Design, installation, and operation of the PLUTO observatory, Isla de Canales de Tierra, Panama

PLUTO (Panama LJL Underwater Tropical Observatory) is a cabled observatory deployed in January 2006 in approximately 20 m of water 1.5 km to the south west of the Liquid Jungle Lab, Isla de Canales, on the Pacific coast of Panama. The observatory consists of an underwater node with 1000 BT Ethernet connectivity to a shore station over an electro fiber-optic cable. The node provides diver accessible connectivity to four Ethernet ports, four 120 AC power outlets, and eight ports with serial RS232 communications and 12 and 24 VDC. All power and communications ports are under relay control from shore. Sensors currently installed include salinity, temperature, pressure, water current speed and direction (ADCP), chlorophyll, turbidity, oxygen, down-welling light at surface and two depths (PAR), and an array of temperature sensors extending from the bottom to near surface. A high definition pan and tilt camera was installed in April, 2008 to allow remote observations of fish community structure in relation to high frequency internal waves that propagate through the region during the dry season (Dec-April). The shore station houses three computers for data acquisition and processing. All data are distributed via satellite back to WHOI and made available on the web (http://4dgeo.whoi.edu/panama/). The first 29 months of operation of PLUTO has provided unprecedented opportunities to study the relative roles of offshore ocean forcing (tides, upwelling of nutrient-rich, cool, salty deep water, large scale currents), surface forcing (winds, insolation, and precipitation - evaporation), and terrestrial runoff of nutrients and particulates in a relatively undisturbed tropical region. While it is well known that plankton production in the Bay of Panama is limited by upwelling of nitrate during the dry season, no such upwelling exists to the north in the Gulf of Chiriqui where the Isla Canales de Tierra is located. However, during the dry season, observations from PLUTO show influx of cold (14-16degC) water along the bottom followed by depression and strong vertical oscillations of the thermocline, which we interpret as internal waves. These features disappear several weeks following cessation of upwelling in the Bay of Panama as seen from AVHRR satellite data. At the PLUTO site in 2006, chlorophyll reached a maximum 20 days following the most intensive period of internal wave activity while acoustic backscatter (zooplankton and particulates) from the 1.2 mHz ADCP peaked 25 days following the chlorophyll maxima (data available on the PLUTO website). A similar sequence of events occurred in 2007. During the 2006 wet season variability in hydrograpghy, chlorophyll, and backscatter were low until late in the season when a second bloom occurred concomitant with the lowest salinity, presumably due to nutrients from runoff. PLUTO is an example of a relatively inexpensive underwater observatory, hand deployed on a remote tropical island, that is providing unique, high frequency, long term observations of events, which would otherwise go undetected.