SHALLOW-WATER PHOTOTROPHIC CARBONATE SEDIMENT FACTORIES IN THE TEMPERATE SOUTHERN OCEAN

Neritic Southern Ocean carbonate depositional environments on the continental margins of southern Australia and New Zealand are warm-temperate in the west and cool-temperate in the east. The two main shallow marine, open-ocean benthic sediment factories are macroalgal rocky reefs and seagrass meadows. Biomass in both factories is dominated by phototrophs, but their occurrence as community-dominant factories is determined by temperature and substrate. Both factories occur together in warm-temperate western southern Australia; macroalgae on the rocky reefs and broad-leaf seagrasses on intervening sand flats. By contrast macroalgal rocky reefs dominate in cool-temperate southern Australia and New Zealand; broad leaf seagrasses are inconsequential. Macroalgae across the region are overwhelmingly phaeophytes (brown algae) and rhodophytes (red algae) with large bull kelp prominent in very shallow cool-temperate environments. Sediment from the macroalgal reefs is mostly produced by gastropods and geniculate coralline algae living beneath a macroalgal canopy on illuminated upward-facing substrates and by a wide variety of invertebrates such as small benthic foraminifers, bryozoans, sponges, gastropods, bivalves, echinoderms, barnacles, and octocorals growing in shaded cryptic habitats. These biofragments are shed off the reefs to accumulate as an apron on the surrounding seafloor. Broad-leaved seagrasses that only thrive in warm–temperate environments support an additional community of gastropods, large benthic foraminifers, geniculate coralline algal, and infaunal bivalves. Sediments in such warm-temperate settings are thus a blend of particles from both macroalgal reef and seagrass factories. By contrast it is only macroalgal forests in cooler environments that contribute biofragments. The contribution from two factories in warm-temperate settings together with lower dissolution rates leads to overall higher accumulation rates in this depositional realm.