Factors controlling seasonality in leaf-litter breakdown in a Mediterranean stream

Abstract

Litter breakdown is a pivotal ecosystem function in headwater streams, where it fuels food webs and controls C flux. Breakdown rates depend on environmental characteristics and can display strong seasonal variation, particularly in intermittent streams. To identify the environmental factors driving seasonality of litter breakdown, we ran 5 breakdown experiments with poplar leaves during the wet phase (November–August) in a 3rd-order intermittent Mediterranean stream. We assessed the contribution of decomposers and detritivores to total breakdown seasonality by measuring total (coarse-bag) and microbial (fine-bag) breakdown and estimating invertebrate-mediated breakdown rates (difference between coarse and fine mesh). Breakdown rates (k) increased from autumn to early summer when expressed as k/d and decreased during the drying phase. However, when expressed k/degree-day (dd), rates peaked in early spring and subsequently decreased. The high fine-mesh/coarse-mesh ratio (0.70) indicated that microbes drove total breakdown. Hierarchical partitioning (HP) analyses in k/d showed that temperature was the most important environmental factor for microbial breakdown and affected invertebrate breakdown. Temperature presented strong synergistic effects with other variables. Following removal of the temperature effect, total breakdown was related mainly to current and conductivity, microbial breakdown was related to water quality (conductivity, pH, and O2), and invertebrate-mediated breakdown was related only to current. The relation of invertebrate breakdown with current variation might be explained by strong seasonality in total invertebrate and shredder densities, which seemed to be linked to seasonality in discharge. Climate change can profoundly affect stream ecosystems, but the responses of invertebrates and microbes in terms of litter processing and C fluxes can be difficult to predict in intermittent Mediterranean streams.