Draft - WAMSI 2 - Dredging Node - 5.2 - Defining thresholds and indicators of primary producers - Field Study 2: Mechanisms of seagrass recovery following disturbance

Brief description

The objectives of Project 5.2 Field Study 2 are to determine whether recovery following disturbance is by sexual or asexual means and the relative importance of each, hence to determine the capacity, timeframes and mechanisms of recovery from light and sediment deposition effects.

Lineage

Statement: For each seagrass species, a single plot-based, clearance and recovery experiment will be conducted to determine whether recovery following disturbance is by sexual or asexual means and the relative importance of each. The experiment will not parameterize the rate of recovery following dredging disturbance - that will be done in the laboratory experiments. Instead, these will confirm the relative importance of regrowth from existing plants (e.g. new shoot production or invasion by adjacent rhizomes), recovery from new propagules and recovery from the seed bank. This will provide context for interpretation of the laboratory experiment results and will also provide a fundamental insight into the recovery of these species in natura! settings following disturbance. The experiment will be conducted at a single site for each species. The design will be based on that of similar studies undertaken by Rasheed and co-workers (e.g. Rasheed 2004) in northern QLD. At the site, 12 plots will be completely cleared of all vegetative seagrass material. To determine how recolonisation is influenced by asexual reproduction, six of the cleared plots will have plastic borders sunk into the sediments to isolated the plot treatments from asexual colonisation through rhizome extension from surrounding meadow. The other six cleared plots will have not border and 6 additional uncleared plots will be used as controls. To investigate if recolonisation is influenced by the availability of sexual propagules, recovery of seagrass will be compared among plots with and without intact seed banks. Seeds will be removed in one of two ways, subject to the outcomes of pilot studies. The first methods will remove seeds form 12 plots by sieving the sediment through a <0.2 mm sieve. The seeds will be retained and the sediments returned to the plots. A control for the disturbance caused by sieving will be established by returning the sieved seeds back to six of the sieved plots. In the event that this method proves too difficult given the volume of sediment involved, a second approach may be used. The sediment will be removed from de-seeded plots and replaced with sediment with similar characteristics but clean of seeds. The dean sediment will be collecting from a location with no seagrasses nearby. Sub-samples of the clean sediments will be used to check that there are no seeds using a density separation technique with Ludox 40% and centrifugation. If no seeds are detected then this sediment is then added to the plots. To ensure that the sediment can support seagrass growth (controls). we could trial some plots with the same technique that allows seagrass to grow on the sediment (i.e. remove barriers and/or add seeds at a similar density found at the site). Shoot density will be monitored periodically over 6-9 months as a non-destructive measure of regrowth into the experimental plots.