Overview of impacts

No part of the ocean has today completely escaped the impact of human pressures, including the most remote areas. One clear example of this is the universal presence of stratospheric fall-out from atmospheric nuclear-weapons testing, but many other pressures on the marine environment are nearly as widespread.

Human pressures impact on the ocean in many and complex ways. They can take effect directly (as when an oil spill kills sea-birds and sessile benthic biota) or indirectly (as when climate change results in changes to the stratification of seawater, with an adverse effect on the nutrient cycle and the production of the plankton on which fish feed). Equally, the effects can be seen both on the natural environment (as when populations of sea turtles are reduced by tourist development on or near their breeding beaches) as well as on human society and economic activities (as when the collapse of a fish stock removes the economic base of coastal communities). Human pressures can also vary widely in their intensity and spread. Sometimes they have a concentrated impact: for example, the annual expansion of a large dead zone in the Gulf of Mexico, resulting from the high level of inputs of nitrogen compounds in the run-off from the Mississippi and other catchments. Sometimes the effects of human pressures have a very widely distributed effect: for example, the diffusion of persistent organic pollutants over the Arctic zone by airborne volatilization (for both examples, see Chapter 20 on land-based inputs) (Halpern, 2008).

Summarizing the impacts

An analysis of the overall impact of all the human pressures examined in this Assessment has to start by looking at the direct impacts and collateral effects of each pressure and to examine where those impacts and effects are found. However (as argued below), although this is an essential first step, it is not enough. In addition, any review of the effects of human pressures on the marine environment has to look both at the effects on the marine environment and at the consequences for human society and economies.