ROLE OF DETRITUS IN THE PRODUCTIVITY OF A ROCK‐OUTCROP COMMUNITY IN A PIEDMONT STREAM

Abstract

A rock outcrop community in a typical southern Piedmont stream was studied to determine its trophic structure and productivity. Podostemum ceratophyllum was the primary producer component of the community and also provided a place of attachment and shelter for a rheophilic fauna consisting largely of insects. The primary consumer organisms derived 66% of their energy from allochthonous organic matter consisting largely of leaf material. Productivity by phytoplankton in the river water was insignificant as was respiration in the shifting sand bottom. The river behaved as a heterotrophic stream because of suspended organic detritus in the water. A higher and more variable quantity of particulate detritus was present in the water during summer than winter. This seasonal difference was attributed to stream discharge—stream bed relationships and a more rapid decomposition of organic fragments at higher summer temperatures. The net annual productivities, determined by the cropping method, in cal/cm 2 for trophic groups were: Podostemum 434, filter feeders 16.8, herbivores 6.51, detritus feeders 1.68, herbivore and detritus feeders 2.78 (total primary consumer 27.8), and carnivore 3.66. The turnover of biomass increased with increasing productivity by individual species but this relationship does not hold true for all communities. A comparison of trophic level production efficiencies and productivities in several communities suggests there are self‐regulating mechanisms in natural communities which function between producer and consumer groups. Community stability is an important factor for the realization of maximum productivity in flowing water environments. A theory of stream succession has been proposed which is based on the graded stream concept.