St Helena Bay Research Articles

Role and efficacy of marine protected areas for the South African rock lobster, Jasus lalandii

Protected areas for the South African rock lobster, Jasus lalandii, were sampled by using divers, traps and ringnets at sites within and adjacent to four protected areas (St Helena Bay, Saldanha Bay and Table Bay rock lobster sanctuaries and the Betty’s Bay marine reserve), over two years. Virtually no rock lobsters were found in St Helena Bay sanctuary, probably because of periodic harmful algal blooms. Abundance was greater in Saldanha Bay sanctuary than in adjacent fished areas, but only once in two years. Sizes were, however, larger in this sanctuary than the fished areas. By an order of magnitude, fewer and smaller rock lobsters were caught within the Table Bay sanctuary than in adjacent areas. Only at Betty’s Bay were rock lobster sizes and abundance consistently greater inside than outside the reserve. Fecundity was similar among sites, with females in protected areas contributing no more to egg production than would be expected by the proportional area occupied by protected areas. Rock lobsters do benefit from protection in Betty’s Bay reserve, but the west coast sanctuaries appear poorly located and seemingly contain large areas of unsuitable substrate. They clearly need relocation to be effective.

Observations on the Diel Vertical Distribution of Hydromedusae in the Southern Benguela

Diel vertical migration (DVM) of six common species of hydromedusae was investigated during two drogue studies conducted in St Helena Bay on the west coast of South Africa in February 1995. Clytia spp., Obelia spp. and Bougainvillia macloviana, were largely confined to surface waters and did not appear to display any DVM. By contrast, Leuckartiara octona and Euphysa aurata displayed clear DVM and, like Proboscidactyla menoni, were found at greater depths than the other species. The depth distribution and amplitude of migration varied between surveys for most species, so definitive interpretations of patterns could not be made.

Taxonomy of the <i>Passerina filiformis</i> complex (Thymelaeaceae)

Revision of the genus Passerina L. indicated a new delimitation of taxonomic entities within the Passerina filiformis L. complex. Evidence from leaf anatomy greatly assisted in the recognition of taxa. P. filiformis is here divided into two sub­species. namely P. filiformis subsp. filiformis and P. filiformis subsp. glutinosa (Thoday) Bredenkamp A.E.van Wyk. and a new species. P. montivagus Bredenkamp A.E.van Wyk, which is also described. The new taxa are geographically separated: subsp. filiformis ranges from Piquetberg in the north through the Cape Peninsula in the south, where it is quite com­mon. to Attaquaskloof in the southwestern Cape: subsp. glutinosa occurs along the coast between Vredendal and St Helena Bay; and P. montivagus has a wide distribution from Mossel Bay and Oudtshoom in the south through Eastern Cape and along the Great Escarpment northwards to Zimbabwe, with outliers in Tanzania.

Camões: Ambiguous Imperialist

Camões:Ambiguous Imperialist NICHOLAS MEIHUIZEN Luis Vaz de Camões depictedone ofthefirst recorded meetings between Europeand southern Africa inhismaritime epicaboutVascoda Gama's voyagetoIndia,Os Lusíadas.He presented Africa ina predictable enough way froma postcolonialpoint of view - we see the Portuguese complacently assuming their superior cultural positioning inthefaceofthe extraordinary backwardness of thelocals. That is, theonlyworld-view presented isa Europeanone,theAfrican onecannotevenbe imagined, let alone creditedwith any value, beingsimplydegenerate by European standards. It is interesting to turnto Camoes's mostimportant surviving source,a diarywritten probably byÁlvaroVelho,1 a member ofthecrew oftheSão Rafael,one ofVascoda Gama's fleet.2 Putting thesourceand Camõessidebyside,onebecomes awareofvariances inattitude thatmight be ascribed, forinstance, to thediffering aimsofthetwowriters interms ofbothendsandmeans.Oneisa diarist, recording events ina factual, nondramatic way.The other isan artist, adapting events inas dramatic a way as possible.The differences between thetwoworks, though, also highlight theabove-noted blindarrogance inCamões,as a comparative analysisin thefirst sectionofthisessaywillshow.The important pointto bearin mind,however, is thatCamões was notan unreflecting patriot, and this fact needs to be emphasizedto counterpoiseany monodimensional conception ofthepoetinitially suggested. I willtherefore consider inthe secondpartof thisessaytheimplications attending theepistemological aesthetics underlying notionsofempire, as suggested byMichelFoucault in The OrderofThings, whileinthefinal sectionI willexaminea strong current of thoughtin Camões thatis explicitly criticalof aspectsof imperialism. * * * 1 Vasco da Gama: The DiaryofHis TravelsThroughAfricanWaters1497-1499, trans,byEric Axelson(Somerset West:StephenPhillips,1998),p. 19. 2 A sixteenth-century copyofthediarycame to lightintheconventofSanta Cruz in Coimbra, afterwhichitwas takento theBibliotecaPublica of Porto.It was publishedin Lisbon in 1838 underthemisleadingtitleRoteiroda Viagemque em Descobrimentoda India pelo Cabo da Boa EsperançafezDom Vasco da Gama. The titleis misleadingbecause thebook is nota roteiroor rutter, thatis, a work thatgivessea-routes.In 1998 the South AfricanhistorianEric Axelson broughtout a new translation ofthework,Vasco da Gama: The Diary ofHis TravelsThrough AfricanWaters1497-1499. I draw on thisversionin the presentessay ratherthan on E. G. Ravenstein'sbetterknownHackluytSocietytranslation of 1898.In thecase ofCamões I use the highly readableThe Lusiads,trans,byLandegWhite(Oxford:OxfordUniversity Press,1997)- CAMÕES: AMBIGUOUS IMPERIALIST Z$ Earlyin theVelhoDiarythereis a passagethatpertains to da Gama's landingat St Helena Bay,on thesouth-west coast of Africa, some 160 kilometres north ofwhatisnowCape Town: On thenext dayafter wehadanchored, which wasThursday [9November 1497], we wentashorewiththeCommander-in-Chief [Vascoda Gama]andtookoneofthose men.He was smallofbodyand lookedlikeSanchoMexia. He was goingabout gathering honey intheheath, for thebeesofthat landplaceitatthefoot ofthescrub. WetookhimtotheCommander-in-Chief 's ship, whoplacedhimathistable, andhe ateofeverything that weate.Thenext daytheCommander-in-Chief clothed himvery wellandordered himto be putashore.On thefollowing dayfourteen or fifteen of them cametowhere wehadthevessels. The Commander-in-Chief wentashoreand showed them many trade-goods tolearn ifthere weresuchgoodsinthat land;andthe goodswere cinnamon andcloves, seed-pearls andgold,andother things aswell.They did notknowthosetrade-goods at all; it seemedtheyhad neverseenthem.The Commander-in-Chief gavethem little bellsandrings oftin.ThiswasonFriday, and thesameon thefollowing Saturday. On Saturday there cameaboutforty orfifty of them, andwe,after dining, went ashore. With ceitils that wecarried with uswetraded for shells which they woreintheir ears, which lookedasifthey hadbeensilvered over, andfox-tails they carry fastened tosticks with which they fantheface.HereI traded a sheath, which oneofthem woreonhisgenitals for a ceitil.3 Thecorresponding passageinCamõesreadsas follows. Da Gamaspeaks: Wewent ashore atanopenstretch, Where ourmen quickly scattered To reconnoitre this welcome land Where nooneseemed tohaveventured; ButI,eager toknowwhere I was, Stayed onthesandy beachwith thepilots To measure thesun'sheight, anduseourart To fix ourbearing onthecosmic chart. Wefound wehadlongagoleft behind Thesouthern Tropic ofCapricorn, Being between itandtheAntarctic, Thatleast-known region oftheworld. Atthis, my companions returning, I sawa stranger with a blackskin Theyhadcaptured, making hissweet harvest Ofhoney from thewildbeesintheforest. He looked thunderstruck, likea man Never placedinsuchanextreme; He couldnotunderstand us,norwehim Whoseemed wilder than Polyphemus. I began byshowing himpuregold Thesupreme metal ofcivilisation, Thenfine silverware andhotcondiment: Nothing stirred inthebrute theleastexcitement. 3Vascoda Gama,pp.23-24. 26 NICHOLAS MEIHUIZEN I arrangedto showhimsimpler things: Tinybeads oftransparent crystal, Some littlejingling bellsand rattles, A redbonnetofa pleasingcolour; I saw at oncefrom nodsand gestures That thesehad madehimveryhappy. I freedhimand lethimtakehispillage, Smallas itwas, tohisnearbyvillage. The nextdayhisfellows,all ofthem Naked, and blackerthanseemedpossible, Trooped downtheruggedhillsidepaths Hopingforwhattheirfriend had obtained.(V.26) Thus,intheDiarythefirst event ofimportance after thePortuguese land is the'taking'of one of thenatives.In Camões thismoment of human contact is delayedbythedetailssurrounding thenavigational expertise of the Portuguese.If an immediateengagement with other humans is emphasized intheDiary ',thisbusiness-like absorption inworkis emphasizedinCam...

Interannual variation in the composition of the assemblages of medusae and ctenophores in St. Helena Bay, Southern Benguela Ecosystem

The assemblages of medusae and ctenophores were examined from samples collected each winter from St Helena Bay, over the 10-year period 1988-1997. A total of 50 hydromedusae, 1 scyphozoan and 2 ctenophore species were identified from 243 samples. Although the data set is characterised by great interannual variability, two main assemblages could be identified each year. These were characterised by either holoplanktonic medusae (e.g. Liriope tetraphylla, Aglaura hemistoma) or meroplanktonic medusae (e.g. Mitrocomella millardae, Chrysaora hysoscella) and ctenophores (e.g. Pleurobrachia pileus). The holoplanktonic medusae were typical of samples at the southern edge of the Bay, and were positively associated with both depth and temperature. Their abundances tended to increase during warm years (1992, 1993 and 1997) as warm surface water flooded the Bay. The meroplanktonic medusae and ctenophores were typical of samples collected within the Bay, and the density of species tended to be negatively correlated with temperature and depth. In spite of the eurythermal nature of the meroplanktonic species, they were more common during cold years (1990 and 1995). This paper represents the first Bay-wide, interannual study of any zooplankton group, and contributes important base line information on the structure of regional pelagic assemblages.

Harmful algal blooms of the southern Benguela Current: a review and appraisal of monitoring from 1989 to 1997

The Benguela upwelling system is subjected to blooms of harmful and toxic algae, the incidence and consequences of which are documented here. Red tides are common and usually attributed to members of the Dinophyceae, most of which are non-toxic. The incidence of these blooms varies spatially, with most blooms confined to the area west of Cape Agulhas. Cape Point forms the natural divide for species that dominate blooms of the west coast of South Africa as opposed to those that dominate the South Coast. Blooms occur most commonly from January to May, during the latter half of the upwelling season. Each red tide is associated with synoptic weather patterns, which dictate the onshore and offshore movement of dinoflagellate-dominated frontal blooms. There is also interannual variation, thought to be related to weather pattern changes. The harmful effects of highbiomass, non-toxic blooms include die-offs resulting from anoxia or hypoxia. Other effects of high biomass blooms include those that may cause mechanical or physical damage or those that may alter the foodweb. Recently, a bloom of the very small pelagophyte, Aureococcus anophagefferens, referred to as brown tide, in Saldanha Bay and Langebaan Lagoon resulted in growth arrest in both oysters and mussels. Toxic species cause mass mortalities of fish, shellfish, marine mammals, seabirds and other animals. Human illness is caused by contaminated seafood when toxic phytoplankton are filtered from the water by shellfish that accumulate toxins to levels that are potentially lethal to humans and other consumers. Of these shellfish poisoning syndromes, Paralytic (PSP) and Diarrhetic Shellfish Poisoning (DSP) are common in the Benguela. Confirmed cases of PSP have been attributed to the dinoflagellate Alexandrium catenella. Although shellfish are usually only marginally affected, in extreme cases of poisoning, mussel mortalities have been observed, and in most instances these have been attributed to blooms of A. catenella. Sardine Sardinops sagax mortalities in St Helena Bay have also been attributed to the ingestion of this PSP-producing dinoflagellate. Monitoring has revealed the presence of Dinophysis acuminata, D. fortii, D. hastata, D. tripos and D. rotundata, all of which have been reported to cause DSP. The dinoflagellate Gymnodiniun cf. mikimotoi, has been implicated in a type of Neurotoxic Shellfish Poisoning and human skin and respiratory irritations have been attributed to aerosol toxins produced by this species.

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.

Physical processes contributing to harmful algal blooms in Saldanha Bay, South Africa

The study synthesises current understanding of the predominant physical processes responsible for the seasonality of harmful algal blooms, notably Alexandrium catenella and Dinophysis spp., in the nearshore environment of Saldanha Bay on the west coast of South Africa. Saldanha Bay is one of the few naturally sheltered areas on the South African coastline suitable for in situ shellfish farming and is the major site for the production of black mussel Mytilus galloprovincialis in South Africa. Mussel farming started there in 1985 and the present level of production is some 2 700 tons per annum. Since 1994, disruption of harvesting as a result of the presence of harmful algal species has been a regular late-summer phenomenon. Toxic blooms that are ultimately advected into the bay develop on the continental shelf to the north between 32°S and St Helena Bay, a region characterized by favourable conditions for dinoflagellate growth and circulation patterns that facilitate build-up of intense blooms during late summer. Offshore dinoflagellate populations are advected shorewards and polewards in response to relaxation of upwelling at the Namaqua cell to the north. Dinoflagellate blooms are advected south from the southern Namaqua shelf during upwelling relaxation. Under such conditions, the gyre south of Elands Bay moves offshore and a barotropic flow past Cape Columbine is established. Evidence suggests that the nearsurface component of the flow occurs as a sudden “flood” event. These dinoflagellate-containing shelf waters are in turn advected into Saldanha Bay when upwelling relaxes, when the density gradient between the bay and the shelf drives surface inflow and bottom water outflow. These flows are reversed with the resumption of upwelling over the shelf, resulting in intrusion and entrainment of bottom water and surface outflow. Entrainment dictates that the bay acts as a net importer of bottom water and net exporter of surface waters over a synoptic cycle. This system of exchange between Saldanha Bay and the shelf curtails the duration and severity of toxic episodes in the bay relative to the shelf.

Changes in the composition of the non-copepod zooplankton assemblage in St Helena Bay (southern Benguela ecosystem) during a six day drogue study

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 180:111-120 (1999) - doi:10.3354/meps180111 Changes in the composition of the non-copepod zooplankton assemblage in St Helena Bay (southern Benguela ecosystem) during a six day drogue study M. J. Gibbons1,*, N. Gugushe1, A. J. Boyd2, L. J. Shannon2, B. A. Mitchell-Innes2 1Zoology Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa 2Sea Fisheries Research Institute, Private Bag X2, Roggebaai 8012, South Africa *E-mail: mgibbons@uwc.ac.za ABSTRACT: Marked changes in the composition and structure of the non-copepod components of a zooplankton assemblage were observed during the course of a 6 d drogue study in the southern Benguela ecosystem. These changes reflected in part the spiraled nature of the drogue and were linked to the settlement of meroplankton from the surface waters and to the nocturnal recruitment, and subsequent accumulation, of migratory holoplankton and zoobenthos from depth. The presence of large numbers of benthic species in the plankton is hypothesised to reflect their upward migration at night owing to food limitation, since little of the overlying diatom bloom was sedimenting out. The presence of low-oxygen bottom water is hypothesised to account for the failure of the detritivorous zoobenthos to return to the seafloor during the day. The results suggest that the composition of zooplankton assemblages in upwelling areas is not only driven by the dynamism of the physical environment, but that, under certain circumstances, a more subtle interplay between the biological and physical environment may result in strong changes in assemblage composition. KEY WORDS: Zooplankton · Assemblage composition · Southern Benguela · Zoobenthos · Cumacea · Phytoplankton aggregates · Low-oxygen water Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 180. Publication date: May 03, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

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.

CHEMICAL OCEANOGRAPHY AND MARINE GEOSCIENCE OFF SOUTHERN AFRICA: PAST DISCOVERIES IN THE POST-GILCHRIST ERA, AND FUTURE PROSPECTS

SUMMARY In 1895/96. John D. F. Gilchrist was appointed marine biologist to the Cape Colony. During voyages extending as far as Walvis Bay and Maputo, he initiated studies of the marine geology and chemical oceanography of the shelf while mapping substrata for new demersal fishing grounds. The shelf sediments off the East Coast are controlled by wave processes along the inner shelf and by the poleward-flowing Agulhas Current along the outer shelf. In contrast. South Coast sediments of the eastern Agulhas Bank consist of wave-dominated, landward-coarsening modem (Holocene) terrigenous muds to sands on the inner shelf and relict wave-dominated shelly sands on the outer shelf, deposited during Pleistocene lowstands within glacial (hypotbermal) periods. The Agulhas Current also appears to exert a controlling influence over the nutrient chemistry and, hence, primary productivity, on the East and South Coast margins. The surface waters of the Agulhas Current are nutrient-poor and most East Coast areas are consequently considerably less productive than their West Coast counterparts at the same latitude, but the underlying South Indian Central Water (SICW) is nutrient-rich. Recent findings suggest that the Agulhas Current may induce upwelling of nutrient-rich bottom water derived from SICW at sites such as the Natal Bight and off Port Alfred by kinematic upwelling so enhancing the nutrient content of surface waters and increasing the potential for primary production there. A second physical process, which is also thought to be related to interaction of the Agulhas Current and bottom topography, is the dynamic shelf-edge upwelling of SICW onto the shelf along portions of the South Coast where the shelf is wider. It is uncertain whether this is continuous in space or time, but it is possible that the process may, prime bottom waters for wind-induced upwelling in the south-western lee of capes along the South Coast. On the West Coast, the outer-shelf sediment consists of Holocene planktonic-foraminiferal ooze, reflecting the dominating influence of the equatorward-flowing Benguela Current. The middle-shelf sediment often consists of glauconitic sand, whereas the sediment of the inner shelf usually has a landward-coarsening and -thickening wedge of terrigenous muds to sands. Wind-induced upwelling is the dominant West Coast physical process of relevance to the sedimentology and chemistry of the inner shelf and overlying waters. In the southern Benguela this is seasonal, resulting in seasonal variability in the abundance of nutrients and the resultant productivity of surface waters and associated biogeochemical processes, such as the appearance of oxygen-depleted bottom water. There is a northward decrease in the seasonality of these physical and biogeochemical processes along the West Coast, which is reflected in an increase in the reducing nature of the underlying organic-rich sediments between St Helena Bay and Walvis Bay. In the deep-sea environment of the Cape Basin, the clockwise pole-ward flow of both the Antarctic Bottom Water (AABW) and the North Atlantic Deep Water (NADW) is reflected in a major zone of erosion of the sea floor, mantled by abundant ferromanganese nodules, at the foot of the continental rise, which is fed by margin-perpendicular slumps, debris flows and canyon-fed turbidity currents. The currents, driven by Coriolis Force, both swing left (east) into the Agulhas Passage between the Agulhas Bank and the Agulhas Plateau, before parting company in the Transkei Basin, where the AABW is forced eastwards by the northeast-shallowing contours of the Natal Valley. The E-W-orientated Agulhas Drift, a contourite drift, is being deposited on the left (north) side of the AABW. The NADW then heads into the Natal Valley to deposit margin-parallel contourite drifts at the foot of the continental slope as far north as Durban, where the Central Terrace and then the Mozambique Ridge steer the NADW first east and then south back to the mouth of the Natal Valley.

Summer incidence of humpback whales on the west coast of South Africa

A total of 14 sightings of 25 humpback whales was recorded from temperate waters of the west coast of South Africa in summer (December–February). In addition, a 6,45 m individual (estimated age 6½ months) stranded at St Helena Bay in February 1982, and a 6,15 m individual (estimated age 4½ months) was found ensnared in the buoy line of a lobster trap off Saldanha Bay in December 1990. These records suggest that certain individuals, particularly juveniles, may not undertake the summer feeding migration. Although the 6,45 m individual was emaciated when stranded, the 6,15 m individual had been feeding on stomatopods, indicating that some individuals feed opportunistically during the summer off the west coast of South Africa.

‘Making like America’: the industrialisation of the St Helena Bay Fisheries c. 1936‐c. 1956

This paper offers a critique of structuralist theories of the salience of ‘external determinants’ in South Africa's post‐1945 industrialisation by way of a case study of the St Helena Bay fisheries, which underwent rapid industrialisation and subsequent deindustrialisation during and after the Second World War. This process was characterised by the state‐engineered articulation of secondary factory production with a primary fishing sector dominated by small boat owners. The reasons for this articulation are located in the nature of primary production, the rise of trade unionism in the fisheries during the war and Afrikaner nationalism's concern to foster small capital. The boat owning petty‐bourgeoisie which emerged at the Bay comprised a substantial speculative element and was unwilling to invest in new technology capable of freeing fishing from its dependence on environmental constraints. The combination of falling prices on the international market and falling catches at the Bay after 1953 created a se...

The fishery for, and general biology of, the St JosephCallorhinchus capensis(Dumeril) off the South-Western Cape, South Africa

St Joseph Callorhinchus capensis are caught by a directed gillnet fishery off the coast of South Africa's Western Cape and also incidentally by trawl in shallow water on both South and West coasts. The fishery only became established in the early 1980s and landings have subsequently stabilized at approximately 800 tons per year, almost one-third of which is a by-catch of trawling operations. The gillnet fishery is centred in St Helena Bay and takes very little by-catch. Both trawl and gillnet operations take place in nursery areas of C. capensi and some species of true shark. C. capensi occurs widely off the southern African coast, from 1 to 366 m deep, but large numbers are only caught inshore (<50 m), the only area where immature fish are found. The fork length at 50% sexual maturity is 435 mm in males (3,3 years) and 496 mm in females (4,2 years). Mature fish have an annual onshore/offshore migration, most mature fish aggregating inshore in summer to breed and lay eggs. They then disperse to deep water...

Sea surface heat exchange at St Helena Bay and implications for the southern Benguela upwelling system

Measurements of the different components of the heat budget were taken at St Helena Bay, on the west coast of South Africa, from 14 to 21 October 1986 in order to examine heat exchange at the air-sea interface. Cold, upwelled water restricts latent heat loss by the sea surface and determines that only a small amount of sensible heat is gained by the sea. Measurements showed that these two turbulent heat fluxes are roughly equal and opposite and more or less cancel each other out. Therefore, the net radiation provided a good estimation of the total heat flux into the surface waters. During favourable conditions (in summer), the daily mean heat, exchange into the sea can be as high as 315 W·m−2, and it is capable of increasing the temperature of the upper 10 m mixed layer by as much as 0,65°C per day. This has important implications for biological production in the cold upwelled water when it is sun-warmed during its movement offshore.

Vertical distribution and diel vertical migration of Calanoides carinatus (Krøyer, 1849) developmental stages in the southern Benguela upwelling region

Diel vertical distribution patterns of the copepodite and adult stages of Calanoides carinatus were analysed using a 10-day time series of observations under contrasting upwelling conditions at an Anchor Station in St Helena Bay, South Africa. The diel vertical migratory behaviour of C. carinatus is probably controlled by both exogenous and endogenous factors. Changes in the food assemblage associated with upwelling significantly modified the vertical distribution and migration of C. carinatus from nonmigratory behaviour under nonsatiating feeding conditions (microflagellates) to marked diel vertical migrations in an upwelling-induced, improved feeding environment (small diatoms). Intrinsic factors such as age, sex and state of ovary maturity also play an important role in the vertical habitat partitioning of C. carinatus in St Helena Bay. Such differential migratory strategies are believed to be adaptations for optimal food utilisation and population maintenance through age-specific depth partitioning. Other adaptive advantages are maximization of reproductive success by copulating at depth at night, and rapid egg development and maximal survival of the first-feeding nauplii by spawning near the surface.

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.

Mesoscale heterogeneities of the phytoplankton distribution in St Helena Bay, South Africa, following an upwelling event

The effect of the variability of the hydrographic field following an upwelling event on the phytoplankton composition and distribution within the St Helena Bay region is investigated. Phytoplankton samples transecting newly upwelled, maturing and aged upwelled water were examined. Both cell concentrations and species diversity were found to increase with the age of the upwelled water. Several species exhibited intense concentration gradients, with stations of similar hydrological conditions having similar populations. All stations were numerically dominated by the microflagellates. In terms of carbon, different components of the phytoplankton were found to be associated with different stages of the upwelling circulation. The variation and peaks in the biomass were primarily attributable to the diatom fraction of the total carbon, with the microflagellates and dinoflagellates maintaining a relatively stable background level. Phytoplankton carbon: chlorophyll a ratios exhibited a coherent spatial pattern. Chlorophyll a values from different locations and depths were therefore not a comparable index of biomass. Changes in the species composition between newly upwelled and aged upwelled water are thought to have been determined to a large degree by the varying histories of these water masses.

Distribution, population structure and biology of stomatopod Crustacea off the west coast of South Africa

Stomatopods recovered during two trawling surveys off the west coast of South Africa during July 1985 and January 1986 consisted exclusively of the endemic subspecies Pterygosquilla armata capensis. The population was concentrated along a narrow inshore strip, in water > 300 m deep, between St Helena Bay and the border of South West Africa/Namibia. Females from the July 1985 cruise were considerably smaller (100 ± 15,5 mm) and less numerous (1:2,2) than males (115 ± 19,8 mm), males growing to a maximum size of over 200 mm total length. The mean sizes of both sexes from the January 1986 cruise were similar, females being 99 ± 11,4 mm and males 113 ± 19,8 mm. However, the sex ratio in the catches then was quite different at 1:1. Both surveys revealed a marked decline in density with water depth, the population being concentrated in areas where the sediment consists of terrigenous muds. Data on length:mass and wet mass:dry mass ratios, organic content and calorific value are provided, and the diet is shown t...

Population structure, reproductive biology and maintenance of Euphausia lucens in the southern Benguela Current

This study investigates the population structure and reproductive biology of the euphausiid Euphausia lucens in relation to temporal and spatial variations in the abiotic and biotic environment in the St Helena Bay region off the west coast of South Afnca. This is an area of high biological productivity and is important for recruitment to the pelagic fishery. Gravid females (stage IV) and young larvae were collected throughout the year, suggesting continuous breeding. Spawning was most intense from August to October (late winter to early spring) just prior to the onset of upwelling and an associated increase in phytoplankton. Recruitment was high until early summer (August-January) and decreased to a low level throughout autumn and early winter (February-June). The proportion of gravid females increased with distance offshore despite relatively low chlorophyll a concentrations offshore. Multiple spawning is inferred from the high frequency of gravid females in the population. The reproductive strategy and vertical distribution of all developmental stages of E.lucens is related to the current regime of the study site in order to formulate mechanisms most likely to play an important role in maintaining coastal populations of E.lucens in the southern Benguela Current.