One of my favorite Megadeth albums and songs, although that’s for another time. The title also fits for a handful of species from a group particularly close to my heart – seagrasses. Seagrasses made the news recently, as a recent report suggest as many as 10 of the 72 known seagrass species are at risk of extinction. And since this blog is named in honor of seagrass, I thought I might delve in, as I have made many seagrass related posts in the past.
Global Seagrass Distribution - from FLMNH
I probably don’t have to tell those of you who follow my blog how important seagrasses are for coastal ecosystems. Seagrasses can be found in all oceans around the globe – from the tropics and temperate zones into the Arctic (see Dr Fred Short’s website for more info and a number of useful powerpoint slides). Everywhere they occur, they provide a variety of ecosystem services – from oxygenation of the water and sediments, stabilization of the sediments, dampening of flow, help to maintain water quality, provide nursery and spawning grounds for numerous species, and are a direct food resource for a number of species. Most of my attention has been on the habitat value of seagrasses – important for a number of commercially important finfish, such as cod, and shellfish, such as my study organism the bay scallop. And much of the literature has focused on these aspects as well.
Causes for Seagrass Decline - From Orth et al 2006
However, despite the overall importance of seagrasses, they are continuing to decline on a global scale, mainly due to anthropogenic causes. Worldwide, seagrasses have declined by at least 30%, a rather alarming figure. Seagrasses typically grow close to shore – so these areas are dynamic to begin with. However, these near coastal zones are typically sites of development and use. While there are natural threats to seagrasses – overgrazing, storm events, disease – the majority of seagrass loss comes at the hands of humans either directly or indirectly. Some direct sources of loss include reckless boating, destructive fishing practices (such as trawling, dredging, raking and tonging), shoreline hardening, docks, and channel dredging. However, perhaps more important are indirect – nutrient overloading and eutrophication shifts ecosystems to water column production, increasing phytoplankton which in turn shades out seagrass. Likewise, overharvesting of filter feeding bivalves has reduced filtration capacity of basins, leading similarly to high phytoplankton and low light. Additionally, climate change, and the resultant increase in water temperature, can cause severe stress in many seagrass species.
Consequences of eutrophication on seagrass
Because seagrasses are so important, it has been the focus of our lab’s research. While I have focused much of my research on the impact of changing eelgrass landscape, as well as alternative habitats, on bay scallops, my first ever research project was investigating the ability of hard clams to facilitate eelgrass growth in a light-stressed environment. In a series of experiments, we tested the degree in which light and nutrients limit eelgrass productivity along an estuarine gradient and investigated the ability of hard clams to facilitate eelgrass growth in ambient and light limiting conditions. Not surprisingly, the farther away from the ocean inlet we went, the higher the phytoplankton and the lower the light reaching the bottom. This lead to a decrease in eelgrass productivity along this estuarine gradient. However, using an in situ shading and nutrient addition experiment, we were able to show hard clam facilitation of seagrass growth, even under light stress. However, since the grass was artificially shaded, the mechanism for increasing seagrass productivity in the shade treatments was via the clams ability to increase the sediment nutrient pool, which was reflected in the seagrass tissues. We published this work in MEPS.
Hard clams ability to alleviate eelgrass stress
(You can see the rest of the slideshow JCarroll- CommunityEcology-Apr12-845am)
However, many other lab members have also done seagrass research. In fact, we like to call ourselves the Peterson Seagrass Rangers. Current student Amber Stubler, who you may remember did a guest blog post on Chronicles, conducted numerous field and lab experiments investigating multiple stressors on eelgrass growth – light, temperature and sediment sulfide concentration – before she started in her current sponge work. Another current student, Brad Furman, is investigating landscape properties of eelgrass meadows and patch persistence over time. Former student Brooke Rodgers investigated harmful herbicides in groundwater on eelgrass growth and survival. Finally, former student Jamie Brisbin conducted a thorough investigation of eelgrass population genetics of Long Island waters (don’t ask me to give more detail, because I don’t understand genetics all that well). So our collective lab is essentially investigating many reasons for and consequences of decline of seagrasses around Long Island, NY.
That’s why this latest manuscript was so interesting. The Biological Conservation manuscript by Fred Short and others, published online, includes a veritable who’s – who of world seagrass experts. Because seagrasses are so important, numerous studies have investigated their decline worldwide. This new article looks at these studies and determines the probability of extinction using guidelines from the International Union for the Conservation of Nature (IUCN) Using criteria from the Red List of Threatened Species, the authors determined that 10 of 72 seagrass species – 14% – are at risk of extinction.
Method for determining risk from Short et al
Taking a database of existing seagrass literature, the authors used 2 main criteria for determining extinction risk and vulnerability. Criteria A examines population decline over time and Criteria B is based on geographic range. Out of the 72 species investigated, 3 could be listed as endangered already. Seven more species were listed as vulnerable due to increased declines in populations. An additional 5 species were listed as Near Threatened by the two criteria. On the other hand, 48 species were listed as Least Concern, meaning that currently, they are not likely to go extinct. That doesn’t tell the whole story, however, as most of the Least Concern species are wide-ranging. So despite being of Least Concern, many of these species are locally threatened, as declines of many of these species are occurring at a local scale.
There also appears to be global patterns in threatened and declining seagrass species. 22% of the seagrass species found in the temperate North Pacific are threatened, with an additional 17% near threatened – with up to 100% of seagrass species in some areas of this region experiencing decline. The tropical Indo-Pacific has 11% of its species threatened. In contrast, none of the 5 species of seagrass found in the temperate North Atlantic have received a threatened status – but again, this may be misleading. Despite no species listed as threatened, on the local level, many of these species are disappearing.
Distribution and decline of seagrasses - Short et al
The manuscript goes on to name some of the threats to seagrass species – many of which I have already listed. But in order to create more awareness, I will name more here. They found that the most common threat to seagrass is humans – 67 species are affected by some anthropogenic impacts. Again, and I can’t make this point more clear, the MOST COMMON THREAT TO SEAGRASS IS HUMANS. Among them, decreases in water clarity and quality due to nutrient overloading – leading to phytoplankton and nuisance algal blooms – and sediment loading – increasing suspended sediments and siltation. Beds are also destroyed by coastal construction, shoreline hardening, and dredging, destructive fishing practices, and mechanical damage due to boats. Invasive species also pose threats to seagrasses. Oh yeah, and climate change, whose potential impact is just starting to be understood.
So what are the recommendations? After all, seagrasses are vital components of coastal ecosystems, and need to be saved, if possible. Poor water quality/clarity is the major threat. Ameliorating this water quality issue is the best way to decrease stress on seagrass and hopefully reverse the trend. Unfortunately, it is likely easier said than done. Efforts to reduce run-off, nutrient and sediment loading need to be undertaken to help increase water quality. In developed countries, we can combat this by stricter rules on fertilizer use and coastal development, as well as increasing the use of tertiary treatment at sewage treatment plants and restoring filter feeding species. In developing nations, this can be done via education and encouragement of eco-tourism as a revenue source, let them know keeping their environments pristine is a great way to generate money for the economy. Additional conservation and restoration efforts are necessary, like those on Long Island and Virginia.
Scott Marion, seagrass researcher at VIMS
Hopefully, we can save seagrasses from the countdown to extinction.
Short, FT, Polidoro, B, Livingstone, SR, Carpenter, KE, Bandeira, S, Bujang, JS, Calumpong, HP, Carruthers, TJB, Coles, RG, Dennison, WC, Erftemeijer, PLA, Fortes, MD, Freemen, AS, Jagtap, TG, Kamal, AHM, Kendrick, GA, Kenworthy, WJ, Nafie, YAL, Nasution, IM, Orth, RJ, Prathep, A, Sanciangco, JC, van Tussenbroek, B, Vegara, SG, Waycott, M, & Zeiman, JC (2011). Extinction risk assessment of the world’s seagrass species Biological Conservation : 10.1016/j.biocon.2011.04.010
Carroll, J, Gobler, CJ, & Peterson,BJ (2008). Resource-restricted growth of eelgrass in New York estuaries: light limitation, and alleviation of nutrient stress by hard clams Marine Ecology Progress Series DOI: 10.3354/meps07593
ORTH, R., CARRUTHERS, T., DENNISON, W., DUARTE, C., FOURQUREAN, J., HECK, K., HUGHES, A., KENDRICK, G., KENWORTHY, W., OLYARNIK, S., SHORT, F., WAYCOTT, M., & WILLIAMS, S. (2006). A Global Crisis for Seagrass Ecosystems BioScience, 56 (12) DOI: 10.1641/0006-3568(2006)56[987:AGCFSE]2.0.CO;2
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