Coloured Dissolved Organic Matter and Iron Rewire the Marine Plankton Food Web in a Changing Climate

Due to climate change, increasing inputs of allochthonous terrigenous, coloured dissolved organic matter (CDOM) and trace metals like iron (Fe) are predicted to strongly impact coastal ecosystems. We induced predicted near-future levels of CDOM and dissolved Fe (dFe) in the coastal ecosystem by adding hummin feed as CDOM source (browning; + B treatments vs. -B), and deferoxamine B as dFe-speciation source (DFB; + D treatments vs -D) and investigated their effects on plankton dynamics in a 22-day mesocosm experiment. Here we show that increased browning and dFe reshaped the underwater light field and altered the phytoplankton succession. Browning favoured biomass increases in small-sized phytoplankton during the first 8d despite the unfavourable light conditions, due to efficient exploitation of CDOM as a food source and growth compensating for microzooplankton grazing pressure. In contrast, no changes attributed to browning during ca. the 2nd half of the experiment were observed. Both phytoplankton and microzooplankton population crashed due to significant grazing pressure from mesozooplankton. We hypothesise that the dual effect of browning and dFe could favour mesozooplankton vertical migration of to the surface in a darker ocean, rewiring the phytoplankton succession and imposing far reaching consequences for the entire plankton trophic cascade.