Graduate Thesis Or Dissertation
 

Effects of light, nutrients, and grazing on periphyton communities in streams

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  • Relative effects of light, nutrients, and grazing on primary production were investigated in streams in the Cascade Mountains of Oregon and in laboratory streams. Relative magnitudes of primary production and associated processes were examined for one year in a stream that flowed through a clearcut and an old-growth forest in the Cascade Mountains. Diel patterns of nutrients in four streams of different size indicated greater demand for inorganic nitrogen than phosphorus and suggested that uptake by aquatic primary producers was responsible for diel fluctuations of nutrient concentrations. Atomic ratios of N/P supported the conclusion that primary producers were most probably nitrogen limited. In an experiment of light and nitrate addition to a stream in an old-growth forest, primary producer processes were enhanced by addition of light (11000 lux) and by addition of both light and nitrate. There was no effect of nitrate addition on accumulation of algae, gross primary production, net community primary production, community respiration, P/R ratios, or community structure of diatoms at low ambient light levels (1100 lux). Addition of light resulted in greater colonization of algae, gross primary production, net community primary production, community respiration, P/R ratios and altered community structure of diatoms. The watershed was clearcut one year later and the experiment was essentially repeated with artificial shading instead of artificial lighting. Responses of colonization and primary production to light and nitrate in the clearcut were similar to those observed in the previous experiment in the old-growth forest. Interactions between primary producers and grazers in lotic systems were examined in laboratory streams. Different densities of snails were established in laboratory channels; primary production in these channels was compared to production in a channel without grazers. Heavy and intermediate grazing resulted in decreases in standing crop of chlorophyll a, biomass/chlorophyll ratios, biomass of periphyton, gross primary production, and net community primary production. Low grazing densities did not significantly alter the above factors. Gross primary production increased with increasing standing crop of chlorophyll a but assimilation number was much greater at low standing crops; therefore, there was a partial compensatory mechanism for adjustment to grazing pressure. Total primary production was approximately equal at both high and intermediate densities of grazers and was greater at low densities of grazers and no grazing. Taxonomic structure of periphyton was altered by heavy and intermediate grazing but not changed at low grazing pressures. Snail growth increased with increasing biomass of periphyton. Responses of aquatic primary producers to canopy removal were examined in a clearcut and old-growth forest section of a stream in the Cascade Mountains of Oregon. Standing crop of chlorophyll a, biomass of periphyton, community respiration, assimilation number, and respiration per unit weight of periphyton were all significantly greater in the clearcut than in the old-growth forested section. Gross primary production was approximately 2.5 times greater in the open section than in the shaded section. Turnover times of periphyton were shorter in the clearcut than in the old-growth forest site. Photosynthetic efficiency was greater in the shaded site than in the open section. Processes of primary production were generally enhanced in the clearcut section of Mack Creek as compared to the old-growth forested section. Shifts of food bases in streams from allochthonous material to autochthonous primary production may have major effects on structure and function of stream ecosystems. Succession of energy bases is best viewed in terms of overall watershed succession. Aquatic primary producers are an integral component of stream ecosystems and a major determinant of the nature and dynamics of higher trophic levels in streams.
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