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Effect of forest harvesting on biological processes in headwater streams within the flume creek experimental watershed, British Columbia Hiebler-Chariarse, Jennifer Ann

Abstract

Effects of forest harvesting on periphyton biomass (measured as Ash Free Dry Mass (AFDM) and Chlorophyll a), benthic organic matter (measured as AFDM) and invertebrate density and diversity were studied in coastal headwater streams in southwestern British Columbia. These aquatic ecosystem variables were also evaluated along the longitudinal gradient of these streams to examine longitudinal patterns. In addition, photosynthetically active solar radiation (PAR), stream water temperatures and chemical concentrations of streamwater were measured and evaluated in order to determine their role in regulating the aforementioned variables. Six headwater streams (1st and 2nd order), three undisturbed and three harvested (3 years postharvest), were sampled over a six-week period during the early summer (May 31 - July 7, 2002). Each stream had nine study reaches (covering a total stream length of 500-700 meters), with harvested streams having three reaches above the harvested area, three reaches within the harvested area and three below - providing for comparisons between forested and harvested stream reaches within streams. Periphyton biomass sampling occurred once a week over the six-week study, while benthic macro invertebrates were sampled over four days during week 3 of the study. Physical and chemical data were also collected once a week for the six-week study period. No significant longitudinal trends were found within the undisturbed streams for any of the studied variables. However, along the harvested streams, Chlorophyll a and stream temperature increased along the downstream longitudinal gradient as the stream flowed through the harvested area and subsequently decreased as the streams flowed back into forested reaches. In addition, incident solar radiation decreased (however not statistically significant) as the streams flowed further into the forested area below the harvested boundary. Incident solar radiation and stream water temperatures were greater in the harvested reaches than in forested reaches of the harvested streams. Periphyton biomass levels were greater within harvested reaches than in forested reaches of the headwater streams. Incident solar radiation was considered to be a primary factor controlling periphyton biomass in these headwater streams, as periphyton biomass increased with increased incident solar radiation. Increased invertebrate density was positively related to periphyton biomass, suggesting bottom-up controls in the aquatic food web. While richness of aquatic invertebrate orders was greater in forested reaches below the harvested area, overall invertebrate diversity was not altered by previous harvesting activities. No differences were seen in benthic organic matter biomass nor were there significant relationships between nutrient concentrations with the aforementioned research variables during the study period.

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