Copepod biodiversity as an indicator of changes in ocean and climate conditions of the northern California current ecosystem

We evaluated copepod taxonomic diversity as a potential biological indicator of ocean conditions in the northeast Pacific Ocean using data collected biweekly between May 1996 and December 2004 and from 1969 to 1973 and 1983 off Newport, Oregon. During the summer, low copepod biodiversity is accompanied by high biomass, with the opposite patterns prevailing in the winter. High biodiversity, and associated low biomass, is also observed during the summers of major El Nino events (1983, 1997-1998) and during years when the Pacific Decadal Oscillation is in a positive phase. From 1996 to the present, seasonally adjusted monthly anomalies of taxa richness and the Shannon-Weiner diversity index were compared to hydrographic and meteorological variables and to basin-wide climatological indices. Correlations of biodiversity with physical variables were strongest during the summer months for the Multivariate El Nino/Southern Oscillation Index, temperatures at a water depth of 50 in, and the Pacific Decadal Oscillation. During a 4-year cool period (1999-2002), biodiversity was low, likely a result of the increased transport of coastal subarctic waters into the northern California current. In recent years (2002 2005), however, there has been a dramatic increase in biodiversity. These increases may be attributable to the influence of a weak El Nino event in 2003; yet, high biodiversity persisted through the summer of 2005 and at times was higher than during the strongest El Nino events of the 20th century (1983 and 19971998). Our analyses suggest that changes in source waters, driven by remote basin scale forcing and not local environmental events, cause interannual-to-decadal variations in copepod biodiversity in the northern California current.