Regulation of phytoplankton and bacterioplankton in riverine systems: Light, temperature and nutrients

Heterotrophic bacterial production and phytoplankton primary production were assessed monthly during 1998 and 1999 in the Ohio River and two tributary reservoirs (Kentucky Lake and Lake Barkley). Discharge varied seasonally, with spring discharge values 3 fold (reservoirs) to 15 fold (Ohio River) greater than summer base pool (∼500 m3s−1 at all sites). Mean water column irradiance was inversely correlated with discharge and tended to be higher in the reservoirs (∼8 E·m −2·d−1) compared with the Ohio River mainstem (5 E·m−2·d−1). Bacterial production ranged from 25–1700 μg C L−1·d −1 in the reservoirs and 20–1000 μg C L₋₁ ·d−1 in the Ohio River and was correlated with temperature and dissolved organic nitrogen. Primary production averaged 60 μg C L−1·d−1 in Kentucky Lake, 190 μg C L−1·d−1 in Lake Barkley and ∼50 μg C L−1·d −l in both sites of the Ohio River and was positively correlated with temperature and light. Community respiration rates were generally lower in the reservoirs and during low discharge in the riverine mainstem, reflecting reduced DOC loading and utilization of algal-derived substrates. Low heterotrophic:autotrophic and P/R ratios suggested dependency on non-phytoplanktonic organic matter in the Ohio River and during high discharge in the reservoirs. Seasonal and spatial variations of resource limitation of phytoplankton and bacterioplankton were assessed monthly from March to November 1999. Laboratory dilution assays with light and nutrient manipulations indicated light as the dominant factor limiting phytoplankton growth at irradiances below 4–9 E m−2 d−1. During light saturated conditions reservoir phytoplankton were P-limited (Lake Barkley), or shifted from a spring P- to an autumn N-limitation (Kentucky Lake), while silica became limiting to riverine phytoplankton. Temperature and nutrient assays indicated that bacterioplankton shifted from temperature to substrate limitation at temperatures >20°C. Inorganic nutrients were important in low nutrient environment of reservoirs, while riverine bacteria were most frequently limited by carbon. Bacterial growth efficiencies ranged from 0.01 to 0.6 and were negatively related to temperature. Our findings indicate that multiple factors regulate phytoplankton and bacterioplankton and that spatial complexity may arise from differences in discharge and river geomorphology.