Over the past few decades, a range of strategies and techniques has been used to monitor the sea. More recently, the role of monitoring has been expanded to include the use of autonomous underwater vehicles to perform ocean surveys. With these vehicles it is now possible for the scientist to make complex studies on topics such as the effect of metals, pesticides and nutrients on fish abundance, reproductive success and ability to feed, or on contaminants such as chemicals or biological toxins that are transported in particulate form and become incorporated into living organisms (plankton, bivalves, fishes) or become deposited in bottom sediments. The scientist or environmentalist may desire to detect hazardous substances in the ocean such as chemicals from an underwater vent or toxic algae such as red tide. Additionally, the military's detection of mines, biological, chemical or radioactive threats are also very important in the monitoring of the seas.
These considerations explain today's development of new types of autonomous underwater vehicles with integrated sampling equipment that is able to perform a wide-range of fully automated monitoring surveys over extended periods of time. These vehicles survey and monitor the sea environment in a cost-effective manner combining survey capabilities, simultaneous water sampling and environmental data gathering capacities. Included in these types are autonomous underwater gliders that have the ability to glide for long distances and are in some cases able to travel under power. There are currently four classes of underwater gliders: 1) those that use mechanical or electrical means of changing their buoyancy (i.e., drop weights, or electrical power from batteries), 2) those that use the thermal gradient of the ocean to harness the energy to change the vehicle's buoyancy, 3) those that are able to use other means of power such as ocean wave energy, and 4) hybrid vehicles that use standard propulsion systems and glider systems.
Gliders are designed for deep water where the vehicle can traverse large areas with minimal use of energy and are specifically designed for the needs of the Blue Water scientist, which require greater control over the vehicle (the free-drifting profiling Argos floats that scientists often use have no control capabilities beyond descending and rising vertically in the water column. The newest Argos float model cycles to 2000m depth every 10 days, with 4-5 year lifetimes for individual instruments)(Argos, 2008). Some of these glider AUVs have space for multiple scientific instruments and have the ability to obtain water or biological samples. Scientists who perform experiments in shallower water can also use the vehicle for short duration gliding dives or under power if one of the hybrid gliders is used.
The more information scientists are able to accumulate the better they will be able to determine the health of the ocean ecosystem and document the specific ecosystem parameters. Using an AUV glider, pollution of ocean waters can be detected and quantified in an automated way; depending on the glider, water samples can be taken and analyzed to determine water quality as well as any contaminating chemicals. Thus, dangerous substances in the sea can be detected earlier and their harmful effects can be dealt with quicker. Depending on the vehicle's configuration the scientist may have the ability to take fly-by photographs of organisms in the water column.
Initially gliders were targeted for missions that were a combination of three archetypes: time series, transects and roving assistants to research cruises (Sherman et al., 2001) by the scientist for surveying and monitoring the deep-sea environment. This is still true but scientists at various institutions such as Florida Institute of Technology's Department of Marine and Environmental Systems in Melbourne Florida have desired more. A survey was conducted of the opinions of marine scientists (biological, physical, chemical oceanography, marine biology, environmental science, and ocean engineers) and the biological research published on the Internet with respect to which organisms take precedence in ocean studies was analyzed. From these investigations, one of the most important biological groups in the life cycle of higher ocean organisms (e.g., fish), and consequently a very important element in the research of all marine organisms, was found to be the phytoplankton1. Phytoplankton play a fundamental role in the ocean's biological productivity and directly impact the climate. It is important that scientists determine how much phytoplankton the oceans contain, where they are located, how their distribution is changing with time, how much photosynthesis they perform, and what organisms such as marine invertebrate larvae feed upon the phytoplankton (Herring, 2007b).
Next, the marine invertebrate larvae and zooplankton (e.g. krill - Euphausia superba) were found to be a very important biological group that affect the life cycle of higher ocean organisms. Krill are small shrimp like crustaceans that are the most important zooplankton species associated with sea ice and are very important in the Antarctic food web. Krill occur in groups or large swarms and occupy a niche similar to that of the herring in the North Atlantic. Krill attain a size of 6-cm and feed primarily on phytoplankton or sea ice algae. Its feeding apparatus is built to filter phytoplankton out of the water column and to scrape algae from the ice. Krill is the staple food of many fish, birds and mammals in the Southern Ocean. The biomass of Antarctic krill is considered to be larger than that of the earth's human population and krill swarms can occupy an area equivalent to 2.5 times the size of Washington, DC (AWI, 2008a).
Southern Ocean GLOBEC is conducting a study of the Antarctic krill in which they are attempting to define the habitat, prey, predators, and competitors of this species. This organization could make immediate use of such a vehicle as the autonomous underwater glider. In fact, recent evidence indicates that seasonal coverage is necessary to fully understand the linkages between the environment, krill, and top predators.
1 Phytoplankton are microscopic plants that live in the ocean. There are many species of phytoplankton that grow abundantly in oceans around the world and are the foundation of the marine food chain. Since phytoplankton depend upon certain conditions for growth, they are a good indicator of change in their environment making them of primary interest to oceanographers and environmental scientists (Herring, 2007a).
The zooplankton science questions that a glider could help answer are (AWI, 2007b):
1. What is the abundance of krill?
2. How many populations are there?
3. How do krill survive during winter with a minimal food supply?
A third important biological group that affects the life cycle of higher ocean organisms is the algae, which at times is responsible for harmful algal blooms (HAB). HABs occur throughout the world, affecting European and Asian fisheries, Caribbean and South Pacific reef fishes, and shell fishing along the coasts of the United States. These HABs are caused by several species of marine phytoplankton, microscopic plant like cells that produce potent chemical toxins (Mote, 2007).
Research on these and other biological groups requires non-traditional approaches to acquire the needed scientific information. Various institutions are addressing this issue by developing vehicles which implement special biological catching and photographing systems to document small visible species, using a navigation system that will use the scientific data to control the vehicle's movement. In addition to biological investigations, documenting the chemical make-up of all areas surveyed, specifically where samples were taken is important for the scientist to obtain a complete understanding of that region. The chemical layout and the corresponding biological data are normally for a specific transect, but a vehicle's transect might not be on a traditional grid pattern. For example, the scientist may desire to obtain samples within a polluted area with a specific concentration of the pollution. To accomplish this, non-traditional approaches of navigation are required to acquire the desired scientific information. Data from geophysical and acoustic sensors can be combined, analyzed and entered into the navigation system to aid in controlling the vehicle with respect to the chemical information supplied.
To date, most survey AUVs have relied on rudimentary single variable differential gradient navigation systems, external triangulation, or inertial based dead reckoning systems. Research is now being conducted using the changes in various geophysical parameters as navigation cues (i.e., phenomenon based navigation). Some of these parameters are temperature, salinity, turbidity, chlorophyll, rhodamine, fluorescein, and passive acoustic signals. These navigation techniques are expected to provide a better understanding of the geophysical environment where biological samples are obtained, in addition to characterizing the data.
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