Bio-Physical Processes Controlling Plankton Community Structure at Isla Canales de Tierra, Underwater Tropical Observatory (PLUTO)


Scott M. Gallager and Christine Mingione, Department of Biology, WHOI
Steven Lerner, Applied Ocean Physics and Engineering, WHOI

The first 10 months of operation of the Panama Liquid Jungle Laboratory Underwater Tropical Observatory (PLUTO) on Isla Canales de Tierra 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 run off 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 (16°C) 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, 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. During the wet season variability in hydrography, Chlorophyll and backscatter were low until late in the season when a second bloom occurred concomitant with the lowest salinity, presumable due to nutrients from runoff. Thus a variety of processes including coral growth, plankton production, and fish distributions in the Gulf of Chiriqui may be directly linked to upwelling in the Bay of Panama. This project proposes to study the mechanisms controlling seasonal changes in plankton community structure by coupling long term, high frequency observations using a VPR mounted on PLUTO with local process studies designed to understand horizontal and vertical variability in plankton distributions. Observations using a towed VPR will be coupled with the hydrographic survey and modeling study proposed by Ji, Limeburner, and Beardsley to allow a first look at the relative influence of internal waves and terrestrial runoff as sources of nutrients and energy for structuring plankton communities.