Human activities have fundamentally altered the natural flow of
water and biologically essential elements through natural
cycles. For example, humans now appropriate over half the usable
fresh water on the planet and fix more atmospheric nitrogen into
a biologically available form than all natural terrestrial
sources. These profound alterations of natural hydrological and
biogeochemical cycles may change the types of organisms that a
given region supports, fundamentally transforming the ecosystems
upon which we depend for goods and services. The human impact on
mass flows comes from direct inputs through
agricultural/industrial/sewage activities, by changes in runoff
and river discharge, by damming, by alterations in the ability
of wetlands to take up or release nutrients, and by diffuse
input from the atmosphere. Effects of human disturbance become
integrated at the watershed level, and are transferred to the
coastal zone, which often suffers disproportionate impact (such
as toxic blooms, anoxic dead zones,