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 leseuri) 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 P. australis seeds, both unburied and buried, and one year 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 ~ 4cm) 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 one year old seedlings. Sediment bioturbators act as an important but often overlooked, bottleneck to seedling recruitment and recovery of degraded seagrass communities. KEY WORDS: bioturbators, seeds, seedling recruitment, restoration, seagrass, Posidonia australis
within the sediment. Urchins then moved within the sediment for an average distance of 29.3 cm (± 0.3, range: 23 – 35 cm day−1, ~1.2 cm hr−1) within a day. Measurements of sand dollar movement in laboratory conditions and in situ, suggest individual sand dollars could disturb ~0.15 - 0.4 m2 day − 1 respectively, and their populations could disturb between 6-16 m2 of the survey area and 3-8 seeds, or 13-35% of all recorded seeds, daily at the densities recorded at OAC. Individual sea stars moving across the sediment could disturb ~1.35 m2 day – 1 and the sea star population at OAC could disturb ~75 m2 and ~38 seeds, 100% of recorded seeds, per day. Last, heart urchins observed in Shark Bay could disturb ~0.4 m2 per day at their highest densities recorded in this study, however, at higher observed densities of 2-4 urchins m−2 (J. Statton, personal communication), urchins could disturb ~3 m2 per day. If 50 m2 restoration plots at OAC in Cockburn Sound or UL1 in Shark Bay were broadcast with 100 seeds m-2, sand dollars could disturb 600 – 1600 seeds daily of the seeds in the 50m2 plot), sea stars could disturb 7500 seeds daily (100% of the seeds in the 50 m2 plot), and heart urchins could disturb 40-300 seeds daily (0.8 – 6% of the seeds in the 50 m2 plot). Field Bioturbator Experiments – Recently Settled Seed The three species of bioturbators exhibited different movement characteristics, affecting how they disturbed seeds on the surface versus seeds buried within the sediment. All three species of sediment bioturbators examined were capable of dislodging recently settled seeds in both Cockburn Sound and Shark Bay. The odds of a seed being disturbed were significantly higher in lanes with fauna present; sand dollars (odds ratio: 50.4, 95% CI: 8.8-290, p < 0.001),
The relative impact of incidental bioturbator disturbance will thus likely be larger in the short term for seeds without a dormancy period than for dormant seeds which may survive several disturbance events and for whom survival will be dictated over a longer period of time. Seeds with dormancy may, however, encounter additional predation pressure over the length of their dormancy period that also reduces the number of viable seeds in the seed bank and leads to lower seedling establishment rates. But these studied bioturbators are a small subset of the global and diverse bioturbating community that are capable of disturbing seeds and/or seedlings. Larger bioturbators may also disturb the sediment when they forage for infauna. The green crab (Carcinus maenas) disrupted and uprooted Z. marina transplants in New England, USA (Davis et al. 1998). The portunid crab Callinectes sapidus is a known bioturbator of Zostera marina meadows in the Atlantic (Wilson et al. 1990) and a relative, Portunus armatus, is a common bioturbator in Western Australia and was observed disturbing bare sediment during this study (Figure S4). Likewise, elasmobranches are known to dig into both bare and vegetated patches in search of food and generate large escarpments in the sediment (Orth 1975, Townsend & Fonseca 1998). Previous studies indicate these elasmobranch bioturbators may (Orth 1975, Backman 1984, Fonseca et al. 1994, Inglis 2000) or may not (Valentine et al. 1994) be able to disturb adult clones of seagrass species, but these studies have not incorporated seeds and seedlings which would be more susceptible to damage and loss from elasmobranch foraging. The widespread and diverse nature of bioturbators suggests these communities may play a more substantial role in seagrass seedling recruitment than currently acknowledged.