The impacts of human activities on marine organisms and ecosystems, and even directly on human health and well-being, are often clearly visible. A swimmer may fall ill after a day at the beach, or a tourist after a shellfish dinner. A mangrove forest may die after being smothered by an oil slick. Or a turtle may be washed up on shore with plastic waste still tightly wrapped around its neck.
Elsewhere the causes of particular impacts may be more difficult to determine. The deaths of cetaceans in many of the world's oceans are a regular cause for concern. Biopsies often show very high levels of toxic compounds in their bodies, but whether these were the ultimate cause of death, or contributed to the cause of death, or were simply an unrelated phenomenon may be difficult to determine. Multiple stresses may often combine. A decline in diversity or biomass on a coral reef may be linked to any one of a broad range of stresses, or perhaps to more than one.
The fact remains that we are rapidly changing the condition of our oceans. Some of these condition changes have clearly discernible impacts, but others are more subtle. It is useful to look more closely at these impacts from the perspective of human uses of the ocean, and from the perspective of individual species or species groups which may be high profile, or highly susceptible; and also from the perspective of wider ecosystems, where multiple impacts may combine to threaten their stability.