The spatio-temporal development of macroalgae in rivers

Abstract

In shallow clear rivers macroalgae are a diverse component of biota and especially easily recognizable and easily accessible organisms, facilitating studies of spatial and temporal variation across environmental gradients. Their macroscopic form and appearance makes field studies of their limits and requirements in space and time possible. Some taxa occupy spatially restricted microhabitats, particularly in headwaters, while others are abundant along the length of a river. Spatial variation can occur within a site, between different stream types, or across wide regions. Temporal variation in growth or colonization by a particular species at a particular site can be driven by seasonal changes in discharge and nutrients, which may lead to regular or irregular temporal sequences. More stable temporal dynamics are observed in taxa with high adaptive capacities to resist physical disturbances or to re-establish quickly after floods. We examined the most common soft-bodied macroscopic algae (SBM) from two geographically extended datasets of temperate streams, from alpine to lowland regions in Austria and a shallower altitudinal gradient of southeastern New York State. Morphological and functional characters, combined with key environmental variables (based on median and multivariate statistics), are used to analyze general trends and causalities for species-specific spatial and temporal niches. These results provide strong arguments in favor of using a combination of on-site studies of growth form and phenology, with ecophysiological and molecular studies in the lab to improve our understanding of the factors regulating stream macroalgae occurrence in space and time in the future.