Phytoplankton distribution in the upwelling area off NW Africa

In spring 1968, the centre of the upwelling region off NW Africa was investigated using R. V. Meteor. The hydrographic structure in this area is characterized by irregular upwelling in space and time. The complicated distribution of the biological components (phytoplankton and dissolved plus particulate organic substances) is expressed only within the limits given by the resolving power of the system. Therefore, the basic values have been smoothed statistically to suppress accidental irregularities. The problematics of such manipulation are discussed. The phytoplankton distribution shows a pronounced maximum in the Cap Blanc region with extreme values in the range of > 50µg C · l^–1 in the surface layer. A decrease was observed not only in the direction of the open sea, but also along the coast. This distribution pattern is repeated (1) at the depth of the vertical phytoplankton maximum, which is near the surface in the Cap Blanc region; (2) at the large proportion of diatoms characteristic for upwelling water; (3) at the phytoplankton stock in the euphotic zone with maximum values up to > 0.5 g C · m^–2. It is calculated that (minimum) daily production is almost equal to the stock. Processes of decomposition demonstrate the upwelling by regionally different relations between O₂ consumption and PO₄ liberation. An anchor station situated near the centre of the upwelling area indicates another small-scale development of phytoplankton as demonstrated by distribution charts of the total area. Pronounced daily rhythms are revealed after extensive smoothing procedures.     

Estimations of mesozooplankton biomass in a coastal upwelling area off NW Spain

The abundance and several estimators of mesozooplankton biomass(dry weight, ash-free dry weight, displacement volume, carbonand nitrogen) were measured in a coastal upwelling area offLa Coru*****na (NW Spain) at monthly intervals between 1990and 1995. Holoplanktonic copepods dominated in most of the samples,but meroplanktonwere also important near the coast, especiallyduring late spring and through summer. Gelatinous organisms(medusae, siphonophores and larvaceans) reached significantabundances (-10%) in shelf waters by late summer, but also nearthe coast in spring. Log-linear equations were computed betweendry weight, displacement volumeand the other biomass estimators,to allow for interconversion among different measurements. Toprevent the influence of meroplankton and gelatinous zooplanktonin the estimations of biomass, the equations were adjusted usingonly samples in which the abundance of each of these groupswas <10% of total individuals. Our equations produced carbonand ash-free dry weight estimates that are comparable to thoseobtained with the ratiosgiven in other studies in the region.However, the esti mations of mesozooplankton carbon from displacementvolume using the adjusted equation with our data are lower thanthose reported previously for the area, which werecomputed usingequations adjusted with data from a different ecosystem. Theuse of interconversion factors taken from the literature isdiscussed, along with the implications of the estimations ofmesozooplankton biomass in the computations of the flux of carbonthrough the ecosystem.     

Rostock zooplankton studies off West Africa

Since the beginning of the seventies, upwelling research has become increasingly popular in the path of the Canary and Benguela Current, because of economic consideration, particularly in the relation to fisheries and marine geology. Many expeditions were carried out between 1970 and 1977, including 8 cruises of the German R. V. “A. v. Humboldt” operating from Rostock. Measurements covered scales ranging in time from minutes to several years and in space from hundreds of metres to several thousands of kilometres. Zooplankton studies focussed on quantitative, metabolic, taxonomic, and parasitological aspects. Plankton was collected with a WP-2-UNESCO standard net to a maximum depth of 200 m. The epipelagic mesozooplankton consists mainly of copepods, especially calanoids with developmental times of about 20 to 23 days. After an upwelling event, zooplankton is able to double its biomass. This typical biomass increase is independent of coastal distance and depth. The upwelling response lasts about 3 weeks in nearsurface waters, and 6 to 8 weeks in depths below 75m. A relationship was observed between the duration of seasonal upwelling (that means the numbers of single upwelling events) and the cumulative increase of biomass. This net growth rate of zooplankton biomass is most pronounced at the shelf break, the area with the highest fish biomass, and in the upper 25 m. Differences between the expected and the real rate values in conjunction with the known amount of nutritive demands of fishes allow the estimation of the fish biomass in a given area. The near coastal Ekman upwelling, which is an event in the time scale of about two weeks, also shows seasonality in some areas. Off Northwest Africa the largest expansion was recorded in the first half of the year, extending from 10° N to 24° N, more than 400 km offshore and at least down to 200 m. It contracts in the second half of the year to an area between 20°N and 22°N, 100 to 200 km off the coast and in an average depth of 25 m. These zooplankton biomass patterns are superimposed by mesoscale phenomena, originated by other than Ekman upwelling events. Those are, for example, long coastal parallel waves, producing cells of intensified upwelling and downwelling, and eddies, caused by instabilities in a frontal zone parallel to the coast. Different water masses can be distinguished by indicator species, species combinations or the significant absence of species. This was demonstrated for chaetognaths. The calanoidCalanus helgolandicus (Claus, 1863), a typical species of the North Atlantic, indicates North Atlantic Central Water, whereasCalanoides carinatus (Krøyer, 1849) is an indicator of South Atlantic Central Water. Finally, comparisons of near coastal current regimes, transport velocities, and developmental rates of calanoids allow one to conclude that a suitable mechanism is present to maintain plankton in the coastal environment.     

The biochemical composition of phytoplankton in an upwelling region off south Africa

Variations in the concentrations of chlorophyll a, ATP, protein, and carbohydrates in phytoplankton have been investigated in a nearshore upwelling region off the Cape Peninsula. During active upwelling temperatures <10 °C, high nutrient concentrations and low concentrations of the biochemical constituents were measured. When upwelling lessened and conditions stabilized temperatures increased and blooms of phytoplankton appeared. High concentrations of chlorophyll a and ATP and a high protein/carbohydrate ratio were then recorded. At very low nutrient levels chlorophyll a and ATP concentrations were still high but an increase in the acid-soluble carbohydrate content and a corresponding decrease in the protein/carbohydrate ratio was observe. It was concluded that the ratio of protein to carbohydrate was a suitable indicator of the physiological state of a phytoplankton community in the local upwelling region.     

Diatom associations and succession in Lake Kariba,South Central Africa

Diatom Associations formed by 1% or more of species, found by the Thomasson analysis method are discussed in relation to the morphology and limnology of the four basins of Lake Kariba. A brief comparison is made with associations in two stretches of the Zambesi river. A dominant association formed by certain species of Melosira demonstrated a pattern of succession, in relation to the thermal cycle in the lower, middle and upper lake by variations of the percentage occurrence of species forming it: Melosira granulata dominated during homothermy and remained so throughout stratification in the lacustrine conditions at the barrier or wall; M. granulata v. angustissima dominated during overturn and became the dominant species in the riverine conditions of the two uppermost basins. Results from the Central basin illustrated the intermediate position of this area of the lake. The pattern was disrupted in the far N.E sub-basin of the Sanyati basin, which in general showed individuality and where accidental and ambivalent plankters became co-dominant with the Melosira association during homothermy.A second association, formed by species of Cyclotella, Fragilaria and Synedra was a regular feature of the diatom population, it was sub-dominant to the melosira association at the barrier but gradually decreased up-lake and was superseded by accidental and ambivalent planktonic species. Except for the N/E Sanyati sub-basin, the diatom associations showed a gradual change from upper to lower lake, thus basin individuality was not seen.     

Dinoflagellate cyst distribution in marine surface sediments off West Africa (17–6°N) in relation to sea-surface conditions, freshwater input and seasonal coastal upwelling

An organic-walled dinoflagellate cyst analysis was carried out on 53 surface sediment samples from West Africa (17–6°N) to obtain insight in the relationship between their spatial distribution and hydrological conditions in the upper water column as well as marine productivity in the study area.Multivariate analysis of the dinoflagellate cyst relative abundances and environmental parameters of the water column shows that sea-surface temperature, salinity, marine productivity and bottom water oxygen are the factors that relate significantly to the distribution patterns of individual species in the region.The composition of cyst assemblages and dinoflagellate cyst concentrations allows the identification of four hydrographic regimes; 1) the northern regime between 17 and 14°N characterized by high productivity associated with seasonal coastal upwelling, 2) the southern regime between 12 and 6°N associated with high-nutrient waters influenced by river discharge 3) the intermediate regime between 14 and 12°N influenced mainly by seasonal coastal upwelling additionally associated with fluvial input of terrestrial nutrients and 4) the offshore regime characterized by low chlorophyll-a concentrations in upper waters and high bottom water oxygen concentrations.Our data show that cysts of Polykrikos kofoidii, Selenopemphix quanta, Dubridinium spp., Echinidinium species, cysts of Protoperidinium monospinum and Spiniferites pachydermus are the best proxies to reconstruct the boundary between the NE trade winds and the monsoon winds in the subtropical eastern Atlantic Ocean. The association of Bitectatodinium spongium, Lejeunecysta oliva, Quinquecuspis concreta, Selenopemphix nephroides, Trinovantedinium applanatum can be used to reconstruct past river outflow variations within this region.     

Zonation of a coastal grassland in Ghana,West Africa

Zonation of grassland lying behind a coastal lagoon in Ghana is described, and related to seasonal fluctuations in moisture, chlorinity and aeration. The distribution of similar vegetation on the West African coast is discussed.     

Distribution of chlorophyll a and Gymnodinium catenatum associated with coastal upwelling plumes off central Portugal

The composition and the distribution patterns of phytoplankton were studied during the early stages of an upwelling event in Lisbon Bay, in September 1999. The distribution of chlorophyll a and phytoplankton assemblages was asymmetric about the upwelling centre, with the highest abundance of chlorophyll a observed at the inner (coastal) side of the plume. The diatom Cylindrotheca closterium dominated in the upwelling core and the chain forming diatom Proboscia alata dominated at the outer side of the feature within mature oceanic waters. Chain forming dinoflagellates such as Alexandrium affine and the toxin producer Gymnodinium catenatum were most numerous at the inshore side of the upwelling front. These patterns were compared with the distribution of G. catenatum observed in the same area in 1985 and 1994, under conditions of fully developed upwelling when chlorophyll a and G. catenatum maxima were observed extending southwards on the inshore side of an upwelling plume. Different hydrodynamic conditions at each side of upwelling plumes associated with pronounced Capes are evoked to explain the asymmetry on the phytoplankton patterns.     

Food selection by a sit-and-wait predator,the monkfish,Lophius upsicephalus,off Namibia (South West Africa)

Synopsis We investigated prey selection by the monkfish, Lophius upsicephalus. This sit-and-wait predator preferentially selected prey types that would be expected to react to the lure of the illicium. Prey size selection was dependent on predator size, with larger predators feeding on a wider range of prey sizes.     

Nanoheterotroph grazing on bacteria and cyanobacteria in oxic and suboxic waters in coastal upwelling areas off northern Chile

The vertical distribution and abundance of microbial assemblagesand the grazing of nanoheterotrophs upon prokaryotes in oxicand suboxic waters were examined in two coastal upwelling areasoff northern Chile where a shallow Oxygen Minimum Zone (OMZ)is characteristic. Prokaryotic prey included bacterioplanktonand cyanobacteria (Synechococcus); both displayed a bimodaldistribution, with abundance maxima above and within the upperOMZ. Flagellates numerically dominated the nanoplankton andwere mostly concentrated in the oxic layer. Mean ingestion ratesof cyanobacteria by nanoflagellates (vacuole content method)ranged from 0.2 to 1.1 cells flagellate^–1 h^–1 andmean consumption rates (34–160 cells mL^–1 h^–1)were four times higher in the oxic layer. With the selectiveinhibitors technique, specific grazing rates on bacteria werelow (<0.1 h^–1) and consumption did not control bacterialproduction in the surface layer but did so in the suboxic layer(accounting for >100% of bacterial production). With thesame method, the specific grazing rate on cyanobacteria rangedbetween zero and 0.23 h^–1 with no clear differences betweenoxygen conditions; prey growth and production were always higherthan the grazing pressure (accounting for <17% of cyanobacterialproduction). The impact of grazing by nanoheterotrophs in regulatingthe production of prokaryotes in oxic and suboxic waters inthis region is discussed.     

Observations on sound scattering layers in the upwelling off N.W.Africa and in the North Equatorial Current

Observations with a 38 kc echosounder during the NECTAR IIIexpedition in March/April 1982 showed distinct differences inpatterns and densities of the Sound Scattering Layers (SSL)between the upwelling area off N.W.Africa and the waters ofthe North Equatorial Current. The pattern of the SSL in theNorth Equatorial Current resembled that of the tropical NorthAtlantic Ocean, while in the upwelling area a deviating patternwas found. These different patterns reflect different depthdistributions found in both zooplankton and mesopelagic fishfrom RMT 1 + 8 catches. The densities of the SSL were higherin the upwelling area than in the Equatorial Current, showingsimilarities with zooplankton biomass and abundance of mesopelagicfish, in contrast with the abundance of echopatches, usuallyinterpreted as fish shoals, which increased towards the west.     

Ecosystem engineering in the Quaternary of the West Coast of South Africa

Despite advances in our understanding of the geographic and temporal scope of the Paleolithic record, we know remarkably little about the evolutionary and ecological consequences of changes in human behavior. Recent inquiries suggest that human evolution reflects a long history of interconnections between the behavior of humans and their surrounding ecosystems (e.g., niche construction). Developing expectations to identify such phenomena is remarkably difficult because it requires understanding the multi‐generational impacts of changes in behavior. These long‐term dynamics require insights into the emergent phenomena that alter selective pressures over longer time periods which are not possible to observe, and are also not intuitive based on observations derived from ethnographic time scales. Generative models show promise for probing these potentially unexpected consequences of human‐environment interaction. Changes in the uses of landscapes may have long term implications for the environments that hominins occupied. We explore other potential proxies of behavior and examine how modeling may provide expectations for a variety of phenomena.     

Sero-genetic studies on the San of South West Africa.

The San, a physically, culturally and linguistically distinctive people, have been shown by archaeological records anciently to have inhabited the whole of Eastern and Southern Africa. They, in common with the Khoi, the other members of the Khoisan race, are confined now to Southern Africa and principally to Botswana and South West Africa, though a number are also found in Angola. Sero-genetic data concerning seven South West African groups are presented in this study, and confirm a shared overall genetic profile characteristic of the San in general, slightly different from that of the Khoi and in significant contrast with that of the Negroes.     

Summer phytoplankton community structure and growth in a regional upwelling area off hachijo Island,Japan

Phytoplankton community structure both in terms of taxonomic composition and size distribution, and growth were examined in a regional upwelling which occurred in a Kuroshio region in summer. Organic carbon abundance of diatoms was markedly enhanced by the upwelling and they accounted for 87% of the total phytoplankton carbon. Nitzschia pungens Grunow was numerically most prominent accompanied by Chaetoceros and Rhizosolenia species. On the other hand, only a small increase was observed in phytoplankton other than diatoms, which consisted of unicellular cyanobacteria, coccolithophorids, gymnodinoids, and so called monads and flagellates. Division rate of diatoms from the upwelled water was twice that of the other forms. This difference in growth response among phytoplankton groups resulted in a shift of community structure to dominance of diatoms from that of the other forms. The time needed for this shift was estimated to be at least ≈ 3 days.     

Environmental fungal diversity in the upwelling ecosystem off central Chile and potential contribution to enzymatic hydrolysis of macromolecules in coastal ecotones

Even though occurrence of fungi in several marine environments has been documented, their inclusion within the marine microbial loop is not fully recognized. A major constraint is whether fungi in coastal waters are truly marine or represent transient microorganisms transported from terrestrial environments. We approached this issue by analyzing ambient fungal composition and hydrolytic activity of culturable fungi along a nearshore-offshore gradient in the upwelling ecosystem off central Chile, a region of high marine productivity strongly influenced by river discharges. We detected different communities of fungi in nearshore and offshore waters, with near estuary strains hydrolyzing proteins and carbohydrates faster than those from offshore sites. We conclude that coastal waters off central Chile comprise distinct fungal communities representative of offshore and nearshore environments, and provide new evidence for fungi processing organic matter in coastal ecotones, opening a fresh perspective for disappearance of organics carried by rivers in the coastal ocean.     

Notes on the genus Dendroceros in West Africa and South Atlantic Islands

Abstract

Following revision of Dendroceros specimens from West Africa and South Atlantic islands, three species are recognized for this geographical area: Dendroceros adglutinatus, D. africanus and finally D. herasii which is described as new. On the other hand, D. crispatus is excluded from Africa while Folioceros fuciformis (Megaceros lacerus, Anthoceros fuciformis) is excluded from Equatorial Guinea. A preliminary key to the five Dendroceros species known to occur in Africa is included.     

Zooplankton Spatial and Temporal Distribution in a Tropical Oceanic Area off West Africa

Tropical instability vortices are believed to modify the trophic food web by affecting plankton production. Such instabilities have been evidenced in a tuna fishing area of the Atlantic (10°–20° W and 2°–5° N). However, the origin of the tuna abundance remains uncertain as no data on zooplankton communities on which tuna preys are supposed to fed upon are available. This study was focused on short term spatial and diel variations of mesozooplankton communities sampled at 2° N during and after the passage of an instability wave evidenced from satellite imagery, at 4° N and at 0°, i.e. in the Equatorial divergence. Samples were collected with an opening–closing multisampler and with a WP2 net (200 μm). Copepods prevailed (86–92% of the total zooplankton) with Clausocalanidae, Oncaeidae, Corycaeidae, Calanidae, Eucalanidae as dominant families. At 0° and at 2° N during the wave, large-sized organisms constituted over 50% of total dry weight (DW). These percentages decreased after the wave and at 4° N. Zooplankton DW and densities were highest at 0° and lowest at 4° N. At 2° N after the wave, we observed a strong increase in the abundance of zooplankton which surpassed that at 0°. This increase mainly resulted from an increase in small copepods and copepodites. Our results suggest that the high production linked to the upwelling of nutrients caused by the geostrophic divergence is partly advected northward by the instability wave. The advection did not reach 4° N which displayed the characteristics of a typical tropical structure.     

Benthic feeding by krill, Euphausia superba Dana, in coastal waters off West Antarctica and in Admiralty Bay, South Shetland Islands

Diatoms in the food of krill caught in the summer in coastal waters close to the South Shetland Islands and in the summer and three winters in Admiralty Bay, King George Island were investigated. The diatoms were used as indicators of sources of krill food. Principal component analysis divided the diatom taxa into two groups, one being pelagic diatoms and from the food of krill in coastal waters, and the other being benthic diatoms and taxa from the food of krill in Admiralty Bay. All pelagic taxa of diatoms were consumed by krill. In the summer in coastal waters, benthic diatoms in krill food originated from the sublittoral. In the summer in Admiralty Bay, besides phytoplankton, diatoms were noted from the sublittoral and littoral in the food of krill. However, in the winter mostly sublittoral and epiphytic diatoms were present in krill food. Accepted: 20 March 2000