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UBC Theses and Dissertations

The influence of the stream-lake confluence on benthic lake delta communities and the impacts of upstream forest harvest Becu, Mariella Helen Jacoba


The lake delta (LD) is the recipient of nutrient and energy subsidies from upstream habitats. Donor-controlled subsidies can support primary production and consumer densities in recipient LDs. However, LDs may also receive the impacts of upstream disturbance. In coastal British Columbia, small oligotrophic lakes with numerous stream inflows are common, yet forest harvesting (FH) is also pervasive across this region. FH can adversely affect stream biota by altering sediment and nutrient inputs, temperatures, water yields, and food availability. However, downstream contributions, especially to lakes, are rarely measured or considered within FH management. My objectives were to test (1) whether LDs are more productive and/or diverse habitats compared to areas of the lakeshore (LS) not influenced by stream inflows; and (2) whether there are shifts in LD benthic communities in response to upstream FH. I compared LD communities with upstream riparian areas that were harvested (harvested LDs – HLDs) or relatively undisturbed (unharvested – ULDs) within 100 m of the streamside, and LS sites. It was predicted that subsidies would lead to higher periphyton and macroinvertebrate standing stock, and a different community structure in ULDs and HLDs versus LS sites. As nutrient runoff is often higher after FH, higher periphyton measures in HLDs versus ULDs were predicted. Due to adverse effects of FH on aquatic biota, macroinvertebrate abundance and/or diversity and the relative abundance of sensitive versus tolerant taxonomic and functional trait groups were predicted to be higher in ULDs versus HLDs. I found that LDs in British Columbia are hotspots for benthic production; however, upstream FH may reduce periphyton and macroinvertebrate standing stocks by approximately 30 and 50%, respectively. Periphyton and macroinvertebrate standing stocks were 1.5 and 2 times higher on average in ULDs, versus HLDs and LS sites, respectively. The community structure and relative abundance of most taxa and trait groups were similar, and sensitive and tolerant groups showed trends related to food availability. My results suggests that nutrient and energy availability, consumption, and benthic production were likely higher at ULDs. Identifying ULDs as hotspots along the shoreline advances our understanding of FH disturbance in relation to stream-lake connectivity.

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