PLANKTON ECOLOGY AND THE PROTEROZOIC-PHANEROZOIC TRANSITION

Most modern marine ecology is ultimately based on unicellular phytopla nkton, yet most large animals are unable to graze directly on even rel atively large net phytoplankton; the repackaging effected by herbivoro us mesozooplankton thus represents a key link in marine metazoan food chains. Despite the deep taphonomic biases affecting plankton fossiliz ation, there is a clear record of phytoplankton from at least 1800 m.y ago. Proterozoic plankton are represented by small-to medium-sized sp haeromorphic acritarchs and probably do not include many/most of the u nusually large acritarchs that characterize the Neoproterozoic. The fi rst significant shift in phytoplankton diversity was therefore the rap id radiation of small acanthomorphic acritarchs in the Early Cambrian. The coincidence of phytoplankton diversification with the Cambrian ra diation of large animals points compellingly to an ecological linkage between the two, particularly in light of recently discovered filter-f eeding mesozooplankton in the Early Cambrian. The introduction of plan ktic filter feeders would have established the second tier of the Elto nian pyramid, potentially setting off the ''self-propagating mutual fe edback system of diversification'' now recognized as the Cambrian expl osion (Stanley 1973, 1976). By consuming significant percentages of ne t phytoplankton and suspending it as animal biomass and non-aggregatin g fecal pellets, mesozooplankton cause a net reduction in export produ ction; a general introduction of zooplankton would therefore have redu ced carbon burial and moderated the bloom and bust cycle that must hav e characterized Proterozoic populations of net phytoplankton. The effe ct of added trophic levels in Early Cambrian ecosystems can be viewed as a serial application of the trophic cascade process observed in mod ern lakes, whereby the introduction of higher trophic levels determine s the accumulation of plant biomass at the base of the system. As such , the major biogeochemical perturbations that mark the onset of the Ph anerozoic might be considered a consequence, rather than a cause, of t he Cambrian explosion; reduced C export due to zooplankton expansion e xplains the otherwise anomalous drop in delta(13)C at the base of the Tommotian. Cambrian acanthomorphic acritarchs likely derived from plan ktic leiosphaerids exposed to mesozooplanktic grazing pressure, the or namentation effectively increasing vesicle size without compromising b uoyancy or surface-area:volume ratios. Alternatively, they may represe nt an escape into the plankton through a miniaturization of the much l arger Neoproterozoic acanthomorphs. An invasion of small benthic herbi vores into the water column to exploit the phytoplankton accounts for the origin of the mesozooplankton and may have been the key innovation in the Cambrian explosion.