PLANKTON DYNAMICS THROUGH THE NEOPROTEROZOIC-PALEOZOIC TRANSITION: ECOLOGICAL, ENVIRONMENTAL AND EVOLUTIONARY FEEDBACKS

Plankton and plankton dynamics have changed profoundly over the past four billion years, including a fundamental change of state through the Cryogenian-Ediacaran-Cambrian transition. Whereas Proterozoic export production was dominated by small, inherently buoyant cyanobacteria, leading to relatively stratified turbid-water conditions, its Phanerozoic counterpart is represented by large, often biomineralizing eukaryotic phytoplankton associated with rapid export and a well aerated water column. In modern aquatic systems these qualities represent alternative stable states maintained through positive plankton-environment feedbacks, typically mediated by the feeding activities of suspension feeding metazoans. The same was true in the mid-Neoproterozoic, except that metazoans had yet to evolve most of their modern feeding repertoires (and associated co-evolutionary and engineering effects on eukaryotic phytoplankton). The earliest sign of Phanerozoic-style plankton dynamics appears around 750 Ma with the first quantitative occurrence of eukaryotic biomarkers and a distinctive new range of eukaryotic microfossils. This modal shift is intriguingly coincident with recent molecular clock estimates for the first appearance of animals, and there is a good case for recognizing early sponge-grade suspension feeding as an important selective agent for larger, more export prone phytoplankton - leading to fundamental changes in the biological pump, oceanic ventilation, and possibly global climate. Further co-evolutionary innovations in the plankton would have accompanied the muscle-based suspension feeding of cnidarians-grade animals; however, the most profound effects were undoubtedly induced by the explosive radiation of bilaterians in the early Cambrian (ca. 530 Ma). The unique capacity of motile metazoans with a through-gut to build multi-trophic food webs, drive co-evolutionary diversification, and actively trans-locate nutrients between the plankton and benthos revolutionized the marine biosphere.