Remote Assessment of 4-D Phytoplankton Distributions off the Washington Coast

Satellite-based optical measurements were coupled with physical and optical measurements from Seaglider – a long-range autonomous glider – to study interactions between biological and physical processes off the coast of Washington, USA, and to evaluate space-time variability of regional phytoplankton and particle distributions. Using satellite ocean color data variability in near-surface chlorophyll a was characterized across a range of spatial and temporal scales ranging from 1 – 500 km and from days – years to assess region-wide responses by phytoplankton to changes in environmental conditions. Results from 1998 – 2002 revealed both strong negative and positive anomalies associated with lingering effects of the 1997-98 El Niño and an invasion of Subartic water into the California Current System in 2002, respectively. Ocean color satellite data were also used to derive ‘spectral signatures’ for waters associated with the Juan de Fuca Eddy to monitor these waters as they moved southwards towards Washington beaches. Episodic southward transport of these waters may play a role in bloom initiation of the potentially toxigenic pennate diatom Pseudo-nitzschia and the ability to track these waters by remote sensing could help determine when to initiate more intensive sampling for domoic acid along the Washington coast. From April 2002 – December 2005 Seaglider conducted highly resolved (~5 km horizontal spacing, ~1 m vertical resolution, ~15 d temporal resolution) surveys across the northern California Current System. A new matchup procedure minimized discrepancies between Seaglider fluorescence and satellite-derived estimates of chlorophyll a at the surface (r = 0.834) allowing observations from these disparate remote sensing platforms to be fused together to create a quasi-4-dimensional representation of the phytoplankton distribution within a persistent offshore eddy in September and October 2004. Unfortunately daytime fluorescence is quenched at all times of the year in these waters with maximum quenching exceeding 80% during summer making it difficult to compare near-surface fluorescence measurements with satellite-derived estimates of surface phytoplankton biomass. A detailed statistical characterization of mid-day fluorescence quenching in April 2002 and from August 2003 – December 2005 was conducted to constrain the magnitude and variability in mid-day quenching to better use fluorescence as an independent validation for phytoplankton biomass at the surface