GRL - Zooplankton anomalies in the California Current system before and during warm ocean conditions

Abstract

<1> Zooplankton in the California Current had large anomalies in biomass and composition in 2005. The zone most strongly affected extended from northern California to southern British Columbia, where zooplankton biomass was low from spring through autumn, community composition showed reduced dominance by northern origin taxa, and life cycles of some species shifted to earlier in the year. Although similar anomalies have previously been observed over the entire California Current system during strong El Niño events, the 2005 zooplankton anomalies were more localized, initiated by a combination of very warm temperatures (since early 2003), plus weak and late upwelling, and low phytoplankton productivity in spring and early summer of 2005. However, the zooplankton anomalies persisted longer: through the remainder of 2005 and into 2006.

EDIT

<15> When did the 2005 zooplankton anomalies start to develop, and how long did they last? Initial onset may have begun as early as 2003 (Section 3.2 and Figures 1 and 2). Month-by-month plots of 2005 zooplankton biomass and compositional anomalies from central Oregon (Figure 3) show that the signs of the compositional anomalies were already fixed by the start of 2005, when January-March total biomass was near the (low) seasonal norm. In late March or April, all zooplankton anomalies began to strengthen rapidly, and continued to intensify through June. From April onward, 2005 was well below the climatology, even further below the “cool” years 2000–2002, and often below the “warm” years 1996–1998. Low biomass persisted until October, negative anomalies of “Northern” copepods persisted until September, and positive anomalies of “Southern” copepods to the end of 2005 and on into 2006. In contrast, most 2005 atmospheric and oceanographic environmental anomalies ended earlier (arrows in Figure 3). The spring transition to low coastal sea-level and equatorward wind/currents had occurred by late May . Phytoplankton biomass then began to accumulate, and monthly average chlorophyll was high by July . Sea-surface temperature anomalies turned sharply negative in mid-July . Clearly, many of the wind-forced environmental characteristics of the Oregon upwelling system (upper layer temperature, Ekman transport, phytoplankton biomass and productivity) had returned to near-average by mid summer. However, “Northern” copepods were unable to match this return to normal levels off Oregon (nor off southern Vancouver Island), and the “Southern” copepods were not dislodged from the Oregon shelf (nor from the Vancouver Island shelf). We do not as yet know why the zooplankton anomalies persisted longer, but offer two possibilities:

<16> 1. Populations of resident/northern species are constrained by evolved seasonal life history strategies. If they are to have large populations in summer through autumn, reproduction and growth must be strong in spring. Figure 3 shows partial recovery of the “northern” taxa in July and August, but their absolute amount (exaggerated on a logarithmic scale) contributed relatively little total biomass. In contrast, many of the “southern”-origin (warm water) taxa have less seasonal and more opportunistic life history and reproductive strategies. Once established, they continued reproducing in a food-rich environment, even after onset of upwelling turned that environment “cool”.

EDIT

3.4. Summary of Findings and Interpretations

<18> Significant zooplankton anomalies were observed in 2005 in much of the CCS, but were most intense off central Oregon, and gradually weakened and changed character both poleward and equatorward. Off Oregon and southern British Columbia, the zooplankton response (reduced total biomass, greatly reduced biomass of “resident” northern species, greatly increased abundance and biomass of southern-origin species) resembled responses during strong El Niño events, but local details and between-region comparisons suggest forcing in 2005 was primarily by regional weather patterns rather than by coast-wide or basin-scale physical anomalies. Zooplankton anomalies persisted two-to-many months longer than the 2005 environmental anomalies of wind, water properties, and phytoplankton productivity, suggesting significant inertia of zooplankton anomalies once they have become established. Further persistence is as yet unknown, but is an important issue because of the potential consequences of sustained zooplankton anomalies for higher trophic level populations.

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Are they "oh, that's interesting" or "Oh, Fuck" anomalies?