Methodology:

Morphometric data:

Collected for 417 Calanus finmarchicus copepods (stage CV) collected in the Fram Strait, Arctic Ocean. Swimming activity in the laboratory was measured in 225 epipelagic and 192 mesopelagic copepods from two stations (F7 and D6) in August 2019, coinciding with the timing of early diapause in this species. Copepods were photographed under a stereomicroscope after removal from the locomotor activity monitor experiments (see swimming activity dataset). Photographs were taken onboard RRS James Clark Ross. The following measurements were made; body size (prosome areas in mm2) lipid sac area, as well as lateral lipid sac area / prosome area (LA / PA ratios). Data are thus separated for copepods from each activity monitor allowing pairing with the other dataset.

Copepods were sourced from epipelagic and mesopelagic depths for comparisons of diapause indicators at stations F7 and D6 in the Fram Strait (Greenland Sea). Copepods were collected from the water column using the Hydrobios® Mammoth MultiNet system, comprising nine individual nets (1 m2 apertures, 300 µm mesh) that were towed vertically at 10-20 m min-1 (ascent speed). Upper mesopelagic (285-520 m) and epipelagic (5-170 m) depth ranges were sampled. These copepods were first used in activity experiments lasting 6-day periods. They were identified as CV C. finmarchicus using morphological approaches prior to these experiments, and their identifications were since confirmed using molecular approaches. Photographs were taken at 25 objective magnification using a WILD microscope. A calibration ruler allowed pixel-mm conversions. Back on land, the photographs were analysed using ImageJ (FIJI) for the following measurements: body size (prosome areas in mm2), lipid sac area, and lipid sac / prosome area (LA / PA) ratios.

Swimming activity data:

Collected using locomotor activity monitors for 469 Calanus finmarchicus copepods (stage CV) collected in the Fram Strait, Arctic Ocean. Swimming activity in the laboratory was measured in 246 epipelagic and 223 mesopelagic copepods from two stations (F7 and D6) in August 2019, coinciding with the timing of early diapause in this species. Individual copepod swimming activity was monitored every minute (here binned into 30-minute bins for analysis), for six days using modified LAM10 activity monitors (Trikinetics®). These are infrared light beam arrays which detect the movement of individual copepods in test chambers (clear acrylic tubes of 10 mm diameter and filled with 0.2 µm filtered seawater), registering beam break events on a laptop computer using proprietary Trikinetics® software (filescan.exe). Experiments were conducted in darkness at two experimental temperatures in an incubator onboard RRS James Clark Ross. Copepods were sourced from epipelagic and mesopelagic depths for comparisons of diapause indicators at stations F7 and D6 in the Fram Strait (Greenland Sea). Copepods were collected from the water column using the Hydrobios® Mammoth MultiNet system, comprising nine individual nets (1 m2 apertures, 300 µm mesh) that were towed vertically at 10-20 m min-1 (ascent speed). Upper mesopelagic (285-520 m) and epipelagic (5-170 m) depth ranges were sampled. CV C. finmarchicus copepods were then identified in darkness using taxonomic identifications, later verified by molecular tools. Experiments were conducted over 6-day periods. Activity monitors recorded infra-red beam breaks by individual copepods at two positions (middle and bottom) inside acrylic tubes every 60 seconds. Experiments were conducted in darkness at 2.8oC (station F7) and 4oC (station D6), consistent with environmental temperatures at the collection depths.

Oxygen consumption activity data:

Collected using a Loligo microplate respirometry system for 40 Calanus finmarchicus copepods (stage CV) collected in the Fram Strait, Arctic O...(28)

Data collection:

Photographs were taken using a stereomicroscope onboard the ship. Fiji was used as the image analysis software (https://imagej.net/software/fiji).

Swimming activity was measured using modified LAM10 activity monitors (Trikinetics), infrared light beam arrays which detect the movement of an individual copepod in the middle of a test chamber (clear acrylic tubes of 10 milimetre diameter and filled with 0.2 micrometre filtered seawater). Beam break events were registered on a laptop computer using proprietary Trikinetics software (filescan.exe).

Oxygen consumption was measured in 10 individuals from each depth layer (20 individuals total from each station) for 2 days, using a Loligo microplate respirometry system (1700 microlitre well plates containing 0.2micrometre filtered seawater), calibrated at 4 degrees Celsius.