In situ nutrient enrichment experiments with periphyton in agricultural streams
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Four agricultural streams were examined to determine if nitrogen or phosphorus was limiting the growth of the attached algal communities. Experiments were conducted in situ using nutrient-diffusing artificial substrata. Experiments conducted over a 2-year period demonstrated that the benthic algal communities in these streams were seldom limited by nitrogen or phosphorus. Nitrogen was, however, found limiting on two occasions when ambient nitrogen levels were reduced. These experiments were characterized by extended low flows and warm water temperatures. Large algal mats which proliferate during these periods may be responsible for reducing the available nitrogen to a limiting level. Major storm events are thought to interrupt the development of nitrogen limiting conditions by scouring the algae and increasing the nitrogen loading from the watershed. Water temperature was demonstrated as an important factor in controlling periphyton growth rates and may have influenced algal response to nutrient input. Ammonium additions often enhanced algal growth in the absence of nitrate stimulation.
Key wordsnutrient limitation algae periphyton nitrogen stream agriculture
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