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Seagrass meadows are highly productive habitats that can act as “blue carbon sinks” in coastal ecosystems by facilitating sedimentation and trapping particles. However, the magnitude and occurrence of these effects may be species and density dependent. The present study is the first estimation of seagrass sediment carbon sink in the temperate Zostera marina beds in the Baltic Sea. Several descriptors of organic matter characteristics, along with possible organic matter sources in the sediment were compared at vegetated and unvegetated bottoms. The 210Pb dating of the sediment has been used for accumulation rate assessment. The photopigments and POC concentrations in sediments were higher in vegetated bottoms. The SIAR (Stable Isotopes in R) mixing model based on nitrogen and carbon stable isotope values, indicated that higher percentages of organic matter originated from seagrass production in vegetated sediments (40–45%) compared to unvegetated ones (5–21%). The carbon stock in the upper 10 cm of the vegetated sediments ranged from 50.2 ± 2.2 to 228.0 ± 11.6 (g m−2), whereas the annual C accumulation amount from 0.84 ± 0.2 to 3.85 ± 1.2 (g m−2 yr−1). Our study shows that even the relatively weakly developed vegetation of the small temperate seagrass species enhance organic carbon concentration in the sediments. Estimated carbon stock was much lower than those reported for most of the seagrass meadows elsewhere, and the carbon burial rate was the lowest ever reported. Evidently, the global calculations of sediment carbon stock should be reconsidered by taking into account density and species‐related variability.

Gut content examination and trophic markers (fatty acids, stable isotopes of C and N) were combined to delineate the diet of the dominant species of amphipods from Mediterranean Posidonia oceanica seagrass meadows and to highlight trophic diversity among this community. Our results indicate that, although all dominant species heavily relied on macroalgal epiphytes, considerable interspecific dietary differences existed. Carbon stable isotope ratios notably showed that some of the amphipod species favored grazing on epiphytes from leaves or litter fragments (Apherusa chiereghinii, Aora spinicornis, Gammarus aequicauda), while others such as Dexamine spiniventris preferred epiphytes from rhizomes. The remaining amphipods (Caprella acanthifera, Ampithoe helleri and Gammarella fucicola) readily consumed both groups. In addition, SIAR modeling suggested that most species had a mixed diet, and relied on several food items. Fatty acid analysis and gut contents revealed that contributions of microepiphytic diatoms and of benthic and suspended particulate organic matter to the diet of amphipods were anecdotal. None of the examined species seemed to graze on their seagrass host [low 18:2(n-6) and 18:3(n-3) fatty acids contents], but Gammarus aequicauda partly relied on seagrass leaf detritus, as demonstrated by the lesser 13 C-depletion of their tissues. Overall, our findings suggest that amphipods, because of their importance in the transfer of organic matter from primary producers and detritus to higher rank consumers, are key items in P. oceanica-associated food webs.

In recent years, sea ice cover along coasts of East Antarctica has tended to increase. To understand ecological implications of these environmental changes, we studied benthic food web structure on the coasts of Adélie Land during an event of unusually high sea ice cover (i.e. two successive austral summers without seasonal breakup). We used integrative trophic markers (stable isotope ratios of carbon, nitrogen and sulfur) to build ecological models and explored feeding habits of macroinvertebrates. In total, 28 taxa spanning most present animal groups and functional guilds were investigated. Our results indicate that the absence of seasonal sea ice breakup deeply influenced benthic food webs. Sympagic algae dominated the diet of many key consumers, and the trophic levels of invertebrates were low, suggesting omnivore consumers did not rely much on predation and/or scavenging. Our results provide insights about how Antarctic benthic consumers, which typically live in an extremely stable environment, might adapt their feeding habits in response to sudden changes in environmental conditions and trophic resource availability. They also show that local and/or global trends of sea ice increase in Antarctica have the potential to cause drastic changes in food web structure, and therefore to impact benthic communities.

Fjords have been recently recognized as hot spots of organic carbon (Corg) sequestration in marine sediments. This study aims to identify regional and local drivers of variability of Corg burial in north Atlantic and Arctic fjords. We provide a comparative quantification of Corg, δ13C, photosynthetic pigments content, benthic biomass, consumption, Corg accumulation, and burial rates in sediments in six fjords (60–81°N). Higher sediment Corg content in southern Norway reflected longer phytoplankton growth season and higher productivity. Higher contributions of terrestrial Corg were noted in temperate/southern Norway (dense land vegetation and high precipitation) and Arctic/Svalbard (glacial erosion) than in subarctic/northern Norway locations. Benthic biomass and carbon consumption were best correlated to δ13C and photosynthetic pigments content indicating control by quality rather than quantity of available food. Benthic faunal consumption did not seem to affect the variability in Corg burial. Regional environmental factors (water temperature and latitude) combined with local factors (Corg, grain size, and pigment concentration) explained 94% of Corg burial variability. Based on the present study and literature data on Corg content, origin, and burial rates, the fjords were classified into four categories: temperate, subarctic, Arctic with glaciers, and Arctic without glaciers. The variability in marine productivity, terrestrial inflows, and carbon sequestration in fjords must be considered for global estimates of their role in blue carbon storage and for building scenarios of future changes in the course of climate warming.

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14Elucidating predator-prey relationships is an important part of understanding and assessing the 15 structure and function of ecosystems. Sharks are believed to play a significant role in marine 16 ecosystems, although their specific trophic ecology is largely unexplored. Stable isotopes of 17 nitrogen (δ 15 N) and carbon (δ 13 C) are a widely applied tool in food web studies but there is a 18 need to quantify stable isotope dynamics in animals, particularly sharks. In this study, diet-tissue 19 discrimination factors (DTDF = stable isotope in consumer tissue -stable isotope in diet) and 20 turnover rates (time for the isotope to be assimilated into the consumer's tissue) of stable 21 isotopes were estimated in blood, fin, and muscle tissue for the shark species Scyliorhinus 22 stellaris fed two diets with different isotope values. Subsequently, these diet-and tissue-specific 23DTDFs were used in isotopic mixing models to quantify the diet of Scyliorhinus canicula caught 24 in the North Sea and compared with stomach content data. DTDFs for δ 15 N (∆ 15 N) and δ 13 C 25 (∆ 13 C) ranged from -1.95‰ to 3.49‰ and from 0.52‰ to 5.14‰, respectively, and varied with 26 tissue and diet type. Isotope turnover rates in plasma and red blood cells, expressed as half-lives, 27 range from 39 to 135 days. A majority of the variability of DTDFs reported in this and other 28 studies with sharks can be explained by linear relationships between DTDF and dietary isotopic 29 values. From these relationships, we propose a method for isotope mixing models that uses diet-30 specific DTDFs, which improves diet reconstruction estimates of animals, particularly 31 mesopredator sharks that consume a large range of prey types. 32 33

Passive acoustic recording (PAR) systems are non‐invasive and allow researchers to collect data over large spatial and/or temporal scales. As fish sounds are species‐specific and repetitive, PAR can provide a large amount of data about spatio‐temporal variation in fish distribution and behaviors. Ophidion rochei, found in the Mediterranean and Black Seas, is a sand‐dwelling species, meaning that the behavior of this cryptic nocturnal fish cannot be observed in the field. Fortunately, male O. rochei produce long, multiple‐pulsed calls that are easy to identify. The aim of this study was to determine whether or not male calls are linked to reproduction behaviors. If so, PAR would allow a detailed description of the seasonal and daily rhythms in O. rochei reproduction behavior. A hydrophone was deployed from 18 July 2011 to 21 June 2012 and from 7 June 2013 to 2 July 2013 on a sandy area (42°34′48′′ N, 8°43′43′′ E) in front of the STARESO research station (NW Corsica). Male sounds were obtained only at night from late spring to early fall. The annual sound production period corresponds to the reproductive season of O. rochei. Sound production followed diel cycles: it was sustained for the entire night at the beginning of the sound production season but limited to shorter periods in the evening during the second half of the season. These differences in daily and seasonal sound production tempo can be used in future recordings to make inter‐annual comparisons and estimate the physiological state of the fish.