72-1In Situ Sensors for Monitoring the Marine Environment
The marine environment has been affected by growing population and industrial pressures in the coastal zone. Activities such as dumping of waste, construction of harbors, dredging, and extraction processes all contribute to changes in environmental quality. In order to assess the impacts of these activities and instigate appropriate remedial actions where needed, it is necessary to monitor a wide range of properties (i.e., biological, chemical, and physical) of the marine environment. Recently, such monitoring activities are becoming harmonized within regulatory frameworks and international agreements (e.g., within the Europe-specific initiatives under the Helsinki and OSPAR Commissions, the European Union’s Water Framework Directive [200/60/EC], and more recently Marine Strategy Framework Directive [2008/56/EC]). The latter requires for a thematic strategy for the protection and conservation of the marine environment and mandates for the establishment of clear environmental targets and monitoring programs. Most work on developing monitoring methods has been focused on freshwater (e.g., ground and surface water) applications, but now there is an increasing demand to adapt these methods for use in the marine sector. Monitoring the marine environment, however, presents a number of problems that are not encountered with freshwaters. For example, oceans cover very large areas, and there are complex currents and, in coastal systems, tides that can cause large shifts in salinity. Large dilutions of pollutants occur, and so many contaminants can appear at very low, but often biologically significant, concentrations. Boats and ships are usually needed to reach monitoring sites, with samples of water, sediment, and biota transported back to land-based laboratories for analysis or in some cases analyzed onboard ship. The use of sampling coupled with classical biological or chemical analysis is expensive and time-consuming and often fails to provide representative information over changes in time (short term over a tidal cycle and longer term over seasons, years, and decades). An additional problem is that delays between sampling and analysis can compromise sample integrity (especially for 72-2labile compounds), while changes in temperature and pressure can also be an issue. A solution to some of the problems outlined earlier is the use of sensors that are particularly suited for making in situ measurements in the marine environment and can provide continuous or semicontinuous observations and transmit the information telemetrically to shore-based data-gathering facilities. Their use can overcome some of the problems of the undersampling (in both space and time) of coastal waters and the ocean. Many engineering platforms are available on which these sensors can be deployed in the environment and include APEX floats, autonomous underwater vehicles (AUVs), gliders, benthic landers, moorings, remotely operated vehicles (ROVs), nodes attached to cable networks, and even tagged live stock and wildlife (Figure 72.1). This approach thus eliminates the need for expensive (>$15–20,000 day−1) sampling from ships, and there can be real-time acquisition of information where any strategic decisions can be made in a short time scale.