Jun 16, 2019 | Updated: 11:54 AM EDT

Ocean Habitats Are On a Steep Decline

Jun 12, 2019 08:43 AM EDT

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Billions of people worldwide - especially the world's poorest- rely on healthy oceans to provide livelihoods, jobs and food and the range of goods and services that flow from coastal and marine environments. The FAO estimates that fisheries and aquaculture alone assure the livelihoods of 10 to12 percent of the world's population, and 4.3 billion people are reliant on fish -- including freshwater -- for 15 percent of their animal protein intake. Nearly 200 million people depend on coral reefs to protect them from storm surges and waves.

However, some of the key habitats that underpin ocean health and productivity are in steep decline. Coral reefs support more than a quarter of marine life but the world has already lost about half of its shallow water corals in only 30 years. If current trends continue, up to 90 percent of the world's coral reefs might be gone by midcentury. The implications of this for the planet and all of humanity are vast.

What is widely recognized as a crisis for biodiversity also risks becoming a major humanitarian challenge, particularly for coastal areas in South East Asia, Melanesia, Coastal East Africa and the Caribbean where there is strong dependence of communities on marine resources for food and livelihoods.

Tropical seas overheated by climate change have bleached, damaged and killed coral at unprecedented levels. Mass bleaching was first documented in the 1980s and satellite imagery has connected the distribution of bleaching events on Australia's Great Barrier Reef in 1998, 2002 and 2016 with increased sea surface temperatures. In the aftermath of the bleaching event in 2016, extreme, prolonged heat led to catastrophic die-off of fast-growing coral species - which have complex shapes that provide important habitats - and these were replaced by slower-growing groups that shelter fewer sea creatures. This drastically changed the species composition of 29 percent of the 3,863 reefs that make up the Great Barrier Reef. Other threats to coral reefs include overfishing, selective fishing and destructive fishing practices, and pollution from runoff which sullies reef waters, compromising coral health.

Mangroves are a key natural asset for many tropical and subtropical coastlines, providing livelihoods to many millions of coastal families and protecting them from violent storms and coastal erosion. They sequester nearly five times more carbon than tropical forests and provide nurseries to innumerable juvenile fish species that grow to join wider ocean ecosystems. Clearing for development as well as over-exploitation and aquaculture have contributed to a decline in the extent of mangroves by 30 percent to 50 percent over the past 50 years.

Seagrasses, marine flowering plants that include the widely distributed genera Zostera, Thalassia, and Posidonia, also represent important coastal ecosystems that provide critical human benefits including habitat that supports commercial and subsistence fisheries, nutrient cycling, sediment stabilization, and globally significant sequestration of carbon. They are threatened directly by destructive fishing practices, boat propellers, coastal engineering, cyclones, tsunamis and climate change, and indirectly by changes in water quality due to land runoff. In their global assessment, Waycott et al. 2009, found that seagrasses have been disappearing at a rate of 110 km2 per year since 1980 and that 29 percent of the known areal extent has disappeared since seagrass areas were initially recorded in 1879. These rates of decline are comparable to those reported for mangroves, coral reefs and tropical rainforests, and place seagrass meadows among the most threatened ecosystems on Earth.

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