Physiological characteristics of inorganic nitrogen uptake by spatially separate algal communities in a nitrogendeficient lake

1. The physiological characteristics of nitrogen uptake by sublittoral and eulittoral (splash zone) epilithic periphyton as well as epipelic periphyton in N‐deficient Castle Lake, California were determined by evaluating the half‐saturation constants (K_t) and initial slopes (V_max/K_t) of uptake kinetics curves. These results were compared to similar studies of phytoplankton nitrogen uptake in this lake. 2. The strategies of nitrogen uptake differed among the various communities and were largely determined by the proximity of each to pools of available dissolved inorganic N (DIN). 3. The sublittoral algae did not have a high biological affinity for either NH₄ or NO₃ and depended on nitrogen fixation for their N supply. The eulittoral community showed an increased capacity for DIN uptake at low substrate concentration, but not as high as measured for the phytoplankton community. Epipelic algae live immediately adjacent to a large pool of interstitial sediment NH₄ and showed no physiological adaptations for surviving in a N‐deficient environment. 4. K_t values for all benthic communities were approximately two orders of magnitude greater than ambient substrate levels. In contrast, the half‐saturation constants for NH₄+ NO₃ uptake by phytoplankton were very similar to in situ levels of these nutrients.