Southern Benguela System Research Articles

Assessment of the likely sensitivity to climate change for the key marine species in the southern Benguela system

Climate change is altering many environmental parameters of coastal waters and open oceans, leading to substantial present-day and projected changes in the distribution, abundance and phenology of marine species. Attempts to assess how each species might respond to climate change can be data-, resource- and time-intensive. Moreover, in many regions of the world, including South Africa, species may be of vital socioeconomic or ecological importance though critical gaps may exist in our basic biological or ecological knowledge of the species. Here, we adapt and apply a trait-based sensitivity assessment for the key marine species in the southern Benguela system to estimate their potential relative sensitivity to the impacts of climate change. For our analysis, 40 priority species were selected based on their socioeconomic, ecological and/or recreational importance in the system. An extensive literature review and consultation with experts was undertaken concerning each species to gather information on their life history, habitat use and potential stressors. Fourteen attributes were used to estimate the selected species’ sensitivity and capacity to respond to climate change. A score ranging from low to high sensitivity was given for each attribute, based on the available information. Similarly, a score was assigned to the type and quality of information used to score each particular attribute, allowing an assessment of data-quality inputs for each species. The analysis identified the white steenbras Lithognathus lithognathus, soupfin shark Galeorhinus galeus, St Joseph Callorhinchus capensis and abalone Haliotis midae as potentially the most sensitive species to climate-change impacts in the southern Benguela system. There were data gaps for larval dispersal and settlement and metamorphosis cues for most of the evaluated species. Our results can be used by resource managers to determine the type of monitoring, intervention and planning that may be required to best respond to climate change, given the limited resources and significant knowledge gaps in many cases.

Evaluating the effects of climate change in the southern Benguela upwelling system using the Atlantis modelling framework

AbstractThe ocean is affected by multiple anthropogenic stressors including climate change, the effects of which are already evident in many ocean ecosystems. The ABACuS v2 end‐to‐end model together with climate projections from the NEMO‐MEDUSA 2.0 model were used to evaluate the effects of fishing, warming and horizontal and vertical mixing on the southern Benguela upwelling system. Of the drivers examined in this study, warming had the greatest effect on species biomass, with mainly negative effects. The magnitude of the impacts of warming intensified from the RCP 2.6 to the 8.5 emission scenario. Fishing negatively affected demersal and large pelagic fish, which in turn resulted in a biomass increase of forage fish due to a decrease in predation pressure. Water mixing was found to have minor indirect effects on zooplankton biomass and fish. The responses of species and species groups to the combined effects of fishing and warming were approximately equally divided between additive, synergistic and antagonistic. Interpretation of our model results suggests that the southern Benguela system is likely to be affected by climate change, including substantial changes in the abundance of some species important to the region's fisheries. Future planning for fisheries needs to take this into account, including through management that strives to maintain the resilience of key species and the system as a whole. In line with previous studies on the southern Benguela, the results reinforce the importance of including consideration of the indirect and combined impacts of climate change and fishing in management and planning.

An Atlantis model of the southern Benguela upwelling system: Validation, sensitivity analysis and insights into ecosystem functioning

Ecosystem models are valuable tools for evaluating the effects of different environmental stressors on an ecosystem and to increase system understanding. The Atlantis modelling framework is an end-to-end model that includes information and processes from the abiotic environment to the human component of a system. An application of Atlantis for the Benguela and Agulhas Currents was updated and validated against time series of biomass and catch (1990–2013). Five univariate skill metrics were used to evaluate model performance of biomass data of selected species. A sensitivity analysis, focused on the growth rate of small and large phytoplankton and large zooplankton, was conducted to evaluate the influence of these parameters on model outputs. A total of 26 alternative model parameterisations based on a variation of ±20% around the growth parameter of the selected groups were compared to the baseline model. Skill metrics suggest that the model captures the main emergent properties of the southern Benguela, the magnitude and trends in biomass and catch of most modelled groups. Large variations in biomass were observed among the sensitivity runs compared to the baseline. The most marked differences in relative biomass among runs seem to be attributed to changes in the growth rate of large zooplankton. The Atlantis model presented here has the capability to be used as a tool to provide strategic information for fisheries management purposes in the southern Benguela system.

The role of the Agulhas in the Benguela Current system: A numerical modeling approach

AbstractA modeling approach is used to investigate the influence of the Agulhas on the southern Benguela system. Two climatological ROMS simulations are run that are identical except that in one of them the effect of the Agulhas is removed. Comparing their outputs allows for a clear indication of the role of the Agulhas on both the large‐scale and shelf dynamics. About 15 Sv of the mean transport of the Benguela Current is shown to be contributed by Agulhas influx, with the most intense flow being associated with extreme turbulence and its meandering nature is a reflection of the passage of Agulhas Rings. The injection of warm water is particularly evident beyond the shelf‐edge, producing a perennially intense cross‐shelf density front that is enhanced during upwelling season. This gradient drives a jet that is fastest at the shelf‐edge but that extends from the midshelf to at least 100 km beyond it and is associated with dynamic uplift via vortex squashing. Similarly generated is the Good Hope Jet that extends northwestward from the western Agulhas Bank. Turbulence associated with Agulhas leakage increases rapidly beyond the shelf‐edge causing the upwelling front in the southern Benguela to be subject to intense mixing, leaving a relatively uniform front there. Locally generated regions of high turbulence exist in the vicinity of the shelf‐edge jets as well as further north in association with the large filaments that originate from the perennial Lüderitz upwelling cell, with or without the influence of the Agulhas.

Humpback whale "super-groups" - A novel low-latitude feeding behaviour of Southern Hemisphere humpback whales (Megaptera novaeangliae) in the Benguela Upwelling System.

Southern Hemisphere humpback whales (Megaptera novaeangliae) generally undertake annual migrations from polar summer feeding grounds to winter calving and nursery grounds in subtropical and tropical coastal waters. Evidence for such migrations arises from seasonality of historic whaling catches by latitude, Discovery and natural mark returns, and results of satellite tagging studies. Feeding is generally believed to be limited to the southern polar region, where Antarctic krill (Euphausia superba) has been identified as the primary prey item. Non-migrations and / or suspended migrations to the polar feeding grounds have previously been reported from a summer presence of whales in the Benguela System, where feeding on euphausiids (E. lucens), hyperiid amphipods (Themisto gaudichaudii), mantis shrimp (Pterygosquilla armata capensis) and clupeid fish has been described. Three recent research cruises (in October/November 2011, October/November 2014 and October/November 2015) identified large tightly-spaced groups (20 to 200 individuals) of feeding humpback whales aggregated over at least a one-month period across a 220 nautical mile region of the southern Benguela System. Feeding behaviour was identified by lunges, strong milling and repetitive and consecutive diving behaviours, associated bird and seal feeding, defecations and the pungent “fishy” smell of whale blows. Although no dedicated prey sampling could be carried out within the tightly spaced feeding aggregations, observations of E. lucens in the region of groups and the full stomach contents of mantis shrimp from both a co-occurring predatory fish species (Thyrsites atun) and one entangled humpback whale mortality suggest these may be the primary prey items of at least some of the feeding aggregations. Reasons for this recent novel behaviour pattern remain speculative, but may relate to increasing summer humpback whale abundance in the region. These novel, predictable, inter-annual, low latitude feeding events provide considerable potential for further investigation of Southern Hemisphere humpback feeding behaviours in these relatively accessible low-latitude waters.

Modelling the effect of food availability on recruitment success of Cape anchovy ichthyoplankton in the southern Benguela upwelling system

Cape anchovy Engraulis encrasicolus adapted its reproductive strategies to the southern Benguela system by spawning over the Agulhas Bank, an area of low productivity that is located upstream of the predominant upwelling system. Frontal jet currents transport eggs and larvae toward the west coast of South Africa, where recruitment takes place. To characterise the recruitment dynamics of Cape anchovy ichthyoplankton, we used an individual-based model forced by a coupled hydrodynamic–biogeochemical model. The results show the importance of food (especially diatoms and copepods) dynamics on the spatial and temporal patterns of recruitment success, and also confirm the importance of the spawning area, timing and water depth on the recruitment success of Cape anchovy larvae.

Early life-history strategy of Cape hake in the Benguela upwelling region

The spatial and temporal variability of the egg and larval abundance of Merluccius spp. in the Benguela Jet Current was analysed. The Cape hakes, occurring in waters off South Africa, are important for the commercial fisheries in the southern Benguela system, but information on their early life-history strategy is sparse. Data and samples, for this study, were collected during bimonthly cruises of the South African Sardine and Anchovy Recruitment Project (SARP) from August 1995 to July 2003. Samples were collected with a mini-bongo net (300 μm mesh) on a transect crossing the Benguela Current off the Cape Peninsula at 34.15°S, 18.29°E extending 58 nm offshore. Eggs and larvae were commonly found in currents with north-westerly flow, suggesting that the Benguela Jet Current transports eggs and larvae of Merluccius spp. to the northern nursery areas. Spawning grounds of Merluccius spp. appeared to be on the western Agulhas Bank during August 1996 to July 1998. Two possible spawning grounds in different water depths were identified, suggesting spawning by both Cape hake species, M. capensis and M. paradoxus. Peak abundances of eggs and larvae of Merluccius spp. were found from June to October, a time of reduced offshore transport. Abundances of eggs and larvae were low in austral summer and autumn. Spawning intensity was higher during times of reduced upwelling as shown by correlations of abundances of early life stages with the upwelling index. This leads to the assumption that the early life-history strategy of Merluccius spp. is well adapted to the variability in upwelling intensity.

Species‐specific genetic markers for identification of early life‐history stages of Cape hakes, Merluccius capensis and Merluccius paradoxus in the southern Benguela Current

The correct identification of fish egg and larval stages is crucial for inferring spawning areas and subsequent dispersal routes for marine fishes. The authors use species‐specific mtDNA polymerase chain reaction to estimate proportions of Merluccius capensis and Merluccius paradoxus eggs and larvae and to elucidate early life stage distribution patterns in the southern Benguela system.

Iodine distribution in the Southern Benguela system during an upwelling episode

Iodate and total iodine concentrations have been measured at eight stations along the coast of South Africa, between Cape Town and Cape Columbine, during a period of upwelling. At most, iodine uptake from the water amounted to 0.020 μM, and appeared to be entirely iodate; iodate reduction to iodide was negligible. Meanwhile the profiles of the major plant nutrients and chlorophyll- a showed that the water column was heavily influenced by processes of productivity. Overall then, the system reflected minimal iodate reduction accompanied by extensive uptake of nutrients. This challenges the conventional view, gained from the study of tropical and sub-tropical waters, that iodate reduction in the oceans is coupled closely to nutrient uptake and primary production. The implications of this are explored.

Dissolved Iodate and Total Iodine During an Extreme Hypoxic Event in the Southern Benguela System

Dissolved iodate and total iodine were studied in St Helena Bay, South Africa, during a period of acute hypoxia, following upwelling off Cape Columbine. Despite the generally high concentrations of chlorophyll α (10–30mgm−3) total iodine concentration was essentially constant in the main part of the Bay, and similar to that found elsewhere in the oceans. Occasional, lower concentrations of total iodine (0·28 to 0·42μM) were found with exceptionally high chlorophyll α concentrations (500mgm−3) in shallow waters. In contrast, iodate was found to be reduced to iodide at both the surface and the bottom of the Bay. The implications of these changes are discussed, given that the surface waters reflect sustained eutrophication while the bottom waters are hypoxic as a result of the organic-rich sediment from the waters above.

Thermal Stratification in Saldanha Bay (South Africa) and Subtidal, Density-driven Exchange with the Coastal Waters of the Benguela Upwelling System

A study was conducted to understand the mechanisms driving observed subtidal variability in the stratification of Saldanha Bay, located in the southern Benguela system. It was found that the 6–8 day period variability in bay stratification was caused by the inflow and outflow of cold upwelled water driven by changing baroclinic pressure gradients between the coastal and bay domains. The direction and magnitude of the pressure gradients were governed by coastal upwelling activity and a lag in the response of the bay to changes in density structure in the coastal ocean. When the pressure gradients were bayward and cold water was being driven into the bay the cycle was termed to be in an ‘active phase’ and the reverse was termed the ‘relaxation phase’. The upwelling-favourable equatorward wind stress impacted the bay stratification in two ways: on the regional scale, wind drives upwelling and governs the inflow–outflow of cold upwelled bottom water, which strengthens stratification; conversely, on the local bay scale, wind drives vertical mixing, which weakens stratification. A four-phase model is used to describe the observed variability in stratification in the bay. The associated density-driven exchange flows are capable of flushing the bay in 6–8 days, about one-third of the time for tidal exchange alone (c. 25 days). These inflows of cold bottom water are ecologically critical as they supply nutrients to the bay and thus impose a control on new production within the bay environment. Further ecological implications of this bay–ocean exchange include export of phytoplankton new production to the coast, limitation of the risk of harmful algal blooms (HABs) and the division of the system into two distinct ecosystems (bay and lagoon).

A model of copepod population dynamics in the southern Benguela upwelling region

A simple population dynamics model was constructed to simulate temporal variability in the biomass of a dominant copepod Calanoides carinatus (Copepoda: Calanoida) along the West Coast region of South Africa. Calanoides carinatus is extensively preyed upon by the commercially important anchovy Engraulis capensis, thus variability in zooplankton production may serve as a useful predictor of variability in anchovy recruitment levels. The model developed here circumvents the need to include a large number of parameters because it uses satellite-derived estimates of chloro- phyll a concentration and sea surface temperature as primary inputs. Abundance estimates necessary to initialize the model are readily obtainable from biannual research cruises. The model successfully simulates observed features of a copepod population's response to pulses of upwelling and is robust with respect to most of its parameters because minor changes in their values result in predictable changes in model output. The model showed greatest sensitivity to parameters that are difficult to determine empirically, such as predator-induced mortality rates. Gaps in our present understanding of the nature and scale of processes affecting copepod egg abundance, survival and viability in the southern Benguela system were identified as the dominant impediment to simulating copepod popu- lation dynamics in the region.

Carbon pathways and export associated with the southern Benguela upwelling system: a re-appraisal

A network of six NO3-N-driven carbon pathways between the ocean, shelf and sediments was described in a paper published in 1992. A budgetary approach at the annual scale meant that, if three of the pathways were quantified, solutions could be found algebraically for those remaining. The network of pathways in the present study remains unchanged in principle, but in respect of long-term carbon sequestration, there has been a shift in emphasis from ocean to continental shelf. This results from an adjusted estimate for carbon exported seawards of the continental shelf, mainly owing to a re-examination of the typical offshore penetration of upwelling-derived water. Whereas the 1992 paper used a study based on grey-scale contrast, from Meteosat imagery, to designate a region up to and occasionally beyond the 2 000 m isobath where water of upwelling origin was present on a quasi-permanent basis, the present study used actual sea surface temperatures (SSTs) from NOAA imagery and found comparatively little water continental shelf sediments and 4% is lost below the offshore permanent thermocline. The remaining 30% is re-cycled over shorter time-scales within the southern Benguela system.

Long-term trends in the abundance and community structure of coastal zooplankton in the southern Benguela system, 1951–1996

Long-term trends in the abundance and species composition of zooplankton are described. These trends are based on retrospective analysis of zooplankton samples collected in the St Helena Bay area off the South African west coast since the development of the pelagic fishery in the 1950s. Samples were selected from seven sampling programmes (1951–1967, 1974, 1978, 1983, 1984, 1987 and 1988–1996) that monitored the main commercial fishing grounds during peak recruitment (March–June). Abundance data are presented for size-based taxonomic groups, which include cyclopoid and small, medium and large calanoid copepods, cladocerans, euphausiids, amphipods, chaetognaths and pelagic tunicates. Despite discontinuity in the time-series and the use of different samplers, all taxa showed a significant long-term increase in their abundance by at least one order of magnitude between 1951 and 1996. Total zooplankton abundance increased from 4.2×103 to 7.2×105 ind·m−2 over the same period. Multiple regression analysis of the log-transformed data indicates size-based differential rates of increase in population levels among the crustacean size classes, with a maximum rate in cyclopoids (slope = 0.052) and a minimum rate in euphausiids (slope = 0.035). This resulted in a significant shift through time in the crustacean zooplankton community structure, which may reflect differential size-selective predation during periods of anchovy Engraulis capensis or sardine Sardinops sagax dominance. The observed long-term increase in zooplankton abundance could be a response to a long-term decrease in predation pressure following decreases in stocks of pelagic fish, which in turn could be caused by increased predation by top predators. Another mechanism contributing to the long-term increase in zooplankton is the observed long-term intensification of coastal upwelling, which could enhance primary and secondary production, and increase advective input of zooplankton populations into the study area and augment their retention.

The influence of the Benguela upwelling system on Namibia's marine biodiversity

Namibia's marine environment falls within the Benguela system, an eastern boundary current upwelling system in the south eastern Atlantic Ocean. Conditions within much of this environment change continuously as a consequence of the upwelling of nutrient-rich water into the surface zone. In addition, irregular anomalies in temperature, oxygen concentration and salinity occur, particularly in shelf waters. These fluctuations, which are inherent in the functioning of the Benguela system, tend to favour the persistence of few, generalist species, while at the same time high productivity supports large abundances. This trend is evident in all the major marine habitats off Namibia, where diversity is often lower than in comparable habitats in the southern Benguela system off the west coast of South Africa. Namibia's marine environment is considered 'relatively pristine', although threats to biodiversity are posed by both natural and anthropogenic phenomena.

δ 15 N as a tool to demonstrate the contribution of fish-waste-derived nitrogen to an Ulva bloom in Saldanha Bay, South Africa

This study uses stable isotope ratios of nitrogen (δ15N) to test the hypothesis that a bloom of the green seaweed Ulva lactuca, which occurred in Saldanha Bay, South Africa, in summer 1993/94 was linked to an adjacent discharge of nitrogen from pelagic fish processing waste. It is suggested that only two significant sources of new nitrogen were available to the Ulva: the natural nitrate flux from coastal upwelling and the fish factory nitrogen effluent. A significant difference (1.9‰) was found in the mean δ15N values between Ulva samples from a control site at Langebaan Lagoon (8.9‰) and those from Saldanha Bay (10.8‰). The latter value, which is relatively enriched in 15N, is consistent with the view that the nitrogen taken up in Saldanha Bay originated from a trophic position corresponding to pelagic fish in the southern Benguela system. The δ15N values from Ulva at the control site are consistent with nitrogen originating from the natural oceanic nitrate pool. It is suggested that this stable isotope method is useful in linking the causes and effects of eutrophication.

Spatial dynamics of coccolithophore communities during an upwelling event in the Southern Benguela system

The spatial dynamics of coccolithophore communities were investigated in the top 100 m of a longitudinal transect off Hondeklip Bay, Southern Benguela region, during a period of active upwelling in late June 1993. This transect crossed five hydrological domains that were identified from the vertical sections of temperature and salinity, namely the upwelling zone, the upwelling front, the intermediate domain, the offshore divergence, and the oceanic region. Total coccolithophore standing crops ranged from less than 10 × 10 3 cells 1 −1 to a maximum of 278 × 103 cells 1-1. Highest values were recorded in the top 25 m in the upwelling zone and at the offshore edge of the upwelling front. Fifty different taxa were identified with Emiliania huxleyi and Syracosphaera epigrosa being the dominant forms in the standing stock. The offshore divergence and upwelling front effectively isolated assemblages with distinct species compositions and diversity trends. The most successful assemblage in terms of both species diversity and standing crop was found in the intermediate domain where oceanic and upwelled water mix. Equally important was the control of the water-masses motion on the vertical distribution of coccolithophore communities. Shallow-dwelling communities thrived in the inshore upwelling and offshore divergence areas, whereas deep-dwelling communities were found in the intermediate domain, where surface waters downwell. No clear relationship was found between the concentrations in major nutrients, and the species diversity and total standing crop values. S. epigrosa was the only recorded species whose absolute abundance showed a positive relationship with the nitrate and phosphate surface-water concentrations.

Copepod production in the southern Benguela system

Copepod production in the southern Benguela system

Towards predicting recruitment success of anchovyEngrauus capensisGilchrist in the southern Benguela system using environmental variables: a rule-based model

The construction of a rule-based model aimed at assisting in the management of the Cape anchovy Engraulis capensis by predicting the success of recruitment is described. Of the many environmental variables which, potentially, may give an indication of future recruitment, the fundamental physical indices of sea surface temperature and wind were selected for inclusion in the model. From simple regression and correlation analysis, qualitative rules were constructed relating recruitment success to wind frequency, wind velocity and sea surface temperature in one or more of the spawning, transport and recruitment (nursery) areas of the anchovy. By calibration of the rules, and the consideration of annual spawner biomass, it was possible to obtain a good estimate of observed recruitment. The model was then used to examine the implications of a number of possible environmental scenarios on annual recruitment. Results from the statistical analyses and those from the model indicate that transport of larvae may be o...

The dynamics of phytoplankton populations, including a red-tide bloom, during a quiescent period in St Helena Bay, South Africa

Phytoplankton species distribution and production in relation to hydrology were examined in St Helena Bay in the southern Benguela system during December 1987. The presence of a red-tide bloom of the dinoflagellate Ceratium furca provided the opportunity to examine the conditions required for the development of such an event. Hydrographic parameters indicated the intrusion of warm oceanic water adjacent to St Helena Bay whereas, within the Bay, aged upwelled water was evident following relaxation of upwelling and stabilization of the water column. C. furca was recorded in surface waters at a concentration of 1,3 × 106 cells·ℓ−1 and productivity estimates exceeded 500 mgC·m−3·h−1. Productivity indices were lowest within the red-tide bloom, indicating that the dominance of C. furca cannot be explained by rapid growth. The bloom appears rather to represent the accumulation of slow-growing ungrazed populations subject to concentration by physical entrainment and vertical migration. Phytoplankton succession is less likely to be disrupted in St Helena Bay because water masses are retained there for longer periods than outside the Bay. The prevalence of late successional forms such as dinoflagellates is therefore more likely within the Bay.