Gut evacuation rate and grazing impact of the krill Thysanoessa raschii and T. inermis

Gut evacuation rates and ingestion rates were measured for the krill Thysanoessa raschii and T. inermis in Godthåbsfjord, SW Greenland. Combined with biomass of the krill community, the grazing potential on phytoplankton along the fjord was estimated. Gut evacuation rates were 3.9 and 2.3 h⁻¹ for T. raschii and T. inermis, respectively. Ingestion rates were 12.2 ± 7.5 µg C mg C⁻¹ day⁻¹ (n = 4) for T. inermis and 4.9 ± 3.2 µg C mg C⁻¹ day⁻¹ (n = 4) for T. raschii, corresponding to daily rations of 1.2 and 0.5 % body carbon day⁻¹. Clearance experiments conducted in parallel to the gut evacuation experiment gave similar results for ingestion rates and daily rations. Krill biomass was highest in the central part of the fjord’s length, with T. raschii dominating. Community grazing rates from krill and copepods were comparable; however, their combined impact was low, estimated as <1 % of phytoplankton standing stock being removed per day during this late spring study.