Coral Reefs and Climate Change
Impacts on surrounding ecosystems

Loss of coral reefs will have serious impacts on surrounding ecosystems, as reefs play an important role in the entire tropical “seascape”, which consists of mangrove forests, seagrass beds, coral reefs, and the open ocean (Moberg & Folke 1999). These ecosystems provide important services to each other in order to maintain balance and diversity in the coastal environment. Thus, the negative impacts of climate change on coral reefs will be felt throughout the seascape, as neighboring habitats will lose the important ecological services provided by reefs. Below is a diagram representing the connections between mangrove, seagrass, and coral reef habitats (from Moberg & Folke 1999), and further explanation of these relationships.

Reefs provide physical protection to seagrass and mangrove habitats that are further inshore by dissipating energy from ocean currents, waves, and storms. This buffering action creates a calm environment inshore that facilitates sedimentation and development of these other ecosystems (Moberg & Folke 1999)

Some herbivorous reef fish and invertebrates like sea urchins are regular visitors to nearby seagrass beds, where they play an important role in regulating the plant community structure and nutrient cycling. They feed on seagrass blades and can drastically reduce their abundance (Birkeland 1985). While feeding, they may provide a food source to predators and excrete nutrients, adding energy to the ecosystem (Moberg & Folke 1999).

Many reef animals also use mangroves (right) or seagrass habitats (below right) as a nursery, spending time there as juveniles before living on the reef as adults (Birkeland 1985). As juveniles in these ecosystems, they play an important role in nutrient cycling and the food web. A loss of reef organisms would therefore result in a decline in nutrient input and biomass in neighboring seagrass beds and mangrove forests.

Reefs also export nutrients and organic compounds to other ecosystems. High production of photosynthetic materials bysymbiotic zooxanthellae in corals can result in an excess of organic compounds. As coral colonies grow larger, the surplus of photosynthetic products also grows. These high energy compounds, such as mucus, wax esters, and dissolved organic carbon, are released into the surrounding water column (Hatcher 1988). Nitrogen is also fixed at a considerably higher rate on coral reefs than in surrounding ecosystems, resulting in excess nitrogen being released to surrounding waters. Furthermore, many reef organisms have pelagic larvae (below) that are a food source for many commercially important fishes (Moberg & Folke 1999). The export of these organic compounds, nutrients, and plankton from reefs plays an important role in the productivity of the pelagic environment (Sorokin 1999 in Moberg & Folke 1999).

As reef ecosystems decline due to global climate change, the rest of the dynamic seascape will decline as well. Mangroves and seagrass beds will lose the physical barrier that reefs provide for them, without which they may not be able to exist. They will also lose important controls on their community structure; without herbivorous reef fish and urchins, seagrass beds may be overgrown by benthic algae (McGlathery 1995). The pelagic food web will lose the significant input it receives from reefs, resulting in a general decline in ocean productivity.

 

Photo credits:
seagrass: www.ncl.ac.uk/tcmweb/remote/images/seagrass.jpg
plankton: Australian Institute of Marine Science
mangroves: Andy Lewis