Browsing by Author "Brown, Penelope Cranstoun"

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    Phytoplankton production studies in the coastal waters off the Cape Peninsula, South Africa
    (1980) Brown, Penelope Cranstoun; Field, John G; Hay, C
    Procedures for measuring phytoplankton production in the southern Benguela upwelling region were investigated. A 125 ml incubation bottle is recommended for routine primary production measurements. Exposure of production samples to high light intensities during manipulations on deck was found to inhibit rates of production near the bottom of the euphotic zone particularly below a thermocline. Simulated in situ rates of oxygen production were only slightly higher than in situ rates. When ships time is limited, the simulated in situ method can safely be used in local waters. Definite diel periodicity in rates of production at the sea surface was demonstrated and appears to be linked to light levels and nutrient concentrations, as well as to the ratios of light to dark hours. Since diel variation probably decreases with depth, its effect on integrated daily production is reduced. Nevertheless, the time of incubation should be chosen to minimize the effect of periodicity on daily production estimates. The period spanning noon is recommended for local use. Changes in phytoplankton production and biomass are linked with physical and chemical changes in the upwelling system off the Cape Peninsula. Extremely active upwelling was found to limit primary production and from these measurements the annual net primary production in the Cape Peninsula upwelling region is estimated for the first time to be approximately 1.13 kgC.m⁻ ³.y⁻¹.
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    The development and decline of phytoplankton blooms in the southern Benguela upwelling region
    (1986) Brown, Penelope Cranstoun; Field, John G; Hutchings, Laurence
    [page ii, 27 missing] These "Redfield productivity 14 estimates" were similar to C-uptake productivity but lower than estimates obtained from changes in particle volume. If the period of maximum nutrient decrease was used for the calculations, the "Redfield productivity 14 estimates" lie between the C-uptake and particle volume estimates. 14 Daily rates of C-uptake water column productivity ranged between 0.94 and 14.01 g C.m- 2 .d- 1 (mean 3.80 g C.m- 2 .d- 1) and were similar to or higher than productivity estimates reported for other upwelling areas. Phytoplankton biomass in the upper 50 metres ranged between 8 and 506 mg phll �- m- 2 (mean 208 mg chll �.m- 2); on average, about half the biomass occurred below the 1% light level indicating that self-shading is an important factor limiting primary production in the Benguela upwe!ling system. The temporal scale of phytoplankton bloom development was investigated in terms of changes in chlorophyll � concentrations in the euphotic zone. The build up and decline of the primary phytoplankton (diatom) bloom in newly upwelled water occurred within 6-8 days. The initiation of blooming appears to be controlled by the stability of the water body (vertical and horizontal), and the decline of the bloom was usually associated with reduced nutrient levels and is considered to result mainly from phytoplankton cells sinking out of the surface layers. Dispersive processes may also contribute to bloom decline. Zooplankton grazing made little impact on the phytoplankton community. The wide coastal band of chlorophyll-rich water seen in satellite images to well beyond the outer limit of the drogue tracks, suggests that both regenerated nutrients and new nutrients (entrained into surface waters after the primary bloom), maintain primary production in shelf waters at moderate levels than the 6-8 days suggested by this study.