Our last venture to a die back site began at one site located in the Central Marls. Here, we sampled leaves for grazing patterns, collected any insects we could find, and took salinity measurements (see here and here for our previous findings). Recently, we had the opportunity to view another die-off site located in the Southern Marls. These sites were very different in terms of the scale of the die back, namely that the Central Marls site was on a much greater scale.
Some observations at the Marls (Central vs. Southern) sites:
- At the Southern site, there was still a great deal of activity (i.e. fiddler crabs were present and not all trees were barren) along the die-off zone. At the Central site, the death was overwhelming and the only organism commonly found was the small snail, Batillaria.
- At the Southern site, there was a clear delineation where the die-off ended, and where green dwarf Red and Black Mangroves persisted. At the Central site, there was no distinct line to distinguish where the die-off ended.
- Unlike the Central site, the Southern site had a slight elevation to it, which enabled more water flow between the mangroves. At the Central sight, there was no visible elevation change; the water was stagnant and extremely hot.
- At both sites, many organisms inhabited the dead prop roots (i.e. amphipods and grubs), however, they are not likely the culprits for the die-off itself. Rather, they are detritovores taking advantage of the decaying root materials.
- Other coastal plant species (i.e. Buttonwoods, salt grass and glasswort) were present in higher densities at the Central site, which had greater dieback than the Southern site.
- The pore water salinities (~15cm below surface) at the sites were very different. At the Central site, we measured readings as high as 65 ppt (parts per thousand) while the Southern site had readings around 45 ppt.
At the Central site, it seems as though the die-off is a result of some abiotic factor, such as salinity. One of our current hypotheses stems from the major difference in elevation and water flow we noted between these sites. It could be possible that a change in sedimentation lead to a change in the tidal flow across the mangrove plain at the Central site, thus causing the stagnant water to build up there. The build up of stagnant water may have resulted in increased salinity values as fresh water evaporates and salt is then deposited onto the sediment. However, the Southern site may have a different story. The fact that fairly low salinities, fiddler crabs and “healthy” dwarf mangroves were present at the Southern site may make this particular die-off story more complex. We are continuing to examine the grazing patterns on the leaves, branches and roots in addition to factors such as salinity, tidal fluctuation and sedimentation at both sites.
If you’ve found this interesting, see here for more knowledge regarding mangrove forests in general.