By Jessica Wurzbacher, MSc
Earl, Irene, Katrina, Floyd, Bob, Katia… they have all caused major destruction, loss of life and devastation to many. It is extremely likely that you know the devastating impacts hurricanes can have on our coastal communities, but what goes on beneath the waves as the thunderous beasts unleash on the shore line?
The impacts of hurricanes on invertebrate marine life (e.g. coral reefs):
Coral reefs are fragile biological structures, composed of the calcium carbonate skeletal remains of many generations of coral polyps. Corals species grow in different shapes and sizes and dominate in different areas of the reef (Figure 1).
Figure 1: Some of the different growth forms of corals (Photos: Jessica Wurzbacher)
Corals harbor symbiotic algae called zooxanthellae, therefore they are dependent on light for photosynthesis. The plate-like growth forms are found at deeper depths where light levels are lower, the higher surface area to volume ratio allows them to capture light more efficiently. The "massive" (boulder- or mound-shaped) corals (e.g. Porites spp.). are more robust and as a result are often found in areas exposed to regular ocean swells.
The branching corals (e.g. Acropora spp.) are the most susceptible to wave damage, but are often found in shallow areas as they are able to use this fragmentation as a method of dispersal. These corals can reattach if they settle on a suitable substrate, and they have a relatively fast rate of growth (~20cm/yr).
Hurricanes can have a devastating impact on coral reefs crushing these delicate structures. (see the last scene of BBC's Blue Planet: Coral Seas). The waves can also propel rubble, and other debris (e.g. artificial reefs not securely anchored) onto the small coral polyps crushing and killing them. Coral polyps are members of the phylum Cnidaria, and resemble inverted jellyfish embedded in calcium carbonate corallite cups with many tentacles extended. A small impact can crush them.
Figure 2: Coral polyps with tentacles extended for night time feeding(Photo: Jessica Wurzbacher)
A meta-analysis study conducted by Gardner et al (2005) on 20 years of data, found that in the Caribbean, coral cover was reduced on average by about 17% in the year following a hurricane. The size of this loss increased with hurricane intensity and the time elapsed since the last impact (Gardner et al, 2005). It was also determined that coral cover at impacted sites declined at a significantly faster rate (6% per year) than at nonimpacted sites (2% per year) (1980's) (Gardner et al, 2005). In more recent years (1990's) there has been a more similar decline in coral cover at impacted and non-impacted sites, suggesting a larger role from other stressors (Gardner et al, 2005), and corals are now dealing with problems of multiple stressors.
The wave action also increases turbidity and sedimentation, this lowers light penetration to the photosynthetic algae in the coral's tissues, and also directly kills the coral through smothering as the substrate particles clog cilia on their tentacles. The heavy rainfall associated with hurricanes alters the salinity of the water, which is an additional stress for the fragile marine life (Morton, 2002).
Many invertebrate are affected by the increased wave action associated with hurricanes, e.g. breakage of hard coral, sponges, seafans, seawhips, etc, but in general, large vertebrate are minimally affected. Many organism can sense the impending weather system and show great escape tactics. Fish are able to detect waves of pressure or mechanical disturbances in the water through neuromasts in their lateral line (Figure 3). Blacktip reef sharks (Carcharhinus limbatus) were recorded responding to the drop in barometric pressure as Tropical Storm Gabrielle approached, and fled into deeper water ahead of the storm (Heupel et al, 2003)
Figure 3: The Lateral Line system in fish
The resulting pollution from hurricanes 29th August, 2005 Hurricane Katrina made land fall in Buras, LA, the destruction was immense. By 9th September 2005 NOAA and the Environmental Protection Agency (EPA) had been notified of 200 hazardous materials released and more than 70 salvage problems with a pollution threat (Pine, 2006). In addition, 46 offshore oil platforms were destroyed, a further 20 were damaged and nearly 50 oil spills were reported in the near-shore environments (Figure 4) (Pine, 2006). Figure 4: Oil spill in Breton Sound, LA., following hurricane Katrina in 2005 (Photo: NOAA
The impacts of hurricanes on the coastline
Hurricanes can cause significant coastal erosion in a short amount of time, exacerbated by the elevated sea levels association with the storm surge. The powerful force of the waves can easily move large amounts of sand, for example Ocracoke Island, NC suffered severe coastal erosion following Hurricane Irene last month.Figure 5: May 6, 2008 (upper, pre-storm) and August 31, 2011 (lower, post-storm). The yellow arrow in each image points to the same cottage. A breach was carved through the barrier island, severing NC Highway 12. The storm surge was approximately 2 m high on the sound-side and was less on the ocean-side. (Photo: USCG)
The impacts of hurricanes on sea temperatures:
By the end of the summer the nutrients in the surface water have been depleted by the photosynthetic phytoplankton, productivity is low. A strong hurricane will cause ocean mixing, bringing up cooler deeper water to the surface. This deeper water is rich in nutrients, and coupled with the late summer sunlight it generates a large plankton bloom (Figure 6). It is too premature to conclude that human activity (particularly global warming) has had a detectable impact on the Atlantic hurricanes. Model simulations indicate that the 21st century warming may lead to a larger number of intense hurricanes and higher hurricane rainfall rates, but fewer hurricanes overall (NOAA).
Figure 6: Graph showing primary productivity in temperate oceans throughout the year, with a "Spring Bloom" and a "Fall Overturn" as phytoplankton levels peak with high levels of nutrients and sunlight.
What you can do?
References and further reading: