| Summary: | Aspects of the biological oceanography of the Strait of Georgia have been studied
periodically since the 1930s, although very few studies have measured nutrient concentrations,
the timing and magnitude of the spring phytoplankton bloom, and the timing and magnitude of
the spring zooplankton peak biomass. This thesis reports nutrient concentrations and
phytoplankton and zooplankton biomass during spring of 1996-1998 in the central Strait of
Georgia. Sampling was conducted biweekly from February to June, for nitrate, phosphate,
ammonium, and urea concentrations, chlorophyll concentrations, zooplankton dry weight, and
Neocalanus plumchrus stage composition.
Nitrate and phosphate concentrations were high in late winter but began to decline in late
March reaching a minimum in June. Ammonium and urea concentrations were low in late winter
and increased in late March reaching a maximum in April. Concentrations then declined and
reached a minimum in June. Two large chlorophyll peaks were observed in the Strait in March
and June, bracketing the large peak in zooplankton biomass that occurred in late April.
These results were then compared with results from previous studies in an effort to
determine i f the timing and the magnitude of spring bloom are controlling the timing and
magnitude of the zooplankton peak biomass. Since the mid-1960s there have been nine years
where sampling took place in the spring for both chlorophyll and zooplankton biomass. These
data revealed in years when there was tight coupling between phytoplankton and zooplankton,
zooplankton biomass was not always greater than years with poor coupling. This suggests that
this relationship is more complex and that there are other factors that determine the magnitude of
the spring peak in zooplankton biomass.
The observed peak in zooplankton biomass in the 1990s occurred approximately one
month earlier than had previously been reported for the Strait in the 1960s and 1970s. Because
N. plumchrus is the dominant zooplankton species in these waters at this time, it was proposed
that this shift in timing was due to a change in the developmental timing of this copepod. When
historical N. plumchrus stage composition data were collected it was determined that there has
been a shift in developmental timing on the order of 25 days. Three hypotheses were proposed to
explain this change. Presently, the favoured hypothesis is that a gradual warming o f the surface
layer in the Strait over the past 30 years has induced this change, possibly due to a shortening of
each copepodite developmental stage.
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