Evaluation of incubation volume effect on in-situ Euphausia pacifica grazing rates with natural food assemblages

Abstract

Euphausia pacifica is a key player to link between low and high trophic in the Yellow Sea. Their feeding ecology can be used as the basic information for understanding the food-web dynamics. In-situ feeding experiment as traditional approach to study the feeding ecology provides the direct feeding rate. However, experimental conditions (incubation volume, number of individual, diet concentration, etc.) must be cautiously designed to get more realistic values. Therefore, we conducted the in-situ feeding experiment for E. pacifica under the three different incubation volumes to determine the optimal incubation volume. Live adult krill were sampled in spring and summer 2013-2014 from the Yellow Sea. They were maintained in filtered seawater (GF/F filter) for 6 hours to empty their gut. The 2, 4, and 10 krill individuals were set into bottles with three different volumes (2, 4, and 10L), respectively, filled with pre-screened seawater (200㎛) from the depth where the krill were sampled. Then they were incubated with natural food assemblages under dark and in-situ water temperature for 12 hours. Food assemblages and biochemical components (nutrient, POC/N, Chl-a) were examined at the end of incubation. No incubation volume effects on grazing rate and food selectivity were observed. However, the small volume (2L) experiment with a few individuals has a risk due to their mortality. Furthermore, a large volume (10L) experiment to study the feeding ecology provides the direct feeding rate. However, experimental conditions (incubation volume, number of individual, diet concentration, etc.) must be cautiously designed to get more realistic values. Therefore, we conducted the in-situ feeding experiment for E. pacifica under the three different incubation volumes to determine the optimal incubation volume. Live adult krill were sampled in spring and summer 2013-2014 from the Yellow Sea. They were maintained in filtered seawater (GF/F filter) for 6 hours to empty their gut. The 2, 4, and 10 krill individuals were set into bottles with three different volumes (2, 4, and 10L), respectively, filled with pre-screened seawater (200㎛) from the depth where the krill were sampled. Then they were incubated with natural food assemblages under dark and in-situ water temperature for 12 hours. Food assemblages and biochemical components (nutrient, POC/N, Chl-a) were examined at the end of incubation. No incubation volume effects on grazing rate and food selectivity were observed. However, the small volume (2L) experiment with a few individuals has a risk due to their mortality. Furthermore, a large volume (10L) experiment needs a large incubation space and seawater volume. Therefore, we choose the incubation volume (4L) with 4 individuals as the optimal condition for in-situ feeding experiment.