Recently upwelled water from the southern Benguela upwelling system was incubated in a 60 l microcosm for 43 d under simulated in situ conditions, to follow the development and activity of the heterotrophic microplanktonic community associated with phytoplankton growth and decay. The initial bacterial population (40 pg C 1-l), dominated by small rods (V = 0.198 pm3) and large cocci (P = 0.142 l~n73), with Vibrionaceae as the domlnant plateable strain, exhibited slow turnover times for added I4C-labelled substrates (R = 5.7 h 10%ells-l). Net bacterial growth was exponential (0 016 h-') during phytoplankton growth (12 pg C I-' h-', Days 0 to 4). At maximum phytoplankton and bacterial biomass (1330 and 136 pg C 1-' respectively, Day 4) Pseudomonadaceae dominated the plateable isolates; bacterial turnover times for 14C-substrates were rapid (glucose: 1.5 h 106 cells-', alanine: 0.49 h 106 cells-', glutamate: 0.29 h 106 cells-'), suggesting a close coupling between phytoplankton growth and the ab~lity of bacteria to utilise dissolved organic carbon (PDOC) substrates. Bacterial biomass was reduced to < l 5 pg C 1-' by Day 9, due to diminished availability of PDOC during phytoplankton senescence and predation by microflagellates which developed in the microcosm (<5 pg C 1-' up to Day 4, 96 pg C 1-' on Day 8). After phytoplankton senescence (Day 10) detrital carbon stimulated exponential growth (0.021 h-') of a second bacterial community (max. biomass: 231 pg C 1-' on Day 25) dominated by small cocci (P = 0.009 pm3) and large rods (V = 0.672 with Flavobactenaceae as the dominant plateable bacteria. As this community exhibited no uptake of added I4C-labelled substrates, we surmise that it was exploiting POC which dominated carbon resources at this time. Estimates of bacterial production calculated from net growth rates were ca 50 to 97 % higher than values based on ~m e t h~l -~~] thymidine incorporation (TTI). These differences may be due to inadequate DNA extraction procedures, large numbers of bacteria without thymidine transport systems, or isotope dilution. Empirically determined conversion factors to correct for these differences fell within the range of 1.6 to 46 X 106 cells mol-' TTI.

A general s~z e -b a s e d model IS used here to p r e d~c t the complex tempoial successlons observed in planktonic communities after an upwelling event In the southern Benguela a n d to lesolve a number of fundamental questions regarding the t r o p h~c dynamlcs ot the p e l a g~c food web The model was not based on field data, and simulat~on results do not exactly mimic field and laboratory results Nonetheless, the s~mulation model makes a major contr~butlon towards our understdnding of the dynamlcs of the planktonic food web after up\uell~ng The model predicts r a p~d growth ot d phytoplankton community dominated by nanophytopldnkton-sized cells and d ldter netphytopldnkton bloom After nitrate depletion the bloom 1s dominated by ndnophytoplankton dependent upon regenerated nltrogen Analysls of C and N flows showed thdt resplrat~on and graLlng w e i e largely responsible for the decline of the phytoplankton bloom, accounting for 47 and 44 "., respectively of the total C fixed by phytoplankton over the 20 d period Mesozooplankton grazed 62 Yu of the decllnlng bloom (Days 10 to 201, but only 18 'Yu of the total C flxed This was due to the dominance of nanophytoplankton primary producers which were unavailable to the larger mesozooplankton but were consumed by miclozooplankton The m~croblal food web played an important role In N cycllng and in the production of mesozooplankton throughout the simulat~on period To determine the ~e l a t i v e Importance of the classical dlatom-dom~nated food chaln versus the microbial food web during one upwelling event, network analys~s was used to assess C and N flows In the model foodweb The total dependency coeffic~cnts showed that mesozooplankton depend malnly on netphytoplankton (76 "c,) for C dunng the first 10 d of the phytoplankton bloom but depend equally on netphytoplankton ( 7 0 % ) and microzooplankton (69 %) for N Dunng the last 5 d of the bloom the biomass of both major prey items is low, and mesozooplankton depend equally (21 to 22 "$,) on netphytoplankton and microzooplankton for both C and N Food chains are longer and trophic effic~ency decreases The frequency of upwell~ng in the southern Benguela may thus be an impoltant fact01 determining the relative dominance of short diatom-based food chains versus the m~crobial food web and therefore the annual y~e l d of p e l a g~c fish

Temporal changes in the biomass relationships and community structure of the planktonic food web during the development of a plume of upwelled water in the southern Benguela were investigated during 2 consecutive drogue studles. Three distinct water masses of increasing age were encountered. Primary production was highest (1 g C m-' h-') at the start of the study and decreased as the plume moved offshore. The planktonic community was initially characterised by a high biomass of bacteria (40 to 60 mg C m-"), a diatom-dominated phytoplankton community ( > 5 mg chl m-3), and a mesozooplankton community (30 to 86 mg C dominated by the copepod Calanoides carinatus. At the start of the second drogue, phytoplankton production and biomass were lower (ca 0.5 g C m-2 h-', 2 to 5 mg chl m-3), and the phytoplankton community was dominated numerically by nanoplanktonic flagellates (2 to 20 pm). Bacterial biornass estimates (190 mg C m-3) were the highest recorded thus far in sltu in the southern Benguela. Further offshore, primary production rates were < 0.2 g C m-'h-' and the biomass of phytoplankton and bacteria decreased to < 2 mg chl and ca 20 mg bacterial C m-3 C. carinatus dominated the copepod biomass throughout the study period, and showed a general inverse relationship with phytoplankton biomass. Grazing impact may have contributed significantly to the decline of the bloom, with copepods ingesting S to 10 % of phytoplankton biomass in maturing upwelled water, and up to 38 % towards the end of the bloom. Results suggest that herbivorous copepods become food-limited during the quiescent phase of the upwelling cycle or when the phytoplankton community is dominated by small nano-and picoplanktonic cells. Microheterotrophic pathways appear to be an important component of the pelagic food web in the southern Benguela. The relative dominance of the classical diatom-mesozooplankton food chains versus longer microbial food webs may have important implications for the population dynamics of pelagic fish.

A comparison was made of ingestion rates of the copepod Calanoides cannatus estimated from the disappearance of chlorophyll (by fluorometry and HPLC), disappearance of cells, egg production, and from the gut fluorescence technique Three measures of gut turnover time were also compared. Our primary goal was to determine whether the gut fluorescence method underestimated ingestion and if so, to determine if the problem was due (i) to pigment digestion or ( U ) to incorrect estimates of gut passage time. Experiments used as food the diatom Thalassiosira weissflogii at 3 concentrations. The first 4 methods ylelded the same result: grand means were 2.7 11g chl d-' at 8000 cells ml-', 1.5 ~i g chl d-' a t 3000 cells ml-' and 0.84 pg chl d-' at 1500 cells ml-l. The gut fluorescence technique underestimated ingestion, yielding rates of 1.0, 0.92 and 0.58 pg chl d-' when calculated following the traditional approach of multiplying gut pigment content by gut evacuation rate (GER). The underestimate was not due to pigment digestion because pigment budgets showed that all pigment was accounted for. 106 "/ o on average. Thus we suggest that the problem was associated with using GER as a measure of gut passage time (GPT). GPT estimated from the GER was 16.7 min, but when estimated from (i) measurements of fecal pellet poduction rates and from (ii) the ratio, plgnlent egestion r a t e h e a n gut pigment content, results averaged 10.2 min. Using this value, ingestion rates were equal to the other techniques at the intermediate and low food concentrations, and 80 O/O of the other techni.ques at the high concentration

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