Environmental future of estuaries

INTRODUCTION Estuaries are biologically productive coastal ecosystems where fresh water from land drainage mixes with seawater (Kennish 1986, 2001 a). They are coastal indentations with restricted connections to the ocean and remain open at least intermittently (Kjerfve 1989). The system can be subdivided into three regions, namely (1) a tidal river zone characterized by lack of ocean salinity but subject to tidal rise and fall of sea level, (2) a mixing zone, the estuary proper, characterized by water mass mixing and strong gradients of physical, chemical and biotic components, reaching from the tidal river zone to the seaward location of a river mouth or ebb-tidal delta, and (3) a nearshore turbid zone in the open ocean between the mixing zone and the seaward edge of the tidal plume at full ebb tide. The global coastal population may approach 8 billion people by 2025 (Chapter 1) and anthropogenic activities affect nearly all estuaries (Weber1994; Hameedi 1997). Developing countries widely lack government environmental regulations or have less stringent controls than most developed countries, and their technology to mitigate environmental impacts in estuarine embayments is also poor. However, currently the most significant human impacts are concentrated along heavily populated industrialized coasts (Fig. 13.1). Anthropogenic impacts on estuaries include point and non-point source pollution, habitat loss and alteration, shoreline development, altered hydrologic regimes, sea-level rise, land subsidence, energy/resource operations, boating and shipping, overfishing, introduced/invasive species and sediment input/turbidity (Table 13.1).