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Littoral zone primary production in a coastal reservoir ecosystem Beer, Julie Ann

Abstract

There has been little research examining littoral productivity in freshwater ecosystems. Previous studies have focused on pelagic production, largely because pelagic production was viewed to be the predominant source of carbon in aquatic ecosystems. More recently there has been some research to suggest that the productive capacity of littoral zones may be significant, especially in nutrient-poor ecosystems, where the transfer of carbon through the food chain is driven by microbial activity at the base of the food web. This may be particularly important in reservoir ecosystems, as there are concerns that water level fluctuations resulting from reservoir operations may seriously undermine aquatic function in the littoral zone. To assess the impacts of fluctuating water levels, a field study was designed to measure primary productivity in the littoral zones of two coastal temperate hydro-electric reservoirs located in British Columbia. Both located on the Stave River, Stave Reservoir exhibited a pronounced annual water level fluctuation, while immediately downstream Hayward Reservoir maintained a relatively constant water level. Measurements of periphyton biomass were conducted to estimate primary productivity in the littoral zones of both reservoirs. Three sampling transects were established in Stave Reservoir and one in Hayward. Periphyton samples were collected by scuba divers from Plexiglas plates along transect gradients in each reservoir to a depth of 20 m. Samples were analysed using standard lab techniques to estimate and characterize production using measurements of Ash Free Dry Mass (accrual) and chlorophyll-a concentrations to quantify biomass, and species composition to quantify abundance, biovolume and dominance of the algal communities. Primary productivity was estimated by integrating periphyton biomass accrual measurements over depth for each sampling period. Three year average production in Stave was 5.3 gC/m2/year and in Hayward was 12.3 gC/m2/year. Differences in production between the Stave and Hayward reservoirs were evaluated statistically using a two-way analysis of variance (ANOVA) to look for a lake effect and a seasonal effect. The results from the ANOVA found production in Hayward Reservoir to be significantly higher than Stave Reservoir (p = 0.05). A seasonal analysis indicated winter production was significantly lower than all other seasons in both reservoirs. Measurements of light, temperature, nutrients and water level were used to interpret spatial and temporal variability in periphyton production. Littoral and pelagic productivity measurements were used to estimate total aquatic production in Stave and Hayward reservoirs. Analysis results indicate that the littoral contribution to total aquatic production in the fluctuating reservoir environment (Stave) was only 4%, which is lower but still comparable to other oligotrophic temperate lakes. In Hayward (stable water-level environment) the littoral contribution was approximately 50% of overall aquatic production. Reseach findings comparing Stave and Hayward suggest that littoral production is negatively impacted by reservoir operations but the importance of this is questionable when the littoral contribution is so low. Primary productivity in a large, power generating reservoir like Stave, appears to be pelagically driven. Nutrients dynamics tend towards recycling but are compliated by flow in the old river channel in the lower basin. Although it is a managed system, the contribution of littoral production was approximately 5%, which is comparable to other oligotrophic BC lakes. Littoral primary production in Hayward was high compared to oligotrophic temperate lakes. This is likely a function of the fast flushing, riverine character of this reservoir. This study did not examine other factors such as light, temperature and nutrient regimes in sufficient detail to directly compare these factors between the two systems. A clear indication of the effect of water level fluctuations on littoral zone productivity is complicated in this case by the fact that the two reservoirs studied exhibited significant differences in character which may also have accounted for differences in littoral zone productivity.

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