Fragile coastal habitats, including seagrass beds, are destroyed for marinas and ports. Tropical reefs are dynamited for fish and aquarium trade. Millions of acres of mangroves are destroyed for development or wood chips. Increased runoff is considered a potential cause of the ever-increasing outbreaks of toxic algal blooms that threaten shellfish and human health, and, possibly, the existence of the approximately 500 remaining monk seals. Land-based activities including dredging, dam construction, and forestry alter the input of silt and sediments.
Dams can deprive coastal areas of needed sediments.
Conversely, some forestry practices can increase rates of sedimentation and smother coastal ecosystems. Since 1951, plankton abundance and productivity have been declining off Southern California, and increased temperature is the suggested cause. Predation, for instance, can enhance diversity by keeping competitively dominant species from excluding other organisms. Hurricanes have devastating effects on coral reefs, but they play a crucial role in maintaining reef diversity. They provide space for new and fast-growing species to invade and reefs subject to normal hurricane regimes tend to have the highest diversity.