Time filter

Source Type

Malauene B.S.,University of Cape Town | Malauene B.S.,Instituto Nacional Of Investigacao Pesqueira | Shillington F.A.,University of Cape Town | Roberts M.J.,Oceans and Coasts Research | Moloney C.L.,University of Cape Town
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Direct in-situ observations from a shallow underwater temperature recorder on the continental shelf and from a shipboard oceanographic survey, were combined with MODIS satellite data (sea surface temperature and chlorophyll-. a) to assess the temporal and spatial variability of temperature and chlorophyll-. a in the Mozambique Channel near the coastal town of Angoche, 16°S. Intermittent, relatively cool surface water and elevated chlorophyll-. a signatures were found, indicating upwelling near Angoche over an area between 15°S and 18°S. A 5-year (2002-2007) analysis of temperature (from both in-situ and satellite) revealed two distinct periods: (1) the August-March period with highly variable intermittent "cool water" events and (2) the April-July period with little temperature variability. Generally, periods of cooling occurred at about 2 months intervals, but shorter period occurrences (8-30 days) of cool coastal events were also observed. Two possible forcing mechanisms are discussed: (1) wind derived coastal upwelling (using satellite blended sea surface wind derived from NOAA/NCDC) and (2) the effect of passing transient southward moving eddies (using sea level anomalies from AVISO altimetry). It is suggested that the cool surface, elevated chlorophyll-. a waters are primed and formed by favourable wind-driven Ekman-type coastal upwelling, responding to alongshore northeasterly monsoon winds prevailing between August and March. These waters are then enhanced in chlorophyll-. a and advected further offshore by anti-cyclonic/cyclonic eddy pairs interacting with the shelf. © 2013 Elsevier Ltd.

Barlow R.,Bayworld Center for Research and Education | Barlow R.,University of Cape Town | Lamont T.,University of Cape Town | Lamont T.,Oceans and Coasts Research | And 3 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

An investigation of phytoplankton pigment and absorption characteristics was undertaken during three research cruises in the Mozambique Channel to elucidate community structure and examine the adaptation of populations to mesoscale features at the surface and the deep chlorophyll maximum (DCM). Total chlorophyll a concentration (TChla) at the surface was determined to be greater in cyclonic eddies than in anticyclones, while TChla in divergence and shelf zones were similar to cyclones, with frontal zones being slightly lower. TChla at the DCM was similar for all categories, although there was a tendency for anticyclones to have lower TChla. Prokaryotes were the most significant phytoplankton group at the surface, with small flagellates also being of secondary importance, while flagellates dominated at the DCM. A few shelf stations, and frontal and shelf stations close to the shelf, displayed high TChla and diatom domination, particularly at the DCM. Absorption properties and photopigment indices revealed that prokaryote dominated communities had high chlorophyll-specific absorption coefficients, a large range in the proportion of TChla within the total pigment pool and a high proportion of photoprotective carotenoids. Diatoms had low chlorophyll-specific absorption, a relatively high proportion of TChla, and elevated proportions of photosynthetic carotenoids and chlorophyll c. Flagellate dominated communities had intermediate chlorophyll-specific absorption, a lower proportion of TChla, elevated photosynthetic carotenoids and intermediate chlorophyll c. © 2013 Elsevier Ltd.

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.,Oceans and Coasts Research | Lamont T.,University of Cape Town | Barlow R.G.,Bayworld Center for Research and Education | Barlow R.G.,University of Cape Town | And 2 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Variability of hydrographic characteristics and phytoplankton distribution associated with mesoscale eddies in the Mozambique Channel was investigated during four cruises in September 2007, December 2008, November 2009, and April/May 2010. Satellite altimetry was used to map the location of mesoscale features, and together with ETOPO1 bathymetry, was used in a Discriminant Function Analysis to classify in situ sampling stations into five categories, namely cyclonic (C), anti-cyclonic (A), frontal (F), divergence (D), and shelf (S). Fluorescence profiles were integrated through four depth ranges in the upper 200. m and used to determine the depth of the maximum chlorophyll a concentration and to model the euphotic zone depth. At a depth of 100. m, distinctly different hydrographic characteristics were observed between mesoscale features with cyclonic eddies consisting of Subtropical Surface Water and anti-cyclonic eddies containing Tropical Surface Water from the Indian Ocean. Hydrographic properties at divergence and frontal stations reflected a mixture of these water masses, while shelf stations showed considerable variability as a result of the interaction of eddies with the continental slope. Chlorophyll a concentrations in the surface waters were found to be low, with subsurface levels being significantly greater. Phytoplankton biomass in cyclonic and anti-cyclonic eddies was relatively low and not significantly different. The interaction of mesoscale eddies with the continental slope on the western side of the Channel caused upwelling of cooler, nutrient-rich water, which resulted in elevated phytoplankton biomass in the shelf regions. Strong currents at the perimeters of these eddies produced offshore advection of the high biomass into the frontal regions. © 2013 Elsevier Ltd.

Huggett J.A.,Oceans and Coasts Research | Huggett J.A.,University of Cape Town
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Recent studies have shown the Mozambique Channel to exhibit high mesoscale variability, but how mesoscale features influence the zooplankton community in this region is not known. The biovolume, biomass, vertical distribution and community composition of mesozooplankton (>200μm) associated with mesoscale eddies in the Mozambique Channel was investigated during four cruises in September 2007, December 2008, November 2009 and April/May 2010. Stations were categorized according to their location in cyclonic (cold-core) or anticyclonic (warm-core) eddies, frontal, divergence or shelf regions. Mean mesozooplankton biovolume in the upper 200m was 0.33mlm-3, with zooplankton largely concentrated in the upper 100m during all four cruises (weighted mean depth=66.6m). Sampling depth was the most important predictor of biovolume, which was greatest for net samples with a mid-depth of 0-40m, but declined deeper in the water column. Biovolume at the shelf (0.37mlm-3), divergence and cyclonic eddy stations (0.31mlm-3) was significantly greater than at frontal and anticyclonic eddy stations (0.20mlm-3). Mean biovolume was significantly higher during 2008 and 2010 compared to 2007 and 2009, and was also significantly higher for samples collected at night (and twilight) than during the day. The mesozooplankton community in 2007 was strongly dominated by small copepods (~70-80% abundance) followed by appendicularians (10%), ostracods (8%) and chaetognaths (7%). The most abundant copepods were the Paracalanids, Oncaea spp., Oithona spp. and Corycaeus spp. Multivariate analysis showed that the communities in 2007 and 2008 were most strongly structured by depth, but classification (cyclonic/anticyclonic) was also important in 2007 when mesoscale features were more strongly developed. Zooplankton assemblages showed a high degree of homogeneity, with differences between mesoscale features largely due to differing abundances of similar taxa. These observations suggest that mesoscale eddy and shelf interactions play a fundamental role in shaping the Mozambique Channel pelagic ecosystem through the concentration, enhanced growth and redistribution of zooplankton communities. Although frontal areas between eddies were poor in zooplankton biomass, the extensive inter-eddy divergence areas were as rich in biomass as the small cyclonic eddy core regions. These patterns are important for understanding the favorability of observed foraging areas for higher trophic levels. © 2013 Elsevier Ltd.

Barlow R.,Bayworld Center for Research and Education | Barlow R.,University of Cape Town | Lamont T.,University of Cape Town | Lamont T.,Oceans and Coasts Research
African Journal of Marine Science | Year: 2012

Phytoplankton absorption and pigment characteristics of a red tide were investigated in coastal waters of the southern Benguela. Diagnostic indices indicated that dinoflagellates were the dominant phytoplankton group, with diatoms and small flagellates being of secondary importance. Very high biomass was observed close to the coast where chlorophyll a concentrations of up to 117 mg m -3 were measured. Both measured (a ph) and reconstructed pigment absorption (a pig) displayed an increasing trend with chlorophyll a, while the package effect index (Q * a) decreased, indicating increased packaging with an increase in biomass. Proportioning of the total pigment absorption between 400 and 700 nm revealed that chlorophyll a accounted for 39-65% of the absorption, while photosynthetic carotenoids (15-30%) and chlorophyll cs (15-30%) were also prominent in absorbing light for photosynthesis. © 2012 Copyright NISC (Pty) Ltd.

Ternon J.F.,Institute Of Recherche Pour Le Developpement | Roberts M.J.,Oceans and Coasts Research | Roberts M.J.,Rhodes University | Morris T.,Bayworld Center for Research and Education | And 3 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

Circulation and the related biological production have been studied during five cruises conducted in the Mozambique Channel (MZC) between 2005 and 2010. The circulation in the MZC is known to be highly turbulent, favouring enhanced primary production as a result of mesoscale eddy dynamics, and connectivity throughout the Channel due to the variable currents associated with migrating eddies. This paper presents the results of in situ measurements that characterize the horizontal and vertical currents in the surface and subsurface layers (0-500. m). The in situ data were analysed together with the geostrophic eddy field observed from satellite altimeter measurements. Different circulation regimes were investigated, including the "classical" anticyclonic eddy generated at the Channel narrows (16°S), the enhancement of southward migrating eddies by merging with structures (both cyclonic and anticyclonic) formed in the east of the Channel, and the presence of a fully developed cyclonic eddy at the Channel narrows. Comparison between in situ measurements (S-ADCP and velocities derived from surface drifters) and the geostrophic current derived from sea surface height measurements indicated that the latter can provide a reliable, quantitative description of eddy driven circulation in the MZC, with the exception that these currents are weaker by as much 30%. It is also suggested from in situ observation (drifters) that the departure from geostrophy of the surface circulation might be linked to strong wind conditions. Finally, our observations highlight that a-geostrophic currents need to be considered in future research to facilitate a more comprehensive description of the circulation in this area. © 2013 Elsevier Ltd.

Marsac F.,University of Cape Town | Barlow R.,Bayworld Center for Research and Education | Barlow R.,University of Cape Town | Ternon J.F.,IRD Montpellier | And 3 more authors.
Deep-Sea Research Part II: Topical Studies in Oceanography | Year: 2014

The MESOBIO programme investigated mesoscale dynamics using an integrated ecosystem approach, linking physical and biogeochemical processes with different trophic levels. Observation and modeling were used in combination to explain the main processes occurring in the mesoscale eddy field. The particular shape of the Mozambique Channel, composed of two basins interconnected through a narrow zone, favours the generation of mesoscale eddies and increases the opportunity for eddy-shelf interactions. Phytoplankton abundance peaked in areas of nutrient enrichment that are often found in the core of cyclonic eddies, as well as on the continental shelf. Grazers in zooplankton communities exhibited high biovolume in cyclonic eddies, but their abundance was lower in fronts and divergence zones, with lowest biovolume in anticyclones. Biovolume was highest at shelf stations, but very variable and similar to phytoplankton. Age of eddies, their subsequent maturation stage and the dynamics of the eddy field played a major role effecting zooplankton abundance. Micronekton presented abundance patterns coherent with zooplankton distribution, however this was only demonstrated by acoustic methods, whereas mid-water trawl collection and predators stomach contents (predators being used as biological samplers) did not reveal significant relationships with mesoscale features. For upper trophic levels, the average density of foraging seabirds was lowest in anticyclones, highest in cyclones and at intermediate levels in divergence, shelf and frontal zones. However, multifaceted behavioral responses were observed in such a highly variable environment. Swordfish was clearly associated with divergence zones, and to a lesser extent with fronts, suggesting that the higher density in divergences was related to the presence of its main prey, essentially large squids. Although tunas tended to be more abundant in areas with weak geostrophic currents, their relationship to mesoscale features was not straightforward as adult tunas caught by longline have the ability to explore different foraging habitats over a broad range of depths. Several suggestions for advancing eddy-related research from the current state of knowledge are proposed in the second part of the paper. © 2013 Elsevier Ltd.

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

The pattern of surface circulation in the Mozambique Channel was elucidated from the trajectories of 82 satellite-tracked drifters over the period 2000-2010 and complementary satellite-derived altimetry. Overall, the trajectories indicated that anticyclonic activity was mostly observed on the western side of the Channel, with cyclonic activity being more prevalent in the east. A lack of eddy activity was noted in the southeast corner of the Channel (i.e. SW of Madagascar). Drifter behaviour illustrated that surface water from the Comoros Basin, entrained into anticyclonic eddies during formation, can be retained and isolated for months whilst being transported southwards through the Channel. During a tropical cyclone weather event, a drifter was observed to switch between counter-rotating eddies indicating that horizontal mixing of the Ekman layer does occur. The drifters also illustrated and emphasised the flow field and transport between eddies (i.e. the interstitial flow) in the Mozambique Channel. Despite the dominance of southward propagating anticyclones, drifters were able to move north and south through the Channel in the frontal flow field between eddies within periods of 51-207 days. Cross-channel transport in both directions between the Madagascan and Mozambique shelf regions was similarly observed, with time spans of 19-30 days. Surprisingly, drifters from the southern limb of the East Madagascar Current were transported westward across the channel to the Mozambique shelf. This transport was similarly facilitated by the frontal flow field between eddies. It is hypothesised that the frontal zones between eddies and interstitial waters play an important role in distributing biota in the Mozambique Channel. © 2013 Elsevier Ltd.

Kirkman S.P.,Oceans and Coasts Research | Kirkman S.P.,University of Cape Town | Yemane D.,Fisheries Management | Yemane D.,University of Cape Town | And 7 more authors.
ICES Journal of Marine Science | Year: 2013

Kirkman, S. P., Yemane, D., Kathena, J., Mafwila, S. K., Nsiangango, S. E., Samaai, T., Axelsen, B., and Singh, L. 2013. Identifying and characterizing demersal fish biodiversity hotspots in the Benguela Current Large Marine Ecosystem: relevance in the light of global changes. - ICES Journal of Marine Science, 70: 943-954.This study made use of distribution and abundance data of demersal fish and cephalopod species targeted during trawl surveys off Angola, Namibia and the west coast of South Africa, to determine species richness patterns including the location of diversity hotspots in the Benguela Current Large Marine Ecosystem. The reliability of alternative techniques for determining species richness patterns over the study domain, including geostatistical and non-geostatistical interpolation methods and regression type modelling, was tested using a cross-validation method. Generalized additive models were found to be the most effective method and were used to generate horizontal maps of species richness for different periods in each country. Despite changes in community structure that have been documented during the study period and which may be associated with climatic changes, this study showed the presence of consistently predictable hotspot areas over a 20-30-year study period (depending on country). The relationship between species richness and physical/environmental variables was inconsistent between countries, but generally hotspots of species richness were associated with greater depths and cooler bottom temperatures. Range shifts of species associated, for example, with warming of temperatures could conceivably affect the spatio-temporal persistence of hotspots in the long term. © 2013 © 2013 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup. com.

Loading Oceans and Coasts Research collaborators
Loading Oceans and Coasts Research collaborators