Encroachment of Seagrass Meadow Leads to Home Loss

Author: Anita Pandey, Edited By: Emilyann Ashford

A Chronicle of Seagrass Meadow

Seagrass meadows are like any other meadow, filled with green flowering plants, but you won’t see us picnicking or picking daisies and dandelions, precisely because it is under water. These underwater meadows serve as a home base for fish, sea stars, manatees, and a host of other aquatic creatures that frolic there. Or this seemed to be the case in the past, until rise in global sea temperature associated with increased anthropogenic CO2 emission (1,3), rise in sea level (4), and natural disasters (2) began to hinder seagrass growth, leading to reduction of the meadow area. 

Figure 1: A School of Fish in the Seagrass meadow in clear water due to light irradiance. Credit:Shane Gross

Figure 1: A School of Fish in the Seagrass meadow in clear water due to light irradiance. Credit:Shane Gross

What are the ecological roles of the Seagrass?

On top of serving as home base for aquatic creatures, the seagrass trap runoff sediments and filter suspended sediments (3). If any aforementioned environmental stresses arose on seagrass, they would not be able to keep up with cellular energy demands––photosynthesis, respiration and synthesis of necessary enzymes to repair wear and tear.  Therefore, seagrass functioning to clarify water may become secondary to keeping themselves alive under extreme environments, such as a rise in seawater temperature.  

How has increased temperature affected the Seagrass meadow?  

Seagrass species function best at their optimal temperature, but they have been known to tolerate temperatures above and below this optimum (4). Unfortunately, even though seagrass can survive a range of temperatures, its growth has declined (5) which has been attributed to rises in sea temperature. For instance, Zostera marina (eelgrass) showed a decrease in photosynthesis-to-respiration rate (P:R) in high temperature conditions (4). Z. marina thrive in cooler months, but during hotter months, their photosynthetic and respiratory ability decline. When this species is already near its thermal tolerance, a slight increase in temperature affects its seed germination which in turn can impact its abundance and distribution (4). In extreme cases, Z. marina has been reported to die out in the hotter summer (4). This decline in seagrass species leads to gradual decrease in the overall seagrass meadow area, which has been observed since 1930 and continues till the present time (5). This reduction in P:R ratio in seagrass leaves them vulnerable to diseases and predation in the face of rising temperatures that could lead to decline in seagrass meadow (5). Furthermore, regression of seagrass meadow area has domino effects on marine ecosystems because algal species attach themselves on seagrass to grow. This algal growth on seagrass limits light and carbon availability consequently affecting seagrass’s growth (4). Additionally, algae are known to bloom uncontrollably, so seagrass are easily outcompeted when their environment becomes unfavorable to them.

How has Sea level rise impacted the Seagrass meadow?

One cause of sea level rise is melting of polar ice caps, increasing in an oceanic elevation and consequently increasing height of tides, depth, and salinity distribution, all of which can affect the seagrass population (4). The tides specifically can damage seagrass beds and increased depth reduces the light availability, in turn affecting the productivity of these seagrasses (4). Other factors such as increased algal growth and water turbidity limits light penetration to the bottom (3). Therefore, water turbidity caused by suspension of sediments has a loop relationship (i.e., turbidity can reduce seagrass growth while little seagrass means reduced filtering capacity). Thus turbid waters may lead to a decrease in the meadow area, which means loss of home and food sources for aquatic creatures, herbivores and carnivores alike as the food chain is so interwoven. Impacts of sea level rise are not as immediately conspicuous as that of natural disaster, which is occurring more frequently than in the past due to global warming (9).

How have Natural Disasters impacted the Seagrass meadow?

Natural Disasters, such as cyclones, can wipe out the seagrass since some of the species of seagrass live in the shallow water. Cyclone occurrence has been predicted to increase as the effect of global warming, and can result in fragmentation of the seagrass meadow, if it does not uproot the seagrass and pile them on the shoreline (2).  Because seagrass areas support fish, providing nutrients and shelter, this large scale and rapid disturbance can impact the economy of coastal regions in developing countries when the livelihood of people is dependent upon commercial fishing (2). One study reported a greater decline in fish species density which was linked to seagrass decline after a cyclone compared to before cyclone (2). Furthermore, these impacts are not only under seas, they manifest in the economy, causing not only our underwater environment to be affected, but also our social human environment (2). In a way, people also depend on seagrass for their livelihood because they fish where fish are, at the seagrass meadow. Unfortunately the anthropogenic dependence can lead to unintentional hinderance on the seagrass growth as well, from destruction by boat propellers and/or anchor chains. 

Are humans helping or agitating the Seagrass further?

Figure 2: An anchored boat and fishing net. Fishing nets and anchors can slash seagrass and wound them, but the minor wounds can heal as long as they make food from the sunlight. Credit: Illustration

Figure 2: An anchored boat and fishing net. Fishing nets and anchors can slash seagrass and wound them, but the minor wounds can heal as long as they make food from the sunlight. Credit: Illustration

Seagrass meadow areas decline due to anthropogenic development––such as fish farming facilities, artificial beaches, ports, and discharge of chemical wastes in coastal regions (1). For example, reclamation of land for ports can lead to increase in turbidity which in the long run can decrease seagrass growth rate as they require light for photosynthesis. Another factor for turbidity is the increase in inorganic and organic particles, which coupled with competition for light with algal species, causes further degradation of seagrass growth (1). Boats’ propellers are known to damage seagrass but if they are healthy, they can repair minor wear and tears with a little extra expenditure of energies than the usual. On the other hand, if these seagrasses are in an unfavorable environment, even minor wear and tear can have physically high impact.

What can be done?

Restoration of seagrass is possible. Regression of seagrass meadow has been declining since the 1970s due to better sewage management strategies (1). Once the sewage treatment is placed, the seagrass begins to recover (1). As many do the trees, seagrass can be planted since they play an important role in the aquatic ecosystem that extends to a nation’s economy. Florida Fish and Wildlife Conservation Commission has active projects for restoring seagrass. One of the techniques they use is micropropagation––sterilely cloning seagrass in the laboratory because it can be made in stocks, then planting one at a time by hand (6). The benefit of restoration of seagrass is not only home for fish, but it can also sequestrate carbon because according to the Ocean Foundation, seagrass take up 11% organic carbon while they only use 0.1% of the seafloor (7) when just the Mediterranean sea is 0.8% ocean area (8) and Earth is ~70 % water. For the small areas the seagrass occupies, they do a lot of work to maintain marine ecosystems and sink carbon, so it seems fair to maintain the seagrass population and remedy regression of seagrass meadows.

Citations

  1. Boudouresque, C. F., Bernard, G., Pergent, G., Shili, A., & Verlaque, M. (2009). Regression of Mediterranean seagrasses caused by natural processes and anthropogenic disturbances and stress: a critical review. Botanica Marina, 52(5). doi: 10.1515/bot.2009.057

  2. Côté-Laurin, M.-C., Benbow, S., & Erzini, K. (2017). The short-term impacts of a cyclone on seagrass communities in Southwest Madagascar. Continental Shelf Research, 138, 132–141. doi: 10.1016/j.csr.2017.03.005

  3. IPCC. (2012). Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. Retrieved from https://www.ipcc.ch/report/managing-the-risks-of-extreme-events-and-disasters-to-advance-climate-change-adaptation/

  4. National Geophysical Data Center. (2009, March 27). Volumes of the World's Oceans from ETOPO1. Retrieved from https://www.ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html

  5. Pergent, G., Bazairi, H., Bianchi, C. N., Boudouresque, C. F., Buia, M. C., Calvo, S., … Verlaque, M. (2014). Climate change and Mediterranean seagrass meadows: a synopsis for environmental managers. Mediterranean Marine Science, 15(2), 462. doi: 10.12681/mms.621

  6. Seagrass Restoration. (n.d.). Retrieved from https://myfwc.com/research/habitat/seagrasses/projects/active/restoration/

  7. Seagrass. (2019, August 21). Retrieved from https://oceanfdn.org/seagrass/

  8. Short, F. T., & Neckles, H. A. (1999). The effects of global climate change on seagrasses. Aquatic Botany, 63(3-4), 169–196. doi: 10.1016/s0304-3770(98)00117-x

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

Emilyann Autumn