PD Dr. Ced­ric Meu­nier: Top-down and bot­tom-up pro­ces­ses as­so­cia­ted with mi­cro­zoo­plank­ton du­ring phy­to­plank­ton blooms

Top-down and bottom-up processes associated with microzooplankton during phytoplankton blooms

Glo­bal chan­ge puts co­as­tal ma­ri­ne sys­tems un­der enor­mous pres­su­re, threa­ten­ing com­mu­ni­ty struc­tu­re and func­tio­n­ing. In par­ti­cu­lar, plank­ton com­mu­nities which play a cru­ci­al role in the over­all func­tio­n­ing of ma­ri­ne food webs, are par­ti­cu­lar­ly sen­si­ti­ve to si­mul­ta­neous chan­ges in tem­pe­ra­tu­re, pH, and dis­sol­ved nut­ri­ent con­cen­tra­ti­ons. The bio­mass, di­ver­si­ty, as well as the tro­phic in­ter­ac­tions that take place bet­ween plank­to­nic or­ga­nisms are all ex­pec­ted to be im­pac­ted by glo­bal chan­ge, but chan­ges in eco­sys­tem func­tio­n­ing re­main lar­ge­ly un­k­nown sin­ce most stu­dies thus far have fo­cu­sed on sin­gle stres­sor ef­fects most stu­dies. Through a mul­ti­ple-dri­ver ap­proach, we tested the im­pact of dif­fe­rent IPCC sce­na­ri­os and rea­lis­tic nut­ri­ent re­gimes shifts on dif­fe­rent tro­phic le­vels and their in­ter­ac­tions by con­duc­ting in­door mi­cro­cosm and field me­so­cosm ex­pe­ri­ments with North Sea plank­ton. Over­all, our re­sults in­di­ca­te that ma­ri­ne co­as­tal plank­to­nic food webs may shift from being me­so­zoo­plank­ton-do­mi­na­ted towards a do­mi­nant role of the mi­cro­bi­al loop in re­s­pon­se to glo­bal chan­ge. Such shifts in bot­tom-up and top-down pro­ces­ses are not an­ody­ne. Whi­le mi­cro­zoo­plank­ton are a na­tu­ral tro­phic link bet­ween phy­to­plank­ton and bac­te­ria on the one hand, and me­so­zoo­plank­ton on the other hand, in­ten­si­fied tro­phic pa­thways through mi­cro­zoo­plank­ton may di­mi­nish en­er­gy trans­fer ef­fi­ci­en­cy to hig­her tro­phic le­vels, the­r­e­by crea­ting a ‘tro­phic sin­k’ for pro­duc­tion in the food web.