Simulation and field measurements of phytoplankton-bacteria-zooplankton interactions in the southern Benguela upwelling region

Abstract

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