Turbulent Times

Today, I am going to be talking about the impact of storms and rainfall on reef communities. To start, I am going to cover storms.  Gardner et al. (2005) states an interesting idea that storms, whether it be a hurricane or tropical storm, are the "most obvious and frequent natural disturbances" affecting coral reef communities. Initially, after reading this I thought that couldn't be true, especially after reviewing a whole load of other impacts that seem to be far more frequent and in my mind more obvious. However, from reading this paper through, I can see where Gardner et al. (2005) is coming from, and if anything, this impact is becoming far more important. The climate is warming and so is the sea. Increased warmth in our oceans causes increased storm events due to a greater rate of latent heat exchange amongst other factors (NOAA). The GBRMPA states that this will cause an increase in frequency and severity of these storms. However we don't really need to someone to state this is happening because we can see this with our very own eyes, especially most recently with Hurricane Sandy. The Wikipedia page for this storm is actually very good; more specifically the 'relation to global warming section'. I agree with the NCAR senior climatologist Kevin Trenberth who is quoted in this section, that global warming does not cause these storms to occur but it increases the likelihood that they can occur. Anyway, that's enough of that... but there is no denying that storm frequency and severity is increasing. Here is a graph to illustrate this...

Bars depict number of named systems (open/yellow),
hurricanes (hatched/green), and category 3 or greater (solid/red), 1886-2004 

Now that we know the frequency and severity of storms is increasing, I can address the impacts they have on reef communities. In a detailed paper by Fabricius et al. (2008), storms are described as the "most severe form of mechanical disturbance" for corals. They are right for saying this. There have been numerous papers detailing how storms act as determinants of reef structure and function through their impacts (references found in Gardner et al. 2005). Simply, the GBRMPA states three impacts: 1. Coral breakage 2. Dislocation 3. Degradation. Furthermore, it is noted that the effects of these storms can have legacies of up to centuries (Connell, 1999). Fabricius et al. (2008) provides are great case study on the impact of tropical cyclone Ingrid that hit the most Northern part of Queensland. This paper looked at the impact on inshore and offshore reefs and highlights the significant perturbations on marine ecosystems. The worst affected inshore reefs amounted to extensive rubble fields whilst the worst affected offshore reefs were stripped and only their solid substratum was left. Their are some statistics presented in this paper that are truly astonishing. For example, on a devastated inshore reef, the coral cover decreased by 800%, diversity 250% and density 30%. Fabricius et al. (2008) clearly illustrates with these figures the role of storms and their ability to break and dislodge corals. Interestingly, from large scale survey analysis, the key factor driving the devastation was the wind speed over a 10 minute average but storm duration was also another significant variable. 

So, thoughts for the future? Well, future projections are somewhat uncertain as they are linked to projections of ocean temperature increase. Sriver and Huber (2006) state that the potential increase in destructiveness could range from 0% to 60%, whilst Knutson and Tuleya (2004) predict an increase of 6-12% in maximum wind speeds by 2090 which equates to half a category on the Saffir Simpson Scale. Relating back to the Great Barrier Reef, a potential increase by half a category in terms of intensity would have dramatic consequences; consequences far greater than ever observed before.

Moving on to rainfall and corals, you may be thinking 'hmmmmmmmm what's going on here?'. Well I thought exactly the same thing, so don't you worry. With storms comes rain and usually these are heavy rainfall events that lead to fresh water inundation in the inshore reefs. The concept is pretty straightforward (and corals are thought to have a low tolerance to changes in salinity). The fresh water reduces the salinity of the coral environment leading to bleaching i.e. the loss of zooxanthellae (please see previous posts for information). A relatively old paper by Nyawira et al. (1987) explored the effects of salinity stress on a specific hermatypic coral. The paper found that this coral species was actually relatively tolerant to salinity change. Normal salinities in the coastal waters that this coral inhabits is no greater than 30% but it is able to acclimate to 42%. However, any change greater than 15% both up or down equalled decreases in respiratory and photosynthetic rates due to a respective decrease in chlorophyll pigments per algal cell and leads to coral demise. Despite this, this evidence (be it slightly old!) has a good point. It shows that corals can adapt and survive with a greater ability to resist prolonged or sudden changes in salinity. But, and this is a big BUT, the combination of other variables makes nothing certain. Therefore we cannot be overly confident and state that corals have a greater threshold than first thought. This would be of no benefit to corals. We as learned individuals have to, dare I say it, prepare for the worst, as modification of our climate is not going to suddenly cease and revert to how it was. 

Over and out,

Seb 

References:

Connell, J. H. 1997. Disturbance and recovery of coral assemblages. Coral Reefs 16 (Suppl): 101-113. Devantier, L., G. De'ath, T. Done, E. Turak, and K. Fabricius. 2006. Species richness and community structure of reef-building corals on the nearshore Great Barrier Reef. Coral Reefs 25: 329-340.

Knutson, T., and R. Tuleya. 2004. Impact of CO2-induced warming on simulated hurricane intensity and precipitation: sensitivity to the choice of climate model and convective parameterization. Journal of Climate 17: 3477-3495.

Sriver, R., and M. Huber. 2006. Low frequency variability in globally integrated tropical cyclone power dissipation. Geophysical Research Letters 33, L11705, doi:10.1029/2006GL026167.