Abstract:The concept of fatty acid trophic marker is based on the fact that the fatty acids of primary producers may be transferred conservatively to, and can be identified in, primary consumers. Recently, fatty acid markers have been widely used to trace or confirm predator-prey relationships so as to illustrate the key trophic linkages in the marine ecosystem. To verify the validity of fatty acid as markers in the trophic relationships between marine algae and the copepod Calanus sinicus, feeding experiment was carried out with this dominant herbivorous copepod from Jiaozhou Bay. During the experiment, Prorocentrum micans and Skeletonema costatum were offered as diets, and starvation was preceded as control. The fatty acid compositions of both the algae and the copepods before and after the experiment were analyzed. Over all, P. micans and S. costatum exhibited different fatty acid patterns. P. micans was characteristic of 18 ∶ 4ω3 and 22 ∶ 6ω3, indicating a typical feature of dinoflagellate. S. costatum was characterized by high amounts of 16 ∶ 1ω7 and 20 ∶ 5ω3, suggesting a systematic status in Bacillariophyceae. Additional, P. micans was found to be much more abundant in the content of total fatty acid than S. costatum.The fatty acid composition of C. sinicus was significantly different from the two algae. Firstly, C. sinicus was rich in both 20 ∶ 5ω3 and 22 ∶ 6ω3, while only one of them was found in large amount in P. micans and S. costatum. Secondly, 20 ∶ 1 and 22 ∶ 1 were identified in C. sinicus, demonstrating an herbivorous feeding of this copepod. Although the incorporation and turnover rates of dietary fatty acids were different from each other, the feeding experiment could still provide clear evidence for the potential of specific fatty acids as trophic markers. Polyunsaturated fatty acids which cannot be synthesized by copepods decreased obviously during the starvation experiment, while structural fatty acids with more conservative chemical properties were consumed in a lesser degree. It could be inferred that in the process of starvation, fatty acids related to the process of energy metabolism were consumed first. 18 ∶ 4ω3, 22 ∶ 6ω3, 18 ∶ 4ω3/16 ∶ 1ω7, ∑18/∑16 and 22 ∶ 6ω3/20 ∶ 5ω3 increased in different degrees at the end of the feeding experiment when C. sinicus was offered with P. micans. However, 22 ∶ 6ω3 was widely accepted as an important structural fatty acid, the stable chemical properties of which made it unsuitable as a marker to indicate the ingestion of dinoflagellate. When the copepod was fed with S. costatum, only 22 ∶ 6ω3 and 16 ∶ 1ω7/18 ∶ 4ω3 increased at the end of the experiment. Considering the special properties and the low content of 22 ∶ 6ω3 in S. costatum, the increase of 22 ∶ 6ω3 might not be a result of the ingestion of this alga. Based on overall considerations of the variations of these fatty acid markers and the Pearson correlation analysis, we deemed that 18 ∶ 4ω3, 18 ∶ 4ω3/16 ∶ 1ω7 and ∑18/∑16 were useful markers to reflect the ingestion of P. micans by C. sinicus, and only 16 ∶ 1ω7/18 ∶ 4ω3 could be used as marker for the ingestion of S. costatum.