Dataset: Copepod Density

During the summer of 2017, three sets of experiments were conducted on August 1, September 6, and September 9, to observe the effect of density gradients (of differing strengths) on the vertical distribution and behavior of the copepod Calanus pacificus. Each experiment consisted of three treatments: a control (without a density gradient), a tank with a weak gradient (with the density of the top layer about 0.0020 kg/m3 less than that of the bottom layer), and a tank with a strong gradient (with the density of the top layer about 0.0040 kg/m3 less than that of the bottom layer).
 

C. pacificus was collected using a small boat near Scripps Canyon in La Jolla, CA (32° 51’ 23.8” N, 117° 16’ 00.1” W) 6-7 days before each experiment. With a 300 µm mesh plankton net (0.5 m diameter mouth), 5-6 oblique tows were taken per sampling trip to a depth of at least 40 m and for a duration of 3 to 5 minutes. Samples were sorted in the lab to isolate late-stage copepodites (C4 and C5 stages) and adults of C. pacificus. Copepods were maintained with regular water changes in an incubator in the dark at 18 degC until the experiment and fed a mixed diet of Thalassiosira weissflogii and haptophytes (Tisochrysis sp. and Pavlova sp.).
 


Prior to each experiment, the top layer fluid was mixed by diluting filtered seawater with distilled (DI) water. The density of the top layer fluid was measured immediately before each experiment, using a handheld density meter (DMA 35, Anton Paar), and was kept roughly consistent between experiments. Copepods were starved for 24 hours prior to each experiment by transferring 20-25 C. pacificus individuals (late-stage copepodites and adults) into a 1 L beaker, and acclimated to the top layer fluid for each treatment. Each beaker was wrapped in aluminum foil to maintain darkness, and was kept at room temperature. Copepods were inspected to ensure a normal swimming behavior before being transferred to the experimental tank.
 

The experimental tank had a square base (10 cm × 10 cm) and a height of 50 cm. The non-stratified control treatment was set up by pouring ~5 L of filtered seawater (hereafter referred to as bottom layer fluid) into the tank. For the stratified treatments, density gradients were established by filling the tank with ~2.5 L of bottom layer fluid, followed by 2.5 L of less dense top layer fluid. Top layer fluid was slowly pumped on top of the bottom layer fluid through a diffuser to minimizing mixing.
 

Once the tanks were set up, 15 - 17 copepods were chosen for each treatment, and were transferred into a 50 mL beaker by pipette.
 

Once image recording had initiated, copepods were slowly poured into the top of the tank, with care so as to not disturb the density gradient. For each treatment, copepods were observed in the tank using two high-resolution cameras (Grasshopper3 4.1 MP Mono USB3 Vision, Pt. Grey) set up perpendicularly to one another to allow for 3D imaging. To image without visible light, a 730 nanometer near-infrared light-emitting diode (M730L4 730 nm, 515 mW Mounted LED, Thorlabs), collimated using a Fresnel Lens, illuminated the tank from below through an acrylic panel installed in the table supporting the experimental tank. Preliminary experiments demonstrated that this light source caused no noticeable heat effects within the tank. Each treatment was recorded for ~30-35 minutes at 12 frames per second. The field of view of each camera was ~30 cm x ~10 cm (with the horizontal section cropped to the width of the tank), allowing observation of copepods ~15 cm above and below the center of the tank.
 

Vertical profiles of conductivity and temperature were taken after each treatment using a conductivity probe (MSCTI, Precision Measurement Engineering, Encinitas, CA).