Linking the metabolic rate of individuals to species ecology and life history in key Arctic copepods

This study compares the active and resting metabolic rates of species and life stages of copepods during the Arctic winter. Measurements were taken on individuals, and rates were described as functions of body mass. Differences in metabolic rate between species and life stages with the same mass were taken as evidence of distinct lifestyles. Copepod species that opportunistically feed during winter had higher active metabolic rates than species in a dormant state, but their resting metabolic rates were similar. Furthermore, metabolic rate increased more rapidly with mass for males compared to females and juvenile copepods with the result that the largest males had much greater rates. Whether the copepod was active or at rest had the greatest effect on metabolic rate, followed by body mass, species, and life stage having the least effect. Individual measurements on copepods can facilitate comparisons of metabolic rate across mass and further enable the measurement of rates when the individuals are active or at rest. The plasticity in metabolic rate found in the present study enables winter active species to have as low metabolic rates as dormant species and then suddenly mobilize and increase metabolism, likely as an adaptation to the scarce and random food availability during winter

A year-round study on digestive enzymes in the Arctic copepod Calanus glacialis: implications for its capability to adjust to changing environmental conditions

The biomass of zooplankton communities in Arctic shelf regions is dominated by the calanoid copepod Calanus glacialis . This species spends the winter in deep water, and then, metabolic rates are low. In late winter, it migrates to the surface where the spring generation develops. To date, it is not fully understood what regulates the activity of the copepods and how it coincides with food availability. To fill this gap, we sampled C. glacialis, mainly copepodite stage V, in a high-Arctic fjord in monthly intervals for 1 year and determined proteinase and lipase/esterase activities in relation to food availability and depth distribution of the copepods. By substrate SDSPAGE(sodium dodecyl sulfate-polyacrylamide gel electrophoresis),we tackled changes in specific isoforms. We found a clear seasonal enzyme activity pattern. Activities in winter were reduced by at least 75 % as compared to spring. Substrate SDS-PAGE showed high heterogeneity of lipolytic enzymes, which could reflect extensive accumulation and metabolization of internal lipids. Only one band of proteolytic activity was found, and it intensified with the onset of the algal blooms. In late winter/spring, we sampled females and CIV, which also showed high digestive enzyme activities in surface water and low activities in deep water. High enzyme activities were related to the ice algal and phytoplankton blooms in spring. In autumn, the copepods descended although food was still available. C. glacialis could thus benefit from an early ice breakup and early algal blooms, but not from long-lasting phytoplankton availabilit

Effect of light and food on the metabolism of the Arctic copepod Calanus glacialis

Reduction in sea-ice thickness and cover is expected to lead to earlier underwater light penetration and thus earlier onset of the spring bloom and a longer open water production in the Arctic. The goal of this study was to understand how these climate-induced changes in light and food regimes may impact the key zooplankton grazer Calanus glacialis CV. We studied this copepod's metabolic response to starvation (filtered sea water; FSW) and algal food (Food) under two different light regimes (light vs. dark) when it was in dormancy (diapause) in winter (November) and active in summer (July). Respiration was measured as indicator of metabolism and was measured for 9 days for copepods exposed to: Dark+FSW, Dark+Food, Light+FSW and Light+Food. The in situ respiration in winter was three times lower than in summer. In winter, light was the main factor to increase the copepod's metabolism to a level comparable with that of active copepods in summer, but respiration only remained high if food was present. In summer, it was the combined effect of light and Food that increased the respiration, although Food seemed more important than light with time. Copepods reduced their metabolism with time when food was absent, regardless of the light regime, probably preparing for diapause. These results suggest that C. glacialis can quickly adapt to a changing light and food regime in the Arctic, being able to wake up from diapause if light and thus food appear and postpone its diapause if the food availability is still favorable

Lipid and Fatty Acid Turnover of Arctic Zooplankton Organisms Revealed by Stable Isotope Analyses

High latitude marine ecosystems are characterized by strong seasonality in incoming light and thus primary production and food availability. Polar zooplankton organisms have developed the ability of storing large amounts of lipid reserves to face this variable environment. Lipids are composed of fatty acids, which are transferred from unicellular algae via zooplankton to higher trophic levels. In our experiments, a 13C labeled diatom-flagellate mix was fed to key zooplankton species (copepods and thecosome pteropods) over some days to a couple of weeks to follow the fatty acid carbon assimilation and possible de novo synthesis of fatty acids and alcohols. Fatty acid and fatty alcohol compositions were determined by gas chromatography. The 13C incorporation was monitored using compound specific isotope ratio mass spectrometry. Among the small sized copepods Pseudocalanus minutus and Oithona similis, maximum lipid turnover occurred in P. minutus, which exchanged 2.6% day-1 of total lipid, whereas 0.5% day-1 were exchanged in O. similis. In P. minutus, the diatom markers 16:1(n-7), 16:2(n-4), and 16:3(n-4) were almost completely renewed from the diet within 21 days, while 15% of the flagellate markers 18:2(n-6), 18:3(n-3) and 18:4 (n-3) were exchanged. In O. similis, 15% of both flagellate and diatom markers were renewed within 21 days. Thecosome pteropods, in contrast, are less lipid-rich and less studied, although they can contribute with more than 20% to the zooplankton biomass in Arctic waters. The daily turnover rate of lipid was between 0.15% day-1 in L. helicina and 1.3% day-1 in L. retroversa. High carbon assimilation was found in both diatom and flagellate markers in L. helicina accounting for 0.8% over 6 days. In L. retroversa, 0.8% of the diatom markers were exchanged after 6 days while 13.9% were renewed in flagellate markers. Our methods allow us to estimate lipid and fatty acid turnover rates of specific Arctic key organisms to better understand the carbon und energy flux through the high latitude marine ecosystems

Indication for the timing of diapause – Seasonal development of metabolic activity in the Arctic copepod Calanus glacialis

Calanus glacialis strongly contributes to the energy fluxes in Arctic shelf ecosystems. During its life cycle, this copepod species overwinters at depth with physiological adjustments which are yet not fully understood. We therefore assessed key metabolic activities and extracellular pH (pHe) in the haemolymph, seeking to identify the mechanisms which characterize this so called diapause state. For one year we sampled copepods in a high Arctic fjord and found that their enzymatic activities and pHe followed a clear seasonal trend. Both, activities and pHe were high when the copepods were feeding in surface waters in spring. In winter at depth, metabolic enzyme activities and pHe were low (pHe<6), while lipid catabolism on internal reserves increased. Metabolic activity matched the pHe pattern suggesting that low pH supports metabolic depression. Metabolic enzyme syntheses decreased and copepods descended before food had disappeared. In late winter, metabolic activity increased and the copepods migrated to the surface long before the spring bloom. Our data support that diapause timing is regulated internally, with an early enzyme synthesis that prepares the copepods for activity before spring

Maline Daase: How much for the night? - Energetic costs of overwintering for the Arctic copepod Calanus glacialis

The copepod Calanus glacialis comprises up to 80% of the zooplankton biomass in Arctic shelf seas and plays a key role in Arctic marine ecosystems. It is primarily a grazer, accumulating essential polyunsaturated fatty acids from its algal diet as well as converting low-energy carbohydrates and proteins in algae into high-energy wax ester lipids. It is able to survive long periods without food by descending to depth and lowering its metabolism to a minimum, a state referred to as diapause. Although C. glacialis may be in this physiological state for up to 8 months each year we know very little about the energetic costs required during diapause. We therefore initiated an extensive field campaign in a high-Arctic fjord, sampling the local population monthly from June 2012 to July 2013. Monthly carbon demand was estimated by measuring respiration, image analysis was used to analyse variability in lipid content over the season. The carbon demand during winter differed among C. glacialis CIV, CV, females and males, with CV and adults being active much earlier previously assumed. Lipid reserves in CV and females remain largely untouched throughout autumn but decrease from January on, most likely to fuel moulting and maturation. The C. glacialis population declined steeply from January to May suggesting that individuals may run out of energy stores during winter. Of the verwintering stages, only IV seems to stay in diapause over an extended period, utilizing little of its lipid storage from fall through winte