The drivers and bottlenecks of sexual reproduction in seagrasses are a crucial element in their conservation and restoration, determining resilience over ecological and evolutionary timescales. We collected flowering, seed production, and seedling establishment data for the seagrass Posidonia australis annually between 2013-2018 in meadows at six locations around Rottnest Island, Western Australia. We present data on inflorescence and vegetative shoot density, reproductive effort, flower and seed density, seed to ovule ratio, seed predation, and seedling survival. We found variable annual rates of flowering and seed production among meadows and between years. Some meadows, however, flowered more intensely and produced more seeds across the years of the survey. Inter-site and inter-annual variation in seed production, the stochastic nature of weather during seed release, and the large, but variable, impact of seed predation are likely the principle drivers of successful recruitment into established meadows and in colonising unvegetated sands. We propose that for the long-lived and persistent P. australis, variable annual reproductive investment increases the probability of low levels of continuous recruitment from seed in this seagrass, despite high rates of abiotic and biotic disturbance at seedling, shoot and patch scales.

Shoot density, inflorescence density, and seed production

Investment in sexual reproduction, vegetative shoot, inflorescences, flower and fruit (seed) densities were determined within P. australis meadows among the six locations before fruit release in November between 2013 and 2018. Vegetative shoot density in seagrass meadows was assessed using 10 x 0.04 m2 randomly sampled quadrats. Inflorescence density was assessed from five replicate 10 m x 1 m (10 m2) belt transects. Flower and fruit (seed) production per inflorescence were determined from the random collection of 12 inflorescences from transects at each site. Inflorescences consisted of a stem (petiole) bearing several spikes (3 - 12) with 3 - 5 hermaphrodite flowers on each spike, which ripen and release pollen in late Austral winter, July to August. Following successful pollination, fruits develop for approximately 12 weeks. For each inflorescence, the number of fully developed fruit, undeveloped fruit and remains of flowers that had not been pollinated were counted on each spike. Total number of flowers per inflorescence were derived from the sum of all fruit, undeveloped fruit and remains of flowers. Seed to ovule ratio was determined from the total number of mature fruit (1 fruit = 1 seed) divided by the total numbers of flowers (1 flower = 1 ovule).

Seed predation

Seed predation was assessed over two distinct time periods between 2001 and 2016 (2001, 2003, 2004, and then again in 2013, 2014, 2016) at Parker Point, Rottnest Island, to determine whether predators may limit seed survival. Details on the tether or seed board methods can be found in [11] and [20], respectively. Ten tethered seeds were placed in a continuous meadow of P. australis, approximately 2 m apart and one to two meters from the sand edge, as well as in sand also 1 - 2 m from the seagrass meadow. Seed boards were used at three sites in 2013 and 2014 spaced approximately 20 m apart. At each of the three sites, three seed boards were placed 3 m from each other at 1 - 2 m from the sand-seagrass edge in each habitat. Seeds were assessed for signs of predation every 24 hrs for 7 days and replaced if missing or partially eaten. The experiment was conducted for only three days in 2016 due to weather and logistical constraints. 

Recruitment of seedlings

In order to quantify seedling recruitment and survivorship at Rottnest Island, we surveyed sparsely vegetated, 300 m-2 plots adjacent to well-established P. australis meadows to track the emergence and survival of recruiting seedlings through time. We counted and photo-tagged all seedlings at Parker Point, Stark Bay and Thompson Bay in November 2014 and marked their position with a Garmin GPS with a position accuracy of approximately 50 cm (Garmin Corp. Models GPSmap78 and etrex20). We also counted the number of shoots for each seedling to determine shoot and age-based recruitment. We returned the following March, 2015, just prior to winter storms, known to cause mortality in recruiting seedlings [6], to recount and measure all seedlings, including the new four month old 2014 cohort.  We repeated this process in November 2015 – 2018, and March 2016 - 2018.