In this article the environmental issues of both the energy conversion concepts using the temperature gradients that exist between water masses in the ocean at low latitudes (OTEC) and between sea or river water masses in the ocean at high latitudes (AWTEC), are examined. Both OTEC an AWTEC use a degraded solar energy resource that does not consume irreplaceable natural resources nor generate specific pollutants. Yet building and operating plants will inevitably disturb the natural environment in their close surrounding and - to some extend - at the regional and global levels. The AWTEC and OTEC threat for the environment depends upon the very processes of the energy conversion themselves, and upon the size and the design of the installations. As for the environmental consequences they are depending upon the characteristics of the site and its surrounding ecosystem.