Introduction

Water in the form of lakes, rivers or the ocean, and the sediments that accumulate below water, are inevitably impacted, either directly or indirectly, by many of the changes that have affected the land and the atmosphere. In some cases, water simply provides one of the major links in biogeochemical cycling. Often, as in the case of carbon, it provides, along with accumulating sediments, one of the major sinks. Irrespective of the role it fulfils with respect to any given process within the Earth system, its quality, distribution and availability are altered by many of the changes already described in the two previous chapters. Since water is one of the essentials for the maintenance of life on Earth, as well as a vital ingredient in a vast range of industrial and domestic processes, the changes to lakes, rivers and the oceans brought about by human activity are a key part of global change. Anthropogenic transformations of both quantitative and qualitative aspects of the hydrological cycle are of vital concern.

Changes in lake-water chemistry

Cultural eutrophication

Eutrophication can be the outcome of natural processes and many lakes are naturally eutrophic. Cultural eutrophication is the chemical enrichment of the water body through human impacts on nutrient supplies. It generally leads to increased productivity, hence higher quantities of organic carbon, which can result in major changes in the structure and functioning of the lake ecosystem, especially where the decomposition of the additional carbon involves high levels of oxygen demand by decomposer organisms.