The abundance of mixotrophic algae drives the carbon isotope composition of the copepod Boeckella gracilipes in shallow Patagonian lakes

The importance of allochthonous versus authochthonous carbon in the diet of zooplankters has been discussed at length in the recent literature, the more so for lakes with dissolved organic matter (DOM) concentrations between 3.5 and 25 mg L-1. Here, we investigated the sources of carbon for Boeckella gracilipes (Copepoda, Calanoida) in three Patagonian shallow lakes that have different but relatively moderate DOM concentrations (<5 mg L-1). We linked their delta C-13 values with parameters such as water color (absorbance 440 nm, (abs440)), dissolved organic carbon (DOC) concentrations and the ratio Color(abs440):Chlorophyll a (Chl a). We also determined the isotopic composition of seston and phytoplankton composition. Our results showed a direct relationship between the delta C-13 values of B. gracilipes and seston (r(2) = 0.64, P < 0.001), and we demonstrated the importance of phytoplankton to B. gracilipes as its delta C-13 signal was strongly related to the abundance of the mixotrophic nanoflagellate Chrysochromulina parva (r(2) = 0.86, P < 0.001). We further found an inverse relationship between DOM and the ratio Color(abs440): Chl a and B. gracilipes delta C-13 values (r(2) = 0.67 and 0.56, respectively), which also supported the importance of authochthonous carbon for the copepod. We conclude that phytoplankton, particularly mixotrophic algae, rather than allochthonous carbon, supports the copepod B. gracilipes in shallow Patagonian lakes.