Ecophysiological studies on Aureococcus anophagefferens blooms in Saldanha Bay, South Africa

Abstract: The detection of brown tide dominated by the pelagophyte Aureococcus anophagefferens in Saldanha Bay in 1997 was the first occurrence of such blooms outside the north-east coast of the USA. Repeated brown tides in a semi-isolated aquaculture facility in the bay have resulted in poor oyster yields during bloom periods threatening the economic viability of the farming venture. The dramatic economic and ecological consequences of brown tides in the USA initiated a considerable research effort for over two decades that has revealed a number of ecophysiological adaptations of Aureococcus proposed to confer an advantage over competing phytoplankton. The aim of the present study was to investigate certain physiological and ecological characteristics of natural brown tide assemblages to provide a comparison with findings from elsewhere. Investigations were conducted on a number of occasions between 1998 and 2003 during visible brown tide events in a semi-isolated oyster culture facility within Saldanha Bay. Microscope counts revealed that Aureococcus was the most abundant phytoplankton species present during these blooms amounting to 96.7% of total phytoplankton and other nanoplankton numbers. Size fractionated experiments confirmed that primary productivity was dominated by the <10μm fraction (87.4%). Nitrogen (N) uptake rates based both on calculations from kinetic parameters and on direct measurements using 15N isotopes show reduced N as NH4 and urea to be far more important than oxidised N as NO3 (f-ratio between 0.04 and 0.24). Organic N as urea provided an important N resource for these brown tides. Uptake affinity estimated from the initial slope of the Michaelis–Menten equation did not reveal any marked preference for either N source; the small contribution from NO3 being dictated largely by low environmental concentrations. There was a marked separation of primary production and N uptake in the vertical such that C/N assimilation ratios ranged from 28.5 at the surface to 1.6 at depth. Depth integrated values more closely approximated the Redfield ratio. Studies investigating the effect of irradiance on primary productivity and N uptake were consistent with a low-light adapted species with light saturation parameters of 71–102μmolm−2 s−1 for photosynthesis and 14–68μmolm−2 s−1 for N uptake. Dark uptake of reduced N was in excess of 50% of maximum rates in the light. The findings of this study support the idea of a species well adapted to growth in turbid waters that are poor in dissolved inorganic nutrients, particularly as NO3, though with significant supplies of organic N. A salient characteristic of Saldanha Bay brown tides was the co-occurrence of a Chrysochromulina sp. that may have exerted some degree of grazing control over Aureococcus. [Copyright &y& Elsevier]