Abstract Spatial patterns of planktonic production and respiration in the surface mixed layer were examined in eight Texas, USA reservoirs to test the hypothesis that P:R ratios are lowest in upreservoir inflow zones and highest in downreservoir open-water zones, as predicted by the heuristic reservoir zonation model. We measured summer planktonic metabolism with light–dark bottles and physical–chemical condi- tions in epilimnetic water at 85 sites distributed among sixteen longitudinal transects within the eight reservoirs