Assessing the ecological integrity after nutrient inputs in streams: The relevance of the observation scale
Keywords:
Algae, bacteria, macroinvertebrates, colonization, monitoringAbstract
Two different streams receiving high nutrient content were compared in relation to evidence provided by structural (biomass) and functional (community metabolism) descriptors. The first (Gaià) was an unshaded agricultural stream, where high net community metabolism and chlorophyll concentration were only modified by flood episodes which caused the sloughing of the benthic community. The second stream (Fuirosos) was forested and nutrients were added for 44 days. In this case, bacteria and macroinvertebrates were also observed and were compared to a control upstream stretch with low nutrient content. Even though there was an increase of chlorophyll because of the nutrient addition, the macroinvertebrate community density as a whole was not affected. Moreover, metabolism (net community metabolism and respiration) did not experience significant changes. An unexpected effect of nutrient addition was detected using a finer scale of observation: during a colonization experiment, chlorophyll and bacterial density became uniform for the different substrata analyzed (sand, cobbles) after 44 days. These structural changes implied a loss of structural heterogeneity which may be associated with significant modifications of stream functioning (such as the nutrient retention) for these systems in the longer term.
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