Trends in tuna carbon isotopes suggest global changes in pelagic phytoplankton communities

First record of mercury concentrations and stable isotopes (13C & 15N) in albacore (Thunnus alalunga) from the Western Equatorial Atlantic Ocean

This paper reports the first record of total mercury (THg) concentrations in albacore (Thunnus alalunga), one of the main tuna species caught from the Western Equatorial Atlantic Ocean and presents a preliminary comparison with other regions and tuna species. Mean, standard deviation and range of concentrations in T. alalunga (515 ± 145 ng g-1 ww; 294-930 ng g-1 ww) with 92 % being of methyl-Hg, are higher than in albacore from other Atlantic Ocean subregions despite their smaller body size. These concentrations are similar to those from the Pacific and Indian oceans, but lower than in the Mediterranean. Compared to other sympatric tuna species, concentrations are higher than those in T. albacares and similar to T. obesus. These results are discussed considering the potential differences in stable isotope values (13C and 15N) of T. alalunga populations from multiple oceanic areas and compared to other tuna species worldwide.

Effects of lipid extraction on stable isotope ratios of carbon and nitrogen in muscles of freshwater fish

The extraction of lipids by the Folch method from the muscles of all the fish studied led to statistically significant differences in the values of δ15N. At the same time, lipid extraction led to a statistically significant increase in δ13C in pike and roach, and to a statistically insignificant decrease in δ13C in perch and bream. Thus, lipid extraction cannot serve as a universal method of sample preparation for the analysis of the isotopic composition of carbon (13C/12C) and nitrogen (15N/14N) in fish muscles. The differences between the δ13C values in the samples before and after lipid extraction were statistically investigated by different models. It is shown that mathematical correction method models can be used, but the results are depending on the fish types.

A century of mercury: Ecosystem-wide changes drive increasing contamination of a tropical seabird species in the South Atlantic Ocean

Mercury (Hg) is a highly toxic metal that adversely impacts human and wildlife health. The amount of Hg released globally in the environment has increased steadily since the Industrial Revolution, resulting in growing contamination in biota. Seabirds have been extensively studied to monitor Hg contamination in the world's oceans. Multidecadal increases in seabird Hg contamination have been documented in polar, temperate and subtropical regions, whereas in tropical regions they are largely unknown. Since seabirds accumulate Hg mainly from their diet, their trophic ecology is fundamental in understanding their Hg exposure over time. Here, we used the sooty tern (Onychoprion fuscatus), the most abundant tropical seabird, as bioindicator of temporal variations in Hg transfer to marine predators in tropical ecosystems, in response to trophic changes and other potential drivers. Body feathers were sampled from 220 sooty terns, from museum specimens (n = 134) and free-living birds (n = 86) from Ascension Island, in the South Atlantic Ocean, over 145 years (1876-2021). Chemical analyses included (i) total- and methyl-Hg, and (ii) carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes, as proxies of foraging habitat and trophic position, respectively, to investigate the relationship between trophic ecology and Hg contamination over time. Despite current regulations on its global emissions, mean Hg concentrations were 58.9% higher in the 2020s (2.0 μg g^-1, n = 34) than in the 1920s (1.2 μg g^-1, n = 107). Feather Hg concentrations were negatively and positively associated with δ¹³C and δ¹⁵N values, respectively. The sharp decline of 2.9 ‰ in δ¹³C values over time indicates ecosystem-wide changes (shifting primary productivity) in the tropical South Atlantic Ocean and can help explain the observed increase in terns' feather Hg concentrations. Overall, this study provides invaluable information on how ecosystem-wide changes can increase Hg contamination of tropical marine predators and reinforces the need for long-term regulations of harmful contaminants at the global scale.

Isotopic Niche Analysis of Long-Finned Pilot Whales (Globicephala melas edwardii) in Aotearoa New Zealand Waters

Simple Summary

Isotopic niche analyses can elucidate a species’ foraging ecology. Using isotopic values of δ¹³C, δ¹⁵N and δ³⁴S, the isotopic niche of long-finned pilot whales (Globicephala melas edwardii) in Aotearoa New Zealand was investigated for animals that stranded in six different events across two locations between 2009 and 2017. Generalised additive models revealed that stranding event was a stronger predictor for δ¹³C and δ¹⁵N values than body length, sex, or reproductive status, indicating that spatiotemporal differences explained isotopic variation of G. m. edwardii in New Zealand waters better than ontogenetic factors.

Abstract

The quantification of a species’ trophic niche is important to understand the species ecology and its interactions with the ecosystem it resides in. Despite the high frequency of long-finned pilot whale (Globicephala melas edwardii) strandings on the Aotearoa New Zealand coast, their trophic niche remains poorly understood. To assess the isotopic niche of G. m. edwardii within New Zealand, ontogenetic (sex, total body length, age, maturity status, reproductive group) and spatiotemporal (stranding location, stranding event, and stranding year) variation were investigated. Stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N) were examined from skin samples of 125 G. m. edwardii (67 females and 58 males) collected at mass-stranding events at Onetahua Farewell Spit in 2009 (n = 20), 2011 (n = 20), 2014 (n = 27) and 2017 (n = 20) and at Rakiura Stewart Island in 2010 (n = 19) and 2011 (n = 19). Variations in δ³⁴S values were examined for a subset of 36 individuals. General additive models revealed that stranding event was the strongest predictor for δ¹³C and δ¹⁵N values, whilst sex was the strongest predictor of δ³⁴S isotopic values. Although similar within years, δ¹³C values were lower in 2014 and 2017 compared to all other years. Furthermore, δ¹⁵N values were higher within Farewell Spit 2017 compared to any other stranding event. This suggests that the individuals stranded in Farewell Spit in 2017 may have been feeding at a higher trophic level, or that the nitrogen baseline may have been higher in 2017 than in other years. Spatiotemporal differences explained isotopic variation of G. m. edwardii in New Zealand waters better than ontogenetic factors.

Trophic Dynamics and Feeding Ecology of Skipjack Tuna (Katsuwonus pelamis) off Eastern and Western Taiwan

The skipjack tuna (Katsuwonus pelamis) is a mesopredator fish species with seasonal abundance in waters off Taiwan. Regional ecological and life-history information has been historically lacking for this species. In recent years, stable isotope analysis (SIA) of carbon and nitrogen has been used to assess predator feeding ecology and broader ecosystem trophic dynamics. This study evaluated comparative skipjack feeding ecology in distinct regions off Taiwan, combining traditional stomach content analysis with SIA of individuals off western (n = 43; 2020) and eastern (n = 347; 2012–2014 and n = 167; 2020) Taiwan. The stomach content analysis showed the most important prey to be ponyfish (Photopectoralis bindus) in western Taiwan and epipelagic squids (Myopsina spp.) and carangids (Decapterus macrosoma;) in eastern Taiwan from 2012 to 2014 and epipelagic carangids (Decapterus spp.) and flying fishes (Cheilopogon spp.) in eastern Taiwan in 2020, suggesting that the skipjack tuna is a generalist predator across regions. In contrast, time-integrated diet estimates from Bayesian mixing models indicated the importance of cephalopods and crustaceans as prey, potentially demonstrating more mesopelagic feeding in less productive waters during skipjack migrations outside the study regions. Skipjack off western Taiwan had a slightly higher estimated trophic position than in the waters off eastern Taiwan, potentially driven by the varying nutrient-driven pelagic food web structures. Skipjack SI values increased with body size off eastern Taiwan but not in western waters, suggesting that opportunistic predation can still result in different predator–prey size dynamics between regions.

Empirical estimation of marine phytoplankton assemblages in coastal and offshore areas using an in situ multi-wavelength excitation fluorometer

Phytoplankton assemblages are essential for understanding the quality of primary production in marine ecosystems. Here, we describe the development of a methodology for monitoring marine phytoplankton assemblages using an in situ multi-wavelength excitation fluorometer (MEX). The MEX recorded the fluorescence excited with nine light-emitting diodes, temperature, and sensor depth. We prepared reference datasets comprising MEX fluorescence and plant pigment-based phytoplankton assemblages of nine chemotaxonomy groups (diatoms, dinoflagellates, cryptophytes, chlorophytes, haptophytes type 3, haptophytes type 4, prasinophytes, cyanophytes, and prochlorophytes). Conversions from the MEX fluorescence to the phytoplankton assemblages were conducted with two processes. First, target MEX fluorescence was decomposed using a linear inverse model for calculating coefficients. Second, pigment-based chemotaxonomy of the target MEX fluorescence was reconstructed using the coefficients and the chemotaxonomy assemblages of the reference data. Cross-validation analyses indicated good estimation of the proportion of diatoms, dinoflagellates, cryptophytes, cyanophytes, and prochlorophytes with MEX, and when chlorophytes, haptophytes and prasinophytes were summarized as other eukaryotes, the positive correlation was seen between proportions estimated with MEX and pigments as same as other five chemotaxonomy groups. Repeated MEX observations were conducted in the Kuroshio, the Sea of Japan, the Oyashio, and the Okhotsk Sea. The water-column integrated biomass indicated that the diatoms were an important primary producer in the Oyashio and the Okhotsk Sea, while eukaryotes were important in the Sea of Japan and prochlorophytes were important in the Kuroshio. Our method with the MEX will be a powerful tool to understand and estimate the chemotaxonomy-level assemblages and biomass in the ocean.

Rapid temporal decline of mercury in Greenland halibut (Reinhardtius hippoglossoides)

Mercury (Hg) pollution in the ocean is an issue of global concern, however bioaccumulation regimes of this ubiquitous pollutant in marine apex predators have important knowledge gaps. Our fish length and stable isotope (δ¹⁵N and δ¹³C) normalized data of Greenland halibut (GH) (Reinhardtius hippoglossoides) showed that Hg bioaccumulation in fillet tissue decreased by ~35-50 %, over a ten-year period from 2006 to 2015 (n = 7 individual sampling years). Hg was predominantly in the methylmercury form (>77 %). Results from a Bayesian information theoretic model showed that GH Hg concentrations decreased with time and its associated declines in Hg air emissions, estimated trophic position, and a potentially lower degree of demersal prey use as indicated by temporal trend shifts in nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope values. GH trophic shifts accounted for about one third of the observed temporal reduction in Hg. Our study demonstrates the importance of simultaneously considering Hg emissions, food web dynamics and trophic shifts as important drivers of Hg bioaccumulation in a marine, deep water fish species and highlights the effectiveness of Hg regulations on ocean apex predator Hg concentrations and overall seafood safety.

Lipid-free tuna muscle samples are suitable for total mercury analysis

Tropical tunas are largely consumed worldwide, providing major nutritional benefits to humans, but also representing the main exposure to methylmercury, a potent neurotoxin that biomagnifies along food webs. The combination of ecological tracers (nitrogen and carbon stable isotopes, δ¹⁵N and δ¹³C) to mercury concentrations in tunas is scarce yet crucial to better characterize the influence of tuna foraging ecology on mercury exposure and bioaccumulation. Given the difficulties to get modern and historical tuna samples, analyses have to be done on available and unique samples. However, δ¹³C values are often analysed on lipid-free samples to avoid bias related to lipid content. While lipid extraction with non-polar solvents is known to have no effect on δ¹⁵N values, its impact on mercury concentrations is still unclear. We used white muscle tissues of three tropical tuna species to evaluate the efficiency and repeatability of different lipid extraction protocols commonly used in δ¹³C and δ¹⁵N analysis. Dichloromethane was more efficient than cyclohexane in extracting lipids in tuna muscle, while the automated method appeared more efficient but as repeatable as the manual method. Lipid extraction with dichloromethane had no effect on mercury concentrations. This may result from i) the affinity of methylmercury to proteins in tuna flesh, ii) the low lipid content in tropical tuna muscle samples, and iii) the non-polar nature of dichloromethane. Our study suggests that lipid-free samples, usually prepared for tropical tuna foraging ecology research, can be used equivalently to bulk samples to document in parallel mercury concentrations at a global scale.

Stable isotope signatures in historic harbor seal bone link food web-assimilated carbon and nitrogen resources to a century of environmental change

Anthropogenic climate change will impact nutrient cycles, primary production, and ecosystem structure in the world's oceans, although considerable uncertainty exists regarding the magnitude and spatial variability of these changes. Understanding how regional-scale ocean conditions control nutrient availability and ultimately nutrient assimilation into food webs will inform how marine resources will change in response to climate. To evaluate how ocean conditions influence the assimilation of nitrogen and carbon into coastal marine food webs, we applied a novel dimension reduction analysis to a century of newly acquired molecular isotope data derived from historic harbor seal bone specimens. By measuring bulk δ¹³ C and δ¹⁵ N values of source amino acids of these top predators from 1928 to 2014, we derive indices of primary production and nitrogen resources that are assimilated into food webs. We determined coastal food webs responded to climate regimes, coastal upwelling, and freshwater discharge, yet the strength of responses to individual drivers varied across the northeast Pacific. Indices of primary production and nitrogen availability in the Gulf of Alaska were dependent on regional climate indices (i.e., North Pacific Gyre Oscillation) and upwelling. In contrast, the coastal Washington and Salish Sea food webs were associated with local indices of freshwater discharge. For some regions (eastern Bering Sea, northern Gulf of Alaska) food web-assimilated production was coupled with nitrogen sources; however, other regions demonstrated no production-nitrogen coupling (Salish Sea). Temporal patterns of environmental indices and isotopic data from Washington state varied about the long-term mean with no directional trend. Data from the Gulf of Alaska, however, showed below average harbor seal δ¹³ C values and above average ocean conditions since 1975, indicating a change in primary production in recent decades. Altogether, these findings demonstrate stable isotope data can provide useful indices of nitrogen resources and phytoplankton dynamics specific to what is assimilated by food webs.

Stable Isotope Analysis of Specimens of Opportunity Reveals Ocean-Scale Site Fidelity in an Elusive Whale Species

Elusive wildlife are challenging to study, manage, or conserve, as the difficulty of obtaining specimens or conducting direct observations leads to major data deficiencies. Specimens of opportunity, such as salvaged carcasses or museum specimens, are a valuable source of fundamental biological and ecological information on data-deficient, elusive species, increasing knowledge of biodiversity, habitat and range, and population structure. Stable isotope analysis is a powerful indirect tool that can be used to infer foraging behavior and habitat use retrospectively from archived specimens. Beaked whales are a speciose group of cetaceans that are challenging to study in situ, and although Sowerby's beaked whale (Mesoplodon bidens) was discovered >200 years ago, little is known about its biology. We measured δ13C and δ15N stable isotope composition in bone, muscle, and skin tissue from 102 Sowerby's beaked whale specimens of opportunity collected throughout the North Atlantic Ocean to infer movement ecology and spatial population structure. Median δ13C and δ15N values in Sowerby's beaked whale bone, muscle, and skin tissues significantly differed between whales sampled from the east and west North Atlantic Ocean. Quadratic discriminant analysis that simultaneously considered δ13C and δ15N values correctly assigned >85% of the specimens to their collection region for all tissue types. These findings demonstrate Sowerby's beaked whale exhibits both short- and long-term site fidelity to the region from which the specimens were collected, suggest that this species is composed of two or more populations or exhibits a metapopulation structure, and have implications for conservation and management policy. Stable isotope analysis of specimens of opportunity proved a highly successful means of generating new spatial ecology data for this elusive species and is a method that can be effectively applied to other elusive species.

Differences in metabolic rate between two Atlantic cod (Gadus morhua) populations estimated with carbon isotopic composition in otoliths

The isotopic composition of inorganic carbon in otoliths (δ13Coto) can be a useful tracer of metabolic rates and a method to study ecophysiology in wild fish. We evaluated environmental and physiological sources of δ13Coto variation in Icelandic and Northeast Arctic (NEA) cod (Gadus morhua) over the years 1914-2013. Individual annual growth increments of otoliths formed at age 3 and 8 were micromilled and measured by isotope-ratio mass spectrometry. Simultaneously, all annual increment widths of the otoliths were measured providing a proxy of fish somatic growth. We hypothesized that changes in the physiological state of the organism, reflected by the isotopic composition of otoliths, can affect the growth rate. Using univariate and multivariate mixed-effects models we estimated conditional correlations between carbon isotopic composition and growth of fish at different levels (within individuals, between individuals, and between years), controlling for intrinsic and extrinsic effects on both otolith measurements. δ13Coto was correlated with growth within individuals and between years, which was attributed to the intrinsic effects (fish age or total length). There was no significant correlation between δ13Coto and growth between individuals, which suggests that caution is needed when interpreting δ13Coto signals. We found a significant decrease in δ13Coto through the century which was explained by the oceanic Suess effect-admixture of isotopically light carbon from fossil fuel. We calculated the proportion of the respired carbon in otolith carbonate (Cresp) using carbon isotopic composition in diet and dissolved inorganic carbon of the seawater. This approach allowed us to correct the values for each stock in relation to these two environmental baselines. Cresp was on average 0.275 and 0.295 in Icelandic and NEA stock, respectively. Our results provide an insight into the physiological basis for differences in growth characteristics between these two cod stocks, and how that may vary over time.

Stable mercury concentrations of tropical tuna in the south western Pacific ocean: An 18-year monitoring study

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.

Decadal shift in foraging strategy of a migratory southern ocean predator

Rapid anthropogenic environmental change is expected to impact a host of ecological parameters in Southern Ocean ecosystems. Of critical concern are the consequences of these changes on the range of species that show fidelity to migratory destinations, as philopatry is hypothesized to help or hinder adaptation to climate change depending on the circumstances. Many baleen whales show philopatry to feeding grounds and are also capital breeders that meet migratory and reproductive costs through seasonal energy intake. Southern right whales (Eubalaena australis, SRWs) are capital breeders that have a strong relationship between reproductive output and foraging success. The population dynamics of South Africa's population of SRWs are characterized by two distinct periods: the 1990s, a period of high calving rates; and the late 2010s, a period associated with lowered calving rates. Here we use analyses of stable carbon (δ13 C) and nitrogen (δ15 N) isotope values from SRW biopsy samples (n = 122) collected during these two distinct periods to investigate foraging ecology of the South African population of SRWs over a time period coincident with the demographic shift. We show that South African SRWs underwent a dramatic northward shift, and diversification, in foraging strategy from 1990s to 2010s. Bayesian mixing model results suggest that during the 1990s, South African SRWs foraged on prey isotopically similar to South Georgia/Islas Georgias del Sur krill. In contrast, in the 2010s, South African SRWs foraged on prey isotopically consistent with the waters of the Subtropical Convergence, Polar Front and Marion Island. We hypothesize that this shift represents a response to changes in preferred habitat or prey, for example, the decrease in abundance and southward range contraction of Antarctic krill. By linking reproductive decline to changing foraging strategies for the first time in SRWs, we show that altering foraging strategies may not be sufficient to adapt to a changing ocean.

Decadal changes in blood δ13C values, at-sea distribution, and weaning mass of southern elephant seals from Kerguelen Islands

Changes in the foraging environment and at-sea distribution of southern elephant seals from Kerguelen Islands were investigated over a decade (2004-2018) using tracking, weaning mass, and blood δ13C values. Females showed either a sub-Antarctic or an Antarctic foraging strategy, and no significant shift in their at-sea distribution was detected between 2004 and 2017. The proportion of females foraging in sub-Antarctic versus Antarctic habitats did not change over the 2006-2018 period. Pup weaning mass varied according to the foraging habitat of their mothers. The weaning mass of sub-Antarctic foraging mothers' pups decreased by 11.7 kg over the study period, but they were on average 5.8 kg heavier than pups from Antarctic foraging mothers. Pup blood δ13C values decreased by 1.1‰ over the study period regardless of their sex and the presumed foraging habitat of their mothers. Together, these results suggest an ecological change is occurring within the Indian sector of the Southern Ocean with possible consequences on the foraging performance of southern elephant seals. We hypothesize that this shift in δ13C is related to a change in primary production and/or in the composition of phytoplankton communities, but this requires further multidisciplinary investigations.