Copepods are among the most abundant and diverse groups of mesozooplankton in the world's oceans. Each species has a certain depth range within which different individuals (of the same life stage and sex) are found. Lipids are accumulated in many calanoid copepods for energy storage and reproduction. Lipid content in some species increases with depth, however studies so far focused mostly on temperate and high-latitude seasonal vertically migrating copepods and compared lipid contents among individuals either from coarse layers or between diapausing, deep-dwelling copepods and individuals found in the photic, near-surface layer. Here we examined whether lipid contents of individual calanoid copepods of the same species, life stage/sex differ between finer depth layers within the upper water column of subtropical and Arctic seas. A total of 6 calanoid species were collected from samples taken at precise depths within the photic layer in both cold eutrophic and warm oligotrophic environments using SCUBA diving, MOCNESS and Multinet. Measurements of lipid content were obtained from digitized photographs of the collected individuals. The results revealed significant differences in lipid content across depth differences as small as 12–15 meters for Mecynocera clausi C5 and Ctenocalanus vanus C5 (Red Sea), Clausocalanus furcatus males and two clausocalanid C5s (Mediterranean Sea), and Calanus glacialis C5 (Arctic). We suggest two possible explanations for the differences in lipid content with depth on such a fine scale: predator avoidance and buoyancy.

A large amount of warm Atlantic water (AW) enters the Arctic as a boundary current through Fram Strait (West Spitsbergen Current [WSC]) and is the major oceanic heat source to the Arctic Ocean. Along the north‐western Svalbard shelf, the WSC splits into the shallow Svalbard Branch, the Yermak Branch that follows the slope of the Yermak Plateau, and the Yermak Pass Branch flowing across the plateau. The WSC has previously been studied using moorings, dedicated oceanographic transects, and models. In this study, we mapped the circulation patterns and AW flow around Svalbard using Vessel‐Mounted Acoustic Doppler Current Profiler data from multiple surveys during four consecutive summers (2014–2017). Despite the scattered nature of this compiled data set, persistent circulation patterns could be discerned. Spatial interpolation showed a meandering boundary current west of Svalbard and a more homogeneous AW flow, centered around the 1,000‐m isobath north of Svalbard. In all summers, we observed a northward jet between 79 and 80°N and the 1,000‐ and 500‐m isobaths, before the WSC divided into the three branches. North of Svalbard, the shallow Svalbard Branch reunited with the Yermak Pass Branch between 10 and 15°E and a part of the AW circulated within Hinlopen Trench. The calculated volume transport of 2 Sv in the upper 500 m compares well with model results and previous observations. Our results further show that the Yermak Pass Branch can be as important as the Svalbard Branch in transporting AW across the Yermak Plateau during summer.

A comparative study of lipid profile of the daubed shanny Leptoclinus maculatus postlarvae from fjords of different domains (arctic-boreal Kongsfjord and high-arctic Rjipfjord) of Svalbard Archipelago waters in summer was performed. A more efficient accumulation of lipids in the fishes from Kongsfjord compared to Rjipfjord due to triacylglycerols and wax esters was established. These differences may be related mainly to the trophic conditions of the habitat (species composition, abundance, and availability of food items). The higher level of cholesterol esters than wax esters at the L1 stage of development of the daubed shanny postlarvae compared to older fishes pointed to their active feeding mainly on phytoplankton. The differences in the content of certain classes of phospholipids may indicate adaptive changes at the level of biomembranes, contributing to the maintenance of cell homeostasis in response to abiotic environmental factors, and reflect their additional intake with food at the early stages of fish development.