Periphyton metabolism along a nutrient gradient in a desert river (Truckee River, Nevada, USA)

Periphyton metabolism was studied at five sites along a 70 km unshaded stretch of the Truckee River below the City of Reno, Nevada (USA). Sites differed with respect to concentrations of soluble reactive phosphorus and total dissolved inorganic nitrogen because a water pollution control facility discharged its treated wastewater through a small tributary into the river downstream of site 1 (most upstream located site). Unglazed tiles were incubated at each site from late June to August (summer) and from September to November (autumn) 1986. At the end of each incubation period, oxygen metabolism of the periphyton communities growing on tiles was measured in transparent flow-through respiration chambers for 24 h under near natural light and temperature conditions. In August, when biomass and metabolism were positively correlated with nitrogen and phosphorus, mean chlorophyll a ranged from 53 (site 1) to 290 mg chl a m(-2) (site 2, 3 km downstream of the wastewater input). Periphyton gross primary production varied between 3.3 +/- 0.8 (site 1) and 9.1 +/- 1.4 g O-2 m(-2)d(-1) (site 2), and respiration between 2.1 +/- 0.4 and 10.1 +/- 1.3 g O-2 m(-2)d(-1). P/R ratios ranged from 0.9 at site 2 to 1.8 about 65 km downstream from the wastewater input. In November, chlorophyll a was significantly higher (99 +/- 29 to 509 +/- 155 mg chl a m(-2) and gross primary production and respiration on average 50 and 65% lower, respectively, than in August. Hence, colder temperatures appeared to reduce respiration more than primary production and significantly increased P/R. Neither gross primary production nor respiration were correlated with autumn nutrient concentrations.