Spring and summer phytoplankton community dynamics and comparison of FRRF- and ¹³C-derived measurements of primary productivity in Rivers Inlet, British Columbia

• Main Author: Shiller, Victoria Jade
• Language: English
• Published: University of British Columbia 2012
• Online Access: http://hdl.handle.net/2429/43570

Spring and summer phytoplankton community dynamics were monitored in the temperate coastal fjord, Rivers Inlet, British Columbia, to understand their impact on the growth of juvenile sockeye salmon. Spatial patterns in timing and magnitude of the diatom-dominated spring bloom appeared to be controlled by differences in mixing and stratification. At the sheltered head of the inlet, riverine input stratified the water column, the phytoplankton bloom appeared earlier and more intense. At the well-mixed mouth, where currents diluted the phytoplankton seed population, the bloom appeared delayed and reduced in intensity. The spring bloom was terminated by nitrate depletion except where salinity was low and phosphate became limiting due to oligotrophic freshwater input. While the spring community was diatom-dominated, the summer community was more diverse, with increased abundance of flagellates and ciliates. Primary productivity measurements using fast repetition rate fluorometry (FRRF) correlated well (~1:1) with estimates derived from 2-hour ¹³C-uptake incubations when phytoplankton were healthy in the lab. In contrast, under high light or low nutrient conditions, FRRF underestimates primary productivity. Community composition may also influence FRRF estimates of primary productivity. We calculated annual FRRF-derived primary productivity of 550 – 1100 g C · m⁻² · yr⁻¹. Potential sockeye production was estimated using primary productivity estimates derived from our ¹³C data. These calculations suggest that the sockeye carrying capacity of the fjord and lake are well matched and that current levels of primary production could support the ~10-fold higher historical sockeye returns. This implies that contemporary and historical levels of primary production are similar and are not the cause of the sockeye decline. More likely, a timing mismatch between trophic levels is negatively impacting sockeye smolts. Our results suggest that future sockeye production in Rivers Inlet may be negatively impacted by increased freshwater input and stratification, both of which may be influenced by climate warming. This physical forcing may precipitate a shift to a flagellate-dominated phytoplankton community, a longer food chain, and reduced energy transfer to smolts. Continued monitoring of phytoplankton dynamics is critical for refining predictions of ecosystem change and facilitating improvements in sockeye stock management policies.