Perhaps the simplest method of monitoring is manual observation, either with measuring instruments or with the naked eye. An example of this type of monitoring is a marine biologist who manually counts the number of whales surfacing off the coast in a particular spot in a particular time. The advantages of this type of monitoring are that it requires little investment in equipment, and has the highest degree of flexibility – if the biologist sees something else of interest at the survey site, he or she can make direct, first-hand observations without the need to redesign or redeploy instrumentation. Human observers may also be able to fix errors in the monitoring protocol “on the fly” in a way that a more instrument-focused approach cannot. For example, if a site survey has a particular set of GPS coordinates and the human observer arrives at those coordinates and realizes that the site location is off, the observer can fix the mistake and carry out the survey. The primary disadvantages of manual observation are that recurring observations quickly become very expensive, and there may also be reliability issues. For example, a graduate student can forget to come in to conduct a set of observations. Manual observations may also be subject to greater biases and subjectivity effects.
Another fairly simple, though powerful, technique for monitoring is the deployment of fixed sensors, whether singly or as part of a sensor network. An example of this type of monitoring is the global network of fixed data collection buoys and shore stations maintained by the National Oceanic and Atmospheric Administration’s National Data Buoy Center. This network of more than 1300 stations collects ocean temperature, current, conductivity, wave height, and wave frequency data, along with meteorological data, for use in weather forecasting, tsunami detection and alerts, and scientific and commercial research. The advantages of a fixed sensor network are in the acquisition of historical data sets which can be continuously compared to new data, the relative ease of maintenance of a network of fixed sites, and the ability to develop a comprehensive network. Indeed, the NDBC’s network has achieved global reach, making it one of the most valuable and useful fixed sensor networks in existence. The disadvantage of fixed sensors is that, being fixed, they cannot easily be redeployed to cover a new area of the aquatic region being monitored.
A more flexible approach to sensor deployment is the use of drop sensors, or temporary sensors for a particular location, which are then collected for re-use or allowed to degrade in place. An example of this type of sensor deployment would be the temporary placement of water-quality sondes along a flooding river system; the data collected from the sondes would be used for a limited time and then the sondes would be picked up and put back into storage or used on a different project. The advantage of this type of deployment is that it is extremely flexible; the sensors can be placed at the specific points where data collection is most important, and easily relocated as needs shift or if the initial deployment was less than optimal. The disadvantages of a drop sensor approach are that the sensors are generally not in a hardened or protected structure and are subject to theft, vandalism, or damage or destruction from environmental causes, and there is also potential for data loss owing to the ad hoc nature of the network infrastructure. For example, a set of inexpensive temporary buoys might be deployed to record ocean currents over a period of time, with the data to be collected at the end of the study period, only to have a major storm hit the network on the last day before collection, sinking half of the buoys and compromising the entire project.
The most sophisticated approach to monitoring the aquatic environment is to place the survey instruments on a ship, boat, or other craft and take the desired measurements from that vessel. For example, an aquaculture facility might have small craft with water quality sensors mounted in the hull to patrol the edge of the fish pens, testing the level of contaminants in the water. This approach has the advantages of most of the other approaches described here; it is flexible, it is protected from the environment, it has human observers on-site to address problems or concerns, etc. The main disadvantage of this approach is that ships and boats tend to be very expensive. The NBDC would need a much larger budget if it wanted to keep 1300 crewed ships on station to watch for tsunamis!
One way of greatly reducing the costs of this approach is to utilize ROVs as the data collection platforms. As ROVs are small, often portable by just one person, they can be deployed very flexibly in any aquatic environment, from water tanks to the deep ocean. Since they are controlled by a human operator, they maintain great flexibility and ability to adapt on the spot to problems that may arise, and are also protected against theft or vandalism by the proximity of the operator. When adverse weather or environmental conditions arise, the ROVs can be pulled out of the water, reducing the risk of data loss or loss of the vehicle itself owing to those kinds of conditions. Most importantly, ROVs can be equipped with a very wide array of top-quality scientific sensors, enabling them to monitor aquatic conditions just as well as sensor buoys or shore stations.
Every data collection mission has its own objectives and special needs, and finding the right type of monitoring solution for the mission requires judgment. ROV-based data collection is one option among many, but it is an option that is very flexible and has the potential to keep costs low without compromising the data collection process.