A species · Plant
Posidonia seagrass
The gulf floor's slow-growing meadow-builder — a true flowering plant, not a weed, that nurseries the gulf's seafood and buries carbon in the dark, and is easily lost to murky water and slow to win back.
- On the gradient
- The submerged floor of the gradient — rooted below the tideline, seaward of the samphire coast
- Rock
- Shallow carbonate-sand seabed of the St Vincent Basin
- Soil
- Fine carbonate sand bound by a spreading rhizome mat, turning anoxic and carbon-rich just below the surface
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What you'll take away
- Identify Posidonia as a true flowering plant rooted in sand, not seaweed or algae, and distinguish it from the wirier Amphibolis it commonly grows alongside.
- Explain the meadow's three jobs — fish-and-crustacean nursery, sand-stabiliser, blue-carbon store — and why each depends on clear water.
- Account for the scale of Adelaide's metropolitan seagrass loss since the 1930s, and explain why Posidonia, though the hardier of the gulf's two seagrasses, still recovers at a crawl.
Say the word “seagrass” and most people picture something limp and green, washed up on the tideline after a storm: a kind of glorified seaweed, not much to think about further. Posidonia is nothing of the sort. It is a true flowering plant, complete with roots and veins, descended from land plants that returned to the sea tens of millions of years ago and never gave up the habit of rooting in sand. Across the shallow, sheltered floor of Gulf St Vincent it does this on a grand scale, its strap-shaped leaves rising from a spreading mat of rhizome that creeps forward, node by node, at a pace better measured in decades than in seasons — which is precisely what makes a Posidonia meadow so easy to lose and so slow to win back (Tanner et al. 2014).
Under that unhurried canopy the meadow runs two businesses at once. Above the sediment it is a nursery, the shelter in which juvenile King George whiting, western king prawns and blue swimmer crabs grow large enough to risk the open gulf. Below it, in sediment gone airless and dark within centimetres of the surface, it is quietly burying carbon in root and rhizome debris that local microbes cannot get enough oxygen to break back down, banking it away for centuries — the same “blue carbon” trick worked, hectare for hectare, at a rate that can out-save a standing forest (Macreadie et al. 2017).
Of the gulf’s two dominant seagrasses, Posidonia is the tougher, markedly more tolerant of the nutrient and sediment loading a growing city washes into the sea than its wirier neighbour Amphibolis, the branching wire-weed it commonly shares the seabed with (Tanner et al. 2014). Tougher was not the same as invulnerable. Since the 1930s more than five thousand hectares of metropolitan seagrass — Posidonia among it — have vanished off the Adelaide coast, as decades of treated sewage and stormwater fed the algae and drifting weed that clouded the water and starved the meadow of the light it needs (Fox et al. 2007). And because a Posidonia rhizome advances at the pace it does, no amount of good intention can replant a lost meadow quickly; recovery lags decades behind the speed of the loss (Tanner et al. 2014).
So read a patch of bare, sun-bright sand on the gulf floor for what it more often than not is: not an empty seabed, but a felled forest, still trying, a few centimetres of rhizome at a time, to grow back.
Go deeper — the mechanism & the evidence
The mechanism behind Posidonia's double life is the same waterlogged chemistry throughout. Its rhizome mat sits in sediment so starved of oxygen that the microbes which would ordinarily respire dead leaf and root material back into carbon dioxide are throttled to a crawl, so the carbon simply stays, banked in the mud rather than breathed back to the sky — the same anoxic-burial trick documented at roughly 165 tonnes of organic carbon per hectare in the top metre of Australian tidal-marsh soils, adding on the order of half a tonne more each year (Macreadie et al. 2017), and shared by seagrass sediment under the same physiology. That slow rhizome growth cuts both ways: it is what lets an established meadow bank carbon for centuries, and what makes losses so hard to reverse, since Posidonia cannot resow itself onto bare sand the way an annual weed would (Tanner et al. 2014). Its relative tolerance of nutrient and turbidity loading, compared with the more sensitive wire-weed Amphibolis it commonly grows alongside, bought Gulf St Vincent's Posidonia meadows time — but not immunity, as the metropolitan loss figures show (Tanner et al. 2014; Fox et al. 2007).
Sources for this guide · 3
Every claim here is traceable. Each citation links to its entry in the bibliography.
- Tanner, J.E., Irving, A.D., Fernandes, M., Fotheringham, D., McArdle, A. & Murray-Jones, S. (2014). Seagrass rehabilitation off metropolitan Adelaide: a case study of loss, action, failure and success. Ecological Management & Restoration 15(2): 168–179. DOI: 10.1111/emr.12133. https://onlinelibrary.wiley.com/doi/10.1111/emr.12133 — Posidonia/Amphibolis composition, comparative resilience to nutrient and turbidity loading, and rehabilitation pace off metropolitan Adelaide.
- Fox, D.R., Batley, G.E., Blackburn, D., Bone, Y., Bryars, S., Cheshire, A., Collings, G., Ellis, D., Fairweather, P., Fallowfield, H., Harris, G., Henderson, B., Kämpf, J., Nayar, S., Pattiaratchi, C., Petrusevics, P., Townsend, M., Westphalen, G. & Wilkinson, J. (2007). Adelaide Coastal Waters Study, Final Report, Volume 1: Summary of Study Findings. CSIRO, Adelaide, for the SA Environment Protection Authority. https://www.epa.sa.gov.au/files/477350_acws_report.pdf — Scale of metropolitan Adelaide seagrass loss since the 1930s (Adelaide Coastal Waters Study).
- Macreadie, P.I., Ollivier, Q.R., Kelleway, J.J., Serrano, O., Carnell, P.E., Ewers Lewis, C.J. et al. (2017). Carbon sequestration by Australian tidal marshes. Scientific Reports 7: 44071. DOI: 10.1038/srep44071. https://doi.org/10.1038/srep44071 — Blue-carbon stock and accumulation rate in Australian tidal soils — the anoxic-burial mechanism Posidonia meadows share.
Concepts this teaches — follow a thread
The inverse estuary (the backwards sea)Blue carbon (the gulf's quiet climate work)
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Part of these pathways
Test yourself →
Under a Posidonia meadow the sediment turns airless and dark within centimetres of the surface. Why does that matter for the carbon locked up in the dead leaf, root and rhizome material piling up there?
The mechanism is the same anoxic-burial trick documented across Australian tidal-marsh soils: oxygen-starved sediment throttles the microbes that would ordinarily respire dead plant matter back into carbon dioxide, so the carbon simply stays in the mud rather than cycling back to the sky. That is measured at roughly 165 tonnes of organic carbon per hectare in the top metre of Australian tidal-marsh soils, with roughly half a tonne more banked each year (Macreadie et al. 2017) — the physiology a Posidonia meadow shares, hectare for hectare, with a standing tidal marsh.