MANGROVE DIE-BACK IN THE ABACO MARLS: THE FACTS
For some time now, there has been understandable concern about the increasing evidence of mangrove die-back in the Abaco Marls and elsewhere in Abaco waters. Scientific investigations are ongoing and you will find some of the survey results so far on the excellent Abaco Scientist interactive map HERE. You’ll find other relevant and authoritative mangrove material if you check out the BLOG menu of the website.
The ‘200 sq. miles’ in my map is debatable, depending what one includes. Other estimates are of 300 or even 400 sq. miles. Whichever, the Marls cover a massive area of mangroves, islets, flats, channels and wonderfully diverse wildlife. A large proportion of the many species – fish, birds, turtles etc – depend on the complex ecology of the mangroves for food, shelter and breeding. Depletion of the mangroves from whatever cause will have a direct effect on the creatures of the Marls.
Ryann Rossi, a PhD student with North Carolina State University, has been researching the worrying phenomenon of mangrove die-back in the Marls this summer. She has written an interesting and informative account (conveniently in the RH ‘Facts about…’ style) that was published in Abaco Scientist last week. The blue links will take you to the ABSCI site for further information on each topic. I’m grateful to Ryann and ABSCI for permission to use the material.
Five Things to Know About the Mangrove Die-back in The Marls (at this point, anyway)
1. This die-back appears to be the result of multiple stressors acting together. Think of it in the sense of our own body – when our immune system is down, we are often more susceptible to getting sick. The same thing is likely happening to the mangroves.
2. It appears as though a fungal disease may be taking advantage of already stressed mangroves and causing die-back. We did preliminary surveys across Abaco and found fungal lesions nearly everywhere. However, the fungus was present in different densities in different areas. In the die-back area nearly all the leaves remaining on trees have lesions. We think that this pathogen capitalized on the mangroves being weakened by other stressors such as hurricanes, which cause extensive leaf drop, change in the movement of water, change in sedimentation and erosion.
3. We are still working on identifying the pathogen associated with the lesions we’ve found. We are confident that it is a fungus and are currently growing fungal cultures in the lab to examine defining morphological characteristics in addition to using DNA sequencing to identify the culprit.
4. We have documented the presence of the Robust Bush Cricket (Tafalisca eleuthera) in the die-back areas as well as other areas with high densities of lesions. These crickets are documented to consume Red and White mangrove leaves. As such, we were concerned about their potential role in die-back. We set out a caging experiment to exclude the crickets from certain dwarf Red mangrove trees to see just how much grazing they may be doing in the die-back area. This experiment is ongoing.
5. The take home: there is likely more than one causal agent of the die-back in The Marls. Many factors govern mangrove productivity and functioning: nutrient availability, salinity, sedimentation rate, herbivory, and disease are just a few of the factors that contribute to overall mangrove function making it very difficult to pin point which factors may be driving the die-off. On the bright side, we are confident that we have a lead on the causes and we are working hard in the field and laboratory to fully understand what is going on in The Marls.
Mangrove Die Off on Abaco by Ryann Rossi, NCSU
This summer Stephanie Archer and I continued research efforts focused on determining the cause of the mangrove die-off in The Marls (work funded by the National Science Foundation). Our efforts were predominantly focused on the fungal pathogen we found associated with the die-off site. We created a small citizen science and outreach project to document the presence or absence of the pathogen across Abaco. This project consisted of short surveys and leaf collections. In total, 92 areas were surveyed including locations from Abaco and San Salvador. We also took this outreach project to the annual Bahamas Reef Environment Education Foundation (BREEF) teacher training conference. There we disseminated survey packets to teachers from islands throughout The Bahamas who will help us collect more data on the presence (or absence) of this pathogen on other Islands.
Three men on a skiff – Abaco Marls
On Abaco, we constructed an experiment to investigate the role of grazing and the presence of fungal lesions on Red mangroves. We simulated grazing using crafting scissors to cut small sections on 600 leaves in 4 different mangrove creeks. We observed the leaves for 28 days to determine if cutting leaves predisposed leaves to fungal infection. At these sites we also trapped for insects to gain an idea of what kind of grazers may be chewing on the leaves. We also did a series of disease incidence surveys that will be routinely monitored for disease progress over the next 2 years. These surveys will allow us to systematically track the progress of the disease. In addition to our field work, we spent many hours in the laboratory isolating fungi from leaves to grow in culture. These cultures were brought back to North Carolina State University and will be sequenced in order to help us identify the fungal pathogen responsible for making the lesions on the mangrove leaves.
Credits: Source material Ryann Rossi; Abaco Scientist; all photos © Keith Salvesen @ Rolling Harbour except those by Ryann / ABSCI in the main article and Melinda Riger’s cool shark
ROLLING HARBOUR ABACO 
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