Seagrass is Sacred

Seagrass is a much vaunted and universally protected marine habitat, but should all seagrasses and seagrass meadows be protected at all costs?

Seagrasses evolved from land plants while the dinosaurs were still running the show on land¹. In the 1930s a disease outbreak killed approximately 90% of the seagrass Zostera marina in Europe and North America causing major changes in coastal ecosystems².  This loss and subsequent ecosystem changes demonstrated the important ecosystem services provided by seagrasses such as nursery habitats for fish, crabs and prawns, high primary productivity supporting grazers and detritivores, as well as coastal protection through their interaction with overlying waters lessening wave energy and thereby reducing coastal erosion³. More recently, their role in carbon sequestration has become a major focus as we continue to hope that absorption of carbon dioxide by plants will offset the continually growing amount released by human activities⁴.

Recent anecdotal reports from Bay custodians indicate concern about a loss of seagrasses in some areas. Certainly, my observations using the aerial imagery resource from Nearmap Pty Ltd validates such concerns (Fig. 1).

Sand encroachment is quenching probably important subtidal seagrass beds, and some process seems to be causing loss of intertidal processes [report to come if a recently submitted paper passes peer review]. Options to thwart these sand waves include placing shellfish reef structures to slow their progress and protect remaining habitat. Work needs to be done to determine whether this would work.

Regardless of this emerging challenge and potential solution, there is great interest in restoring shellfish reefs elsewhere in the Bay (see shellfish reef restoration). But where to put them?  Given the iconic nature of seagrasses, one might think “Not on them!”, but where else?

Shellfish reefs tend to do poorly if they are placed subtidally (R. Porter pers. comm.). Mangroves, another marine icon, live too high on the shore to be realistically and effectively substituted for shellfish reefs habitat, so that leaves us with what? …. mud?

Mud, sand, muddy sand, and sandy mud are all terms we use to describe such habitats. To the causal observer they are unvegetated and often appear relatively barren, but a closer look [and, I mean down on your knees, staring intently through a magnifying glass] reveals an abundance of small critters. Indeed, some major fisheries might depend on these seemingly bare habitats⁵.

So, in my mind, the matter of seagrass being exclusively sacred is untenable as seagrasses differ in the ecosystem services they provide³. I am not suggesting that we only situate shellfish restoration structures on seagrass beds, but the Bay’s seagrass beds and unvegetated habitats now lie where once lay extensive oyster reefs, and sedimentation in the Bay has changed many of its former habitats beyond recognition. At some point they must be considered fair game.

My point is that we cannot hold any habitat scared and protect it all costs in an ever-changing Bay. Nuanced discussions are urgently needed if we are to arrive at informed decisions regarding Quandamooka. Seagrasses must be in the mix.

Ian Tibbetts (Director The Moreton Bay Foundation)

References

1. Larkum AWD, Waycott M, Conran JG (2018) Evolution and biogeography of seagrasses. In: Larkum A, Kendrick G, Ralph P. (eds) Seagrasses of Australia. Springer, Cham. https://doi.org/10.1007/978-3-319-71354-0_1

2. Muehlstein LK (1989) Perspectives on the wasting disease of Zostera marina. Diseases of Aquatic Organisms 7, 211-221

3. Nordlund LM, Koch EW, Barbier EB, Creed JC (2016) Seagrass ecosystem services and their variability across genera and geographical regions. PLoS ONE 11: e0163091. https://doi.org/10.1371/journal.pone.0163091

4. Lovelock CE, Reef R (2020) Variable impacts of climate change on blue carbon. One Earth 3, 195-211. https://doi.org/10.1016/j.oneear.2020.07.010

5. Giaroli ML, Chargulaf CA, Gilby BL, Tibbetts IR. (2023) Tidal migrations of juvenile Sillago spp. in a subtropical intertidal nursery seascape. Marine and Freshwater Research 74, 1193-1210. https://doi.org/10.1071/MF23033

6. Moore T (2022). Moreton Bay copped at least five times more mud, silt than during 2011. Brisbane Times (March 15).

Fig 1. Changes in the extent of intertidal seagrass (light olive) and subtidal seagrass (dark green) and the advance of sand waves south of Amity, Minjerribah. Nearmap aerial imagery sourced using UQ license. Southern end of Amity Camping Park visible at top right for scale. North at top.