12.2. Coral Reefs
12.2.1: General Coral Reef Ecosystem
12.2.2: Optimal Environment for Coral
12.2.3: Effects of High Temperature on Coral
12.2.4: Phase Shifts from Coral to Macroalgae
12.2.1: General Coral Reef Ecosystem
Coral reefs are characterized by the coral structures that provide habitat for the fish and invertebrate species that make up a significant amount of the ecosystem. Hard corals create the reef itself and they typically consist of a layer of colonial polyps that live on the surface of a calcium carbonate skeleton that is secreted by the coral polyps. Corals rely on a symbiotic relationship with the zooxanthellae algae which can be found in the gastrodermis in the "stomach" of coral polyps. Zooxanthellae photosynthesize while residing inside their host, and provide the necessary nutrients and energy for the invertebrate host, transferring 95% of their production.(Muscatine, 1990). In return, corals supply zooxanthellae with nutrients essential for photosynthesis such as ammonia and phosphate from their waste metabolism. These nutrients seem to be essential for the survival of the zooxanthellae because the water column in the tropics is usually devoid of essential inorganic compounds (Trench, 1979).
12.2.2: Optimal Range of Temperature, Salinity, Nutrients, Aragonite Saturation State, and Light levels for Coral Growth
Temperature, salinity, nutrients, aragonite saturation state, and light are among the most important factors in controlling the geographic distribution of shallow-water coral reefs (Couce et al., 2012) (Kleypas et al., 1999). The global, annually averaged tolerance limits for coral reefs are 21.7—29.6 °C for temperature, 28.7—40.4 psu for salinity, 4.51 μmol L-1 for nitrate, 0.63 μmol L-1 for phosphate, and 2.82 for aragonite saturation state. The averaged minimum light intensity in coral reefs is 450 μmol photons m-2 s-1 (Guan et al., 2015).
12.2.3: Effects of Higher Temperatures on Coral
Coral reefs have become one of the most vulnerable ecosystems to climate variation and change. Corals, the building blocks of carbonate reefs, have a restricted thermal tolerance, resulting in ‘bleaching’ events (loss of symbiotic algae) when sea surface temperatures rise above a given threshold (Graham et al., 2008). Sea temperatures in tropical regions have increased by 1°C over the past century. These increases in temperature can trigger events known as coral bleaching, where the thermal tolerance of corals and their photosynthetic symbionts, zooxanthellae, are exceeded (Guldberg, 1999)
12.2.4 Corals Macroalgae Phase Shifts
Other substantial influences for coral reef degradation also exist. Specific factors such as eutrophication, increased sedimentation, tourism, and increased fishing pressures may interact with climate change to produce negative synergistic effects (Wilkinson and Buddemeier, 1994). Degrading reefs undergo a phase shift in which the abundance of corals decline and switches to an increase in abundance of larger fleshy macroalgae (Done, 1992). The main drivers that have been cited to explain such a shift is primarily from eutrophication (Lapointe, 1997) and reduction in herbivory (Hughes, 1994). Eutrophication is mainly caused by high nitrogen and phosphorous runoff from agricultural lands that seep into the ocean. The increased nutrient load creates an optimal environment for the production of macroalgae, which is a direct competitor with coral reefs since they decrease the total amount of available light for zooxanthellae to photosynthesize. Increased fishing pressure has reduced the number of herbivorous fish by orders of magnitude, estimated at around a 60 % decrease (Jackson, 1997) (Bellwood et al., 2004). Due to this reason it is essential to conserve species specializing in herbivory if our goal is to preserve coral ecosystems (Adam et al., 2015).
Youtube Video: Parrotfish Keeping Macroalgae in Check Through Herbivory
Youtube Video: Jennifer Smith from Scripps Explaining The Basics of Coral Reefs
Youtube Video: Coral Bleaching Explained
Youtube Video: Heron Island Marine Research Station University of Queensland Coral Reef Climate Change Experiment
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