Dynamics in the trophic structure of the macroinvertebrate community in a Mediterranean, temporary stream

Studies on the ecology of resource availability and its relation to hydrological regime are essential for aiding the understanding of stream functioning. This paper assesses the intrinsic role of potential food resources and hydrological variables in determining the functional structure of the macroinvertebrate community in a temporary, spring-fed stream. Quantitative estimates of food resources and aquatic macroinvertebrates, together with estimates of the hydrological variables, were collected during an annual surficial flow period. Standing stocks of benthic organic matter were moderately high (annual mean of 157.7 g AFDM m(-2)) and remained relatively constant over time. Moreover, allochthonous material was dominated by coarse material, reflecting the high retention but slow decomposition rates of Holm oak leaves, the major constituent of allochthonous inputs. Mean levels of periphyton biomass were high and decreased over the flow period (annual mean of 17 mg chl a m(-2)). The functional structure of the benthic macroinvertebrate community varied little over the study period and was consistently dominated by grazers and collectors. Conversely, predators and shredders were poorly represented and increased over the flow period. These observations were attributed to the impoverished invertebrate community that frequently characterizes temporary streams and the poor food quality associated with sclerophyllous leaf inputs at the beginning of the flow period. The small amplitude in discharge during most of the annual flow period and the high temperatures that distinguish the Mediterranean climate might allow algal food to be always available for consumers. This result may explain why autochthonous material was the most important food resource controlling food web dynamics in the studied temporary stream.