Turnover of trophic markers and lipid carbon in Arctic marine food webs : the contribution of key zooplankton species

In the context of climate warming, severe shifts in the phyto- and zooplankton communities, and thus changes in trophic interactions, are expected. It is therefore essential to better understand the lipid and fatty acid turnover in the in the lipid-driven Arctic food web. This study aims at evaluating the role of zooplankton in the transfer of lipids from primary producers to higher trophic levels. It combines field observations and experimental work to fill the gaps of knowledge in the ecology and lipid biochemistry of Arctic zooplankton key species, i.e. the copepods Calanus glacialis, Pseudocalanus minutus and Oithona similis, the thecosome pteropods Limacina helicina and L. retroversa and the gymnosome pteropod Clione limacina

Lipid storage consumption and feeding ability of Calanus glacialis Jaschnov, 1955 males

Calanus is one of the best studied genera of Arctic zooplankton, but still we know very little about the males since they are short-lived and mainly present in winter. Their short life-span compared to females is assumed to be a combination of high mating activity, no feeding and consequential depletion of lipid stores. In this study we tested 1) if the life span of male Calanus glacialis is limited by their lipid storage reserves and 2) if males are capable of feeding and utilize food if present. We ran two separate experiments from January to March; one on starvation and one on feeding. In the 39-days long starvation experiment we followed the lipid sac size of individually incubated males until their time of death. On average the total lipid (TL) content decreased by 2.6 to 4.5 μg day−1, but despite this males had substantial amounts of lipids left (131.4 μg, SD 44.0) when they died. This strongly suggests that the depletion of lipid reserves is not the main reason for males' short life span which in this study was measured to be up to 73 days. In the feeding experiment, we fed both C. glacialis males and females ad libitum with 13C labelled microalgae. Both males and females were capable of feeding and assimilate the diatom monoculture, but females responded faster to the sudden favourable food conditions, and produced more and larger fecal pellets than the males. Assimilation of 13C labelled 20:5(n-3), an essential polyunsaturated fatty acid (PUFA), from the diatom diet was traceable in both males and females on day 21, and then with a higher enrichment in females than males. Morphological investigations of the feeding appendages showed some differences between sexes, suggesting males to be more omnivorous than females. In conclusion, lipid storage depletion is not the cause of death for male C. glacialis, and males may even compensate for some of the mating energy costs by feeding. In future, we recommend further studies on the role of essential fatty acids (FA) for sperm formation and aging as determining factors for males' relatively short life span.publishedVersio

Lipid turnover reflects life-cycle strategies of small-sized Arctic copepods

This study aimed at understanding how life-cycle strategies of the primarily herbivorous Pseudocalanus minutus and the omnivorous Oithona similis are reflected by their lipid carbon turnover capacities. The copepods were collected in Billefjorden, Svalbard, and fed with 13C labeled flagellates and diatoms during 3 weeks. Fatty acid (FA) and fatty alcohol compositions were determined by gas chromatography, 13C incorporation was monitored using isotope ratio mass spectrometry. Maximum lipid turnover occurred in P. minutus, which exchanged 54.4% of total lipid, whereas 9.4% 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. P. minutus exhibited typical physiological adaptations of herbivorous copepod species, with a very high lipid turnover rate and the ability to integrate FAs more rapidly from diatoms than from flagellates. O. similis depended much less on lipid reserves and had a lower lipid turnover rate, but was able to ingest and/or assimilate lipids with the same intensity from various food sources, to sustain shorter periods of food shortage

Year-round population dynamics of Limacina spp. early stages in a high-Arctic fjord (Adventfjorden, Svalbard)

The thecosome pteropods Limacina helicina and L. retroversa are important contributors to the zooplankton community in high-latitude environments but little is known about their distribution and life cycle under polar conditions. We collected the early life stages (< 1 mm) of the thecosome population in 2012 and 2013 at a bi-weekly to monthly resolution in fjord highly influenced by Arctic waters as well as Atlantic inflows (Adventfjorden, Svalbard, 78°N), together with environmental parameters. L. retroversa only occurred episodically, in association with the inflow of Atlantic water, with low numbers and random size distributions. This suggests that this boreal species does not fulfill its life cycle in Adventfjorden. In contrast, young specimens of L. helicina were present during the entire study. Veligers hatched in late summer/autumn and measured 0.14 mm on average. They grew with rates of 0.0006 mm day− 1 over the 10–11 months of development. Only thereafter, growth accelerated by one order of magnitude and maximal rates were reached in autumn (0.0077 mm day− 1). Our results indicate that L. helicina reaches a size of 1 mm after approximately 1.5 years in Adventfjorden. We therefore suggest that L. helicina overwinters the first year as a small juvenile and that it needs at least 2 years to reach an adult size of 5 mm in Adventfjorden. This reveals an complex and delicate aspect of the life-cycle of L. helicina and further research is needed to determine if it makes the population especially vulnerable towards climate changes

Radiological detection of nephrocalcinosis in farmed Atlantic salmon Salmo salar L.

Nephrocalcinosis is a common disorder in nurseries in Norway (Klykken, Reed, et al., 2022) and was reported as one of the main welfare challenges in farmed salmon by The Norwegian Fish Health Report of 2019 (Sommerset et al., 2020). Nephrocalcinosis is described as deposits of minerals within the kidneys (Bruno, 1996), that can disturb kidney function, which in turn can have dramatic consequences on fish performance and survival. The aetiology is most likely related to environmental factors, and suboptimal water quality has been indicated in several studies as the main risk factor (Fivelstad et al., 1999; Fivelstad et al., 2003; Khan et al., 2018; Fivelstad et al., 2015; Good et al., 2010; Lewisch et al., 2013; Chen et al., 2001). Newly conducted research suggested that osmoregulatory stress may be the trigger for nephrocalcinosis (Boissonnot et al., 2022). Regardless of the severity of the condition, fish rarely present external signs, and it is thus challenging to monitor its prevalence and development. Present diagnostic methods require euthanasia as they consist of visually scoring the accumulation of deposits and the severity of lesions. Macroscopic assessments of necropsied fish are often imprecise, since small deposits are rarely visible to the naked eye, and histopathology is therefore considered as the best existing diagnostic method (Klykken, Boissonnot, et al., 2022). Research on, and monitoring of, nephrocalcinosis has been greatly hampered by the lack of non-invasive methods of assessing the presence and severity of this condition, as it is not possible to follow the development of the disease in single individuals, and as the number of sampled fish is limited due to ethical reasons. Radiology has previously been used for assessing vertebrae deformities in Atlantic salmon (Drábiková et al., 2021; Holm et al., 2020), based on the classification scheme developed by Witten et al. (2009), and there has been a rapid development of the technology (Ou et al., 2021) including portable systems, which allow efficient in situ diagnosis. Nephrocalcinosis in Atlantic salmon is mainly identified as amorphous carbonate apatite, a calciumdominated mineral (Klykken, Reed, et al., 2022), and it has previously been demonstrated that this mineral composition is suitable for xray detection (Smith & Lehr, 1966). Radiology is non-invasive and can be performed on anaesthetized fish, enabling assessment of nephrocalcinosis without euthanazing the fish. A non-invasive method for assessing nephrocalcinosis would allow for monitoring of the condition over time on an individual level and would be ethically and economically preferable. We have therefore explored radiology as a possible tool to detect and evaluate the severity of nephrocalcinosis, comparing it with histological scoring

Effect of food and light on the development of the Arctic copepod Calanus glacialis during the winter-spring transition

We investigated potential impacts of earlier sea ice retreat and timing of the spring bloom on overwintering copepodites (CIV) of Calanus glacialis, the main primary consumer in Arctic shelf seas. By field studies and laboratory experiments, we following the feeding and growth of C. glacialis under different light (dark/light) and food (starved/fed) conditions, spring 2013, in high-Arctic Svalbard. We carefully monitored the molting rate, lipid content and fatty acid composition in field and laboratory specimens, simultaneously, over a 2 months period. Field results indicated that CIV were in dormancy (diapause) until the light returned. Both light and food appeared to be important “wake-up” triggers from diapause. Growth and development, however, were strongly regulated by food. In field, the main molting from CIV to CV occurred 2-3 weeks after the onset of the spring bloom (June) which corresponded well with the experimental results where the fed ones molted end of April after 3 weeks of intensive feeding, while the starved ones not. Our results suggest that C. glacialis quickly respond to external cues, being able to adapt to a changing Arctic

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

Assimilation and turnover rates of lipid compounds in dominant Antarctic copepods fed with 13 C-enriched diatoms

The study revealed species- and stage-specific differences in lipid accumulation of the dominant Antarctic copepods, the primarily herbivorous Calanoides acutus (copepodite stage V (CV), females) and the more omnivorous Calanus propinquus (females) storing wax esters and triacylglycerols, respectively, which were collected in summer (end of December). Feeding carbon-labelled diatoms to these copepods, 13C elucidated assimilation and turnover rates of copepod total lipids as well as specific fatty acids and alcohols. The 13C incorporation was monitored by compound-specific stable isotope analysis (CSIA). CV stages of C. acutus exhibited an intense total lipid turnover and 55% of total lipidswere labelled after 9 days of feeding. By contrast, total lipid assimilation of female C. acutus and C. propinquus was lower with 29% and 32%, respectively. The major dietary fatty acids 16:0, 16:1(n − 7) and 20:5(n − 3) had high turnover rates in all specimens. In C. acutus CV, the high rates of the de novo synthesized long-chain monounsaturated fatty acids and alcohols 20:1(n − 9) and 22:1(n − 11) indicate intense lipid deposition, whereas these rates were low in females. The differences in lipid assimilation and turnover clearly show that the copepod species exhibit a high variability and plasticity to adapt their lipid production to their various life phases. This article is part of the theme issue ‘The next horizons for lipids as ‘trophic biomarkers’: evidence and significance of consumer modification of dietary fatty acids’