Biofilm nutrient limitation, metabolism, and standing crop responses to experimental application of salmon carcass analog in Idaho streams

Abstract

Dramatic declines of Pacific salmon (Oncorhynchus spp.) populations have decreased delivery of marine-derived material to Pacific Northwest streams where juvenile salmon reside. Managers use artificial nutrient additions to increase juvenile salmon growth and survival and typically assume nutrient-driven increases in biofilm production are an important pathway by which nutrients become available to higher trophic levels. To evaluate how biofilms respond to additions of salmon carcass analog, a pasteurized, processed form of nutrient mitigation materials, we quantified biofilm nutrient limitation, benthic and whole-stream metabolism, and biofilm standing crops before and following experimental additions in tributaries of the Salmon River, Idaho, USA. Biofilm nutrient limitation did not change and standing crop did not increase in response to analog additions at two different levels (low, 30 g·m−2; or high, 150 g·m−2) within 1 month of addition. In contrast, whole-stream and benthic primary productivity and respiration increased in a high-analog treated segment, but did not increase in a low-analog treated segment. Together, our results suggest that metabolism may be a more appropriate tool for assessing the ecosystem effects of nutrient additions than biofilm standing crop or nutrient limitation, which are constrained by a variety of abiotic and biotic factors like hydrology and grazing.