Central Namibia Research Articles

Nutrient dynamics and oceanographic features in the central Namibian upwelling region as reflected in δ<sup>15</sup>N-signals of suspended matter and surface sediments

The study deals with the modern situation of the northern Benguela Upwelling, directing particular attention to the shelf region off central Namibia (21 to 24° S). At the centre of the investigation is the comparison of δ<sup>15</sup>N-records in surface sediments (δ<sup>15</sup>N<sub>sediment</sub>) with suspended particulate matter (δ<sup>15</sup>N<sub>SPM</sub>) from the surface ocean. In addition to that, water column profiles (including hydrographic data) provide an insight into changes of δ<sup>15</sup>N<sub>SPM</sub> with depth and elucidate potential offsets between δ<sup>15</sup>N<sub>SPM</sub> and δ<sup>15</sup>N<sub>sediment</sub>. The parallel spatial trend of δ<sup>15</sup>N<sub>sediment</sub> and surface ocean δ<sup>15</sup>N<sub>SPM</sub> shows that secondary processes are not so pronounced as to obliterate the signal generated in the surface waters. Highest δ<sup>15</sup>N-signatures are found right off the coast where water temperatures are lowest. Concomitantly high productivity rates and low bottom oxygen suggest the upwelling of denitrified source waters. With increasing distance offshore, δ<sup>15</sup>N declines unexpectedly, reaching a minimum above the shelf break. Beyond that, the trend reverses to "normal" with δ<sup>15</sup>N-signals continuously increasing towards the mesopelagic ocean. The decrease in δ<sup>15</sup>N<sub>sediment</sub> and surface ocean δ<sup>15</sup>N<sub>SPM</sub> with increasing distance to the coast disagrees with the concept of Rayleigh fractionation kinetics, viz. the progressive <sup>15</sup>N-enrichment of the nitrate pool as it is gradually used up by phytoplankton growth. On the basis of the available evidence, the downward trend of δ<sup>15</sup>N results from decreased relative nitrate consumption, resting on a combination of reduced primary production and the existence of an ulterior source of nutrients. Nutrient replenishment seems to occur via an additional upwelling front at the edge of the shelf as well as tapping of subsurface nitrate through sufficiently deep penetration of wind- and wave-induced mixing over large areas of the shelf. Both mechanisms are considered capable of working against the expected nutrient drawdown (i.e. <sup>15</sup>N-enrichment) as surface waters travel offshore. It is important to keep these caveats in mind when interpreting δ<sup>15</sup>N-variations in sediment cores retrieved from this area. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201100005" target="_blank">10.1002/mmng.201100005</a>

Nutrient regime and upwelling in the northern Benguela since the middle Holocene in a global context – a multi-proxy approach

The last 5500 years of climate change and environmental response in the northern Benguela Coastal Upwelling are reconstructed by means of three sediment cores from the inner shelf off central Namibia. The study is based on nutrient (δ<sup>15</sup>N, δ<sup>13</sup>C) and productivity proxies (accumulation rates of total organic carbon; AR<sub>TOC</sub>). Reconstructed sea surface temperatures (alkenone-derived SST) and temperatures at subsurface depths (<i>T</i><sub>δ18O</sub>; based on tests of planktonic foraminifers) reflect the physical boundary conditions. The selection of proxy indicators proved a valuable basis for robust palaeo-climatic reconstructions, with the resolution ranging from multi-decadal (NAM1) over centennial (core 178) to millennial scale (core 226620). The northern Benguela experienced pronounced and rapid perturbation during the middle and late Holocene, and apparently, not all are purely local in character. In fact, numerous correlations with records from the adjacent South African subcontinent and the northern hemisphere testify to global climatic teleconnections. The Holocene Hypsithermal, for instance, is just as evident as the Little Ice Age (LIA) and the Roman Warm Period. The marked SST-rise associated with the latter is substantiated by other marine and terrestrial data from the South African realm. The LIA (at least its early stages) manifests itself in intensified winds and upwelling, which accords with increased rainfall receipts above the continental interior. It appears that climate signals are transferred both via the atmosphere and ocean. The combined analysis of SST and <i>T</i><sub>δ18O</sub> proved a useful tool in order to differentiate between both pathways. SSTs are primarily controlled by the intensity of atmospheric circulation features, reflecting changes of upwelling-favourable winds. <i>T</i><sub>δ18O</sub> records the temperature of the source water and often correlates with global ocean conveyor speed due to varying inputs of warm Agulhas Water. It seems as though conveyor slowdown or acceleration not only affected the temperature of the source water but also its nutrient content. This relationship between source water quality and conveyor speed is already known from glacial times. <br><br> doi:<a href="http://dx.doi.org/10.1002/mmng.201100006" target="_blank">10.1002/mmng.201100006</a>

Nutrient regime and upwelling in the northern Benguela since the middle Holocene in a global context - a multi-proxy approach

Abstract. The last 5500 years of climate change and environmental response in the northern Benguela Coastal Upwelling are reconstructed by means of three sediment cores from the inner shelf off central Namibia. The study is based on nutrient (δ15N, δ13C) and productivity proxies (accumulation rates of total organic carbon; ARTOC). Reconstructed sea surface temperatures (alkenone-derived SST) and temperatures at subsurface depths (Tδ18O; based on tests of planktonic foraminifers) reflect the physical boundary conditions. The selection of proxy indicators proved a valuable basis for robust palaeo-climatic reconstructions, with the resolution ranging from multi-decadal (NAM1) over centennial (core 178) to millennial scale (core 226620). The northern Benguela experienced pronounced and rapid perturbation during the middle and late Holocene, and apparently, not all are purely local in character. In fact, numerous correlations with records from the adjacent South African subcontinent and the northern hemisphere testify to global climatic teleconnections. The Holocene Hypsithermal, for instance, is just as evident as the Little Ice Age (LIA) and the Roman Warm Period. The marked SST-rise associated with the latter is substantiated by other marine and terrestrial data from the South African realm. The LIA (at least its early stages) manifests itself in intensified winds and upwelling, which accords with increased rainfall receipts above the continental interior. It appears that climate signals are transferred both via the atmosphere and ocean. The combined analysis of SST and Tδ18O proved a useful tool in order to differentiate between both pathways. SSTs are primarily controlled by the intensity of atmospheric circulation features, reflecting changes of upwelling-favourable winds. Tδ18O records the temperature of the source water and often correlates with global ocean conveyor speed due to varying inputs of warm Agulhas Water. It seems as though conveyor slowdown or acceleration not only affected the temperature of the source water but also its nutrient content. This relationship between source water quality and conveyor speed is already known from glacial times. doi: 10.1002/mmng.201100006

Knowing, farming and climate change adaptation in North-Central Namibia

The agro-ecological knowledge held by Ovambo farmers in North Central Namibia has, for centuries, given them resilience to high levels of climate variability and associated impacts. New research, conducted in North Central Namibia, suggests that knowledge co-production between farmers and agricultural extension workers may, in addition, strengthen adaptive capacity to future climate change. However, this useful kind of knowledge co-production is far from automatic, and indeed the conditions which make it more likely to happen are not well understood. This paper explores agro-ecological knowledge in North Central Namibia as adaptive capacity, and suggests avenues for better conceptualising and understanding the conditions for adaptive capacity-enhancing knowledge co-production.

The biology, life history and management needs of a large sciaenid fish, Argyrosomus coronus, in Angola

The West Coast dusky kob Argyrosomus coronus is an understudied yet important fishery species in Angola. During a five-year study (2005–2009), the species was recorded in all fishery sectors, but was most important in the inshore recreational fishery in southern Angola (Cunene Estuary to Namibe). Early juveniles (<300 mm total length, TL) were captured in the offshore artisanal fishery at Praia Pinda, whereas juveniles (300–600 mm TL), subadults (600–870 mm TL) and adults (>870 mm TL) were captured in all fisheries as far north as Namibe, and shoals of large adult fish (>1 000 mm TL) were occasionally captured in the offshore purse-seine commercial fishery between the Cunene Estuary and Lucira. Because some Argyrosomus species are morphologically cryptic, a DNA barcoding method was used to confirm the taxonomic status of the biological samples used in this study. The male:female sex ratio of examined samples was 1:1.4 (n = 225). The length-at-50% maturity was 823 mm and 904 mm TL for males and females respectively. Age-at-50% maturity was 4.4 and 4.3 years for males and females respectively. The periodicity of otolith ring formation was confirmed to be one year using a marginal zone and a chemical marking analysis. Growth (in mm TL) was best described by: Lt = 1 826(1—e−0.12(t + 1.60)). Argyrosomus coronus fed predominantly on fish, mainly Sardinella aurita (62% frequency of occurrence). Early juveniles appeared to frequent the offshore zone (50–100 m depth), moving into the inshore region at approximately 300 mm TL. Juveniles and subadults were resident (57% recaptured at the same site) and were particularly abundant around the mouth of the Cunene Estuary as well as in central and northern Namibia. Adults undertake migrations that correspond with the movement of the Angola–Benguela frontal zone, moving north as far as Gabon in winter and returning to southern Angola in spring, when spawning appeared to take place offshore. There are currently no catch restrictions on A. coronus in Angolan waters. However, declining catches and increasing fishing effort suggest that some management intervention is required, commencing with a proposed closure of the Cunene Estuary mouth region to fishing.

Mapping Bush Encroaching Species by Seasonal Differences in Hyperspectral Imagery

Bush encroachment is a form of land degradation prominent worldwide, but particularly present in semi-arid areas. In this study, we mapped the spatial distribution of the two encroacher species, Acacia mellifera and Acacia reficiens,in Central Namibia, based on their different phenological behavior. We used constrained principal curves to extract a one dimensional gradient of phenological change from two hyperspectral images taken in different seasons. Field measurements of species composition and cover values were statistically related to bi-temporal differences in hyperspectral vegetation indices in a direct gradient analysis. The extracted gradient reflected the relationship between species composition and cover values, and the phenological pattern as captured by the image data. Cover values of four dominant plant species were mapped and species responses along the phenological gradient were interpreted.

The geochemistry and evolution of Palaeogene phonolites, central Namibia

Phonolites of Palaeogene age occur at two different localities in central Namibia, occurring as clusters of eroded monogenetic lava domes. Six phonolites at Aris collectively exhibit a narrow compositional range and represent a single magma system of highly evolved phonolite with high concentrations of Na 2O, Rb, Th, Zr, Zn, Pb, and REE and very low abundances of MgO, P 2O 5, Sr and Ba. No mafic rocks are associated with the Aris phonolites and their ultimate petrogenetic origin remains obscure. The more abundant Staalhart phonolites exhibit more variable but scattered compositional variation consistent with fractional crystallization dominated by sanidine (and nepheline) and pyroxene. The behaviour of REE is decoupled from Zr, Nb, and Th in this suite indicating a controlling role for minor phases in their evolution. The least evolved phonolites have initial 87Sr/ 86Sr ≈ 0.7043, which is identical to a small occurrence of associated plagioclase-bearing nephelinite indicating a possible petrogenetic link. Higher isotopic ratios in more evolved phonolites suggest their evolution was accompanied by some crustal interaction. Comparisons of phonolites world-wide indicate that they all have similar trace element abundance patterns when normalized to associated and possibly parental mafic compositions, regardless of proposed petrogenetic origins (melting, fractional crystallization, AFC, etc.). These patterns of similar pronounced enrichments of Rb, Th, Zr, strong depletions in Ti and P, and Yb N > Gd N (i.e. depletion of middle REE relative to heavy REE) confirm the suggestion that fractional crystallization is the dominant process by which phonolites evolve from mafic alkaline compositions. Despite similar published values of D Eu for sanidine in silicic and phonolitic compositions, highly evolved phonolites develop only modest negative Eu anomalies compared to evolved silicic rocks. This might reflect differences in melt structure, fO 2, halogen complexing or fractionating mineral assemblages in phonolite and silicic liquids. Sparse data on Eu partition coefficients in phonolite bulk compositions does not provide insight into the role of these controls of Eu partitioning in phonolite.

Origin of Mineralizing Fluids of the Sediment-Hosted Navachab Gold Mine, Namibia: Constraints from Stable (O, H, C, S) Isotopes

The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ 18O values of 14 to 15 per mil (V-SMOW). The total variations in δ 18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ 18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ 18O and δ 13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ 13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid ( δ 18O = 12–14‰ δ D = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area.

Perceptions and Measurements: The Assessment of Pasture States in a Semi-Arid Area of Namibia

How farmers perceive the state of their pastures is an important component of their management decisions and affects natural resources in arid and semi-arid regions. In an explorative study conducted in the Rehoboth farm area of central Namibia, we addressed the question whether the judgments of commercial farmers on pasture conditions are consistent with a botanical assessment of these pastures based on measurements. The perceptions were inferred from the comparative statements of farmers on the pasture quality of adjacent farm pairs. For the botanical assessment, biomass and plant species frequency counts of the same pastures were used. The results of the statistical analysis show a large agreement between perceived and measured pasture states of corresponding farm pairs, thus pasture quality dimensions perceived by the farmers agreed with the measured pasture quality variables. We also discuss the problems of designing more elaborate studies of this type.

Combining vegetation indices, constrained ordination and fuzzy classification for mapping semi-natural vegetation units from hyperspectral imagery

Vegetation mapping of plant communities at fine spatial scales is increasingly supported by remote sensing technology. However, combining ecological ground truth information and remote sensing datasets for mapping approaches is complicated by the complexity of ecological datasets. In this study, we present a new approach that uses high spatial resolution hyperspectral datasets to map vegetation units of a semiarid rangeland in Central Namibia. Field vegetation surveys provide the input to the workflow presented in this study. The collected data were classified by hierarchical cluster analysis into seven vegetation units that reflect different ecological states occurring in the study area. Spectral indices covering vegetation and soil characteristics were calculated from hyperspectral remote sensing imagery and used as environmental variables in a constrained ordination by applying redundancy analysis (RDA). The resulting statistical relationships between vegetation data and spectral indices were transferred into images of ordination axes, which were subsequently used in a supervised fuzzy c-means classification approach relying on a k-NN distance metric. Membership images for each vegetation unit as well as a confusion image of the classification result allowed a sound ecological interpretation of the resulting hard classification map. Classification results were validated with two independent reference datasets. For an internal and external validation dataset, overall accuracy reached 98% and 64% with kappa values of 0.98 and 0.53, respectively. Critical steps during the mapping workflow were highlighted and compared with similar mapping approaches.

Do inter-colony differences in Cape fur seal foraging behaviour reflect large-scale changes in the northern Benguela ecosystem?

The northern Benguela ecosystem adjoining Namibia has undergone considerable changes in recent decades, with reductions and northwards shifts of key prey species that have had severe implications for marine top predator populations. We investigated how such environmental variability may impact foraging behaviour of the Cape fur seal Arctocephalus pusillus pusillus, using satellite telemetry on animals in northern, central and southern Namibia. We expected that seal foraging behaviour would reflect a gradient of deteriorating feeding conditions from north to south. Results showed that foraging trips were shorter in the central region, where seals fed over the continental shelf, than in the north or south, where seals fed at the shelf edge. However, whereas seals in the north showed strong fidelity to the colony at which they were tagged and to persistent, clustered foraging areas, seals in the south showed weak fidelity both to the colony at which they were tagged and to foraging areas, which were scattered and variable. Seals in the south also tended to migrate northwards to other colonies while concurrently adapting their foraging behaviour to local conditions. Flexible use of foraging space and colonies of Cape fur seals during the three-year period (2002-2004) demonstrates that the species is adapted for variable environments over time and space.

Global dispersal of Pb by large-volume silicic eruptions in the Parana-Etendeka large igneous province

Volcanism from large igneous provinces is often inferred to have had a significant global environmental impact, but demonstrating synchronicity between the volcanism and sedimentary records of environmental change is not straightforward. Several studies use sediment Pb isotopes to fingerprint the timing of input of volcanic Pb from a particular large igneous province. Published data on Ocean Drilling Program Site 1149B sediments in the Pacific Ocean, associated with the Valanginian δ 13 C excursion that has been linked to the Parana-Etendeka volcanism in central South America and Namibia, require input of radiogenic Pb. The Pb isotope analyses of Parana-Etendeka lavas show that this radiogenic Pb is uniquely associated with the large-volume silicic eruptions rather than the effusive basaltic lavas. This strengthens the case for a temporal link between the Parana-Etendeka volcanism and the Valanginian δ 13 C excursion. Although basaltic magmatism dominates the province (>95%), there is no evidence for Pb associated with volatile metal degassing from such eruptions in the Site 1149B sediments on the other side of the globe, suggesting that eruption columns produced by the basaltic eruptions did not reach the stratosphere. Instead, the widespread global dispersal of Pb is apparently linked to the ability of large-volume (>1000 km 3 ) silicic eruptions to loft aerosols into the upper stratosphere.

The potential of plant biomarker evidence derived from rock hyrax middens as an indicator of palaeoenvironmental change

Hyrax middens are unique environmental archives with the potential to provide unprecedented high-resolution palaeoenvironmental records, particularly in the arid regions of southern Africa. This study provides the first detailed characterisation of the organic matter composition of hyraceum and aims to identify biomarker evidence capable of providing new or supplementary palaeoenvironmental data from these novel archives. Pyrolysis gas chromatography mass spectrometry reveals hyraceum to be dominated by nitrogen-containing aromatic compounds, notably benzamide. This is almost certainly derived directly from the hyrax urine and is probably the main source of nitrogen as measured in bulk δ 15N measurements. Solvent-extractable lipids comprise homologous suites of long-chain n-alkanes (C 24–C 34) and n-alkanols (C 16–C 26), characteristic of higher plant leaf waxes, along with an abundance of animal-derived sterols, higher plant sterols and terpenoids; as well as the ubiquitous benzamide. n-alkane distributions and compound specific δ 13C clearly differentiate samples from the C 3 vegetation dominated Cape Floristic Region, and the more arid, C 4 grass-rich savannas of central Namibia (Klein Spitzkoppe). Distinct changes in n-alkane distribution and δ 13C are observed within the Spitzkoppe midden; most notably the mid to late Holocene period (c. 6000–2000 cal yr BP), which records a progressive reduction in grass-derived organic matter inputs. Based on the animal's feeding behaviour and contemporary site ecology, a phase of increasing aridity is inferred, which is consistent with other proxy data from this site (e.g. bulk δ 15N). The excellent preservation of plant and animal biomarkers suggests that there is significant potential for midden-derived biomarkers to provide long-term palaeoenvironmental proxies.

Rare earth and trace element geochemistry of termite mounds in central and northeastern Namibia: Mechanisms for micro-nutrient accumulation

Rare earth element (REE) and trace element concentrations of ten termite mounds and adjacent topsoil from central and northeastern Namibia were used to investigate the processes underlying the alteration of soil chemical and physical properties by termites ( Macrotermes spp.). Accumulation of micro-nutrients in the mounds was of particular interest because of the ecological implications of the enhanced availability of these scarce elements. The absolute concentrations of REE and trace elements, including nine micro-nutrients (B, Fe, Mn, Ni, Cu, Zn, Se, Mo and Cd), were greater in termite mounds compared to topsoil, suggesting a possible external supply of enriched materials or accumulation of in situ weathering products of the underlying bedrock. Similarities between the chondrite normalized REE patterns of the samples and those of the average upper continental crust (UCC) and Post-Archaean Average Australian Shale compositions indicated that the mounds and topsoil at each site originated from the same host bedrock. Coherent behavior between two incompatible elements (Th and La) in both topsoil and mound samples suggested that the two sets of samples were exposed to two different weathering processes. The relatively low Zr/Sc ratios, fine particle sizes and the persistence of positive Eu anomalies of the mounds relative to UCC were interpreted as reflecting (1) a lack of grain sorting effects and (2) active transport of materials rich in primary clay minerals and trace elements from the deeper regolith to the surface by termites. Lower permeability of mound materials may also prevent leaching of primary minerals and trace elements. Titanium and Zr enrichment in most mound samples (8 samples) was attributed to the decomposition of biotite by Macrotermes with the concomitant release of Ti and Zr into the mounds, suggesting the termites' ability to transform soil minerals and enhance chemical weathering. This was further corroborated by a substantial increase in Th/U ratios in most mounds relative to topsoil. Because of the oxidizing conditions of the mounds, Ti and Zr are likely to be adsorbed onto Fe-oxides. Positive Ce anomalies relative to UCC and the high pH values in the mounds indicate calcrete aggradation or mound aeration as a result of burrowing activities of the termites and evaporation from mounds. The impacts of the changes in environmental conditions and the termite activities on the mound geochemistry are highlighted by heavy REE (HREE) as well as Fe and Mn retentions in the mounds. Heavy REE and light REE (LREE) are likely to coprecipitate with carbonate complexes and Fe- and Mn-oxyhydroxides, and in the process, scavenge a series of micro-nutrients such as Zn, Cu, Cd and Co. A substantial enrichment of Se in mounds, despite the oxidizing conditions of the mounds, could be attributed to active transport of Se-enriched materials from deep soil horizons. The study demonstrated the suitability of REE and trace element geochemistry for assessing the influence of Macrotermes on the physico-chemical properties of semi-arid soils.

Does using species abundance data improve estimates of species diversity from remotely sensed spectral heterogeneity?

Abstract Different approaches for the assessment of biodiversity by means of remote sensing were developed over the last decades. A new approach, based on the spectral variation hypothesis, proposes that the spectral heterogeneity of a remotely sensed image is correlated with landscape structure and complexity which also reflects habitat heterogeneity which itself is known to enhance species diversity. In this context, previous studies only applied species richness as a measure of diversity. The aim of this paper was to analyze the relationship of richness and abundance-based diversity measures with spectral variability and compare the results at two scales. At three different test sites in Central Namibia, measures of vascular plant diversity was sampled at two scales – 100 m2 and 1000 m2. Hyperspectral remote sensing data were collected for the study sites and spectral variability, was calculated at plot level. Ordinary least square regression was used to test the relationship between species richness and the abundance-based Shannon Index and spectral variability. We found that Shannon Index permanently achieved better results at all test sites especially at 1000 m2, Even when all sites where pooled together, Shannon Index was still significantly related with spectral variability at 1000 m2. We suggest incorporating abundance-based diversity measures in studies of relationships between ecological and spectral variability. The contribution made by the high spectral and spatial resolution of the hyperspectral sensor is discussed.

Use of environmental parameters to explain the variability in spawnerrecruitment relationships of Namibian sardine Sardinops sagax

This study attempts to explain the variability in recruitment of sardine in the northern Benguela and to develop potential models by including environmental information to predict recruitment. Two different recruitment and spawner number datasets were available: a VPA-developed dataset, for the period 1952-1987, and data from a simple age-structured model for 1992-2007. In all, four environmental indices were used: the degree of the intrusion of the warm Angola Current into northern Namibia, termed the Angola-Benguela front index; the extent of the upwelling area off central Namibia; average sea surface temperature (SST) over the northern and central Namibian shelf; and wind stress anomalies at Luderitz as an indicator of upwelling strength. Contrary to general belief, it was found that extremely high recruitment can happen at low spawner levels. This occurred in years in which a large upwelling area existed in association with the minimum southward intrusion of the Angola Current. These effects override the normal negative linear relationships with SST and the positive linear relationship with wind. However, when the area of upwelling is average or small, the effects of spawner biomass, SST and wind become important factors in the variability of recruitment. To estimate exceptional recruitment, the upwelling and front indices were included in the model. To measure medium and weak recruitment, spawner numbers and the SST and wind anomaly formed part of the model. These models can be used simultaneously to predict recruitment before annual acoustic surveys take place and thus aid management decisions.

Hydrogen sulphide and methane emissions on the central Namibian shelf

Hydrogen sulphide occurs frequently in the waters of the inner shelf coastal upwelling area off central Namibia. The area affected coincides with hatching grounds of commercially important pelagic fish, whose recruitment may be severely affected by recurring toxic sulphidic episodes. Both episodic biogenic methane gas-driven advective and molecular diffusive flux of hydrogen sulphide have been implicated as transport mechanisms from the underlying organic-matter-rich diatomaceous mud. To test hypotheses on the controls of hydrogen sulphide transport from the sediments on the inner Namibian shelf, water column and sediment data were acquired from four stations between 27 and 72 m water depth over a 3 year long period. On 14 cruises, temperature, salinity, dissolved oxygen, nitrate, methane, and total dissolved sulphide were determined from water column samples, and pore water dissolved methane, total dissolved sulphide, biomass of benthic sulphide-oxidising bacteria Beggiatoa and Thiomargarita, and bacterial sulphate reduction rates were determined from sediment cores. Superimposed on a trend of synchronous changes in water column oxygen and nutrient concentrations controlled by regional hydrographic conditions were asynchronous small-scale variations at the in-shore stations that attest to localized controls on water column chemistry. Small temporal variations in sulphate reduction rates determined with 35S-labeled sulphate do not support the interpretation that variable emissions of sulphide and methane from sediments are driven by temporal changes in the degradation rates of freshly deposited organic matter. The large temporal changes in the concentrations of hydrogen sulphide and the co-occurrence of pore water sulphate and methane support an interpretation of episodic advection of methane and hydrogen sulphide from deeper sediment depths – possibly due to gas bubble transport. Effective fluxes of hydrogen sulphide and methane to the water column, and methane and sulphide concentrations in the bottom waters were decoupled, likely due to the activity of sulphide-oxidising bacteria. While the causal mechanism for the episodic fluctuations in methane and dissolved sulphide concentrations remains unclear, this data set points to the importance of alternating advective and diffusive transport of methane and hydrogen sulphide to the water column.

Processes of high-T fluid–rock interaction during gold mineralization in carbonate-bearing metasediments: the Navachab gold deposit, Namibia

The Navachab gold deposit in the Damara belt of central Namibia is hosted by a near-vertical sequence of amphibolite facies shelf-type metasediments, including marble, calc-silicate rock, and biotite schist. Petrologic and geochemical data were collected in the ore, alteration halos, and the wall rock to evaluate transport of elements and interaction between the wall rock and the mineralizing fluid. The semi-massive sulfide lenses and quartz–sulfide veins are characterized by a complex polymetallic ore assemblage, comprising pyrrhotite, chalcopyrite, sphalerite, and arsenopyrite, native bismuth, gold, bismuthinite, and bismuth tellurides. Mass balance calculations indicate the addition of up to several orders of magnitude of Au, Bi, As, Ag, and Cu. The mineralized zones also record up to eightfold higher Mn and Fe concentrations. The semi-massive sulfide lenses are situated in the banded calc-silicate rock. Petrologic and textural data indicate that they represent hydraulic breccias that contain up to 50 vol.% ore minerals, and that are dominated by a high-temperature (T) alteration assemblage of garnet–clinopyroxene–K-feldspar–quartz. The quartz–sulfide veins crosscut all lithological units. Their thickness and mineralogy is strongly controlled by the composition and rheological behavior of the wall rocks. In the biotite schist and calc-silicate rock, they are up to several decimeters thick and quartz-rich, whereas in the marble, the same veins are only a few millimeters thick and dominated by sulfides. The associated alteration halos comprise (1) an actinolite–quartz alteration in the biotite schist, (2) a garnet–clinopyroxene–K-feldspar–quartz alteration in the marble and calc-silicate rock, and (3) a garnet–biotite alteration that is recorded in all rock types except the marble. The hydrothermal overprint was associated with large-scale carbonate dissolution and a dramatic increase in CO2 in the ore fluid. Decarbonation of wall rocks, as well as a low REE content of the ore fluid resulted in the mobilization of the REE, and the decoupling of the LREE from the HREE. The alteration halos not only parallel the mineralized zones, but may also follow up single layers away from the mineralization. Alteration is far more pronounced facing upward, indicating that the rocks were steep when veining occurred. The petrologic and geochemical data indicate that the actinolite–quartz– and garnet–clinopyroxene–K-feldspar–quartz alterations formed in equilibrium with a fluid (super-) saturated in Si, and were mainly controlled by the composition of the wall rocks. In contrast, the garnet–biotite alteration formed by interaction with a fluid undersaturated in Si, and was mainly controlled by the fluid composition. This points to major differences in fluid–rock ratios and changes in fluid composition during alteration. The alteration systematics and geometry of the hydrothermal vein system are consistent with cyclic fluctuations in fluid pressure during fault valve action.

Foraging opportunities drive interspecific associations between rock kestrels and desert baboons

AbstractInterspecific associations can arise for varied reasons including reduced predation risk and improved foraging success. In the case of bird–primate associations, birds typically appear to follow primate groups to harvest insects flushed by primates' movements. However, while previous studies have linked temporal changes in bird–primate associations to environmental conditions, few have assessed the additional effects of bird activity patterns and primate group behaviour and none have disentangled their potentially interdependent effects. Here, we test the hypothesis that foraging opportunities can drive interspecific associations in a previously undescribed bird–primate association between rock kestrels Falco rupicolus and chacma baboons Papio ursinus in central Namibia. Data were collected from two baboon groups and associated kestrels using instantaneous scan sampling during full‐day follows over a 7‐month field period, and analysed using generalized linear mixed models. We found that kestrel associations with baboons vary with season, show diurnal cycles and are more frequent when the baboons are in open desert habitat, engaged in travel foraging and in a large group. These patterns are statistically independent and consistent with the hypothesis that the kestrel–baboon association is driven by the foraging opportunities acquired by the kestrels. As the baboons do not appear to gain any benefits nor incur any costs from the association, we conclude that the kestrels are likely to be commensal with the baboons.

Rainwater harvesting as an alternative water resource in rural sites in central northern Namibia

In this paper, results of the investigations on rainwater harvesting (RWH) in central northern Namibia are described, which are part of the transdisciplinary research project CuveWaters. On the basis of hydrological and technical as well as social and cultural conditions, appropriate solutions for RWH are developed, discussed, and evaluated. The main objective is to analyse their technical and economical feasibility as well as their affordability for future users. In detail, two small-scale RWH systems are examined: roof catchments using corrugated iron roofs as rain collection areas and ground catchments using treated ground surfaces. The current status of local water supply and water use was investigated in the village of Epyeshona. Concepts and techniques of RWH were discussed with the population. Based on the results of these investigations and on the physical conditions, the appropriate RWH design and the required storage capacities could be identified. Local material and labour costs were determined and a dynamic cost analysis was carried out to compare the technological options in terms of amortisation times and prime costs. The feasibility of the RWH systems was assessed in relation to local socio-economic conditions. The calculations reveal that it is economically feasible to apply decentral techniques of RWH in terms of the roof catchment systems. Moreover, the proposed technologies provide comparable benefits to the public water supply. The ground catchment system, however, needs moderate subsidies to obtain the same benchmark.