Euphausiid population structure and grazing in the Indian sector of the Antarctic Polar Frontal Zone, during austral autumn

• Main Author: Bernard, Anthony Thomas Firth
• Format: Master Thesis
• Language: English
• Published: Rhodes University 2005
• Subjects: Euphausiacea > Antarctic Ocean; Euphausiacea > Feeding and feeds; Antarctic; Antarctic Ocean; Austral; Indian; The Antarctic; Antarc*
• Online Access: http://hdl.handle.net/10962/d1015960; https://corycommons.ru.ac.za/vital/access/manager/Repository/vital:5913

The trophodynamics of the numerically dominant euphausiid species within a region of high mesoscale oceanographic variability in the southwest Indian sector of the Antarctic Polar Frontal Zone (PFZ) were investigated during the austral autumns April/May) of 2004 and 2005. During the 2004 survey, sub-surface (200 m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Sub-Antarctic Front (SAF) and the Antarctic Polar Front (APF) bisected the survey area into two distinct zones, the Sub- Antarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll-a (chl-a) biomass was typical for the region (< 25 mg chl-a m⁻²), and was dominated by picophytoplankton. Total euphausiid abundance and biomass ranged from 0.1 to 3.1 ind m⁻³ and 0.1 to 8.1 mg dwt m⁻³, respectively, and did not differ significantly between the stations occupied in the SAZ and AAZ (p > 0.05). The multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, (2) and restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. E. triacantha was found to have the highest daily ingestion rate ranging from 1 226.1 to 6 029.1 ng pigm ind⁻¹d⁻¹, while the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4 to 943.0 ng pigm ind⁻¹ d⁻¹). The total grazing impact of the selected euphausiids ranged from < 0.1 to 20.1 μg pigm m⁻²d⁻¹, corresponding to < 0.15 % of the areal chl-a biomass. The daily ration estimates of autotrophic carbon for the euphausiids suggested that phytoplankton represented a minor component in their diets, with only the sub-adult E. vallentini consuming sufficient phytoplankton to meet their daily carbon requirements. A cyclonic cold-core eddy spawned from the region of the APF located in the southwest Indian sector of the ...