nutrient uptake, productivity) parameters were measured weekly, on a diurnal basis, over the course of the bloom. Nitrogen uptake rates measured using 15N enrichment techniques indicated ammonium and nitrate were utilised in surface-waters, while ammonium was the preferred nitrogen source at mid-water and bottom depths. Absolute uptake rates for nitrate, ammonium and urea ranged between 0.06 - 11.2, 1.0 - 11.8 and 0.03 - 9.3 mgN/L/hr respectively. Absolute uptake of ammonium in bottom waters increased throughout the bloom, coinciding with ambient ammonium depletion. The bloom was dominated throughout by the dinoflagellates Scrippsiella sp and Gymnodinium simplex and the cryptophyte Oxyrrhis marina. Diurnal vertical distribution patterns indicated a marked change in vertical migration patterns over the three-week study, with species migrating to nutrient rich bottom waters as the bloom progressed. Diurnal downward migration coincided with decreases in ambient nutrient levels in surface waters. This study presents evidence that diurnal vertical migration, rather than being an innate behavioural pattern, is an alterable behavioural response that can be modified by changing environmental conditions.
Day: Thursday, Feb. 4
Time: 09:45 - 10:00am
Location: Sweeney Center
Code: CS62TH0945S