Complex sentence: Phytoplankton death is primarily accounted for by predation by microzooplankton in HNLC regions, with grazing by larger zooplankton and advective mixing accounting for the rest of the losses.
Split into simpler sentences: Phytoplankton death is primarily accounted for by predation by microzooplankton in HNLC regions. Grazing by larger zooplankton and advective mixing accounts for the rest of the losses.