Dormancy (diapause) is a key life-history strategy of pelagic copepods that allows them to thrive in highly seasonal environments. Successful dormancy of copepodid stages requires the ability to store energy efficiently (for example as lipids) and to slow down the rate of mobilization of this capital during the dormant period. The physiology of lipids in copepods has been extensively reviewed; however, data about the energetics of dormancy are currently scattered throughout the literature. Thus, we conducted a meta-analysis comparing the metabolism of active and dormant copepods in 15 species that undergo dormancy as copepodids. Linear mixed-effects models showed that the metabolic rate of dormant copepods is about one-fourth of the values for actively growing copepods, a level that remains consistent across a large range of body size or environmental conditions. Based on these metabolic rates, we used a numerical modelling approach to predict dormancy duration as a function of body mass and ambient temperature, and to explain the observed range of body masses at the initiation of dormancy. Our numerical approach also provides explanations for inter- and intra-specific variability in life-history strategies, such as which stages undergo dormancy and the prevalence of lipid-based reproduction in some copepod species.

Imaging techniques are increasingly used in ecology studies, producing vast quantities of data. Inferring functional traits from individual images can provide original insights on ecosystem processes. Morphological traits are, as other functional traits, individual characteristics influencing an organism's fitness. We measured them from in situ image data to study an Arctic zooplankton community during sea ice break‐up. Morphological descriptors (e.g., area, lightness, complexity) were automatically measured on ∼ 28,000 individual copepod images from a high‐resolution underwater camera deployed at more than 150 sampling sites across the ice‐edge. A statistically‐defined morphological space allowed synthesizing morphological information into interpretable and continuous traits (size, opacity, and appendages visibility). This novel approach provides theoretical and methodological advantages because it gives access to both inter‐ and intra‐specific variability by automatically analyzing a large dataset of individual images. The spatial distribution of morphological traits revealed that large copepods are associated with ice‐covered waters, while open waters host smaller individuals. In those ice‐free waters, copepods also seem to feed more actively, as suggested by the increased visibility of their appendages. These traits distributions are likely explained by bottom‐up control: high phytoplankton concentrations in the well‐lit open waters encourages individuals to actively feed and stimulates the development of small copepod stages. Furthermore, copepods located at the ice edge were opaquer, presumably because of full guts or an increase in red pigmentation. Our morphological trait‐based approach revealed ecological patterns that would have been inaccessible otherwise, including color and posture variations of copepods associated with ice‐edge environments in Arctic ecosystems.

The planktonic copepod, Calanus finmarchicus, resides at the southern edge of its subarctic range in the Gulf of Maine (GoM). Here we investigate the population response of C. finmarchicus to record warming in the GoM in 2012. Demographic data from two time series stations and a plankton survey conducted in early autumn 2012 show that C. finmarchicus did not produce an autumn generation as predicted, despite warm summer and overwintering temperatures. On the contrary, we observed high abundances of the overwintering stage CV in the western GoM and a new cohort in early spring 2013 likely originating from egg production during a winter phytoplankton bloom. This spring cohort was the most abundant ever recorded in the 8-year time series. To account for these observations, we hypothesize that production of females originating from the eastern GoM and Scotian Shelf, combined with growth of copepodid stages in the Maine Coastal Current and local egg production, are the primary sources of supply maintaining high abundances in Wilkinson Basin, the primary repository of C. finmarchicus in the western GoM. Predicting fluctuations in abundance or circumstances for disappearance of C. finmarchicus in the northwest Atlantic will need models that address the roles of local production and advection.

Calanus finmarchicus relies on dormancy to thrive in the seasonal environment of the boreal Atlantic. The lipid accumulation window (LAW) hypothesis proposes that a seasonal window of environmental conditions allows developing individuals to store enough lipids for dormancy to be safely initiated. Successful dormancy requires a sufficient amount of lipids to fulfil the reduced metabolic demand of the dormant individual and to sustain the final maturation process. We used a pattern-oriented modelling approach that implements the LAW hypothesis and employs a genetic algorithm for parameter estimation, in order to reproduce the observed phenology and demography of C. finmarchicus populations from the two contrasting regions, the Gulf of St. Lawrence (GSL) and the Gulf of Maine (GoM) in the northwest Atlantic shelf. In the GSL, the model reproduced the timing of dormancy, the abundance and individual condition of late copepodid stages. In the GoM, the model produced a semi-annual dormancy pattern, as no locally produced individual could last the 6–8 months of dormancy inferred from the available observations. Further testing requires extending demographic time series, including lipid condition of late copepodid stages in the GoM, and the implementation of a 3-D modelling framework that would explicitly address the complex interactions between circulation and population dynamics of C. finmarchicus over the entire northwest Atlantic shelf.

Copepod lipids fuel the Arctic marine ecosystem, but information on the fine-scale distribution of copepods and lipids is nonexistent. This study investigated the fine-scale (1 m) vertical distribution of the copepods Calanus hyperboreus, Calanus glacialis and Metridia longa during a Lagrangian drift in the North Water Polynya using the Lightframe On-sight Keyspecies Investigation (LOKI) imaging system. A copepod species- and stage-specific automatic identification model based on machine learning, a subcategory of artificial intelligence, was used to identify images taken by LOKI. Lipids were measured from images of copepods taken over the whole water column (1 m resolution). Diel vertical migration (DVM) in all three species was detected. In C. hyperboreus and C. glacialis C4-females as well as M. longa C5-females lipid load of deep copepod individuals was significantly higher than that of shallower individuals. Vertical distribution profiles and individual lipid loads suggested that individuals with lower lipid load continued DVM, while others with high lipid load ceased migrating, remaining at depth. Calanus hyperboreus individuals seemed to migrate to diapause at lower lipid fullness (50%) than C. glacialis (60%). A bioenergetics model showed that Calanus females had enough lipids to diapause for over a year, highlighting the significant lipid overhead they use for capital breeding.

The life cycle of Calanus finmarchicus includes a prolonged dormancy phase that allows it to avoid the unfavourable environmental conditions typical of the upper ocean from late summer to early spring in the subarctic North Atlantic. Recent demographic, physiological and genetic evidence supports the hypothesis of a crucial role for lipid accumulation and metabolism in the control of dormancy. We present a stage-resolving biomass model of C. finmarchicus, implementing a mechanistic approach of the control of dormancy based on this lipid hypothesis. The dormancy process obeys 2 rules: (1) active copepodite stage Vs (C5s) enter dormancy when the ratio of lipid to total body carbon exceeds some threshold, and (2) diapausing C5s exit dormancy when their lipid storage approaches a lower threshold. We implemented the model into a 1-dimensional water column framework and compared our results to 2 consecutive years of observations of stage-specific copepodite abundances and lipid content of C5 from the Northwest Gulf of St. Lawrence, Canada. The model produced realistic phenology and temporal patterns in lipid content of C. finmarchicus in response to the observed environmental forcing. Interannual variations in the timing of entry and contributions of different generations to the overwintering stock were shown. Our results are consistent with the hypothesis of a lipid-mediated control of entrance and exit from dormancy in C. finmarchicus