The importance of terrestrial subsidies in stream food webs varies along a stream size gradient

Abstract

Cross system subsidies of energy and materials can be a substantial fraction of food web fluxes in ecosystems, especially when autochthonous production is strongly limited by light or nutrients. We explored whether assimilation of terrestrial energy varied in specific consumer taxa collected from streams of different sizes and resource availabilities. Since headwater streams are often unproductive, we expected that inputs from surrounding terrestrial systems (i.e. leaf litter, terrestrial invertebrates) would be a more important food source for consumers than in mid-size rivers that have more open canopies and higher amounts of primary production available for consumers. We collected basal resources, invertebrates, and fish along a gradient in stream size in the Adirondack Mountains (NY, USA) and in Trinidad and Tobago and analyzed all samples for hydrogen isotopes as a means of differentiating biomass derived from allochthonous versus autochthonous sources. We found significant differences in allochthonous energy use within individual consumer taxa, showing that some taxa range from being entirely allochthonous to entirely autochthonous depending on where they were collected on the stream size gradient (grazers and collector-gatherer functional feeding groups), while other taxa are relatively fixed in the source of energy they assimilate (shredder and predator functional feeding groups). Consistent with expectations, allochthonous energy use was positively correlated with canopy cover in both regions for most feeding groups, with individuals from small, shaded streams having a more pronounced allochthonous signal than individuals collected from larger streams with less canopy cover. However, consumers in the shredder/detritivore feeding group did not vary among sites in their allochthonous energy use, and had a mostly allochthonous signal regardless of canopy cover and algal biomass. Our results demonstrate that the importance of energy from terrestrial subsidies can vary markedly but are similar in both temperate and tropical streams, suggesting a widely consistent pattern.

Catchment properties predict autochthony in stream filter feeders

“…Here, the level of autochthony in black fly larvae was also related to distance to nearest upstream lake, as proportion of lake in the catchment and distance to lake were related, but our analyses using all sites indicate that proportion of lake is more important than distance to nearest lake when both factors are taken into account. In this sense, the diet of filter feeders seems to better reflect landscape characteristics and processes occurring at the catchment scale than stream size and characteristics of the local riparian vegetation (sensu Vannote et al, 1980;Collins et al, 2016). Further, although we found no clear correspondence between standing stock of black fly larvae and their level of autochthony, failure to find black fly larvae at all sampling events in the two most allochthonous sites suggests, as one would expect, lowered total secondary production in these sites (Jenkins, 2015).…”

“…However, most aquatic systems are to some extent subsidized by terrestrially derived (i.e., allochthonous) organic matter. While this material is of poorer dietary quality than aquatic primary production, in some systems allochthonous resources constitute the majority of the resource base (Vannote et al, 1980;Collins et al, 2016). For example, in small streams, primary production is often limited by shading from the riparian canopy, which, at the same time, provides energy in the form of litter inputs (Hill et al, 1995;Wallace et al, 1997Wallace et al, , 2015.…”

“…In streams, food-web autochthony is predicted to be lowest in the headwaters and increase downstream (Vannote et al, 1980;Collins et al, 2016), and there should, therefore, be a simultaneous increase in secondary production downstream (Finlay, 2011;Kelly et al, 2014). However, in small, heterotrophic streams, a peak in secondary production-and therefore, consumer allochthony-can occur soon after the seasonal input of riparian plant litter, which therefore, often coincides with an increased abundance of macroinvertebrate detritivores (Richardson, 1991;Wallace et al, 1999).…”

“…Thus, there are several attributes of catchments and embedded aquatic networks that may determine the availability and use of aquatic versus terrestrial resources in streams. How these attributes interact across catchments is not well known, but it is still assumed that downstream changes in stream width and canopy cover exert strong influences on patterns of basal resource use (Vannote et al, 1980;Collins et al, 2016).…”

Spatially cascading effect of perturbations in experimental meta-ecosystems

“…Our findings show that these managementinduced disturbances (or preventions thereof) can directly affect dynamics in spatially coupled ecosystems, such as rivers or lakes, which are fed by resources from these 'green' ecosystems [7,[46][47][48][49]. We experimentally demonstrated, and showed also in a mathematical model, that the direction of the effects depend on the temporal unfolding, the magnitude of disturbance and the type of connected ecosystems (autotrophic versus heterotrophic dominated).…”