GLOBEC 2000: Long-term Changes in California Current Zooplankton Assemblages and Euphausiid Population Dynamic Parameters
M.D. Ohman, E. Brinton, B.E. Lavaniegos (all at SIO/UCSD); G.H. Rau (UCSC and NASA/AMES); and H.R. Harvey (CBL/UMCES)

In our phase 1 study, we have uncovered substantial changes in the California Current zooplankton on interdecadal time scales, (e.g., changes in salp and euphausiid regimes) as well as interannual time scales (e.g., El Niņo-related changes in copepod diversity and zooplankton biomass). These analyses have focused on springtime variations in the last 50 years in GLOBEC Region III of the California Current System (CCS), as sampled by CalCOFI off southern California. In our phase 2 study, we will focus on Region II of the CCS (off central/northern California) to determine whether the pronounced perturbations we have detected are spatially coherent in different regions of the CCS and are linked to zooplankton fluctuations elsewhere in the NE Pacific, specifically in the Oregon/N. California LTOP and the Coastal Gulf of Alaska LTOP. We will test the GLOBEC core hypothesis that: Production regimes in the coastal Gulf of Alaska and California Current System covary, and are coupled through atmospheric and oceanic forcing, through comparative analysis of zooplankton populations and measures of atmospheric and oceanic forcing in different regions of the NE Pacific.

We will further provide, for the first time, estimates of instantaneous, stage-specific mortality rates for euphausiid populations of the NE Pacific. Theoretical and empirical studies have shown the pronounced sensitivity of models of zooplankton populations and pelagic ecosystems to the mortality rate term. Yet this remains one of the least understood terms in zooplankton population dynamics and no empirical mortality estimates are available for NE Pacific modelers. We will employ inverse modeling techniques to estimate stage-specific mortality rates for Euphausia pacifica and Thysanoessa spinifera. We will test for changes in these rates before and after the 1976-77 climate regime shift, to assess whether regime-specific parameters are needed for modeling studies. We will utilize lipofuscin-based measures of euphausiid age (H.R. Harvey) to validate euphausiid age-length curves used for mortality estimation.

We will test for long-term stability in food web structure, as inferred from zooplankton stable isotope content (G.H. Rau). We will analyze long-term variability of the isotopic content to two species of chaetognaths, (obligate carnivores) to complement our current work with two species of suspension-feeding copepods. We will also analyze the seasonal component of variations in zooplankton populations and stable isotope content to ensure that the apparent long-term changes that we have documented from analysis of springtime variations are not due to shifts in seasonal timing.

To understand the mechanisms and consequences of large-scale changes in Pacific zooplankton assemblages, we need to make the best use of the longest-term data sets available.