ALBERT

Description: Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.

Description: Aim: We examined potential environmental drivers of broad-scale spatial patterns in the trophic structure of marine ecosystems as represented by nitrogen stable isotopes in globally distributed marine predators. Additionally, we assessed the effects of spatial scale on the predictive capabilities of environmental variables. Location: Global oceans. Time period: 2000 to 2015. Major taxa studied: Tunas: Thunnus albacares, Thunnus obesus, Thunnus alalunga. Methods: We undertook a global compilation and meta-analysis of the bulk nitrogen stable isotope ratios (δ15N values) of three tuna species (n = 4,281). After adjusting for regional variations in baseline δ15N values using a global ocean biogeochemistry model, generalized additive mixed models were employed to infer global-scale oceanographic controls of trophic structure, using cosmopolitan tuna species as a model. Results: For the three tuna species, variation in trophic position estimated using bulk δ15N values was largely explained by geographical location and the corresponding oxygen minimum layer depth. Tuna trophic positions declined in areas with reduced oxygen at depth. Food-chain length, as captured by maximum trophic position, was longer in areas of the western Pacific Ocean and shorter in the northern Atlantic and eastern Pacific Oceans. Trophic adaptability of the tuna predators, as indicated by intraspecific variability, was highest in the western and central Pacific Ocean and lowest in the northern Atlantic Ocean. Our analysis demonstrated that while tunas share similar functional trophic roles, deeper-foraging tuna species had higher trophic positions globally. The predictive capacity of environmental variables decreased at finer (regional) spatial scales. Main conclusions: Our work suggests that habitat compression resulting from the predicted global expansion of oxygen minimum zones with ocean warming will impact the trophic structure of marine food webs and the corresponding foraging habits of marine predators. Spatial scale analyses highlighted the importance of representing differences in regional ecological dynamics in global-scale trophic and ecosystem models.

Description: Considerable uncertainty remains over how increasing atmospheric CO2 and anthropogenic climate changes are affecting open‐ocean marine ecosystems from phytoplankton to top predators. Biological time series data are thus urgently needed for the world's oceans. Here, we use the carbon stable isotope composition of tuna to provide a first insight into the existence of global trends in complex ecosystem dynamics and changes in the oceanic carbon cycle. From 2000 to 2015, considerable declines in δ13C values of 0.8‰–2.5‰ were observed across three tuna species sampled globally, with more substantial changes in the Pacific Ocean compared to the Atlantic and Indian Oceans. Tuna recorded not only the Suess effect, that is, fossil fuel‐derived and isotopically light carbon being incorporated into marine ecosystems, but also recorded profound changes at the base of marine food webs. We suggest a global shift in phytoplankton community structure, for example, a reduction in 13C‐rich phytoplankton such as diatoms, and/or a change in phytoplankton physiology during this period, although this does not rule out other concomitant changes at higher levels in the food webs. Our study establishes tuna δ13C values as a candidate essential ocean variable to assess complex ecosystem responses to climate change at regional to global scales and over decadal timescales. Finally, this time series will be invaluable in calibrating and validating global earth system models to project changes in marine biota.

Description: A global dataset of carbon stable isotope (δ13C) values from yellowfin, bigeye, and albacore tuna muscle tissue (n = 4275) was used to develop a novel tool to infer broad-scale movement and residency patterns of these highly mobile marine predators. This tool was coupled with environmental models and lipid content (C:N ratio) of tuna muscle tissues to examine ocean warming impacts on tuna ecology and bioenergetic condition across Longhurst provinces. Over a 16-year study period (2000–2015), latitudinal gradients in tuna δ13C values were consistent, with values decreasing with increasing latitude. Tuna δ13C values, reflecting modelled global phytoplankton δ13C landscapes (“isoscapes”), were largely related to spatial changes in oxygen concentrations at depth and temporal changes in sea surface temperature. Observed tuna isoscapes (δ13CLScorr), corrected for lipid content and the Suess effect (oceanic changes in CO2 over time), were subtracted from model-predicted baseline isoscapes (Δ13Ctuna-phyto) to infer spatial movement and residency patterns of the different tuna species. Stable isotope niche width was calculated for each Longhurst province using Δ13Ctuna-phyto and baseline-corrected nitrogen isotope (δ15Ntuna-phyto) values to further quantify isotopic variability as evidence of movements across isoscapes. A high degree of movement—defined as the deviation from the expected range of Δ13Ctuna-phyto values— was evident in three Longhurst provinces: Guinea current coast, Pacific equatorial divergence, and the North Pacific equatorial counter current. The highest level of population dispersal (variability in Δ13Ctuna-phyto values) was observed in Longhurst provinces within the western and central Pacific Oceans and in the Guinea current coast. While lipid content was low in yellowfin and bigeye, high and variable lipid stores in albacore muscle were consistent with seasonal movements between productive foraging and oligotrophic spawning habitats. Our ability to characterize tuna movement patterns without ambiguity remains challenged by uncertainty in trophic discrimination factors and ecological (e.g. diet variability) processes. However, this study illustrates that model-corrected δ13C values are a valuable, relatively cost-effective tool for identifying potential areas of mixing across management zones, particularly when electronic tagging studies are limited or absent. Stable isotope analyses of tuna tissues can therefore be an additional tool for guiding spatial stock assessments on top predator movement, dispersal patterns, and how they may be altered under a changing climate.

Description: Marine particulate organic carbon-13 stable isotope ratios (δ13CPOC) provide insights in understanding carbon cycling through the atmosphere, ocean, and biosphere. They have been used to trace the input of anthropogenic carbon in the marine ecosystem due to the distinct isotopically light signature of anthropogenic emissions. However, δ13CPOC is also significantly altered during photosynthesis by phytoplankton, which complicates its interpretation. For such purposes, robust spatio-temporal coverage of δ13CP OC observations is essential. We collected all such available data sets, merged and homogenized them to provide the largest available marine δ13CPOC data set (Verwega et al., 2021). The data set consists of 4732 data points covering all major ocean basins beginning in the 1960s. We describe the compiled raw data, compare different observational methods, and provide key insights in the temporal and spatial distribution that is consistent with previously observed patterns. The main different sample collection methods (bottle, intake, net, trap) are generally consistent with each other when comparing within regions. An analysis of 1990s mean δ13CP OC values in an meridional section accross the Atlantic Ocean shows relatively high values (≥ −22 ‰) in the low latitudes (〈 30°) trending towards lower values in the Arctic Ocean (∼ −24 ‰) and Southern Ocean (≤ −28 ‰). The temporal trend since the 1960s shows a decrease of mean δ13CPOC by more than 3 ‰ in all basins except for the Southern Ocean which shows a weaker trend but contains relatively poor multi-decadal coverage.

Description: Global anthropogenic mercury (Hg) emissions to the atmosphere since industrialization are widely considered to be responsible for a significant increase in surface ocean Hg concentrations. Still unclear is how those inputs are converted into toxic methylmercury (MeHg) then transferred and biomagnified in oceanic food webs. We used a unique long-term and continuous dataset to explore the temporal Hg trend and variability of three tropical tuna species (yellowfin, bigeye, and skipjack) from the southwestern Pacific Ocean between 2001 and 2018 (n = 590). Temporal trends of muscle nitrogen (δ15N) and carbon (δ13C) stable isotope ratios, amino acid (AA) δ15N values and oceanographic variables were also investigated to examine the potential influence of trophic, biogeochemical and physical processes on the temporal variability of tuna Hg concentrations. For the three species, we detected significant inter-annual variability but no significant long-term trend for Hg concentrations. Inter-annual variability was related to the variability in tuna sampled lengths among years and to tuna muscle δ15N and δ13C values. Complementary AA- and model-estimated phytoplankton δ15N values suggested the influence of baseline processes with enhanced tuna Hg concentrations observed when dinitrogen fixers prevail, possibly fuelling baseline Hg methylation and/or MeHg bioavailability at the base of the food web. Our results show that MeHg trends in top predators do not necessary capture the increasing Hg concentrations in surface waters suspected at the global oceanic scale due to the complex and variable processes governing Hg deposition, methylation, bioavailability and biomagnification. This illustrates the need for long-term standardized monitoring programs of marine biota worldwide.

Description: Nitrogen and carbon stable isotope data sets are commonly used to assess complex population to ecosystem responses to natural or anthropogenic changes at regional to global spatial scales, and monthly to decadal timescales. Measured in the tissues of consumers, nitrogen isotopes (delta N-15) are primarily used to estimate trophic position while carbon isotopes (delta C-13) describe habitat associations and feeding pathways. Models of both delta N-15 and delta C-13 values and their associated variance can be used to estimate likely dietary contributions and niche width and provide inferences about consumer movement and migration. Stable isotope data have added utility when used in combination with other empirical data sets (e.g., stomach content, movement tracking, bioregionalization, contaminant, or fisheries data) and are increasingly relied upon in food web and ecosystem models. While numerous regional studies publish tables of mean delta N-15 and delta C-13 values, limited individual records have been made available for wider use. Such a deficiency has impeded full utility of the data, which otherwise would facilitate identification of macroscale patterns. The data provided here consist of 4,498 records of individuals of three tuna species, Thunnus alalunga, T. obesus, and T. albacares sampled from all major ocean basins from 2000 to 2015. For each individual tuna, we provide a record of the following: species name, sampling date, sampling location, tuna length, muscle bulk and baseline corrected delta N-15 values, and muscle bulk and, where available, lipid corrected delta C-13 values. We provide these individual records to support comparative studies and more robust modeling projects seeking to improve understanding of complex marine ecosystem dynamics and their responses to a changing environment. There are no copyright restrictions for research and/or teaching purposes. Users are requested to acknowledge their use of the data in publications, research proposals, websites, and other outlets following the citation instructions in Class III, Section B.