The relative importance of food and temperature to copepod egg production and somatic growth in the southern Benguela upwelling system

The fecundity and somatic growth rates of Calanus agulhensisand Calanoides carinatus, the dominant large calanoid copepodsin the southern Benguela upwelling system, as well as the fecundityof several other common copepods, were measured between Septemberand March of 1993/94 and 1994/95. Mean egg production of mostcopepods was low at >30 eggs female^-1 day^-1 {Calanoides carinatus23.7, Calanus agulhensis 19.0, Neocalanus tonsus 16.1 and Rhincalanusnasutus 26.1), whereas the mean fecundity of Centropages brachiatuswas significantly greater (83.6 eggs female^–1 day^-1).This study also presents the first comprehensive field estimatesof the fecundity of Nanno-calanus minor (mean: 26.1 eggs female^–1day^–1, range: 0.0–96.2 eggs female^–1 day^–1)and of somatic growth of N6 and all copepodite stages of Calanoidescarinatus (decreasing from 0.58 day^–1 for N6 to 0.04 day^–1for C5). Somatic growth rates of Calanus agulhensis also declinedwith age: from 0.57 day1 for N6 to 0.09 day1 for C5. Data ongrowth rates were used to assess the relative importance offood [as measured by total chlorophyll (Chi) a concentration],phytoplankton cell size (proportion of cells >10 µm)and temperature to the growth of copepods. Multiple regressionresults suggested that fecundity and somatic growth rates werepositively related to both Chi a concentration and phytoplanktoncell size, but not to temperature. Although it was not possibleto separate the effects of Chi a concentration and phytoplanktoncell size, data from previous laboratory experiments suggestthat copepod growth is not limited by small cells per se, butby the low Chi a concentrations that are associated with theseparticles in the field. Despite growth not being directly relatedto temperature, a dome-shaped relationship was evident in somespecies, with slower growth rates at cool (<13°C) andwarm (>18°C) temperatures. The shape of this relationshipmirrors that of Chi a versus temperature, where poor Chi a concentrationsare associated with cool and warm temperatures. It is concludedthat the effect of food limitation on growth of copepods outweighsthat of temperature in the southern Benguela region. Sourcesof variability in relationships between growth and Chi a concentrationare discussed.     

Reproductive response of the copepods Calanoides carinatus and Calanus agulhensis to varying periods of starvation in the southern Benguela upwelling region

Egg production by the calanoid copepods Calanoides carinatusand Calanus agulhensis fed excess Thalassiosira weissflogiiwas monitored in the laboratory following starvation periodsof 1, 3, 5, 7 and 9 days. Following short (1–3 day) periodsof starvation, egg production by C.agulhensis returned to thesatiated rate (51.1 eggs {female} day^–1) more rapidly(after 0.9–2.4 days of excess food) than that of Ca. carinatus(after 2.8–3.1 days). However, following longer (5–9day) periods of starvation, Ca. carinatus regained satiatedlevels of egg production (55.8 eggs {female}^–1 day^–1)more rapidly (after 3.1–4.0 days of excess food) thanC. agulhensis (after 3.8–5.2 days of feeding following5–7 days of starvation). Moreover, many C. agulhensisfemales did not regain normal rates of egg production after9 days of starvation. For both species, the time required foregg production to recover was proportional to the starvationperiod, although only up to 7 days for C. agulhensis, and wasthe same following 4.25 days of starvation. Previously fed Ca.carinatus terminated egg production more rapidly than C. agulhensiswhen starved. The ability of Ca. carinatus to tolerate, andrecover rapidly from, prolonged periods of starvation, combinedwith a comparatively fast development time and high rate ofegg production, provides this species with a strong competitiveadvantage over C. agulhensis in the highly pulsed food environmentof the southern Benguela upwelling region.     

Diel variations in gut pigment content, diel vertical migration and estimates of grazing impact for copepods in the southern Benguela upwelling region in October 1987

The gut fluorescence technique was used to estimate ingestionand filtration rates of the adult female copepods Paracalanusparvus, Cenlropages brachiatus and Calanus austrails, and copepoditestages 3, 4 and 5 of C.australis in the southern Benguela upwellingregion. During the study period chlorophyll concentrations withinthe upper 20 m of the water column were high, 5 µg I^–1in mid-shelf waters and 15–30 µg I^–1 in innershelf waters. Copepod gut pigment content was low and constantduring the day then increased sharply during the first 2 h aftersunset. Gut pigment content was 2–6 times higher duringthe night compared with daytime values. Small non-migratingcopepods (Paracalanus parvus) showed the smallest diel differencein gut pigment content and large migrating copepods (Centropagesbrachiatus and Calanus australis) the largest difference. Eggproduction rates were 20 and 50% of maximum at the mid-shelfand inner shelf stations respectively, suggesting food-limitation.Comparison of ingestion rates calculated from egg productiondata with ingestion rates calculated from gut pigment data suggestedthat the copepods were feeding omnivorously at the inner shelfstations but herbivorously at the mid-shelf stations. Assumingthat all of the phytoplankton was available as food, the nearshorecopepod assemblage grazed {small tilde}1% of the standing cropeach day, and the mid-shelf assemblage grazed 5% day^–1.Because of errors and uncertainties associated with the gutfluorescence technique, the feeding impact could be underestimatedby 2–4-fold. We discuss several approaches which couldlead to more precise estimates of feeding rates. ³Present address: Marine Sciences, SUNY, Stony Brook, NY, 11794-5000,USA     

The use of sea temperature in characterizing the mesoscale heterogeneity of phytoplankton in an embayment of the southern Benguela upwelling system

The role of meteorological events and hydrography in determiningchanges to the phytoplankton community was investigated in anembayment exposed to a narrow band of coastal upwelling. Dailysampling demonstrated the importance of advective processesdriven by meteorological forcing in controlling rapid shiftsin the biomass and species composition of the phytoplanktoncommunity. Samples of similar phytoplankton composition wereassociated with different stages of the upwelling cycle, asdefined by an index of biological ageing of upwelled waters.Relationships between the physical, chemical and biologicalfields were defined from time-senes measurements. The time elapsedfollowing upwelling, required for the determination of biologicalrates, was estimated from the rate of heating. A primary productionestimate of 2.14 g C m^–2 day^–1 was derived fromdetermination of the rate of nutrient depletion, whereas a phytoplanktonbiomass-nutrient consumption equation provided an estimate of3 92 g C m ^–2 day^–1. Both rates were within therange of estimates obtained from in vitro tracer methods.     

Surface-feeding on cephalopods by procellariiform seabirds in the southern Benguela region, South Africa

Cephalopod beaks recovered from stomach samples taken from 14 seabird species in the southern Benguela region off Southern Africa and from one species at Sub-Antarctic Marion Island, were identified as far as possible, counted and the lower rostral lengths (LRLs) measured. Dorsal mantle lengths (DMLs) and body masses of the cephalopods eaten were estimated. The results of analyses by percentage frequency of occurrence and numerical abundance are discussed with reference to present knowledge of the distribution of cephalopods eaten by seabirds in the areas studied. Division of the cephalopod component of seabird diets into species which float, and species which sink, after death indicates that the birds forage on dead or moribund cephalopods on the surface, rather than catching live bioluminescent cephalopods at night.     

Hypoxia tolerance in the copepod Calanoides carinatus and the effect of an intermediate oxygen minimum layer on copepod vertical distribution in the northern Benguela Current upwelling system and the Angola-Benguela Front

There is a growing concern that hypoxic and anoxic areas in the sea spread in extent and intensity, posing a severe risk to marine ecosystems and fisheries. Hypoxia may affect fish stocks directly or via detrimental effects on important prey species, such as zooplankton. A unique feature of the northern Benguela Current upwelling region and Angola-Benguela frontal system is a pronounced intermediate oxygen minimum layer (IOML) at 60-500 m depth with oxygen concentrations ≤ 1.4 mg O₂l^− 1 (minimum < 0.7 mg O₂l^− 1). Field studies during February-March 2002 demonstrated that the abundance of calanoid copepods and the biomass of mesozooplankton in general were severely reduced within the IOML. The dominant copepod Calanoides carinatus showed a bimodal vertical distribution with parts of the population either comprising all developmental stages concentrated in the surface layer (0-60 m), or copepodids C5 diapausing below 400 m depth apparently avoiding the IOML. Accordingly, abundances of other calanoid copepods were higher at the surface and below 300 m than in the centre of the IOML. The scarcity of planktonic life within the IOML raises the question whether this layer represents an effective barrier for zooplankton vertical migrations. Especially in C. carinatus, ontogenetic vertical migration plays a key role in the retention of the population within the productive upwelling region and for the rapid re-colonisation of plumes of newly upwelled water. To address this issue, the hypoxia tolerance of C. carinatus was determined in a series of laboratory-based, closed-bottle experiments in January 2005. Copepods were kept in gas-tight bottles and the decreasing oxygen concentrations were monitored to establish their minimum oxygen demands. Although copepodids survived apparently unharmed at surprisingly low oxygen concentrations of ca. 1.5 mg O₂l^− 1, they could not tolerate oxygen levels < 1.1 mg O₂l^− 1, implying that the core of the IOML, where O₂ concentrations are below this threshold, is uninhabitable for C. carinatus. In contrast, the IOML may represent a refuge from competition and predation for other copepod species specifically adapted to hypoxic environments.     

A population balance model for fish population dynamics

A population balance model of fish population dynamics for batch systems was developed. A growth rate expression was introduced and coupled with the population balance. Solutions of the model provide predictions of such fish size distribution characteristics as average size, standard deviation and coefficient of variation. A growth diffusivity mechanism was found to be inapplicable to systems where a terminal size is reached. A study of the two parameter growth rate expression was conducted, illustrating that conditions conducive to high growth rates also resulted in broadening of size distributions. The model was compared to data found in the literature to demonstrate its predictive capabilities.     

Effect of food composition on egg production and hatching success rate of two copepod species (Calanoides carinatus and Rhincalanus nasutus) in the Benguela upwelling system

We have analysed the daily egg production (EPR) and hatchingsuccess rates of the calanoid copepods Calanoides carinatusand Rhincalanus nasutus as a function of nano- and microplanktonconcentration and composition in the northern Benguela upwellingsystem off Namibia. Food concentration explained 55% (R. nasutus)to 62% (C. carinatus) of the EPR variability. We found no relationbetween the residuals of the food concentration–EPR regressionand the percentage of the different taxonomic components ofthe nano- and microplankton. Nor was there a relation with theproportion of the diatom Skeletonema costatum that dominatedthe major blooms or with the number of nano- and microplanktonspecies. We conclude that food quality differences could notbe attributed to the relative composition of microplanktonicparticles of the different groups (i.e. taxonomic composition).     

浙南地区烟粉虱发生新动态及其防治对策

长期以来,烟粉虱[Bemisia tabaci（Gennadius）]在浙南地区仅是一种未被重视的次要害虫,但近几年该虫在本地区呈突发性暴发态势,危害严重。笔者就烟粉虱在浙南地区发生的新动态及其原因、综合防治措施等进行了调研和综述。     

Vertical migration and habitat partitioning of six euphausiid species in the northern Benguela upwelling system

The vertical distribution of the adults of six species of euphausiids,Thysanöessa gregaria, Nematoscelis megalops, Euphausiaamericana, E.gibboides, E.hanseni, Stylocheiron longicorne,as well as the larval stages of E.hanseni, were studied duringa 48 h cycle at a fixed station in the northern Benguela upwellingsystem. All the species, except T.gregaria and S.longicorne,proved to perform diel vertical migration, but both migratoryand non-migratory species appeared to be segregated in spaceduring night-time hours, regardless of potential prey (phytoplanktonand copepods). It is suggested that water column structure andhydrographic discontinuities caused by a warm, depth-localizedintrusion, as well as the reproductive strategies of particularspecies, are responsible for this pattern. Eggs and larval stagesof E.hanseni were concentrated near the surface, and the effectof the short-term pulses of the intrusion on their abundanceis discussed.     

Growth and production of Euphausia lucens in the southern Benguela Current

The growth rate of a population of Euphausia lucens from thewest coast of South Africa was estimated from laboratory studiesand from monthly size-frequency distributions of samples collectedover a 1-year period. Laboratory studies indicated that growthrates ranged from 0.131 (larvae) to 0.047 mm day^–1 (juveniles),while size-frequency distributions suggested a growth rate of{small tilde}0.026 mm day^–1 for the adults. The mean annualbiomass from the inshore, intermediate and offshore regionsranged from 9.75 to 47.29 mg dry wt m^–3 with the highestbiomass being found in the inshore region. Calyptopis larvaewere present for most months of the year, indicating continuousrecrwtment. The relative contribution of flesh, moults and eggsto the total annual production was estimated separately forall three regions. Production due to growth (P_g) was estimatedto be 92.71–185.60 mg dry wt m^–3 year^–1, whileexuviai production (P_e) varied between 60.01 and 281.38 mg drywt m year Production of eggs (P_r) was estimated to range from5.07 to 12.39 mg dry wt m year the lowest value being obtainedin the inshore region. Moult production represented {small tilde}6times the mean biomass in each region, while the P/B ratio forflesh production varied from 3.92 to 8.91, the highest ratiobeing obtained in the offshore region. Total P/B ratios rangedfrom 10.14 to 16.01.     

A model for aggregation-dispersion dynamics of a population

An important feature that distinguishes the movement of living systems from the random motion of inorganic material is a delicate balance between spreading and concentrating. This movement is based on the kind of interactions which a bacterial colony may establish during migration. Namely, the antagonistic effects of dispersal which take place preferentially down the population gradient and the tendency in grouping together. In this work a model is proposed which considers these effects. The phase plane analysis and the numerical calculations reveal the existence of stable sharp wave front solutions. The speed of the wave front is modulated by the compromise between the tendencies of spreading and aggregating. The results obtained were compared with experimental observations in cultures of Escherichia coli and Streptococcus faecalis. The agreement between both types of results supports the hypothesis on which the model was based.     

Fungal parasitism in a copepod population: the effects of Aphanomyces on the population dynamics of Boeckella dilatata Sars

Growth parameters of a Boeckella dilatata (Copepoda: Calanoida)population in Lake Hayes over 16 months were calculated in thepresence and absence of a fungus, Aphanomyces sp. (Phycomycetes:Saprolegniaceae) which kills the eggs and increases mortalityof the females. Parasitism was highest during winter and thefollowing summer when birth rates were depressed by up to 14.3%.When the incidence of Aphanomyces is high, parasitism couldbe important in regulating the growth of a copepod population.     

A mathematical model for the population dynamics of army ants

A stochastic cellular automata model for the population dynamics of the army antEciton burchelli on Barro Colorado Island in Panama is set up. It is simulated on the computer and shown to give good agreement with biological data. It is analysed using two approximations akin to the mean field approximation in statistical mechanics, and good agreement with the simulations is obtained. Finally, the role of distance between successive statary phase bivouacs is discussed with regard to the rate of colony growth. There are two aspects of the biological system studied here that make it of general importance. First, the population is structured, since the size of each colony of army ants is crucial. Second, the spatial behaviour of the population, as in many others, is not diffusion-like, although it is random. This has implications for the kind of model that is chosen.     

Quaternary paleoceanology of Benguela upwelling based on microfossils (borehole 362 DSDP)

The study of the upper part of the section of borehole 362 DSDP (southeastern Atlantic) allowed the recognition of five zonal units in the Quaternary based on nannofossils and refinement of the UpperLower Pleistocene boundary based on benthic foraminifers. The study of planktonic and benthic foraminifers in sediments display the history of the Benguela upwelling. The temperature of the surface water layer reconstructed based on Upper Pleistocene planktonic foraminifers reaches 17°–19°C and decreases downward in the section to 2°–3°C, suggesting that, in the Early Pleistocene, the upwelling was more extensive in area. The increasing proportion of benthic foraminifers and large nannofossils downward in the section is evidence of more active dissolution of carbonates in the Early Pleistocene and the high content of terrigenous matter suggests that the Benguela Current occupied a westerly position. The existence of the Benguela upwelling throughout the Late Pliocene-Pleistocene was accompanied by a decrease in its activity.     

Indicators quantifying small pelagic fish interactions: application using a trophic model of the southern Benguela ecosystem

Three indicators quantifying interactions between species are developed for an upwelling system to provide useful measures for the comparison of marine ecosystem structure and function. Small pelagic fish are dominant in upwelling systems, and by definition, they are pivotal in a wasp-waist upwelling system. The indicator of interaction strength (IS) quantifies the effect that a change in biomass of one group has on abundance of other groups. The functional impact (FI) indicator quantifies the trophic impacts of species on their own and other functional groups or feeding guilds. The trophic replacement (TR) indicator quantifies the trophic similarity between a species that is removed from an ecosystem and other species in that ecosystem, i.e. it quantifies the ability of one group to trophically replace another. A trophic model of the southern Benguela ecosystem is used as an example for the application of the indicators. The strong similarities in trophic functioning of the southern Benguela ecosystem in the anchovy-dominated system of the 1980s, and the 1990s when there was a shift towards greater sardine abundance, are explained by the mutual trophic replacement abilities of anchovy and sardine. Differences between the proposed indicators and mixed trophic impact assessment are highlighted, mainly resulting from the static versus dynamic nature of the models upon which they are based. Trophic indicators such as those presented here, together with other kinds of ecosystem indicators, may assist in defining operational frameworks for ecosystem-based fisheries management.     

A quantitative model of honey bee colony population dynamics

Since 2006 the rate of honey bee colony failure has increased significantly. As an aid to testing hypotheses for the causes of colony failure we have developed a compartment model of honey bee colony population dynamics to explore the impact of different death rates of forager bees on colony growth and development. The model predicts a critical threshold forager death rate beneath which colonies regulate a stable population size. If death rates are sustained higher than this threshold rapid population decline is predicted and colony failure is inevitable. The model also predicts that high forager death rates draw hive bees into the foraging population at much younger ages than normal, which acts to accelerate colony failure. The model suggests that colony failure can be understood in terms of observed principles of honey bee population dynamics, and provides a theoretical framework for experimental investigation of the problem.     

Chlorophyll concentrations in the southern Benguela current region as determined by satellite (Nimbus-7 coastal zone colour scanner)

A series of ocean colour experiments was undertaken in the southernBenguela Current region during 1978–1980 as part of acoordinated study of the dynamics of the system. The distributionof surface chlorophyll during different seasons and upwellingevents, derived from Nimbus-7 CZCS data is indicative of thecomplexity of the system which may be divided into four quasi-independentregimes. The phytoplankton patchiness was evident and the growthand decay of the blooms which develop offshore of the activeupwelling centres could be followed seasonally, while mesoscaleeddies were characteristic of the frontal region. The chlorophylldistribution and variation was consistent with hypothesisedspawning of the Cape anchovy, Engraulis capensis, the larvaltransport and subsequent recruitment processes. The potentialof the CZCS for environmental monitoring and for fish availabilitystudies is discussed.