Other contaminants such as radioactive waste, pesticides, and other chemicals have cumulative effects, building up within individuals over time, especially within species high on the food chain. Moreover, various contaminants and physical degradation can act together in a cumulative or synergistic fashion. Other contaminants can trigger ecosystem-wide changes, resulting in conditions that are inimical to a range of species. Runoff of sewage from cities and of fertilizers from agricultural areas elevates the levels of nutrients within nearshore waters. Many bloom species produce toxins.
Human health can also be at risk.
Widespread effects are often noted as a result of sedimentation. Soils eroded from deforested areas and poorly managed agricultural lands often end up at sea, reducing light penetration to seagrass bed, coral, and other communities dependent on the productivity of photosynthesizers living on the sea floor. Nontoxic solid wastes and marine debris cause significant mortality among marine species. For example, plastic bags, fishing lines, and other debris can entangle seals, seabirds, and other organisms, causing slow but sure deaths. Bits of plastic and other man-made materials are regularly ingested by sea turtles and other species, often with fatal consequences.