Habitat compression indices for monitoring ocean conditions and ecosystem impacts within coastal upwelling systems

Upwelling ecosystems are characterized by intense seasonal productivity that supports highly dynamic species populations, high diversity of mid and upper trophic levels, and a myriad of important fisheries. Climate variability and long-term change will impact upwelling intensity, timing and persistence, thereby potentially threatening resilience of coastal food webs and stability of ecosystem services. The spatial footprint of cool upwelled waters in the surface mixed-layer supports trophic transfer and ultimately the productivity of fisheries. The spatial area of upwelled water in eastern boundary upwelling systems can vary dramatically in response to both local and remote atmospheric forcing. These variations contribute to dynamic habitats that impact the structure, function, and spatial characteristics of marine ecosystems. We quantified the variability in "cool-water thermal habitat" area as a new ecological indicator, the Habitat Compression Index (HCI), for the nearshore (within 150 km of the coast) waters of the California Current Large Marine Ecosystem (CCLME). The HCI can be easily updated from ocean model products and satellite observations of sea surface temperature. We describe standardization of the HCI, regional variability, and evaluate HCI relationships with other indicators commonly used to inform ecosystem context within the CCLME. Importantly, our approach to calculating the HCI is easily extendable to other upwelling ecosystems. Further, we discuss the management context of the HCI such as assessing risk of whale entanglement in a highly profitable fishery, and implications for monitoring ecosystem shifts in coastal upwelling systems and the fisheries they support.