Spatial patterns of autochthonous and allochthonous resources in aquatic food webs

Although ecologists have recognized the importance of spatial structure within food webs, this aspect of ecosystems remains difficult to characterize quantitatively. Stable-isotope techniques have recently been used to provide evidence of spatial structure within aquatic food webs. Here, I review current literature on spatial patterns of autochthonous and allochthonous resources in aquatic food webs in lakes and rivers. Across various habitats and ecosystems, the factors determining the major resources of aquatic food webs are primarily phytoplanktonic productivity, benthic algal productivity, and amount of subsidization from terrestrial habitats. Autochthonous and allochthonous resource availability in food webs shifts with gradients in water depth, nutrient concentrations, degree of canopy cover, and distance from terrestrial habitats. Size of lake and river ecosystem (i.e., lake volume and stream width) also affects the relative contribution of the resources to the food webs, as this factor determines the system primary productivity and linkage to terrestrial habitats. Human activities have fragmented river and lake ecosystems and have subsequently modified the structure of aquatic food webs. The responses of food webs to anthropogenic effects differ across ecosystems, and stable isotope techniques can help to quantitatively assess the effects of human impacts on aquatic food webs.