Holocene changes in the trophic ecology of an apex marine predator in the South Atlantic Ocean

Combining δ13C and δ15N from bone and dentine in marine mammal palaeoecological research: insights from toothed whales

Stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions of bone and dentine collagen extracted from museum specimens have been widely used to study the paleoecology of past populations. Due to possible systematic differences in stable isotope values between bone and dentine, dentine values need to be transformed into bone-collagen equivalent using a correction factor to allow comparisons between the two collagen sources. Here, we provide correction factors to transform dentine δ¹³C and δ¹⁵N values into bone-collagen equivalent for two toothed whales: narwhal and beluga. We sampled bone and dentine from the skulls of 11 narwhals and 26 belugas. In narwhals, dentine was sampled from tusk and embedded tooth; in belugas, dentine was sampled from tooth. δ¹³C and δ¹⁵N were measured, and intra-individual bone and dentine isotopic compositions were used to calculate correction factors for each species. We detected differences in δ¹³C and δ¹⁵N. In both narwhals and belugas, we found lower average δ¹³C and δ¹⁵N in bone compared with dentine. The correction factors provided by the study enable the combined analysis of stable isotope data from bone and dentine in these species.

Cenozoic megatooth sharks occupied extremely high trophic positions

Trophic position is a fundamental characteristic of animals, yet it is unknown in many extinct species. In this study, we ground-truth the ¹⁵N/¹⁴N ratio of enameloid-bound organic matter (δ¹⁵N_EB) as a trophic level proxy by comparison to dentin collagen δ¹⁵N and apply this method to the fossil record to reconstruct the trophic level of the megatooth sharks (genus Otodus). These sharks evolved in the Cenozoic, culminating in Otodus megalodon, a shark with a maximum body size of more than 15 m, which went extinct 3.5 million years ago. Very high δ¹⁵N_EB values (22.9 ± 4.4‰) of O. megalodon from the Miocene and Pliocene show that it occupied a higher trophic level than is known for any marine species, extinct or extant. δ¹⁵N_EB also indicates a dietary shift in sharks of the megatooth lineage as they evolved toward the gigantic O. megalodon, with the highest trophic level apparently reached earlier than peak size.

A millennium of trophic stability in Atlantic cod (Gadus morhua): transition to a lower and converging trophic niche in modern times

Stable isotope analyses of zooarchaeological material can be used to examine ecological variability in exploited species at centennial to millennial scales. Climate change is a notable driver of marine ecosystem change, although historical fishing is also likely to have impacted past marine systems. Fishing removes the oldest and largest individuals and may thereby result in shorter trophic pathways and reduced niche width of predatory fish species. In the current study we examine the trophic niche of Atlantic cod, haddock and Atlantic wolffish, in the last millennium using δ13C and δ15N values of bone collagen. We report a lower trophic level of Atlantic cod and haddock but higher level of wolffish in present times, following centuries at consistent and higher trophic levels of Atlantic cod. This results in a concurrent converging trophic niche of the demersal fish. We suggest that the current data set provides a valuable historical baseline facilitating interpretation of current variability in the trophic ecology of northern demersal fish.

Predicting habitat use by the Argentine hake Merluccius hubbsi in a warmer world: inferences from the Middle Holocene

Fish skeletal remains recovered from two archaeological sites dated in the Middle Holocene of Tierra del Fuego (Argentina) were analysed to describe habitat use patterns by hake in the past and predict changes in a warmer world. Mitochondrial DNA was successfully extracted and amplified from 42 out of 45 first vertebra from ancient hake and phylogenetic analysis assigned all haplotypes to Argentine hake (Merluccius hubbsi). According to osteometry, the Argentine hake recovered from the archaeological site were likely adults ranging 37.2-58.1 cm in standard length. C and N stable isotope analysis showed that currently Argentine hake use foraging grounds deeper than those of Patagonian blenny and pink cusk-eel. Argentine hake, however, had a much broader isotopic niche during the Middle Holocene, when a large part of the population foraged much shallower than contemporary pink cusk-eel. The overall evidence suggests the presence of large numbers of Argentine hake onshore Tierra del Fuego during the Middle Holocene, which allowed exploitation by hunter-gatherer-fisher groups devoid of fishing technology. Interestingly, average SST off Tierra del Fuego during the Middle Holocene was higher than currently (11 °C vs 7 °C) and matched SST in the current southernmost onshore spawning aggregations, at latitude 47 °S. This indicates that increasing SST resulting from global warming will likely result into an increased abundance of adult Argentine hake onshore Tierra del Fuego, as during the Middle Holocene. Furthermore, stable isotope ratios from mollusc shells confirmed a much higher marine primary productivity during the Middle Holocene off Tierra del Fuego.

Back to the future? Late Holocene marine food web structure in a warm climatic phase as a predictor of trophodynamics in a warmer South-Western Atlantic Ocean

Stable carbon and nitrogen isotope ratios in the skeletal elements of both ancient and modern marine species from the Beagle Channel were used to compare the structure of Late Holocene and modern food webs, and predict potential changes as a result of a Sea Surface Temperature (SST) increase in the region. Complementary, ancient and modern shells of limpets and mussels were isotopically analysed to explore changes in the isotopic baseline and compare marine food webs through time after an appropriate correction for baseline shifts. Results confirmed a declining pattern of marine primary productivity during the Late Holocene in the Beagle Channel. In general, the isotopic niches overlapped largely in the ancient food web in comparison to the current marine one, with the exception of that of cormorants (Phalacrocorax sp.). Our data suggest that all the species that have undergone intense human exploitation (Arctocephalus australis, Otaria flavescens and Merluccius sp.) significantly increased their trophic levels. The most important finding of this work was the very high isotopic overlap between snoek (Thyrsites atun) and hake (Merluccius sp.) during the Late Holocene. Increasing SST as a result of global warming could favour the recolonization of the southern South-Western Atlantic Ocean by snoek from the South-Eastern Pacific Ocean, with a potential impact on the landings of the economically important Argentine and Austral hake. These findings highlight the relevance of using zooarchaeological remains for providing predictions about marine food webs changes in the near future.

Long-term ecological changes in marine mammals driven by recent warming in northwestern Alaska

Carbon and nitrogen isotopes analyses were performed on marine mammal bone collagen from three archaeological sites (ad 1170-1813) on Cape Espenberg (Kotzebue Sound, northwestern Alaska) as well as modern animals harvested from the same area to examine long-term trends in foraging ecology and sea ice productivity. We observed significant and dramatic changes in ringed seal stable isotope values between the early 19th and early 21st centuries, likely due to changing sea ice productivity and reduced delivery of organic matter to the benthos driven by recent warming in the Arctic. These data highlight the importance of the archaeological record for providing a long-term perspective on environmental variation and interpreting recent changes driven by anthropogenic processes.