Stream Geomorphology: Effects on Periphyton Standing Crop and Primary Production

Anthropogenic changes in stream geomorphology have reduced the capacity of many North American streams to trap and retain sediments and organic matter. Significant losses of retentive structures from many small streams have resulted from beaver trapping. We compared algal standing crops and rates of primary production in two adjacent streams in the Zuni Mountains of New Mexico. Castor Creek has been worked by beaver and is a complex, highly retentive system with an extensive floodplain and accumulations of organic debris. Sawmill Creek is moderately incised, free of large organic debris, and poorly retentive. Clay tiles introduced into Castor Creek had average chlorophyll a standing crops 3-9× greater than Sawmill Creek (54.8-257.8 vs. 18.4-44.9 mg/m²). Chlorophyll a on natural substrates averaged 11× greater in Castor Creek than in Sawmill Creek (1679.4 vs. 154.2 mg/m²). Average standing crop of chlorophyll a per 250 m reach was 136× greater in Castor Creek than in Sawmill Creek due to its greater surface area (12.4×) and greater standing crop (11×). Extensive filamentous algal mats and the macro-alga Nitella gracilis in Castor Creek's beaver ponds exhibited the highest rates of primary production (85 mg C m⁻² hr⁻¹) while epilithic samples from both streams were similar (16.6 and 21.2 mg C m⁻² hr⁻¹ for Castor and Sawmill Creeks respectively). Nutrient amendment experiments revealed that both streams were nitrogen limited. Sediment samples taken in 1989 showed Castor Creek had 25× more dissolved NH₄ in the upper 5 cm of sediment than Sawmill Creek (1242 vs. 50 μg/L). We suggest that higher algal production in Castor Creek is linked to greater nutrient availability at the sediment/water interface due to greater retention and processing of organic matter in the hyporheic zone.