Terrestrial support of detritivorous fish populations decreases with watershed size

Abstract

Abstract. Consumers in aquatic food webs can be supported by terrestrial and aquatic primary production. However, we know little about how allochthony, i.e., the proportion of consumer biomass derived from terrestrial organic matter, varies along environmental gradients. Using hydrogen isotopes (deuterium), we quantified allochthony of an ecologically dominant detritivorous fish species (gizzard shad, Dorosoma cepedianum) in reservoir ecosystems, along a gradient of watershed land use (agriculture vs. forest). We predicted that allochthony would decline with an increase in the proportion of watershed land composed of agriculture (% agriculture). This is based on the supposition that as % agriculture increases, so does the export of dissolved inorganic nutrients to lakes, stimulating algal production and reducing the importance of terrestrial organic carbon subsidies. Allochthony accounted for ;34% of gizzard shad production (mean of 11 lakes), although this fraction varied greatly (0-68%) among lakes and isotope mixing model assumptions. Contrary to our hypothesis, we found no relationship between allochthony and % agriculture. However, allochthony was inversely related to total watershed area, as well as the absolute area of the watershed (rather than the percentage) composed of agriculture. We speculate that watershed area and allochthony are negatively correlated because watershed area exerts a strong control on the relative subsidies of dissolved inorganic nutrients vs. particulate organic carbon. Gizzard shad biomass was positively related to phytoplankton primary production but negatively related to allochthony, suggesting that phytodetritus is a higher quality resource than terrestrial detritus. Overall, our results show that both autochthonous and allochthonous carbon fuel the production of this ecologically important detritivore, the relative importance of allochthony decreases with increasing watershed size, and variation in gizzard shad production is closely tied to variation in autochthonous primary production.

Invasive crayfish increase habitat connectivity: a case study in a large boreal lake

“…The importance of terrestrial subsidies to lakes is dependent on lake-specific properties (e.g. surface and watershed area) (Babler et al, 2011;Francis et al, 2011), and we suggest that the most likely reason for the minor consumption of terrestrial material is simply its low overall availability. In large lakes, the ratio between shoreline edge area and total surface area is generally low which leads to a low total input of terrestrial leaf litter (Gasith and Hasler, 1976).…”

Predicting eutrophication status in reservoirs at large spatial scales using landscape and morphometric variables

“…All statistical analyses were performed in R (R Development Core Team 2009). To improve normality, all data were log 10 (x+1) transformed with the exception of land cover percentage, which was arcsine square-root transformed (Knoll et al 2003, Babler et al 2011…”

Monsoons and habitat influence trophic pathways and the importance of terrestrial‐marine linkages for estuary sharks

“…There, local primary production is often less than community respiration (Pace et al 2004, del Giorgio andPace 2008), and subsidies of allochthonous organic matter from terrestrial sources often supplement local autochthonous sources such as phytoplankton (Hoffman et al 2007(Hoffman et al , 2008, Van den Meersche et al 2009, Babler et al 2011. In contrast to temperate regions, however, the origins of the organic matter sustaining food webs and secondary production in most tropical estuaries are largely unknown (Blaber 2000, Barros et al 2010, Wai et al 2011a.…”