Monitoring of harmful algal blooms in the era of diminishing resources: A case study of the U.S. West Coast

Spatial and temporal decorrelation scales in phytoplankton bloom magnitudes are reviewed with the goal of informing the design of efficient and informative observing networks for monitoring of potentially harmful algal blooms (HABs) along the U.S. West Coast. Our analysis of historic MODIS Fluorescent Line Height data shows that, unlike several previous studies, seasonal timing of phytoplankton blooms off the U.S. West Coast propagates from South to North. In situ data show that temporal decorrelation scales are shortest off Northern California (∼4 days) and longest in the Southern California Bight (∼17 days). In a cross-shore direction, we find that variability in the near-shore biomass is decoupled from biomass further offshore (∼2–4 km).Our review of the cloud cover gap statistics suggests that satellite data provide reasonably inexpensive information about bloom events, particularly at seasonal to inter-annual scales, but is insufficient to capture many event-scale blooms. Absent adequate satellite data, in situ monitoring becomes essential. Existing networks of automated monitoring sites from piers and moorings off the California coast is insufficient to describe regional variability in blooms, but is likely informative of bloom magnitudes in the immediate proximity of observation stations. We suggest that a more effective network will have a combination of shore-based stations and a few (5–10) offshore moorings. Shore-based stations should be located in proximity to sensitive HAB targets. This would provide monitoring of existing conditions and guide decision-making about beach closures and aquaculture management practices. Offshore stations can serve as proxy for regional conditions and can be used to issue early warnings of potential HAB conditions developing in a specific region.

► Observational data are used to design a HAB monitoring array for the U.S. West Coast. ► Daily satellite images of ocean color are insufficient to monitor individual events. ► An in situ network is essential to capture the variability of potential HABs. ► An in situ network is proposed consisting of shore and offshore stations.