Regionalization of Station ALOHA

David Karl, Dale Hebel and Luis Tupas

School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822

Abstract

The Hawaii Ocean Time-series (HOT) JGOFS core measurement program was established to observe and interpret physical and biogeochemical processes at a "strategic, " i.e. representative, open ocean location. The value of the data sets collected are, in part, related to our ability to generalize the results obtained at our site to ecosystem processes over much - larger regions of the subtropical North Pacific ocean; i.e., to the successful regionalization of Station ALOHA. Our efforts to date include site survey and assessment of the "island effect," site verification by measurement of onshore-to-offshore gradients in key habitat variables, remote sensing of our study region using AVHRR and TOPEX satellite imagery, and historical and contemporary comparisons of our data sets to previous measurements in the subtropical North Pacific Ocean.

We evaluated several major criteria prior to selection of the site for the HOT oligotrophic ocean benchmark hydrostation. First, the station must be located in deep water (> 4000 m), upwind (north-northeast) of the main Hawaiian islands and of sufficient distance from land to be free from coastal ocean dynamics and biogeochemical influences. On the other hand, the station should be close enough to the port of Honolulu to make the relatively short duration (< 5 days) monthly cruises logistically and financially feasible. A desirable, but less stringent criterion would locate the station at, or near, previously studied regions of the central North Pacific Ocean, in particular Station Gollum.

After consideration of these criteria, we established our primary sampling site at 22 45'N, 158 00'W at a location approximately 100 km north of the island of Oahu, and largely restrict our monthly sampling activities to a circle with a 6 nmi radius around this nominal site. Station ALOHA is in deep water (4800 m) and is more than one Rossby radius (50 km) away from steep topography associated with the Hawaiian Ridge. We also established a coastal station west-southwest of the island of Oahu, approximately 10 km off Kahe Point (21 20.6'N, 158 16.4'W) in 1500 m of water. Station Kahe serves as a coastal analogue to our deep-water site and the data collected there provide a near-shore time-series for comparison to our primary open ocean site.

To date, we have conducted several site validation studies including a series of regional surveys using a grid of stations located north and south of Station ALOHA on the 100-200 km space scale, and a recently completed 700 km transect cruise to the CLIMAX study area (28 N, 155 W). Preliminary results from our ALOHA-CLIMAX transect will be compared to the extensive historical data set collected in the period 1960-1980 by colleagues from Scripps Institution of Oceanography to examine regional coherence in several key biogeochemical parameters.