David Rivers
David Rivers is studying seedling recruitment (regrowth) patterns in a marine region where there are several seagrass species.
A video link of his findings will appear on this site soon.
Many recruitment studies measure seedling settlement and survival rates to determine recruitment success. David is going a step further by examining the environmental factors that affect recruitment.
The pattern and success rate of seedling recruitment varies but is essential for population maintenance and recovery from disturbance for most species. David's results will provide an overall framework of the role of seedling recruitment in south-west Western Australian seagrass meadows.
Findings to date
1.) Settlement and survival of Amphibolis seedlings
Amphibolis antarctica seedlings were observed to recruit to three habitat types: seagrass meadow, seagrass matte and sand. A seedling transplantation experiment examined whether these habitats conferred a survival advantage to seedlings. Seedlings were planted at two densities onto each habitat and survival rates were compared over a two month period. Seagrass matte and seagrass meadow were identified as favourable habitats for short-term seedling survival. Seedlings transplanted to sand were dislodged within two days or alternatively, were completely buried. Planting density had no effect on the total numbers of survivors, suggesting that A. antarctica is not limited by the number of seedlings produced. The experiment showed that A. antarctica recruitment was limited by their ability of seedlings to disperse to suitable, unoccupied habitats.
2.) Seedling recruitment into gaps
Sandy gaps caused by physical disturbance are a common feature in WA seagrass meadows. The role of seedling recruitment in the infilling of these gaps was examined in a field mapping study. Maps of three gaps were created and the location and densities of seedling recruits was compiled over 11 months.
Visual assessments found that the Amphibolis species Posidonia seedlings had clustered settlement patterns falling within a metre of the gap perimeter. Amphibolis seedlings settled mostly in seagrass meadow and seagrass matte substrates, but survived best on seagrass matte. This may be due to a special adaptation of Amphibolis seedlings which have a modified, hardened petal with comb-like teeth at the base. This ‘comb anchor’ may entangle with the fibrous seagrass matte, facilitating recruitment. Posidonia seedlings settled on both sand and seagrass meadow substrates but survived longest in sand.
The spatial clustering of seedling settlement may be explained by preferential settlement onto different substrates. Alternatively, eddies caused by waves passing over the gap may cause seedlings to settle closer to the gap edge.
Unlike recruitment, seedling loss for all species does not have a distinct spatial pattern. The primary cause of seedling loss is dislodgement but seedlings survive best in areas with reduced exposure to physical disturbance.
Conference papers presented
1. School of Plant Biology Rottnest student conference (February 2008):
The role of seedling recruitment in maintaining seagrass populations
2. Western Australian Marine Science Institution (WAMSI) Symposium, CSIRO Floreat (February 2008): The role of seedling recruitment in maintaining seagrass populations
3. Australian Marine Sciences Association (AMSA), WA Division, student conference Rottnest Island (July 2008): Seagrass recruitment into meadow gaps: spatial variation in seedling settlement and mortality
David Rivers is a PhD candidate at The University of Western Australia working on WAMSI Node 1 projects. His supervisor is Professor Di Walker.
He is pictured second from right with his sponsors (from left) Di Walker from UWA, Dr John Keesing from CSIRO and Dr Steve Blake from the Western Australian Marine Science Institution.
David presents a summary of his research on this link. An interview with David can be seen on this link.