The two experiments for which data is presented in this record were conducted in the context of RMFS' PhD work. The objective of the experiments was to quantify and qualify the effects of diet quality, herein manipulated in terms of different species (the diatom Conticribra weissflogii and the dinoflagellate Oxyrrhis marina) grown under different nutrient regimes (nutrient replete and Nitrogen-depleted), on the fatty acid (FA) assimilation and turnover of the copepod Temora longicornis.Experiments used field-collected copepods; sampling for experiments I and II took place on May 17th and 30th, 2016, respectively, with a 500 µm mesh-size CalCOFI net which was towed horizontally for 15 minutes at 5 m depth off the German island of Helgoland (54o11'N, 07o54'E), in the southern North Sea. Samples were immediately taken to the laboratory, where intact and active adult females were sorted under an Olympus SZX16 stereoscopic microscope. A total of 1260 females were sorted for each date, 1080 for the feeding experiment and 180 for the determination of in situ elemental and biochemical compositions. This study was conducted concomitantly with that from Franco-Santos et al. (2018). The feeding experiment was initiated after sorting, and lasted for five days. Females were distributed between triplicate 3L plastic beakers (75 females L-1), which were fitted with a 300 µm meshed-bottom cylinder, and kept in a dark, temperature-controlled room (10 ± 0.3oC, a temperature similar to that recorded in the surface water during sampling).Batch cultures of C. weissflogii were started on a daily basis (prior to starting the experiment) for five consecutive days; a stock solution was diluted with fresh f/2 medium (with and without nitrate additions, modified from Guillard, 1975), which contained 13C-enriched sodium bicarbonate (NaH13CO3, 4 mg L-1), and was grown for five days before being used to feed copepods (details in Franco-Santos et al., 2018). The same protocol was followed to culture the cryptophycean Rhodomonas salina, but bicarbonate was added to a concentration of 12 mg L-1. The algae were then used to feed the cultures of O. marina and, thus, create its different nutrient treatments. The dinoflagellate batches were cultured with the same protocol as the diatoms, except that the stock solution was diluted on a daily basis with labelled food (i.e., R. salina) rather than once at the start of the culture with isotopically-enriched medium. Cryptophycean cell quantities given to dinoflagellates were adjusted so that the former was depleted from the cultures on day 5. Diatom and dinoflagellate diets were provided for copepods ad libitum (> 350 µg C L-1; 8 and 2 * 103 cells mL-1, respectively) on a daily basis for five days. Cell density in the cultures was determined with a BD Accuri C6 Flow Cytometer. Beakers were gently stirred three times a day in order to resuspend dietary cells. Immediately before feeding copepods, a partial (approx. 65%) water exchange was conducted, which removed most of the food from the previous day.Copepods were sampled on days 1 (in situ composition, t0h), 3 (t48h), and 6 (t120h) of the experiment. Females were pooled into 10 and 50 individuals per replicate for elemental (body carbon (C) and nitrogen (N) contents and molar C:N ratio) and biochemical (total FA content and profile, and FA-specific content and 13C isotopic signal) analyses. Sampled copepods were gently washed in distilled water, then placed into pre-weighed tin capsules (5x9 mm, IVA Analysentechnik) or pre-combusted lipid vials (for elemental and FA analyses, respectively). Cultures were sampled daily during the experiment (after food was provided to copepods) for determination of cell elemental (C and N contents and molar C:N ratio) and biochemical (total FA content and profile, and FA-specific content and 13C isotopic enrichment) compositions. Subsamples of 5.2 and 0.4 *106 cells (for diatoms and dinoflagellates, respectively) were filtered through pre-combusted (500oC for 24h) Whatman GF/F filters (0.7 µm pore size, 25 mm diameter). Tin capsules and filters with samples for elemental analysis were dried at 60oC for 48 h; filters were folded inside tin foil, and both capsules and foil were stored in a desiccator until analysis. Filters with samples for FA analyses were placed into pre-combusted lipid vials, and vails containing both copepods and filters were stored at -80oC until analyses.The dry mass (DM) and C and N contents of samples were obtained as per Franco Santos et al. (2018). Lipid extraction (modified after Folch et al., 1957) and subsequent fatty acid methyl ester (FAME) quantification were performed as described in Franco-Santos et al. (2019) (and references therein). Temora longicornis does not have significant energy reserves and exhibits triacylglycerols (TAGs) as its primary neutral lipids (Fraser et al., 1989; Peters et al., 2013). Lipid classes were not separated in this study, and it was assumed that FAMEs were composed of TAGs. The FA-specific 13C isotopic composition of FAMEs was measured according to Boissonnot et al. (2016).Lipid C assimilation and turnover were calculated according to the equations used by Boissonnot et al. (2016) and Franco-Santos et al. (2019). Lipid C assimilation efficiency (AE), the percentage of (isotopically-enriched) dietary content ingested by copepods that was assimilated into FAs, was also calculated for (a) TFA, (b) saturation-specific sums of FAs (saturated, monounsaturated, and polyunsaturated FAs), and (c) each individual FA that was both available from the diet and assimilated by copepods (> 1% TFA in copepods). All the equations necessary for these calculations are described in the data sets contained in this bundled publication.