Coral reefs can be severely degraded with much smaller levels of eutrophication than those needed to impact open waters. As detailed in Estuarine Ecohydrology, coral reefs have a rich biodiversity and they greatly benefit humanity by building islands and atolls, inhibiting coastal erosion, and supporting fisheries and tourism. The destruction of coastal coral reefs is increasing worldwide (e.g., up to 50% in the last 15 years in some Asian countries). There is not one country in the world that has put in place a sustainable coral reef management policy because human activities on land are not incorporated in coral reef management policies. The present coral reef strategy principle relies on drawing a line around selected coral reefs and naming them marine parks or marine protected areas. Corals and the fisheries are protected inside that line. It ignores the fact that the land and the coral reef are ecologically connected through the rivers. As a result this management practice invariably fails where the corals are impacted by rivers that are impacted by human activities in the catchment.
The health of coral reefs fluctuates naturally in time. Historically, coral reefs have been impacted by natural disturbances such as tropical cyclones and river floods. If good water quality prevailed after these occasional disturbances, reefs have invariably recovered. Nowadays, coral reefs are subject to direct human impacts from land runoff from river catchments impacted by human activities; this results in an increase in suspended sediment and nutrient concentration during the recovery period after a natural disturbance, and from global warming that generates increased bleaching events in summer. Coral reefs are weakened, more frequently diseased, and unable to recover between disturbances; they are thus commonly slowly dying out from failure to recover from disturbances.
The physical forcings of coastal coral reefs are river floods and tropical cyclones that are natural disturbances, and the oceanography that enables the exchange of coral planulae between reefs. The biological forcing is mainly the competition for hard substrate space between the coral and the algae. The recruitment of juvenile coral decreases with increasing algal cover on the hard substrate. Human activities on land increase the suspended sediment and nutrient load, and thus help the algae. Algae are preyed upon by herbivorous fish that is preyed upon by carnivorous fish. The harvesting of herbivorous fish by people also helps the algae. The coral is preyed upon by the crown-of-thorns starfish (COTS), whose population dynamics also appears to be helped by human activities on land resulting in increased nutrient that promotes the plankton that supports the drifting COTS larvae. Additionally, global warming results in an increased mortality of adult corals, or poor health making them more susceptible to diseases or attacks by borers.
An ecohydrology model was built that incorporates all these processes. It was applied to the Great Barrier Reef and successfully verified against 20 years of data. The model (Figure 1) suggests that the biodiversity of the Great Barrier Reef is already seriously impacted by human activities and may progressively over a period of
Figure 1 Location map of the 400-km long central region of the Great Barrier Reef. Predictions of the coral density in the same region for (case 1) no human impact, (case 2) present conditions with land use, (case 3) existing land-use practices and with global warming in the year 2050 following the IPCC scenario A2, (case 4) halving of the nutrient and sediment runoff from land-use and with global warming following the IPPC scenario A2 in the year 2050, (case 5) same scenario as for case 4 in the year 2100. The color bar shows the coral density as a fraction of the hard substrate area for each of the 261 reefs in the model domain (e.g., 0.3 = 30%). Adapted from Wolanski E and De'ath G (2005) Predicting the present and future human impact on the Great Barrier Reef. Estuarine, Coastal and Shelf Science 64: 504-508.
tens of years degrade to end up being mainly an algae-covered substrate, such as most degraded coastal reefs in East Africa, Southeast Asia, and Micronesia. Thus, the Great Barrier Reef, the largest coral reef ecosystem on Earth, may become another casualty of the lack of recognition of the need to adopt ecohydrology as a guiding principle in managing human activities. The model suggests that much-improved land-use practices will enable some regions of the Great Barrier Reef to recover, even with global warming. However, the model suggests that if global warming proceeds unchecked only biological adaptation - about which no information is yet available -may prevent a collapse of the Great Barrier Reef health by the year 2100.
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