Use of allochthonous resources by zooplankton in reservoirs

Abstract

Aquatic food webs are supported by primary production from within the system (autochthony) as well as organic matter produced outside of and transported into the system (allochthony). Zooplankton use allochthonous resources, especially in systems with high terrestrial loading and moderate to low internal primary production. We hypothesized that due to high terrestrial loads and remnant submerged terrestrial material, allochthonous resource use by zooplankton would be significant in all reservoirs and would decline along an increasing reservoir age gradient. Using hydrogen stable isotopes and a Bayesian mixing model, we estimated the contribution of allochthonous sources to organic matter pools and crustaceous zooplankton biomass for ten reservoirs. Dissolved organic matter (DOM) in all systems was dominated by allochthonous sources (posterior distribution median [92% allochthonous), while particulate organic matter (POM) composition varied (2-68% allochthonous) and had a lower allochthonous fraction in older reservoirs. There was no relationship between zooplankton allochthony and reservoir age. Crustaceous zooplankton allochthony varied among systems from 26 to 94%, and Chaoborus allochthony, measured in four reservoirs, was similarly variable (33-94%). Consumer allochthony was higher than POM allochthony in some reservoirs, potentially due to terrestrial DOM pathways being important and/or algal resources being inedible (e.g., cyanobacteria). As with many lakes, in the reservoirs we studied, allochthonous inputs account for a significant fraction of the organic matter of basal consumers.

Terrestrial support of lake food webs: Synthesis reveals controls over cross-ecosystem resource use

“…These were suitable alternatives because their isotopic ratios have been shown to be indistinguishable from both litterfall and dissolved OM inflowing into lakes ( 7 , 54 ). Phytoplankton were either collected by net tows during periods of high biomass ( 8 , 11 , 21 ) or isolated from POM by their specific phospholipid fatty acids ( 28 ). However, given the difficulty in isolating pure phytoplankton using these two approaches, we inferred their isotopic signatures by sampling environmental water either on its own or with POM for 40% ( n = 226) of the consumer observations.…”

Expanding the Isotopic Toolbox: Applications of Hydrogen and Oxygen Stable Isotope Ratios to Food Web Studies

“…Streamflow regime has been found to influence the degree of allochthony in rivers, with algae dominating nutrient contributions to macroinvertebrates and fish following low flow periods, while terrestrial plants made higher contributions following high-flow pulses (Roach and Winemiller, 2015). In reservoir ecosystems, the importance of the allochthonous subsidy is variable, but is negatively correlated with watershed size (Babler et al, 2011) and negatively correlated with reservoir age (Emery et al, 2015). The existing literature supports the hypothesis that percentage of terrestrial organic matter in zooplankton should be highest in humic lakes with low phytoplankton biomass, and lowest in either eutrophic lakes, or clear-water lakes (Berggren et al, 2014), but further work is needed to model and predict this variation among ecosystems .…”

Efficiency of crustacean zooplankton in transferring allochthonous carbon in a boreal lake

“…Terrestrial OM has long been considered an unimportant resource for pelagic aquatic food webs and has been often excluded from the calculations of carbon flux supporting primary and secondary consumers. Increased evidence of a significant share of zooplankton biomass having a terrestrial origin (i.e., allochthony; Emery et al , Cole et al , Pace et al , Berggren et al ), questions this view of t‐OM as an unimportant food source. However, as t‐OM lacks essential elements for growth (Taipale et al ), the high abundance of terrestrial carbon in zooplankton tissues is not expected to promote the production of consumer biomass, but rather to result in organisms having a reduced growth capacity (Brett et al ).…”