Alternative pathways of energy transfer guarantee the functionality and productivity in marine food webs that experience strong seasonality. Nevertheless, the complexity of zooplankton interactions is rarely considered in trophic studies because of the lack of detailed information about feeding interactions in nature. In this study, we used DNA metabarcoding to highlight the diversity of trophic niches in a wide range of micro- and mesozooplankton, including ciliates, rotifers, cladocerans, copepods and their prey, by sequencing 16- and 18S rRNA genes. Our study demonstrates that the zooplankton trophic niche partitioning goes beyond both phylogeny and size and reinforces the importance of diversity in resource use for stabilizing food web efficiency by allowing for several different pathways of energy transfer. We further highlight that small, rarely studied zooplankton (rotifers and ciliates) fill an important role in the Baltic Sea pelagic primary production pathways and the potential of ciliates, rotifers and crustaceans in the utilization of filamentous and picocyanobacteria within the pelagic food web. The approach used in this study is a suitable entry point to ecosystem-wide food web modelling considering species-specific resource use of key consumers.

Knowledge of zooplankton in situ diet is critical for accurate assessment of marine ecosystem function and structure, but due to methodological constraints, there is still a limited understanding of ecological networks in marine ecosystems. Here, we used DNA‐metabarcoding to study trophic interactions, with the aim to unveil the natural diet of zooplankton species under temporal variation of food resources. Several target consumers, including copepods and cladocerans, were investigated by sequencing 16S rRNA and 18S rRNA genes to identify prokaryote and eukaryote potential prey present in their guts. During the spring phytoplankton bloom, we found a dominance of diatom and dinoflagellate trophic links to copepods. During the summer period, zooplankton including cladocerans showed a more diverse diet dominated by cyanobacteria and heterotrophic prey. Our study suggests that copepods present trophic plasticity, changing their natural diet over seasons, and adapting their feeding strategies to the available prey spectrum, with some species being more selective. We did not find a large overlap of prey consumed by copepods and cladocerans, based on prey diversity found in their guts, suggesting that they occupy different roles in the trophic web. This study represents the first molecular approach to investigate several zooplankton–prey associations under seasonal variation, and highlights how, unlike other techniques, the diversity coverage is high when using DNA, allowing the possibility to detect a wide range of trophic interactions in plankton communities.

The cyclopoid copepod Oithona similis is one of the most abundant copepods in the oceans and has a potentially important role in pelagic food webs. However, there is a lack of knowledge on aspects of its biology and function in planktonic communities. In the present study, we aimed to assess and compare the species' trophic role in Greenlandic coastal waters during the winter-spring transition, with a focus on its winter behaviour, when large calanoids are not present in the surface layer. Two locations were studied: waters offshore from Godthåbsfjord (Nuuk) in winter, and Qeqertarsuaq (Disko Bay) in spring (bloom and post-bloom period). The potential prey of adult females of O. similis was quantified, and grazing experiments were conducted to determine the feeding rates of adult females on phytoplankton and protozooplankton >10 μm. The abundance, stage composition, and egg production of O. similis was also investigated. Ciliates were the preferred prey for O. similis, which confirms its importance as a link between the microbial food web and higher trophic levels. We observed high egg production rates and efficiencies of O. similis in winter, confirming that it is active and successfully reproductive in food-limited winter conditions. Our results stress that O. similis is a key component of Arctic and subarctic marine communities throughout the year.KEY WORDS: Oithona similis · Trophic role · Egg production · Greenland · Winter · Bloom · Arctic Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 483: [85][86][87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102] 2013 of Greenland (Head et al. 2003, Thor et al. 2005, Madsen et al. 2008, and within the genus, Oithona similis is a cosmopolitan species. Oithona spp. have been described to have year-round presence (Hansen et al. 2004) and reproductive activity (Nielsen & Sabatini 1996, Ward & Hirst 2007, and it has been argued that these copepods can exploit microbial food webs more efficiently than calanoid copepods (Nielsen & Sabatini 1996), characteristics that make Oithona spp. populations more stable in time and space than those of calanoid copepods (Paffenhöfer 1993). Consequently, Oithona spp. could be important food sources for fish larvae and larger zooplankton, especially in those periods of the year when other potential prey are not available.Regarding Arctic marine environments, only few studies on the biology of Oithona spp. have been published , Ward & Hirst 2007, Madsen et al. 2008, Narcy et al. 2009), and even less is known about their feeding ecology. It has been reported that Oithona similis prefer motile prey, especially ciliates (Nakamura & Turner 1997, Lonsdale et al. 2000, Castellani et al. 2005a, although many aspects of their natural diet are still unknown. Thus, our knowledge on their feeding activity in winter in high-latitude environments is almost nonexistent since most of the studies have focused on more productive periods of the year (Atkinson 1996, Møller et al. 2006...

Experiments to determine egg production and feeding rates of Oithona davisae were carried out under controlled laboratory conditions. From copepodite IV stage on, the animals were fed the heterotrophic dinoflagellate Oxyrrhis marina in a wide range of concentrations (from 10 mg C L 21 to 286 mg C L 21 ), and adult females were daily monitored to study different aspects of their fecundity. Both clutch and egg-production rate increased with food concentration, with values from 8 to 20 eggs for the clutch size, and from 1.8 eggs to 6.3 eggs female 21 d 21 for the egg production. In addition, to assess the efficiency of conversion of food intake into egg mass, two feeding experiments were conducted. Maximum weight-specific ingestion rates (< 80% body C d 21 ) and the egg-production efficiency (16%) were lower than those reported for free-spawning calanoid copepods. The fact that satiating food concentrations for feeding and egg production of adult females of Oithona davisae were rather low suggests an adaptation to exploit oligotrophic environments, and might explain the ecological success of the genus in situations when food becomes limiting for other groups of copepods.