Hans M. Verheye

Copepod hatching success in marine ecosystems with high diatom concentrations

Diatoms dominate spring bloom phytoplankton assemblages in temperate waters and coastal upwelling regions of the global ocean. Copepods usually dominate the zooplankton in these regions and are the prey of many larval fish species. Recent laboratory studies suggest that diatoms may have a deleterious effect on the success of copepod egg hatching. These findings challenge the classical view of marine food-web energy flow from diatoms to fish by means of copepods. Egg mortality is an important factor in copepod population dynamics, thus, if diatoms have a deleterious in situ effect, paradoxically, high diatom abundance could limit secondary production. Therefore, the current understanding of energy transfer from primary production to fisheries in some of the most productive and economically important marine ecosystems may be seriously flawed. Here we present in situ estimates of copepod egg hatching success from twelve globally distributed areas, where diatoms dominate the phytoplankton assemblage. We did not observe a negative relationship between copepod egg hatching success and either diatom biomass or dominance in the microplankton in any of these regions. The classical model for diatom-dominated system remains valid.

Mesozooplankton dynamics in the Benguela ecosystem, with emphasis on the herbivorous copepods

Recent research developments on the ecology, dynamics and trophic position of copepods in the Benguela ecosystem are synthesized. Attention is focused on herbivorous species of the southern Benguela and how they cope with the physical and biological variability characteristic of this upwelling region. Copepods constitute on average approximately half of the total zooplankton carbon and. are most abundant during the upwelling season. They are able to maintain large population densities within local coastal upwelling areas by combining ontogenetically based vertical migration behaviour with features of the current system. Some species have developed finely tuned strategies to overcome periods of starvation between upwelling bouts by storing lipid reserves or by entering temporary developmental arrest. In situ measurements of production rates of local species are among the highest recorded for copepods. Despite an apparent excess of food, copepods exert only limited impact on the phytoplankton, removing on a...

Multiple factors affecting South African anchovy recruitment in the spawning, transport and nursery areas

Despite high primary productivity, the yield of pelagic fish in the southern Benguela is relatively low compared to that in the Humboldt system. Part of the constraint may be the ability of pelagic fish to reproduce successfully in a strongly pulsed upwelling environment, where enrichment, retention and concentration mechanisms are less compatible than in Peru-Chile. Anchovy Engraulis capensis spawn upstream of the main upwelling centres on the food-poor, thermally stratified western Agulhas Bank, over a protracted summer season (October–February) when high wind speeds of 7–8 m˙s−1 are prevalent. Eggs spawned farther east, on the central or eastern Agulhas Bank, may be subject to increased cannibalism and advective losses, whereas those spawned farther west could be susceptible to heavy advective losses offshore during periods of strong southerly winds. Copepod concentrations are negatively correlated with spawner biomass on the western Bank and are inversely linked to high rates of gonad atresia in ancho...

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 sizebased 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.

Short-term variability during an anchor station study in the southern Benguela upwelling system: Abundance, distribution and estimated production of mesozooplankton with special reference to Calanoides carinatus (Krøyer, 1849)

Abstract Daytime zooplankton samples collected at a fixed station position in St Helena Bay during a 27-day time series in March–April 1987 were used to describe the variability in the vertical distribution, abundance and population age structure of Calanoides carinatus in response to upwelling-related processes, and to provide an estimate of their production. The vertical distribution of C. carinatus was characterized by ontogenetic layering of copepodites and adults. The thermocline delimited the maximum depth of young copepodites (CI–CIII). Older stages avoided low-oxygen ( The demographic structure of the C. carinatus population showed evidence of a stable age distribution of copepodites and adults during the first of two upwelling cycles observed during the study. Overall mean abundance was 360 animals m−3. However, during the second upwelling cycle their mean abundance was reduced to 183 animals m−3 and adults dominated the population by 54%. These changes in abundance and age structure are discussed in relation to upwelling-induced advective processes. Surface-dwelling young copepodites (CI–CIII) are thought to be transported away from the study site, while re-seeding of the reduced nearshore population probably took place through advection of diapausal pre-adults (CV) in the upwelled water. Daily production of juvenile C. carinatus was estimated at 0.7 mg C m−3d−1 which, combined with an egg production of 1.0mg C m−3d−1 by addult females, is equivalent to 2% of the observed daily primary production. The mean P:B ratio for the copepodite stages was 0.167d−1. The role of this dominant copepod and the mesozooplankton in the carbon budget in St Helena Bay is discussed. Consumption by mesozooplankters was estimated at 22% of the daily primary production indicating that there was a considerable imbalance between primary producers and production by these heterotrophs.

Life history and population maintenance strategies of Calanoides carinatus (Copepoda: Calanoida) in the southern Benguela ecosystem

Analysis of quarterly cross-shelf distribution patterns of juvenile and adult Calanoides carinatus off the Cape Columbine upwelling centre in the southern Benguela ecosystem shows that this copepod has behavioural adaptations which result in clear patterns of inshore-offshore zonation. The combination of differential diel vertical movements of the various stages, seasonal ontogenetic migration and the resultant differential exploitation of cross-shelf advective processes and longshore current regimes play an important role in the maintenance of populations within the coastal upwelling areas of the southern Benguela ecosystem. At lower latitudes, C. carinatus utilizes true diapause in its life-cycle strategy to bridge the 8–10 month period between upwelling seasons. In the Benguela, however, the presence of an inshore active and an offshore resting component enables local C. carinatus populations to maintain a perennial presence. Because of a protracted upwelling season and occasional upwelling in winter, ...

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

Abstract 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 effect of sea temperature and food availability on the spawning success of Cape Anchovy engraulis capensis in the Southern Benguela

Data on the thermal structure, copepod biomass and production, and total number of eggs of the Cape anchovy Engraulis capensis were obtained from monthly surveys during the periods August 1993 – March 1994 and September 1994 – March 1995 on the western Agulhas Bank and off the South-Western Cape, South Africa. Previous work suggested that anchovy spawn on the western Agulhas Bank in temperatures between 16 and 19°C, where they feed predominantly on copepods. This study shows that the western Bank is a more suitable spawning area for anchovy, having greater thermal stability, a larger area of 16–19°C water and a more consistent food environment than off the South-Western Cape. Also, copepod production on the western Bank was highest in 16–19°C water. To identify factors controlling the area of this water mass, a cluster analysis was used on a suite of hydrographic variables. Three periods were identified: winter (August-September), spring (October-December) and summer (January-March), reflecting changes in...

The nature and origin of the crustacean zooplankton of Sahelian Africa, with a note on the Limnomedusa

The zooplankton of the major Sahel river basins Nile, Shari (Chad), Niger, and Senegal, is different from that found in the Sahara and in Equatorial Africa. Similarities and differences between the individual basins are numerous as well. Many species are shared by the Nile and Lake Chad, by Lake Chad and the Niger (plus Senegal), or occur in all four basins, or are restricted (endemic) to only one basin.

11 Developing a basis for detecting and predicting long-term ecosystem changes

Abstract Long-term ecosystem changes in the Benguela region include species alternations and regime shifts, which are sometimes obscured by large intra- and inter- annual variability in the ecosystem. This chapter proposes that no single model or approach can resolve this variability and effectively detect and predict long-term ecosystem changes; a coherent, robust, transparent and reproducible synthesis framework is required. Indicators and models are described that can be used to identify some aspects of the current state of ecosystem structure and to detect and monitor long-term change. A short-term challenge is to synthesize these varied sources of multidisciplinary (and sometimes contradictory) information in a logical and consistent fashion. An expert system approach is proposed to do this, consolidating results of different indicators and models within a dynamic process that uses feedbacks to validate predictions of the expert system, and to improve it. It is suggested that such an approach should be initiated in the short term, even as models and indicators are being developed further. In parallel, multivariate statistical tools should be refined and applied to existing time series, to identify past periods of ecosystem change. Current data gaps should be filled, including time series of primary production and the abundance of gelatinous zooplankton. In the medium term, the expert system model should evolve to a point where its results can be used to inform various management groups about the state of the ecosystem. Part of this evolution requires that ecosystem indicators be presented with error estimates or formal assessments of quality.