Seagrass Ecology In A World Of Climate Change

Seagrasses are known for supporting an extremely biodiverse ecosystem, including the charming but endangered manatee. It is also the home of many fish used in commercial fishing. Therefore, when considering conservation of these species, it is very important to note that seagrass populations are declining overall worldwide. While factors such as physical mechanical damage, coastal development, and pollution no doubt play a role, climate change may be a new rising factor that is certainly worth more consideration.

Underwater oceanic plants are affected the most by extreme temperature increases, which corresponds to future predictions that seagrass populations will experience range shifts out of their normal distribution and begin traveling to higher latitudes, according to the IPCC Climate Change Report for 2018 (IPCC, 2018).

Figure 1. This diagram from the IPCC illustrates the effects of global average surface temperature change relative to pre-industrial levels. According to this graph, seagrasses face severe and widespread impacts stemming from temperature increases. To stop temperatures from rising so high, there must also be paid attention to other climate change related topics, such as atmospheric CO₂ concentration and radiative forcing.(IPCC, 2018)

It has been predicted that by the end of the twenty-first century, the increase of sea surface temperatures (SSTs) will cause poleward shifts of 888 km for seagrass populations in the southernmost part of their normal range, as they tend to already live on the edge of their thermal tolerance (Valle et al., 2014). However, unlike the animals living among seagrass, this plant itself is unable to physically move by itself and must rely on its seeds taking root in higher latitudes successfully. This could cause certain seagrass populations in this region to go locally extinct.

As an example, in Shark Bay, Australia, there was an extreme weather event of a marine heatwave that caused an estimated 22% reduction in the net baseline of the seagrass meadow, as well as changed the aboveground density from very dense, thick seagrass to mere sparse patches (Ortiz et al., 2018). In an area of seagrass that previously stored 45 ± 6 Mg C ha^-1, which is very similar to what terrestrial soil can store, it is estimated that 2-9 Tg CO₂ could have been released back into the atmosphere in the three years following the disturbance. One of the reasons why seagrass is so efficient at storing carbon is due to low deposition rates in oxygen-poor soils as well as the lack of disturbance that normal terrestrial soils experience, such as wildfires (Fourqurean et al., 2012). This release of carbon increases oceanic and atmospheric temperatures, which has significant implications for seagrass and the organisms relying on seagrass.

Figure 2. This diagram illustrates the effect various global mean surface temperature (GMST) would have on various extreme weather events, in this case being hottest days (which deals with heatwaves, terrestrial and marine), change in number of coldest nights, and chance in extreme precipitation. There is a scenario depicted for a change in 1.5℃ (which is what this IPCC report focuses on) and at 2.0℃. It can be seen that there is an increase in temperature for both hottest and coldest nights in both temperature scenarios, and for most of the world, there is an increase in extreme precipitation. (IPCC, 2018)

Overall, 115 marine species relying on seagrass for their survival have been evaluated by the International Union for Conservation of Nature (IUCN), in which 27% of them are in various threatened categories (Short et al., 2011).

This figure shows various taxonomic groups depending on seagrass and how they are categorized on the IUCN Red List. It can be seen that some species have already gone extinct and many are considered either vulnerable or threatened. What is important to remember is that as climate change continues to progress, these species will only experience higher percentages of endangered, critically endangered, and possibly extinct classifications of animals. (Hughes et al., 2008)

Ray-finned fishes, such as seahorses or pipefish, are the most affected taxonomic group, where 30% of all seahorses evaluated by the IUCN depend on seagrass for their survivorship. Nearly ½ of all fish species considered vulnerable to extinction by the American Fisheries Society depend on seagrass (Hughes et al., 2008). For example, there was a case of extreme seagrass decline in Western Bay, Victoria, Australia where 70% of the local seagrass was lost for unknown reasons (Jenkins et al., 1993). Loss of commercial fish species such as leatherjackets was correlated with the loss of seagrass.

In addition, manatees will be and already are affected by declines in seagrass. For example, there was a case in Florida where hypersaline oceanic conditions caused the algal species Resultor spp to persist in Indian River Lagoon, FL, causing a reduction to the light available to seagrass that resulted in a decline of over 50% (Edwards, 2013). This case is relative to climate change because increased SSTs increase the chance of harmful algal blooms, which creates negative consequences for species of seagrass. The effect of this seagrass decline was felt by the Florida manatee population, as the main component of its diet was seagrass.

This is not a climate change issue that can happen in the future- it is already happening. Seagrasses are a functional marine flowering plant that support ecosystems worth of animals, such as the Florida manatee, commercial fish such as the leatherjacket, various crustaceans, and even shore birds. Since 1980, seagrasses are disappearing at a rate of 110 km y^-1, and 29% of the known area for seagrass has already vanished since 1879 (Waycott et al., 2009). These loss rates are higher than those of coral cover and tropical forests. This means it is imperative to begin seagrass conservation as soon as possible. It is estimated that seagrass beds in the US contribute $215,000 per year to the economy based on the organisms they support, but it only costs $1,236 ha^-1 on average to restore them, which is not even 1% of what seagrasses contribute (McArthur & Boland, 2006). There are certain restoration replanting techniques, such as using rhizome fragments with shoots for replanting and planting over 100,000 shoots, that have the highest success rate (Katwijk, et al., 2015). With the sheer amount of animals that seagrass meadows support, it is no doubt they are a worthy site for conservation. By keeping in mind certain planting techniques for successful restoration of already deteriorated seagrass beds, it is possible to keep this habitat and the ecosystem it supports alive.

-Megan Martinez, Intern at Cape May Whale Watch and Research Center

Rutgers University

References

Edwards, H. H. (2013, October 2). Potential impacts of climate change on warmwater megafauna: the Florida manatee example (Trichechus manatus latirostris). Climatic Change, 121: 727-738. https://doi.org/10.1007/s10584-013-0921-2

Fourqurean, J. W., Duarte, C. M., Kennedy, H., Marbà, N., Holmer, M., Mateo, M. A., … Serrano, O. (2012, May 20). Seagrass ecosystems as a globally significant carbon stock. Nature Geoscience, 5: 505-509. https://doi.org/10.1038/ngeo1477

Hughes, A. R., Williams, S. L., Duarte, C. M., Heck Jr, K. L., Waycott, M. (2008, October 10). Associations of concern: declining seagrasses and threatened dependent species. Frontiers in Ecology and the environment, 7(5): 242-246. https://doi.org/10.1890/080041

IPCC. (2018). Global warming of 1.5°C. [V. Masson-Delmotte, P. Zhai, H. O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J. B. R. Matthews, Y. Chen, X. Zhou, M. I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, T. Waterfield (eds.)]. In Press.

Jenkins, G. P., Edgar, G. J., May H. M. A., Shaw, C. (1993). Ecological basis for parallel declines in seagrass habitat and catches of commercial fish in Western Port Bay, Victoria. Bureau of Resource Sciences Proceedings. University of Melbourne, Queenscliff, Australia.

Ortiz, A. A., Serrano, O., Masqué, P., Lavery, P. S., Mueller, U., Kendrick, G. A., … Duarte C. M. (2018, March 19). A marine heatwave drives massive loss from world’s largest seagrass carbon stocks. Nature Climate Change, 8: 338-344. https://doi.org/10.1038/s41558-018-0096-y

Short, F. T., Polidoro, B., Livingstone, S. R., Carpenter, K. E., Bandiera, S., Bujang, J. S., … Zieman, J. C. (2011 July). Extinction risk assessment of the world’s seagrass species. Biological Conservation, 144(7): 1961-1971. https://doi.org/10.1016/j.biocon.2011.04.010

Valle, M., Chust, G., del Campo, A., Wisz, M. S., Olsen, S. M., Garmendia, J. M., Borja, A. (2014 February). Projecting future distribution of the seagrass Zostera noltii under global warming and sea level rise. Biological conservation, 170: 74-85.

Waycott, M., Duarte, C. M., Carruthers, T. J. B., Orth, R. J., Dennison, W. C., Olyarnik, S., … Williams, S. L. (2009, July 28). Accelerating loss of seagrasses across the globe threatens coastal ecosystems. Proceedings of the National Academy of Sciences, 106(30): 12377-12381. https://doi.org/10.1073/pnas.0905620106

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