Zooplankton in the Strait

From  ”State of physical, biological, and selected fishery resources of Pacific Canadian marine ecosystems in 2012

Dave Mackas, Moira Galbraith, and Kelly Young, of Fisheries and Oceans Canada have compiled and analyzed historic zooplankton data dating back to 1990 for the Strait of Georgia.

Many of the dominant zooplankton taxa in the Strait of Georgia make extensive vertical migrations at daily and/or seasonal time scales. For this reason, the authors focused their analysis on the subset of samples that were collected at deep locations using net tows that sampled all or most of the water column. From 1990-1995 and 1998-2010, sampling by DFO and the Universities of Victoria and British Columbia provided a methodologically consistent and year-round set of time series samples. The authors have reported large decadal changes in the SOG zooplankton community.

The location of their sampling points within the Strait are shown in Figure 1. The SoG total zooplankton dryweight biomass is dominated by copepods (~40%) and euphausiids (~29%). Post 1990 time series for these two categories are shown in Figure 2. Along with many other zooplankton taxa, copepods and euphausiids had low biomass in 1994-95 and very low biomass in 2005-07.

Interannual variability of the entire zooplankton community is often summarized using a statistical technique called Principal Components Analysis (PCA). For the Strait of Georgia, nearly 36% of the total community variability projects onto the first principal component (time series shown in Figure 3a). This component had positive coefficients for nearly all of the zooplankton taxa (i.e. a positive PC1 score indicates above-average biomass for most taxa). It also correlated strongly with below average temperature in the Strait, and with a North Pacific climate index called the North Pacific Gyre Oscillation (NPGO) (Figure 3b). 

Zooplankton sampling locations (red circles) in the 1990-2010 Strait of Georgia zooplankton  time series. Area of circle is proportional to the number of samples collected at each site. Bathymetry is  indicated by blue shading. Black contour line is the 200 m isobath.

Figure 1. Zooplankton sampling locations (red circles) in the 1990-2010 Strait of Georgia zooplankton time series. Area of circle is proportional to the number of samples collected at each site. Bathymetry is indicated by blue shading. Black contour line is the 200 m isobath.


Mackas1

Figure 2. Examples of 1990-2010 zooplankton time series from the Strait of Georgia (from Mackas et al. in press). Top panel shows total copepods, bottom panel shows total euphausiids. Squares indicate logscale anomalies relative to average seasonal cycle. Grey circles indicate annual geomean dryweight biomass: small triangles are biomass in individual samples.


Mackas2

Figure 3.Time series of annual scores for the lead zooplankton PC, and for the North Pacific Gyre Oscillation Index (NPGO). Summarized from Mackas et al. in press