Interannual and interdecadal patterns in timing and abundance of phytoplankton and zooplankton in the central straight of Georgia, BC with special reference to Neocalanus Plumchrus

• Main Author: Bornhold, Elizabeth Anne
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/10233

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.