John G. Field

Trophic flows in the southern Benguela during the 1980s and 1990s

Mass-balanced models of trophic flows in the southern Benguela ecosystem suggest a 10% increase in zooplankton biomass between the 1980s and the 1990s, in agreement with observed trends of increased zooplankton abundance off South Africa over the last few decades. Minimum hake biomass in balanced trophic models is substantially larger than survey and other model estimates,suggestingundersamplingofhakesinsurveysandunderestimationofjuvenilehakemortality.Modelbiomassandmean annual production of five important small pelagic fish groups were larger in the 1990s, and total catches were smaller than in the 1980s. Estimates of biomass per trophic level, transfer efficiencies, mixed trophic impacts and many other ecosystem attributes suggest that trophic functioning of the southern Benguela ecosystem was similar in the 1980s and 1990s. Because catches were lowerandmodelzooplanktonandsmallpelagicfishbiomasseswerelargerinthe1990s,theecosystemwaslesstightlyconstrained by predators (including fishers) and food availability than in the 1980s. Fishing took place at low trophic levels compared to other systems. Despite smaller total catches in the 1990s, fishing was ecologically more expensive (from higher trophic levels) during the 1990s than in the 1980s because snoek and hake catches were large. There was greater shared niche overlap of small pelagic fish predators in the 1990s than in the 1980s. Mean transfer efficiency was 12%. Transfer of biomass at trophic levels III–V appears to be more efficient in the southern Benguela than in other upwelling ecosystems. Primary production required to sustain catches in the southern Benguela ecosystem is 4% of total primary production, i.e. more similar to estimates for open ocean and coastal regions than for other upwelling or shelf systems averaging more than double this value. D 2002 Elsevier Science B.V. All rights reserved.

Comparative trophodynamics of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela: are species alternations between small pelagic fish trophodynamically mediated?

The results of detailed morphological, experimental, field and modelling studies on various aspects of the trophic ecology of sardine Sardinops sagax and anchovy Engraulis encrasicolus in the Benguela ecosystem are synthesised, and differences in the trophodynamics of these two species are highlighted. Anchovy possess a relatively coarse branchial apparatus; feed predominantly by particulate-feeding and maximise their net energetic gain by using this feeding mode; show higher weight-standardised clearance rates than do sardine for prey >580μm; are most efficient at assimilating nitrogen from zooplankton and excrete <50% of ingested nitrogen; feed inefficiently on phytoplankton and derive the bulk of their dietary input from larger zooplankton; and maximise their scope for growth on mesozooplankton. In contrast, sardine possess a relatively fine branchial apparatus; feed predominantly by filter-feeding and maximise their net energetic gain by using this feeding mode; show higher weightstandardised clearance rates than do anchovy for prey <580μm; are most efficient at assimilating nitrogen from zooplankton but excrete >50% of ingested nitrogen; are able to feed on phytoplankton but derive the bulk of their dietary input from smaller zooplankton; and maximise their scope for growth on microzooplankton. These differences provide compelling evidence that anchovy and sardine are trophically distinct, and indicate that the two species show resource partitioning based on zooplankton size. The implications of these trophic differences for ecosystem functioning are discussed, and support the hypothesis that species alternations between anchovy and sardine, both in the southern Benguela and in other upwelling ecosystems, are likely to be trophodynamically mediated.

Persistent upwelling shadows and their influence on zooplankton distributions

Physical (temperature and salinity) and biological (zooplankton) properties were sampled over a 15 mo period in 1988 and 1989 to monitor nearshore environmental variability in northern Monterey Bay, California, USA. During the upwelling seasons of 1988 and 1989, there were two distinct bodies of water along the sampling transect. The offshore water body was characterized by recently upwelled water (typically <12°C) while the nearshore body was significantly warmer (approaching 16°C). A sharp thermal gradient (=front) separated the two water bodies. This front persisted 6.5 km from shore throughout the upwelling season of 1988 and was present again in 1989. Zooplankton-assemblage analysis confirmed the presence of two distinct regions in northern Monterey Bay. We refer to this phenomenon as an “upwelling shadow”, which is the result of the advection of recently upwelled water bearing nutrients and larvae past coastal recesses which are equatorward of upwelling centers.

A numerical analysis of changes in the soft-bottom fauna along a transect across false bay, South Africa

Abstract Grab samples of the benthic fauna have been taken at nine stations along a transect across a marine bay. The stations were located on soft sediment and most were sampled at four seasons during the year. Numerical methods were used to analyse the fauna. Czekanowskis coefficient was used with group-average sorting to classify six main faunistic groups of samples. Information statistical tests were used to define classes and to give lists of species which characterize each faunistic group. These results were confirmed using nearest-neighbour sorting and average member comparisons which revealed some relationships between samples that had been lost in the overall picture obtained by group-average sorting. An ordination did not prove informative, possibly owing to abrupt changes in the data and the lack of general trends through several stations. Because the stations were sampled too infrequently little evidence was found of seasonal changes in the composition of the fauna, a few individual species excepted. Two faunistic groups, rather poor in species, were revealed and associated with the ‘turbulent zone’. Both groups are distinct from the deeper fauna, due probably to the direct effect of wave action and its effect on sediments in shallow water. The four deeper faunistic groups are beyond the reach of wave disturbance and the changes in fauna correspond to changes in sediment parameters, although a cause-and-effect relationship is far from proved. The exclusive and most abundant species in each of the six faunistic groups are mentioned and the results supplemented with data from SCUBA diving and dredgings. The communities defined by numerical analysis are compared with those suggested by previous work and it is concluded that the influence of sediment-type in False Bay modifies a more general depth zonation.

The influence of food availability on population dynamics of a supralittoral isopod, Ligia dilatata brandt

Abstract Size frequency analysis of monthly samples of the isopod Ligia dilatata Brandt in the rocky supralittoral region on the west coast of the Cape Peninsula, South Africa, over a 20-month period shows that L. dilatata lives for 2 yr and that annual recruitment occurs from spring until autumn. Females start reproducing at 12 months but, unlike males, probably do not survive to breed twice. The brood period is 5–6 wk. Fecundity is described by the regression equation N = 1.9 L – 3.7 ( N =no. of eggs in brood pouch, L = length of females in mm). Growth is slow during summer but faster in winter when food is more plentiful. Two distinct phases of mortality are evident; a constant slow mortality for the first 9 months after recruitment, and a faster second phase culminating in the elimination of the cohort after a further 11 months. The in situ temperature range is only 6.2°C (mean: 14.6°C) due to the insulating effect of kelp debris and appears to be less important than food availability in influencing the regular annual cycles observed.

VARIATION IN STRUCTURE AND BIOMASS OF KELP COMMUNITIES ALONG THE SOUTH-WEST CAPE COAST

Summary Transects across six kelp beds located between Cape Agulhas and Saldanha Bay are surveyed and the fauna and flora quantified in terms of energy units. Numerical analysis and information statistic tests are employed to identify areas of similarity and their characteristic species. When data from all the transects are pooled and similarly tested there is a separation of plant-dominated inshore zones from offshore stations, which are densely populated by animals. The more southerly transects tend to have higher overall standing crops of kelp—a feature which can be attributed to the greater water clarity in this area. Both mussels and rock lobsters show the reverse trend, being largely replaced by a sponge-dominated community in the south. Calculations of the plant to animal ratios at each site average only 1,27:1, suggesting that the fauna relies largely on nutritional sources other than attached algae. Standing crop figures are used to calculate the total tonnage of kelp between Cape Point and Salda...

Community-level attributes of exploited and non-exploited rocky infratidal macrofaunal assemblages in Transkei

Abstract The rocky intertidal biota along the Transkei coast of southern Africa is subject to intense exploitation by the indigenous coastal inhabitants. To assess the putative ecological impact of such unconstrained foraging activities the infratidal macrofaunal assemblages at three exploited sites were contrasted with those at two adjacent non-exploited sites. Comparisons of community-level attributes were based on a variety of univariate, graphical and multivariate techniques. Two-way nested anovas revealed that trends in species richness and diversity were not consistent with the presence/absence of exploitation. The apparent conflict between these results and expectations based on Connells intermediate disturbance hypothesis appear to be related to differences in the competitive abilities of “early” and “late” successional species on these shores. Macrofaunal biomass was the only univariate parameter to show significant treatment effects. Although the repeated pulse disturbance regime associated with exploitation differs from the sustained press regime imposed on pollution-impacted communities, their respective abundance/biomass curves show similar trends. Multivariate analyses, in the form of dendrograms, multidimensional scaling (MDS) ordinations and SIMPER routines, derived from Bray-Curtis measures of similarity based on abundance and biomass data, provided the most clear-cut discrimination between exploited and non-exploited sites. The difference in community structure appears to result from a reduction in the abundance of sessile filter-feeders, increase in abundance of certain seaweed-associated species, and decrease in biomass of exploited species and their associated fauna. Differences in community structure amongst sites within treatments were attributed to additional environmental variations. The putative effects of exploitation are also contrasted with those resulting from other forms of anthropogenic disturbance elsewhere.

Characterising and comparing the spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem

The spawning habitats of anchovy Engraulis encrasicolus and sardine Sardinops sagax in the southern Benguela upwelling ecosystem were characterised by comparing their egg abundances with environmental variables measured concomitantly during two different survey programmes: the South African Sardine and Anchovy Recruitment Programme (SARP), which comprised monthly surveys conducted during the austral summers of 1993/94 and 1994/95; and annual pelagic spawner biomass surveys conducted in early summer (November/December) from 1984 to 1999. Eggs were collected using a CalVET net. Physical variables measured included sea surface temperature (SST), surface salinity, water depth, mixed-layer depth, and current and wind speeds; biological variables measured included phytoplankton biomass, and zooplankton biomass and production. Spawning habitat was identified by construction of quotient curves derived from egg abundance data and individual environmental variables, and relationships between these variables were determined using multivariate co-inertia analysis. SARP data showed that anchovy spawning was associated with cool water and moderate wind and current speeds, whereas sardine spawning was related to warmer water and more turbulent and unstable conditions (i.e. high wind speeds and strong currents) than for anchovy. SARP data also showed significant differences in selection of spawning habitat of the two species for all environmental variables. The relationship between anchovy egg abundance and salinity was strongly positive, but strongly negative with water depth, phytoplankton biomass and zooplankton production. Sardine egg abundance was strongly positively related to current speed. The spawner biomass survey data demonstrated that the spawning habitat of anchovy was characterised by warm water and high salinity, whereas sardine spawning was associated with cool water and low salinity. The survey data showed significant differences in spawning habitat selection by anchovy and sardine for SST, salinity and zooplankton biomass, but not for the other environmental variables. There was a positive relationship between anchovy egg abundance and SST, salinity and mixed-layer depth, and a negative relationship with water depth, phytoplankton biomass and zooplankton production. For sardine there was a strong positive relationship between egg abundance and current speed and wind speed. Differences in the results between the two survey programmes could be attributable to differences in their spatio-temporal coverage. Spawning habitats of anchovy and sardine appear to be substantially different, with anchovy being more specific than sardine in their preference of various environmental conditions.

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.

Density and ecological importance of bacteria on kelp fronds in an upwelling region

Abstract Bacteria on the surfaces of fronds of two co-occurring species of kelp have been sampled at monthly intervals for a year in the Benguela upwelling region. Counts of viable bacteria were obtained by the spread plate technique. Bacterial counts at frond bases of both kelps followed a seasonal pattern with minima of 10 3 cells·cm −2 in winter and maxima of 10 6 cells·cm −2 in late summer, similar to the bacterio-plankton cycle. At frond tips bacterial densities remained high (≈ 10 7 cells·cm 2 ) throughout the year and were significantly denser than on other parts of the frond. There was no significant difference between kelp species. Rate of frond erosion does not vary significantly at different seasons, and a minimal estimate of average daily contribution of bacteria eroding off frond tips with the tissue is 2.6 × 10 9 cells·m −2 ·day −1 . The large standing stock of benthic suspension-feeding animals probably utilizes this nitrogen-rich resource.