Coral reefs are complex underwater ecosystems supporting diverse marine life. They provide shelter, food, and breeding grounds for over 25% of marine life, despite occupying less than 0.1% of the ocean floor. Beyond biodiversity, reefs protect coastlines and support global fisheries and tourism. Sediment runoff poses a significant threat to these ecosystems.
Sources and Pathways of Sediment Runoff
Sediment runoff is the movement of terrestrial materials like soil, sand, and silt into waterways and the ocean. Human activities drive increased sediment loads. Agricultural practices, like tilling and clearing land, cause soil erosion. Deforestation removes vegetation that stabilizes soil and prevents runoff.
Coastal development, construction, and urbanization also contribute to runoff. These activities disturb land, exposing soil and increasing erosion. Eroded sediment travels through rivers, stormwater drains, and surface runoff to the marine environment. Coastal currents then disperse this sediment to coral reefs.
Direct Physical Harm to Corals
Sediment directly harms corals. Fine particles settling on corals can smother polyps, blocking feeding and respiration. This burial can lead to suffocation and coral death if prolonged. Corals expend energy to remove sediment with mucus and cilia, but high accumulation overwhelms this capacity.
Suspended sediment increases turbidity, clouding water and reducing light. Diminished sunlight impacts symbiotic algae (zooxanthellae) within coral tissues. Zooxanthellae are crucial for coral nutrition, providing up to 90% of energy through photosynthesis. Reduced light hinders this process, leading to coral starvation and impaired growth.
Coarser sediment particles can abrade or scratch coral tissues. These injuries create wounds, making corals susceptible to infections and disease. Smothering, light reduction, and abrasion stress corals, impacting survival and growth.
Indirect Chemical and Biological Effects
Sediment runoff transports harmful chemicals and fosters biological imbalances. Agricultural runoff contains excess nutrients like nitrogen and phosphorus. These nutrients, attached to sediment, enrich reef waters, fueling macroalgae growth.
Macroalgae outcompete corals for space and light, smothering them and shifting reef composition. Some macroalgae release chemicals that inhibit coral growth or cause tissue damage. Sediment also transports other pollutants, including pesticides, herbicides, and heavy metals.
These pollutants stress corals, impairing physiological functions and reducing growth. Exposure also weakens a coral’s immune system, increasing disease vulnerability. Chronic exposure to sediment and pollutants increases coral stress and disease susceptibility, compromising reef health.
Wider Ecological Repercussions
Sediment runoff impacts the entire reef ecosystem. Degraded reefs no longer support diverse fish, invertebrates, and other marine life that rely on them for food, shelter, and breeding. This leads to biodiversity loss, diminishing reef richness and complexity.
Coral death impacts the reef’s physical structure. As corals die, the reef framework erodes and can collapse, reducing habitat complexity. This loss reduces hiding places and foraging areas for reef inhabitants, leading to a less resilient ecosystem.
Sediment runoff also harms coastal habitats like seagrass beds and mangrove forests. These ecosystems serve as nursery grounds for marine species and act as natural filters, maintaining water quality. Their degradation exacerbates challenges for coral reefs. These impacts diminish reef health and functionality, affecting ecosystem services like coastal protection, fisheries, and tourism.