A sediment bioturbator bottleneck to seedling recruitment for the seagrass Posidonia australis

ABSTRACT: Bioturbating animals have the potential to influence the distribution and survival of seagrass seeds and seedlings within unvegetated substrates. Such disturbances could act as demographic bottlenecks, or restrictions, to seedling recruitment and impede population recovery in degraded systems. This study evaluated the influence of sediment bioturbators on seed settlement and seedling establishment for the seagrass Posidonia australis in temperate and subtropical areas of Western Australia (Cockburn Sound and Shark Bay). Initial benthic surveys recorded the density of sediment bioturbators as well as P. australis seeds and seedlings. The abundant bioturbators observed were the sand dollar Peronella lesueuri and the sea star Archaster angulatus in Cockburn Sound, and the heart urchin Breynia desori in Shark Bay. Surveys demonstrated an overlap in habitat use among bioturbators, seeds, and seedlings, and suggested potential negative seed-fauna interactions in areas where bioturbators were present and abundant. To test this relationship, field manipulative experiments measured the rate at which these bioturbators dislodged and moved both P. australis seeds (unburied and buried) and 1 yr old seedlings. Although all bioturbators dislodged and moved seeds on the sediment surface, only sand dollars and heart urchins dislodged and moved seeds (on average ~4 cm) buried within the sediment. Where high densities of bioturbators, such as sand dollars and heart urchins, exist, considerable impact on P. australis seeds would be expected. No animals dislodged 1 yr old seedlings. Sediment bioturbators act as an important, but often overlooked, bottleneck to seedling recruitment and recovery of degraded seagrass communities.