Whole-lake additions of dissolved inorganic C-13 were used to measure allochthony (the terrestrial contribution of organic carbon to aquatic consumers) in two unproductive lakes (Paul and Peter Lakes in 2001), a nutrient-enriched lake (Peter Lake in 2002), and a dystrophic lake (Tuesday Lake in 2002). Three kinds of dynamic models were used to estimate allochthony: a process-rich, dual-isotope flow model based on mass balances of two carbon isotopes in 12 carbon pools; simple univariate time-series models driven by observed time courses of delta(13)CO(2); and multivariate autoregression models that combined information from time series of delta(13)C in several interacting carbon pools. All three models gave similar estimates of allochthony. In the three experiments without nutrient enrichment, flows of terrestrial carbon to dissolved and particulate organic carbon, zooplankton, Chaoborus, and fishes were substantial. For example, terrestrial sources accounted for more than half the carbon flow to juvenile and adult largemouth bass, pumpkinseed sunfish, golden shiners, brook sticklebacks, and fathead minnows in the unenriched experiments. Allochthony was highest in the dystrophic lake and lowest in the nutrient-enriched lake. Nutrient enrichment of Peter Lake decreased allochthony of zooplankton from 0.34-0.48 to 0-0.12, and of fishes from 0.51-0.80 to 0.25-0.55. These experiments show that lake ecosystem carbon cycles, including carbon flows to consumers, are heavily subsidized by organic carbon from the surrounding landscape.

/pdf/ecosystem-subsidies-terrestrial-support-of-aquatic-food-webs-125ei7e82p.pdf

Ecosystem subsidies: terrestrial support of aquatic food webs from 13c addition to contrasting lakes

In many small aquatic ecosystems, watershed loading of organic C exceeds autochthonous primary production. Although this allochthonous organic C has long been thought of as refractory, multiple lines of evidence indicate that substantial portions are respired in the receiving aquatic ecosystem. To what extent does this terrestrial C support secondary production of invertebrates and fish? Do current models adequately trace the pathways of allochthonous and autochthonous C through the food web? We evaluated the roles of allochthonous and autochthonous organic C by manipulating 13 C content of dissolved inorganic C in a small, softwater, humic lake, thereby labeling autochthonous primary production for about 20 d. To ensure rapid and sufficient uptake of inorganic 13 C, we enriched the lake with modest amounts of N and P. We constructed a carbon flow model based on the ambient and manipulated levels of 13

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2002.47.6.1664

Pathways of organic carbon utilization in small lakes: Results from a whole-lake 13C addition and coupled model

Water-column methane oxidation can represent a substantial carbon transformation pathway in lakes, and circumstantial evidence indicates that methane may be a potentially important source of carbon ...

Methane as a source of carbon and energy for lake pelagic food webs

Carbon stable isotopic composition was determined for zooplankton, POM, DOM and, in two cases, phytoplankton from 12 small forest lakes in southern Finland during summer 1997. The lakes were selected to provide a wide range of water colour (12-365 mg Pt l-1) but minimum variability in other limnological characteristics. POM and DOM showed similar stable isotope ratios (δ 13 C around - 28‰, comparable with values commonly reported for terrestrial C3 vegetation) and these showed no relationship to water colour. Zooplankton were consistently depleted in 13 C relative to the bulk POM on which they might feed. This relative 13 C depletion increased significantly with water colour. Therefore the zooplankton must have been feeding selectively on isotopically light food sources and doing so to a greater extent in the more humic lakes. Although phytoplankton were 13 C-depleted relative to POM, their abundance and their variability between lakes did not appear to explain adequately the observed trend in zooplankton stable isotope ratios. We propose that some zooplankton were grazing on methanotrophic bacteria which utilise isotopically light methane derived from allochthonous organic matter decomposed under anaerobic conditions. This process would be more pronounced in coloured lakes with a higher loading of allochthonous organic matter and greater development of hypolimnetic anoxia.

Stable isotope analysis of zooplankton carbon nutrition in humic lakes

Stable isotope analysis has become a crucial tool for aquatic food web ecologists, but a lack of methodological standardization hinders comparisons between studies. One methodological inconsistency in stable isotope food web research is the decision whether to extract lipids before stable isotope analysis. The depletion in zooplankton stable carbon isotope values (d13C) due to fatty acid content and the accuracy of mathematical correction models designed to predict this depletion were examined for a range of zooplankton species from nine lakes of diverse size and productivity. Large differences of up to 5% observed between d13C values of nonextracted and lipid-extracted zooplankton samples correlated with zooplankton fatty acid content. A mass balance d13C correction model for fatty acid content using atomic C : N ratios and directly measured d 13 C values of fatty acids accurately predicted (R 2 5 0.95) lipid-extracted d 13 C values for both copepod and cladoceran zooplankton. Researchers should use mass balance lipid corrections as an efficient method to eliminate bias in comparisons of zooplankton and fish d 13 C values and allow their results to be more easily compared with other studies.

https://aslopubs.onlinelibrary.wiley.com/doi/pdf/10.4319/lo.2007.52.5.2135

A standard protocol for stable isotope analysis of zooplankton in aquatic food web research using mass balance correction models

https://www.int-res.com/articles/ame/12/a012p085.pdf

Westcott Ka

Papers

Productivity and heterotrophy influences on zooplankton delta13C in northern temperate lakes