Time filter

Source Type

Roberts M.J.,Oceans and Coasts Research | Roberts M.J.,Rhodes University | Ternon J.-F.,Institute Of Recherche Pour Le Developpement | Morris T.,Bayworld Center for Research and Education
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Sea Level Anomaly (SLA) data were used to track a southward propagating eddy dipole along the western slope of the Mozambique Channel over some 6 months. In April 2005, this dipole (with the cyclone to the south) was close to the continental slope off southern Mozambique. The contact zone between the contra-rotating vortices and the slope was surveyed by ship using onboard (S-)ADCP and CTD lines. The data showed strong (>1.4ms-1) southward (geostrophic) currents over the slope adjacent to the anticyclone with horizontal divergence over the shelf edge. Significant slope upwelling between the dipole and the shelf was evident, concomitant with enhanced nutrient and chlorophyll levels enriching shelf near-surface waters. Satellite observations depicted a 300km long surface chlorophyll filament extending offshore in the frontal zone between the contra-rotating vortices. A satellite-tracked drifter deployed at the coastal base of this filament confirmed the offshore advection of chlorophyll-enriched shelf water, which ultimately wrapped around the cyclone and filling its centre. The slope upwelling was also clearly evident in hourly temperature data collected by a recorder deployed on a nearby reef (Zambia Reef) in a depth of 18m. According to the SLA data, the dipole took several weeks to pass Zambia Reef causing prolonged bouts of upwelling that finally ceased when it left the continental slope and moved southwards into the open ocean. Further analysis showed that lone anticyclones and cyclones against the Mozambique continental shelf also induce slope upwelling as a result of horizontal divergence created by the radial circulation of the vortex. In the case of cyclones, the divergence occurs north of the contact zone. Overall, this case study confirms that eddies moving southwards along the western side of the Mozambique Channel are the main mechanism for pumping nutrients into the otherwise oligotrophic surface waters, and moreover, provide a vigorous mechanism for shelf-open ocean exchange. © 2013 Elsevier Ltd.

Lamont T.,Branch Oceans and Coasts | Lamont T.,University of Cape Town | Barlow R.G.,Bayworld Center for Research and Education | Barlow R.G.,University of Cape Town
African Journal of Marine Science | Year: 2015

During February 2010, studies of primary production (PP) and physiology were conducted at five selected sites in the KwaZulu-Natal (KZN) Bight of the Agulhas ecosystem as part of a programme to elucidate the influence of major physical driving forces and nutrient inputs on the structure and functioning of biological communities. These sites were located in the vicinity of the Durban lee eddy, in the midshelf region of the central part of the bight, off the Thukela Mouth, and to the north and south of Richards Bay. At four of the sites, chlorophyll a ranged from 0.10 to 1.44 mg m–3 and integrated PP ranged between 0.35 and 2.58 g C m–2 d–1. The highest biomass and PP, which were comparable to those observed in a wind-driven upwelling system, were associated with a diatom community observed at the midshelf site, and varied between 0.26 and 4.27 mg m–3 and 7.22 and 9.89 g C m–2 d–1, respectively. Environmental conditions at each of the sites differed substantially and appeared to be influential in initiating and controlling the development and distribution of phytoplankton biomass and production. Phytoplankton adaptation to variable environmental conditions was characterised by a decreased light-limited slope (αB) and increased rate of photosynthesis (Pm) and light saturation (Ek) with elevated temperatures. The converse (increased αB and decreased Pm and Ek) was observed as irradiance levels declined. Generalised additive models indicated that irradiance, temperature and biomass were important variables influencing photosynthetic parameters and photosynthetic rates. © 2015 NISC (Pty) Ltd.

Henderiks J.,Uppsala University | Winter A.,University of Puerto Rico at Mayaguez | Elbrachter M.,Alfred Wegener Institute for Polar and Marine Research | Feistel R.,Leibniz Institute for Baltic Sea Research | And 3 more authors.
Marine Ecology Progress Series | Year: 2012

Two distinct morphotypes of the coccolithophore Emiliania huxleyi were observed as part of the phytoplankton succession offshore of Namibia, where coastal upwelling created strong gradients in sea surface temperature (SST), salinity, and nutrient conditions. The sampled surface waters hosted a characteristic succession of phytoplankton communities: diatoms bloomed in newly upwelled waters above the shelf, whereas dense coccolithophore communities dominated by E. huxleyi were found farther offshore, in progressively aging upwelled waters. A substantially calcified E. huxleyi morphotype (labeled Type A*) dominated plankton assemblages at stations influenced by upwelling, that immediately succeeded coastal diatom blooms. This morphotype caused a chlorophyll and 19'-hexanoyloxyfucoxanthin (19'-HF) maximum with >1 × 10 6 cells l -1, straddling a pycnocline at 17 m depth where the in situ N:P ratio was ≅13. Farther offshore, within <20 nautical miles distance, populations of Type A* drastically declined, and a more delicate morphotype with thin distal shield elements and open central area (Type B/C) was found. This morphotype was most abundant (~0.2 × 10 6 cells l -1) in high-phosphate, nitrogen-depleted surface waters (N:P ≅ 8), where it co-existed with other coccolithophores, most notably Syracosphaera spp. Extensive surface blooms of coccolithophores observed by satellites in the same region in the past were identified by microscopy as being produced by E. huxleyi and S. pulchra. However, blooms of E. huxleyi at greater depths in the euphotic zone, such as those observed in this study, will go undetected by satellites and thus underestimate coccolithophore biomass and calcification within upwelling regions. © Inter-Research 2012.

Lamont T.,Oceans and Coastal Research | Lamont T.,University of Cape Town | Barlow R.G.,Bayworld Center for Research and Education | Barlow R.G.,University of Cape Town | Kyewalyanga M.S.,University of Dar es Salaam
Deep-Sea Research Part I: Oceanographic Research Papers | Year: 2014

Investigations of primary production (PP) were undertaken in the southern Benguela ecosystem during two research surveys in October 2006 and May 2007. Significant differences in environmental conditions, as well as biomass and PP, were observed between October and May. During October, integrated biomass and PP were significantly higher, ranging from 20.43 to 355.01mgm-2, and 0.71 to 6.98gCm-2d-1, respectively, than in May, where the range was 47.92-141.79mgm-2, and 0.70-3.35gCm-2d-1, respectively. Distribution patterns indicated low biomass and PP in newly upwelled water along the coast, higher biomass and PP in the mid-shelf region, while lower values were observed at and beyond the shelf edge. Latitudinal variations showed consistently higher biomass and PP in the St. Helena Bay region compared to biomass and PP south of Cape Town. During both surveys, phytoplankton communities were comprised primarily of diatoms and small flagellates, with no significant differences. Phytoplankton adaptation to environmental variability was characterised by increased Pm B and Ek under elevated temperatures and irradiance, while no clear relationships were evident for αB. Generalised Additive Models (GAMs) showed that photosynthetic parameters were all significant predictors of photosynthesis rates (Pz), with Pm B being the most important, accounting for 36.97% of the deviance in Pz. However, biomass levels and environmental conditions exerted a much greater influence on Pz, with irradiance explaining the largest proportion (68.24%) of the deviance. Multiple predictor GAMs revealed that 96.26% of the deviance in Pz could be explained by a model which included nitrate, chlorophyll a, and irradiance. © 2014 Elsevier Ltd.

Guastella L.A.,University of Cape Town | Guastella L.A.,Bayworld Center for Research and Education | Smith A.M.,University of KwaZulu - Natal
Estuarine, Coastal and Shelf Science | Year: 2014

Webcams have become popular means of showcasing beach conditions for a wide variety of beach users. However, webcams can also be a useful tool in assessing changes in coastal morphology and coastal processes. This information can be used by managers to assist in planning. A number of fixed-position beach webcams are freely available to the South African public via various tourism, surfing, weather and aviation websites, individual clubs and a cell-phone network provider. The advantages of these public networks are that the information is free and as the webcams are fixed, afford a consistent and comparable view of the beach. The disadvantage is that you are at the mercy of the provider: resolution is generally poor, downtime and communication are out of your control, and you have no influence over the positioning of the webcam or the discontinuity of service. Notwithstanding the above, the existing webcams can still provide valuable information. From the network of beach webcams available in South Africa we analyse imagery from three beach webcams located in the province of KwaZulu-Natal, at Umhlanga, Margate beach and lagoon, and Amanzimtoti beach and lagoon to examine the coastal dynamics. From these case studies we illustrate seasonal beach rotation and lagoon mouth dynamics, specifically why outlets migrate southwards in opposition to regional longshore drift. © 2014 Elsevier Ltd.

Discover hidden collaborations