Climate change will affect many aspects of coastal ecosystems, especially the immensely important marshes and mangrove swamps that fuel many coastal fisheries. However, whether climate change will result in a net gain or loss of these coastal ecosystems remains unclear. In the case of mangrove forests, most research on their future extent has focused on direct destruction from anthropogenic impacts such as development as opposed to the fate of mangroves under projected climactic conditions. However, recent research has begun to address how mangroves will fare under sea level rise, increased atmospheric CO2, and warmer temperatures. Unfortunately this is a rather complicated endeavor as climate change is a multifaceted environmental shift. Below I will try to expose two complicated aspects of predicting mangrove extent by reviewing some emerging research and discussing how these results might be relevant for the Bahamas.
Mangrove areas most vulnerable (orange) and least vulnerable (blue) to climate change (i.e., sea level rise, temperature change, disturbance) across the globe. The Caribbean is shown to be one of the most vulnerable areas to climate change. Adapted from Alongi, 2008.
Hydrology – Sea level rise and surface elevation have an intricate relationship in mangrove ecosystems. Sea level rise can result in several different responses of surface elevation (Figure 1). For instance, a rise in mean sea level could alter tidal dynamics in a way that would increase sedimentation rates by delivering allochonthous material to the mangrove sediments. Increased sedimentation may increase accretion rates, fuel root growth and affect other sub-surface processes in the mangrove forest. In this light, sea level rise fuels an increase in surface elevation, creating a positive feedback. However, sea level rise may also have negative effects. As hydroperiod (the amount of time mangrove roots are flooded) increases, mangrove sediment may become increasingly waterlogged and more anoxic (oxygen poor). This could result in an alteration of sub-surface processes (i.e., root growth) and may even facilitate mangrove death. Several researchers have argued that both processes will be activated with increasing sea level producing a feedback mechanism between sea level rise and surface elevation in which sea level rise first facilitates surface elevation rise, but then the increase in surface elevation reduces hydroperiod and thus further sedimentation (Gilman et al, 2008; Feller et al, 2010). That said, there is little evidence available to evaluate these predictions. However, we may be able to glean some understanding by examining the past.
Historically, mangrove forests have proven rather resilient to sea level rise. For example, mangroves in Belize have maintained their position during sea level rise over the last 7,000 years (McIvor et al, 2013), at a rate of about 3mm/year. However, if rates of sea level rise exceed the rate at which surface elevations increase (i.e., accretion), mangroves will likely be inundated and forced to colonize more inland habitats. This could occur if sea level rise facilitated erosion of the mangrove forest rather than sedimentation or accretion. This presents an interesting dynamic in the marsh-mangrove ecotone, where studies have already shown mangroves encroaching inland into marsh habitats in Florida (Krauss et al, 2011) and Australia (Saintilan and Williams, 1999).
Figure 1. The relationship sea level rise and surface elevation share. Sea level affects the mangrove peat through surface processes and sub-surface processes and these in turn affect surface elevation. Surface elevation then determines the mangrove forest’s ability to keep pace with sea level rise, i.e., surface elevation must either be equivalent or greater than mean sea level rise to avoid the risk of mangrove drowning. Adapted from McIvor et al (2013).
Temperature – A second limit is temperature. Mangroves are limited by frost tolerance and typically do not persist in locations with air and water temperatures less than 16°C and 24°C, respectively (Ellison, 2000). As climate change progresses, air and sea surface temperatures are predicted to increase. Such increased temperatures are likely to have important direct and indirect effects on the future of mangrove forests. A direct effect of temperature on mangroves includes physiological aspects of performance. An increase in air temperatures may be beneficial to mangroves as photosynthesis rates peak between 30-33°C. However, as temperatures exceed 33°C, the rate of photosynthesis typically declines which may impact the productivity of mangroves (Alongi, 2008). An indirect effect may be an increase in the occurrence and/or intensity of storms such as hurricanes and typhoons. Although mangroves are resilient to these natural disasters, alterations to historical storm regimes may affect the ability of mangroves to withstand destruction.
Figure 2. Here is a depiction of the increase/decrease in mangrove area along the East Coast of FL. The size of markers denotes the percent change in mangrove area over the study area: small (10%), medium (50%), and large (100%). Marker color indicates an increase (black) or decrease (red) in area. A clear trend of 50-100% increase is shown in Northeastern FL while a decrease in mangrove area was shown in Southeastern FL. Adapted from Cavanaugh et al, 2013.
A recent paper explored the former scenario; that is the direct effect of temperature on the limits of mangrove forest. The authors used 28 years of satellite imagery of the Eastern Florida coast to argue that mangrove extent on the Northeast coast of Florida is controlled by extreme cold events. Specifically, survivorship is positively correlated with a decrease in extreme cold events (using a temperature threshold of <-4°C), i.e., the more extreme cold events there are, the fewer mangroves there are. The authors then reason that because a decrease in cold events is a consequence of climate change, we will see a net gain in mangrove habitats along the northern boundary as climate change progresses (Figure 2).
Although the findings of this study suggest that temperature thresholds may be a critical factor in predicting future mangrove area, it is not that simple. It is likely that temperature thresholds vary for different mangrove taxa, thus, in order to truly predict mangrove area, research will need to determine temperature thresholds for a variety of mangrove taxa. Additional factors such as drought tolerance, and interspecific interactions may also be important in determining physiological thresholds.
What does this mean for the Bahamas?
The Caribbean region is one of the most vulnerable areas to mangrove habitat degradation from climate change processes. As sea levels rise, hydroperiod within mangrove systems will be extended and mangrove habitats will be susceptible to sedimentation or perhaps erosion. Temperature of both air and sea water also play a major role in the fate of Bahamian mangroves. For example, in the Marls on Abaco, water temperatures vary drastically among mangrove flats. During the summer months, stagnant seawater is layered on mangrove sediments reaching temperatures greater than 33°C.
One key factor to consider in the relationship between climate change and fate of Bahamian mangroves is substrate type. In the Bahamas, many mangrove habitats (i.e., the Marls) are located on calcium carbonate substrates and have much finer sediment with little organic material. This type of sediment is likely much more susceptible to erosion and dispersion than mats of organic peat found in other mangrove habitats, meaning that the pace of accretion may not keep up with the rate of sea level rise. Additionally, the taxa (i.e., Avicennia or Rhizophora species) of mangrove stands in addition to mangrove community type (i.e., dwarf, fringing, etc) are crucial to determining the ultimate impact sea level rise and temperature changes will have on Bahamian mangroves. Unfortunately, little work has documented the impact climate change has on individual mangrove taxa or community types.
Another complication to predicting the future of mangrove extent is the coincident increase in temperature and sea level (e.g., temperatures rise and ice caps melt). How mangroves will fare synchronous change in both of these elements of habitat suitability is not currently known. For example, if the ability of mangroves to migrate inland with rising sea levels is adversely affected by rising temperatures, then the area of mangroves is likely to decline. This is just conjecture at this point, but just as stress can increase human disease vulnerability, mangrove ecosystems subject to multiple stressors (i.e., sea level rise and rising temperatures) may also be more susceptible to destruction. While studies show that climate change will have grave impacts on coastal ecosystems, the ultimate factor in predicting future mangrove area likely remains anthropogenic activity due to the annual 1-2% loss of mangrove habitat worldwide from human activities (Alongi, 2008). Mangrove habitats and coastal ecosystems alike are continuously threatened by coastal development; therefore actions must be taken to curb our impact on these valuable ecosystems in order to address their future.
Cavanaugh, K.C., Kellner, J.R., Forde, A.J., Gruner, D.S., Parker, J.D., Rodrigues, W., Feller, I.C. 2013. Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events. Proceedings of the National Academy of Sciences. www.pnas.org/cgi/doi/10.1073/pnas.1315800111
Krauss, K.W., From, A.S., Doyle, T.W., Doyle, T.J., Barry, M.J. 2011. Sea-level rise and landscape change influence mangrove encroachment onto marsh in the Ten Thousand Islands region of Florida, USA. Journal of Coastal Conservation. 15(4): 629-638.
McIvor, A.L., Spencer, T., Möller, I. and Spalding. M. 2013. The response of mangrove soil surface elevation to sea level rise. Natural Coastal Protection Series: Report 3. Cambridge Coastal Research Unit Working Paper 42. Published by The Nature Conservancy and Wetlands International. 59 pages. ISSN 2050-7941. URL: http://coastalresilience.org/science/mangroves/surface-elevation-and-sea-level-rise