The trophodynamics of the numerically dominant mesozooplankton (200-2000 m) in the vicinity of the Subtropical Convergence (STC) in the Indian sector of the Southern Ocean during austral autumn (April / May) 2007 were investigated as part of the Southern Ocean Ecosystem Variability Study. The survey consisted of six north-south transects each bisecting the STC between 38º to 43ºS and 38º to 41º45’E. In total, 48 stations situated at 30 nautical mile intervals were occupied over a period of ten days. Hydrographic data revealed a well defined surface and sub-surface expression of the STC, which appeared to meander considerably between 41ºS and 41º15’S. Surface chlorophyll-a (chla) concentrations were low, ranging between 0.08 and 0.68 mg chl-a.m-3 and were generally dominated by the picophytoplankton (<2 m) which made up 66.6% (SD±17.6) of the total pigment. Chl-a concentrations integrated over the top 150m of the water column ranged between 11.97 and 40.07 mg chl-a.m-2 and showed no significant spatial patterns (p>0.05). Total integrated mesozooplankton abundance and biomass during the study ranged between 3934.9 and 308521.4 ind.m-2 (mean = 47198.19; SD±62411.4 ind.m-2) and between 239.8 and 4614.3 mg Dwt.m-2 (mean = 1338.58; SD ±1060.5), respectively. Again, there were no significant spatial patterns in the total mesozooplankton abundance or biomass within the region of study (p>0.05). No significant correlations were found between biological (chlorophyll-a concentrations and zooplankton abundance) and physico-chemical variables (temperature and salinity) (p>0.05). The total mesozooplankton community was numerically dominated by copepods of the genera Pleuromamma, Calanus, Oncaea and Oithona. Other important representatives of the mesozooplankton community included the tunicate, Salpa thompsoni, and the pteropod, Limacina retroversa. At the 40% similarity level, numerical analysis identified five distinct mesozooplankton groupings within the survey area. Differences between the groupings were associated with changes in the relative contribution of numerically dominant species rather than the presence or absence of individual species. No groupings were associated with any specific feature of the front within the survey area. The feeding rates of the six most numerically abundant mesozooplankton species (Calanus simillimus, Limacina retroversa, Pleuromamma abdominalis, Clausocalanus breviceps, Oncaea conifera, Salpa thompsoni) accounting for on average 39% of the total mesozooplankton counts, were investigated using the gut fluorescence technique. For all species, the total gut pigment contents during the night time were significantly higher than the daytime values (p<0.05 for all species). The gut evacuation rates (k) for selected mesozooplankton ranged between 0.14 and 0.81 h-1. The ingestion rates ranged between 147.8 and 5495.4 ng(pigm)ind-1.day-1 which corresponded to a daily ration of between 2.4 and 10.9% body carbon. The combined grazing impact of the selected species on the daily phytoplankton standing stock was highly variable and ranged between 1.2 and 174.1% with an average of 27.3% (SD±38.78%) within the survey area. The highest grazing impact (>60%) was typically associated with those stations where the pteropod, L. retroversa, and the tunicate, S. thompsoni, contributed more than 5% of the total mesozooplankton counts. No significant differences were found in the grazing impact of any or all selected species situated either north, south or in the immediate vicinity of the front (p>0.05 in all cases). The lack of defined spatial patterns in the mesozooplankton abundance and community structure suggests that the STC did not act as a significant biogeographic barrier to the distribution of mesozooplankton during the study. It is presumed that the large scale mixing event caused by a storm prior to this study was responsible for the observed lack of elevated biological activity within the region of the STC.

Thesis (MSc)--Stellenbosch University, 2015. / ENGLISH ABSTRACT: The Southern Ocean (SO) is of significant interest in the understanding of the global carbon cycle and therefore many studies have been conducted to determine the limiting factors controlling the biological pump within the region. During photosynthesis phytoplankton require various nutrients such as NO3, PO4, inorganic carbon and the micronutrient Fe. The SO is a High-Nutrient Low-Chlorophyll region, therefore no macronutrient limitation is experienced by resident phytoplankton but instead the micronutrient Fe is a significant limiting factor within these waters due to limited inputs. Due to deep mixed layer depths, ice cover, low sun angles and cloud cover throughout parts of the year, light is also considered a limiting factor in the SO. Fe and light limitation cause a decrease in photosynthetic efficiency and therefore a decrease in carbon fixation capabilities. During this study we conducted five bioassay shipboard incubation experiments during two cruises along the Greenwich meridian between South Africa and the ice edge, SOSCEx during March and SAFePool during January to February, in which we varied Fe concentrations and light levels to determine the effects of Fe and light limitation or co-limitation within resident phytoplankton. Spatial and temporal variations in phytoplankton response were studied to determine varying effects of limitation across water masses and different stages of bloom decline within the study area. The combined addition of Fe and light gave the largest increase in biomass, photosynthetic capacity and nutrient uptake. In support of the hypotheses tested changes in the photosynthetic apparatus led to changes in the photosynthetic efficiency and growth of the SO phytoplankton, as a result of variations in Fe and light availability. Variability was also observed in the response of phytoplankton to Fe and light amendments due to spatial and temporal variation in resident phytoplankton communities. It was therefore concluded that both Fe and light are significant controls in the resident phytoplankton photosynthetic apparatus, photosynthetic capabilities, organic carbon fixation and therefore the biogeochemical cycles within the Atlantic sector of the SO. / AFRIKAANSE OPSOMMING: Die Suidelike Oseaan (SO) is van beduidende belang in die begrip van die globale koolstofsiklus en dus is baie studies gedoen om die beperkende faktore te bepaal wat die biologiese pomp in die streek beheer. Tydens fotosintese benodig fitoplankton verskillende voedingstowwe soos NO3, PO4, anorganiese koolstof en die mikrovoedingstof Fe. Die SO is 'n High-Nutrient Lae-Chlorofil streek, dus word geen makrovoedingstof beperking ervaar deur inwoner fitoplankton maar in plaas daarvan is die mikrovoedingstof Fe 'n beduidende beperkende faktor binne hierdie waters weens beperkte insette. As gevolg van diep gemengde laag dieptes, ysbedekking, lae son hoeke en wolkbedekking deur dele van die jaar, word lig ook beskou as 'n beperkende faktor in die SO. Fe en lig beperking veroorsaak 'n afname in die fotosintetiese doeltreffendheid en dus 'n afname in koolstof binding vermoëns. Tydens hierdie studie het ons vyf biotoets inkubasie eksperimente aan boord die skeep gedoen tydens twee vaarte langs die Greenwich meridiaan tussen Suid-Afrika en die ys rand, SOSCEx gedurende Maart en SAFePool gedurende Januarie tot Februarie, waarin ons Fe konsentrasies en lig vlakke gewissel het om die gevolge van Fe en lig beperking, of medebeperking, binne inwoner fitoplankton te bepaal. Ruimtelike en temporale variasies in fitoplankton reaksie was bestudeer om wisselende gevolge van die beperking oor watermassas en verskillende stadiums van bloei afname in die studie area te bepaal. Die gekombineerde byvoeging van Fe en lig het die grootste toename in biomassa, fotosintetiese kapasiteit en voedingsopname gegee. Ter ondersteuning van die getoetste hipoteses, veranderinge in die fotosintetiese apparaat het gelei tot veranderinge in die fotosintetiese doeltreffendheid en groei van die SO fitoplankton, as 'n gevolg van variasies in Fe en lig beskikbaarheid. Veranderlikheid is ook waargeneem in die reaksie van fitoplankton om Fe en lig wysigings weens die ruimtelike en tydelike variasie in inwoner fitoplankton gemeenskappe. Dus was dit by die gevolgtrekking gekom dat beide Fe en lig beduidende kontrole in die inwoner fitoplankton fotosintetiese apparaat, fotosintetiese vermoëns, organiese koolstof binding en daarom die biogeochemiese siklusse binne die Atlantiese sektor van die SO.