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Collections for this study occurred in Heriot Bay, Strait of Georgia, British Columbia, Canada (50.105°N, 125.215°W; Fig. 1A) on 16 July and 23 September 2019. Bottom depth was 19 m for both sampling dates.

During each sampling event, we collected individual medusae using a dip net, measured bell diameter to the nearest millimeter using a 60 cm fish measuring board (Dynamic Aqua-Supply Ltd.), and rinsed thoroughly with 0.22 µm filtered seawater. Individual medusae were targeted during collection to capture the available range of bell diameters at each sample date. Medusae were placed in individual Whirl-Pak bags (Dynamic Aqua-Supply Ltd.), immediately frozen in liquid nitrogen, and stored at -80°C until processing for SI and FA analyses. In July, Aequorea victoria were present in the same aggregation as A. labiata and two individuals were collected using the same protocol and analysed for SI only.

To characterize the prey available in Heriot Bay at the time of medusa collection, we collected triplicate water samples from 5 m depth using Niskin bottles to determine the SI (4 L) and FA (10 L) profiles of the particulate organic matter (POM). Water samples for POM were not pre-filtered. Plankton were collected using vertical net tows (15 m depth) with a 64 µm mesh and 0.5 m diameter ring net. A total of six net tows were completed on each sampling day with three allocated for SI analysis and three for FA analysis. Samples for SI analysis were size fractionated into five size classes using 64, 125, 250, 500, and 1000 µm sieves, and FA samples were size fractionated into two classes using 64 and 500 µm sieves. In July, Pleurobrachia bachei were also collected in the plankton net. Bulk POM and plankton size fractions were filtered onto pre-combusted 47 mm GF/F filters using a vacuum pump. All filters and P. bachei were secured in Petri dishes, frozen in liquid nitrogen, and stored at -80°C until processing.

Plankton and POM filters were oven-dried (50°C for 48 hr) for SI analysis or freeze-dried directly from frozen state for FA analysis. All A. labiata, A. victoria, and P. bachei were freeze-dried from frozen state for both SI and FA analyses. After drying, samples were ground to a fine powder and stored for 5-10 months at -80°C until analysis. All FA samples were flushed with nitrogen gas and stored in airtight cryovials to minimize oxidation. To meet the minimum sample weight required for analysis, 2-8 small medusae and 4-6 P. bachei were pooled per sample.

We encapsulated whole filters for POM, 1.0-1.2 mg dry weight of plankton, and 6.0-6.5 mg dry weight of medusa or P. bachei per sample for bulk C and N isotope analyses. Samples were analysed commercially at the University of Victoria Isotope Facility using an ECS 4010 analyzer (Costech) coupled via continuous flow to a Delta Advantage isotope ratio mass spectrometer (IRMS; Thermo-Finnigan). Three internal references, calibrated against International Atomic Energy Agency (IAEA) standards, were used to measure δ13C and δ15N relative to Vienna Pee Dee Belemnite (VPDB) and atmospheric N2. Mean standard deviations for reference materials were ±0.1‰ for δ13C and ±0.2‰ for δ15N. Values for δ13C and δ15N were measured in parts per thousand (‰) relative to external standards of VPDB and atmospheric N2. Stable isotope ratios are expressed using δ-notation [Eq. 1].

δX=(R_sample/R_standard -1) [Eq. 1]

Where X is 13C or 15N and R is the isotopic ratio 13C/12C or 15N/14N, respectively. We calculated C:N using the mass of total C and N that was measured by the elemental analyzer. We applied a lipid correction to δ13C for plankton using an equation derived for regional zooplankton [Eq. 2]

δ^13 C_corrected= δ^13 C_bulk+(0.38*〖C:N〗_bulk )-1.85 [Eq. 2]

Lipids were extracted from whole filters for POM, 80-100 mg dry weight of plankton, and 150 mg dry weight of A. labiata or P. bachei per sample using a one-step FA methyl ester method. All identified FAs were summed to obtain the total mass of FAs, then standardised using the sample dry weight to obtain the mass specific FA content (µg of FA per mg of sample dry weight). Proportions of each FA were calculated as a proportion of all identified FAs.