Seasonal variation in δ¹³C and δ¹⁵N of epilithic microalgae in Gokasho Bay

Abstract

Glass plates were immersed in the surface layer (1 m) in Gokasho Bay during a 7 day period every month over an annual cycle to allow periodic collection of epilithic microalgae and to analyze their year-round stable carbon and nitrogen isotope ratios. The bulk of the attached material was diatoms throughout the year. The δ¹³C and δ¹⁵N of epilithic microalgae varied seasonally, ranging from −22.0 to −14.6‰ and from 3.6 to 10.2‰, respectively. The δ¹³C increased during a period from August to November when large microalgal biomasses were found, while the δ¹⁵N decreased after November through January in the next year. Such seasonal variations are discussed from the viewpoints of the algal growth and ambient nutrient concentrations. Comparison of the δ¹³C (yearly mean=−19.8‰) and δ¹⁵N (7.7‰) for epilithic microalgae with the previously reported values for primary producers and organic matter in Gokasho Bay shows that the overlap of the δ¹³C with marine phytoplankton (−20.4‰), which will result in poor resolution of sources.