Seasonal and inter-annual variations in the abundance and biomass of Neocalanus plumchrus in continental slope waters off Oregon

ABSTRACTSeasonal and inter-annual variability in the abun-dance and biomass of copepodid stages of the sub-arctic oceanic copepod, Neocalanus plumchrus, wasstudied during the January–May growth season, usingan 11-yr time series of zooplankton samples collectedover the upper 100 m of the water column. Abun-dance and biomass peaks occur in March⁄April.Abundance and biomass of N. plumchrus were signiﬁ-cantly negatively correlated with sea surface temper-ature and signiﬁcantly positively correlated with seasurface chlorophyll a, salinity, and density above thepycnocline. The seasonal integrated abundance andbiomass of N. plumchrus declined during the warmyears (2003–05), and increased during the cold years(2006–08). The date when 50% of the population hadpassed through stage C5 was signiﬁcantly negativelycorrelated with temperature – earlier in warm yearsand later in cold years. In 3 yr (2003, 2007 and 2008),a second cohort appeared in mid-May, as indicated bythe presence of stages C1 and C2 in the samples.Unusually high abundances of N. plumchrus in thespring of 2007 and 2008 were associated with coolocean temperatures and an early spring transition inthe NCC ecosystem, suggesting that the NCC eco-system has returned to a cold phase. We discuss themerits of a hypothesis that the N. plumchrus popula-tion observed off Oregon is a local population as op-posed to one that is expatriated from the Gulf ofAlaska.Key words: California current, climate variability,copepod abundance, Neocalanus plumchrusINTRODUCTIONThe waters of the Northern California Current (NCC)system have been characterized by a dramatic shiftfrom a warm and relatively unproductive phase (1977–98) to cool and productive ocean conditions in late-1999 (Peterson and Schwing, 2003). This cool periodlasted 4 yr (1999–2002) and was followed by a suddenreversal to the warm phase in September 2002 (Hooffand Peterson, 2006). Beginning in autumn 2007, theNCC began a transition to a cold phase once again.Climate variability on inter-annual and inter-decadalscales signiﬁcantly affects overall biological processesof living marine ecosystems, especially zooplankton(McGowan et al., 1998; Hooff and Peterson, 2006). Along-term zooplankton time series collected off New-port, OR, allows us to track the response of zoo-plankton to climate variability in the NCC andnortheast Paciﬁc (Peterson and Keister, 2003; Hooffand Peterson, 2006). In coastal waters of the NCC system off Oregon,two distinct groups of copepods dominate, dependingupon season and the origin of source water masses(Keister and Peterson, 2003; Morgan et al., 2003;Peterson and Keister, 2003). Cold-water species(Pseudocalanus mimus, Acartia longiremis, and Calanusmarshallae) are predominant in the upwelling zone insummer, and warm water genera (e.g., Mesocalanustenuicornis, Calanus paciﬁcus, Paracalanus parvus,Ctenocalanus vanus, Clausocalanus spp., and Calocal-anus styliremis) dominate in winter during El Nin˜oevents and during summers with weak upwelling.In offshore oceanic waters, the large sub-arcticoceanic species Neocalanus plumchrus and Neocalanuscristatus occur in winter and spring. The presenceand dominance of Neocalanus spp. in the NCC