Coral reef ecosystems - Coral Reef Ecology lecture notes

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Coral reef ecosystems - Coral Reef Ecology lecture notes Powered By Docstoc
					Coral Reef Ecology lecture notes
Dr. Ouida W. Meier
Marine Biology, 29 March 2005

Research slides in 35mm format were shown rather than as a PowerPoint presentation. As
assistance in your studying, following is a summary of the major points of the lecture.

Local example (Florida Keys) but constellation of challenges is global… this story is
repeating itself at many, many reef sites around the world.

Basic facts about coral reefs
   1. Structure of coral reefs based on reef-building scleractinian corals themselves
   2. Hard substrate is a limiting factor in marine ecosystems; scleractinian or reef-building
       corals themselves constitute the substrate of the reef that so many other organisms
       depend upon for settling, finding food, finding refuge from predators, finding mates, and
       producing offspring (including corals themselves - e.g., lunar-cued mass spawning)
   3. Fast rates of growth of some corals in shallow water due to symbiotic algae
       (zooxanthellae: Symbiodinium microadriaticum): algae use nitrogenous wastes from the
       coral animal, coral gets translocated photosynthate from symbiotic algae
   4. Most corals are both heterotrophic (catching zooplankton) and autotrophic (rely on
       photosynthesis from symbiotic algae); relative importance of the two methods reflected
       in coral polyp size (larger with increased heterotrophy)
   5. Coral reef ecosystems are very highly connected; high tropic connectivity is further
       amplified by high degree of non-trophic interactions (interspecific dependence is
       unusually high in coral reef ecosystems)
   6. High degree of trophic and non-trophic interactions mean that network interactions
       within the system are very dense, and perturbation of one part of the system has multiple
       indirect effects on other parts of the system
   7. Coral reef ecosystems have high gross primary productivity but low net productivity
       because biomass keeps bouncing around inside the system [by organisms eating other
       organisms] - very little energy is exported or available outside of the coral reef ecosystem
       itself, except through off-reef animals that come onto the reef to feed
   8. Reefs have evolved under conditions of clear, warm tropical waters and low-nutrient
       conditions - resulting in higher metabolic rates than for cold-water organisms, and a need
       to obtain food without much initial nutrient subsidy (a driving force for multiple origins
       of photosynthetic symbionts)

Impacts of acute disturbance on coral reefs
   1. Hurricane Andrew, Aug. 1992. Fast moving, high winds. At the time, was the most
      expensive natural disaster in US history.
          a. Terrestrial damage on mainland: houses, trailer parks destroyed, palm trees
              decapitated, crops damaged
          b. Coastline and island damage: mangroves and trees completely defoliated,
              structures damages/destroyed
             c. Underwater: some corals scoured (“sandblasted”), buried, fragmented, but
                 damage underwater not as severe as terrestrial (presumably because of high
                 winds, fast movement)
             d. Regional observations concurred with photostation data in representing these
                 types of damage
   2.   Tropical Storm Gordon - subsequent
             a. Slower-moving, wetter than Andrew
             b. Less terrestrial and more underwater damage than Andrew
             c. Coral reef photostations recorded increased fragmentation after Gordon
   3.   Response of a community to disturbance depends upon:
             a. Frequency
             b. Intensity
             c. Predictability
   4.   In the Florida Keys, disturbances such as hurricanes are fairly frequent; variable in
        intensity; not predictable in the short run, but because over time the probability of a coral
        experiencing a hurricane is high, long-term predictability is increased.
   5.   Conditions that are favorable for the development of coral reefs also give rise to
        hurricanes (because of latitude and climate)
   6.   It is reasonable that corals should be well-adapted to acute natural disturbances such as
        hurricanes, and in fact this appears to be so.

Other sources of disturbance to coral reefs
   1. Natural disturbances:
          a. Hurricanes, tsunamis
          b. Predation (e.g., corallivorous snails, starfish, parrotfish)
          c. Competition with other corals
   2. Many natural challenges to corals appear to be accelerated by anthropogenic factors
          a. Competition with macroalgae / blue-green algae (overgrowth / settlement space) -
              accelerated by nutrient input from sewage, fertilizers
          b. Loss of herbivores that keep algal populations reduced
                   i. Loss of herbivorous fish due to overfishing
                  ii. Loss of Diadema antillarum, long-spined sea urchin, from virus (of
                      unknown origin; susceptibility to diseases greater with additional stresses)
                 iii. Potential switches in diet of herbivorous community due to changes in
                      algal populations (e.g., potential increase of corallivory by parrotfish as a
                      result of an increase in unpalatable algal species due to loss of grazing by
          c. Diseases of corals and other reef organisms (fungal, viral, bacterial; white pox
              disease of once-dominant branching corals now known to be a bacterial agent
              derived from the human gut)
   3. Direct anthropogenic disturbances:
          a. Harvesting coral and other reef organisms for decoration, food, building
          b. Sewage input
          c. Pollution (e.g., mosquito spray)
          d. Overdevelopment of coastal areas
          e. Boat/ship groundings
           f. Diving/swimming/boating (mooring buoys a double-edged sword)
           g. Large-scale landuse changes

Severe coral declines observed prior to hurricanes/tropical storms, measured as loss of coral
cover and loss of species diversity; seen at multiple locations. To address the question of how
consistent and widespread the decline was in the Florida Keys, a broad-scale multi-habitat
quantitative survey program (the Coral Reef Monitoring Project) was funded to assess status and
trends of hardbottoms, patch reefs, shallow offshore reefs, and deep offshore reefs along the
entire Floridian reef tract. Within this project, both immediately and over the longer term,
declines in coral cover and species diversity were observed. Additionally, a high and increasing
incidence of coral diseases was observed and quantified.

An example of effects propagating through multiple ecosystems, and an additional source
of multiple stressors on coral reefs: the Florida Bay Hypothesis. Under normal conditions,
Florida Bay receives fresh water flowing into it from the Everglades, and from Lake Okechobee
before then. In recent years, much fresh water has been siphoned off for the use of densely
populated urban areas and extensive agriculture in south Florida, making less water available to
flow into Florida Bay. Florida Bay water is normally about half the salinity of seawater, well-
oxygenated, moderate in temperature because of ongoing flow, and low in nutrients. Reduced
fresh water flow into Florida Bay have rendered that water higher in temperature, lower in
oxygen, high in nutrients (pronounced at times of seagrass dieoffs), and higher in salinity (on
occasion measuring twice normal seawater salinity in some locations). This abnormally
hypersaline and therefore dense water was predicted to flow out over the reefs episodically,
carrying with it low oxygen, high temperature, high nutrient conditions, and in fact inverted
temperature profiles have been measured over the reef tract.

Coral reefs appear to be much more resilient to acute natural disturbances than to chronic
anthropogenic disturbances. In the Florida Keys and in other reefs around the world, corals
reef ecosystems are being impacted by a host of multiple stressors, and are in very serious
decline globally from constellations of local anthropogenic impacts, direct and indirect.