Influence of Rossby waves on nutrient dynamics and the plankton community structure in the North Pacific subtropical gyre

Nitrate concentrations, chlorophyll a (Chl a) fluorescence, radiance, salinity, and temperature were measured on the Hawaii Air-Sea Logging Experiment, A Long-Term Oligotrophic Habitat Assessment (HALE ALOHA) mooring located near the Hawaii Ocean Time-Series (HOT) Program's Station ALOHA (22degrees45'N; 158degreesW). Nitrate concentrations were determined with OsmoAnalyzers deployed at depths of 120 and 180 m. Deployments in 1997 and 1999 captured monthlong events that brought relatively cold high-nitrate seawater up into the euphotic zone. These events were correlated with negative sea surface height (SSH) anomalies measured by the TOPEX/Poseidon satellite altimeter. These nutrient injections at the Hawaii site were predominantly associated with first baroclinic mode Rossby waves. Elevated nitrate concentrations resulted in increased Chl a concentrations, increased primary productivity, and shifts in the phytoplankton community structure, as determined by HPLC analysis of pigment concentrations. The relative increase in pigments associated with phytoplankton that can grow rapidly and exploit nitrate (e.g., haptophytes and pelagophytes) coincided with the passage of Rossby waves in 1997-1999. A long-term combination of satellite remote sensing, moored instrumentation or remote vehicles and periodic ship-based sampling is needed to fully characterize the spatial and temporal variability due to the passage of Rossby waves and their associated biological responses.