Comparison of methods to determine algal δ¹³C in freshwater

Abstract

To accurately assess the flux of mass and energy to higher trophic levels in a food web using stable isotopes, the isotopic signature of basal sources is required. When studying aquatic food webs, it is difficult to obtain a signature for algae because of challenges associated with isolating small organisms from a bulk sample. In this study, we compared freshwater algal δ13C values obtained using five approaches from the literature. Results indicated that the signatures derived from a primary consumer such as Daphnia sp., from particulate organic carbon with a correction for algal biomass, and from isolated algal samples were comparable. By contrast, algal δ13C values based on the signature of carbon dioxide and algal carbon fractionation were significantly lower than those of the other approaches. The inconsistent values produced by this method were likely due to problems in determining fractionation values based on current models and were potentially related to bicarbonate uptake by algae.

Assessing carbon dynamics in natural and perturbed boreal aquatic systems

“…Additionally, the d 13 C signature of phytoplankton measured in the summer by Marty and Planas [2008] in different boreal lakes and reservoirs of Quebec averaged À32.7 AE 1.7‰. The carbon isotopic signatures (d 13 C FPOC , 0.45 to 70 mm) measured for FPOM in this study ranged between À30.4‰ and À27.7‰.…”

Estuarine consumers utilize marine, estuarine and terrestrial organic matter and provide connectivity among these food webs

“…To corroborate the δ 13 C DIC -based estimates, we estimated δ 13 C phyto values using an independent method that yields similar δ 13 C values to isolated algae samples (see Eq. 2 in Marty & Planas 2008). The method is a 2-source, single stable isotope mixing model that estimates δ 13 C phyto by correcting particulate organic carbon δ 13 C (δ 13 C POC ) for the relative algal biomass contribution (percent algae), which is derived from the chl a concentration (C:chl a = 80; Marty & Planas 2008).…”

“…2 in Marty & Planas 2008). The method is a 2-source, single stable isotope mixing model that estimates δ 13 C phyto by correcting particulate organic carbon δ 13 C (δ 13 C POC ) for the relative algal biomass contribution (percent algae), which is derived from the chl a concentration (C:chl a = 80; Marty & Planas 2008). The proportion of algal carbon in the POC pool was calculated according to Canuel et al (1995).…”

Stable carbon isotope biogeochemistry of lakes along a trophic gradient

“…Common methods for determining δ 13 C phyto are the use of the δ 13 C of POC with correction for nonphytoplankton carbon and estimates based on δ 13 C of DIC with an isotope fractionation factor (ε), obtained from experimental studies. Other methods are the use of zooplankton consumers as a proxy for δ 13 C phyto or size fractionation of organic matter and subsequent determination of δ 13 C of different size classes (Marty and Planas, 2008).…”