Chlorophyll a-specific Δ14C, δ13C and δ15N values in stream periphyton: implications for aquatic food web studies

Periphytic algae attached to a streambed substrate (periphyton) are an important primary producer in stream ecosystems. We determined the isotopic composition of chlorophyll a in periphyton collected from a stream flowing on limestone bedrock in the Seri River, central Japan. Stable isotope ratios of carbon (delta C-13) and nitrogen (delta N-15) and natural radiocarbon abundances (Delta C-14) were measured in chlorophyll a (delta C-13(chl), delta N-15(chl) and Delta C-14(chl)) and bulk (delta C-13(bulk), delta N-15(bulk) and Delta C-14(bulk)) for periphyton, a pure aquatic primary producer (Cladophora sp.) and a terrestrial primary producer (Quercus glauca). Periphyton delta C-13(bulk) and delta C-13(chl) values did not necessarily correspond to delta C-13(bulk) for an algal-grazing specialist (Epeorus latifolium). Periphyton Delta C-14(chl) values (-258 parts per thousand in April and -190 parts per thousand in October) were slightly lower than Delta C-14(bulk) values (-228 parts per thousand in April and -179 parts per thousand in October) but were close to the Delta C-14 value for dissolved inorganic carbon (DIC; -217 +/- 31 parts per thousand), which is a mixture of weathered carbonates (Delta C-14 = -1000 parts per thousand), CO2 derived from aquatic and terrestrial organic matters (variable Delta C-14) and dissolved atmospheric CO2 (Delta C-14 approximately +30 parts per thousand in 2013). Delta C-14(chl) values were also close to Delta C-14(bulk) for E. latifolium (-215 parts per thousand in April and -199 parts per thousand in October) and Cladophora sp. (-210 parts per thousand), whereas the Delta C-14(bulk) value for Q. glauca (+27 parts per thousand) was closer to Delta C-14 for atmospheric CO2. Although the bulk isotopic composition of periphyton is recognised as a surrogate for the photosynthetic algal community, natural periphyton is a mixture of aquatic and terrestrial organic materials. Our results indicate that the bulk periphyton matrix at the study site consists of 89 to 95% algal carbon (derived from C-14-depleted DIC) and 5 to 11% terrestrial organic carbon (derived from C-14-enriched atmospheric CO2).