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

Abstract

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.

Spring bloom dynamics in a subarctic fjord influenced by tidewater outlet glaciers (Godthåbsfjord, SW Greenland)

“…The depth‐integrated zooplankton biomass recorded during the same spatial survey (data already reported in Teglhus et al . []; reproduced in Figure ) exhibited a similar spatial pattern, with a peak in krill biomass around GF10 (0.38 g C m −2 ) and maximum in copepod biomass around GF7 (0.84 g C m −2 ) .…”

“…As stratification and light climate conditions are comparable between stations (Table ), a difference in grazing pressure by zooplankton could be an explanation for the absence of a spring bloom in April–May near the glacier front. Data on zooplankton biomass and grazing experiments however show that the grazing impact is low (maximum 15% of phytoplankton community, [ Teglhus , ]), suggesting that physical drivers must play a dominant role. So why did the spring bloom not develop close to glacier, i.e., at station GF17?…”

“…Biological uptake causes a gradual depletion of nutrients in the upper water layer, which then leads to a concomitant decrease in plankton biomass as one move toward the sill region (Figure ). The prolonged and intense phytoplankton bloom in the fjord forms an important food source for higher trophic levels and sustains a high zooplankton biomass [ Arendt et al ., ; Teglhus et al ., ] (Figure ). Therefore, changes in fjord circulation patterns linked to changes in dominant winds and fjord‐ocean exchange in addition to changes in sea ice cover can have large implications on spring bloom dynamics in high‐latitude fjord systems.…”

A Tale of Two Fjords

“…The high biomass contribution of Metridia longa at the subsurface front implies that local conditions favour its growth. This finding is in line with zooplankton community observations in Nuup Kangerlua that follow the outflow of subglacial water from the inner fjord tidewater glacier towards the mouth (Tang et al, 2011;Swalethorp et al, 2015;Teglhus et al, 2015). Each of these case studies reported a peak in the biomass of M. longa in the same area as the front associated with the subglacial water mass, from ∼0.75 g C m -2 in May to ∼2.5 g C m -2 in August.…”

Distributions and seasonal abundances of krill eggs and larvae in the sub-Arctic Godthåbsfjord, SW Greenland

“…They have the same grazing potential as copepods (Teglhus et al 2015), thereby forming a major link between primary producers and marine vertebrate predators, including fish, birds and mammals (Mauchline & Fischer 1969, Astthorsson & Gislason 1997, Rosing-Asvid et al 2013. It is crucial to understand the environmental drivers of krill population dynamics to predict how future changes in marine environments will affect plankton communities.…”