Glacialá¤-interglacial changes in nutrient utilization in the equatorial Pacific Ocean

Elevated Mercury Concentrations and Isotope Signatures (N, C, Hg) in Yellowfin Tuna (Thunnus albacares) from the Galápagos Marine Reserve and Waters off Ecuador

We examined how dietary factors recorded by C and N influence Hg uptake in 347 individuals of yellowfin tuna (Thunnus albacares), an important subsistence resource from the Galápagos Marine Reserve (Ecuador) and the Ecuadorian mainland coast in 2015-2016. We found no differences in total Hg (THg) measured in red muscle between the two regions and no seasonal differences, likely due to the age of the fish and slow elimination rates of Hg. Our THg concentrations are comparable to those of other studies in the Pacific (0.20-9.60 mg/kg wet wt), but a subset of individuals exhibited the highest Hg concentrations yet reported in yellowfin tuna. Mercury isotope values differed between Δ¹⁹⁹ Hg and δ202Hg in both regions (Δ¹⁹⁹ Hg = 2.86 ± 0.04‰ vs. Δ¹⁹⁹ Hg = 2.33 ± 0.07‰), likely related to shifting food webs and differing photochemical processing of Hg prior to entry into the food web. There were significantly lower values of both δ¹⁵ N and δ¹³ C in tuna from Galápagos Marine Reserve (δ¹⁵ N: 8.5-14.2‰, δ¹³ C: -18.5 to -16.1‰) compared with those from the Ecuadorian mainland coast (δ¹⁵ N: 8.3-14.4‰, δ¹³ C: -19.4 to -11.9‰), of which δ¹³ C values suggest spatially constrained movements of tuna. Results from the pooled analysis, without considering region, indicated that variations in δ¹³ C and δ¹⁵ N values tracked changes of Hg stable isotopes. Our data indicate that the individual tuna we used were resident fish of each region and were heavily influenced by upwellings related to the eastern Pacific oxygen minimum zone and the Humboldt Current System. The isotopes C, N, and Hg reflect foraging behavior mainly on epipelagic prey in shallow waters and that food web shifts drive Hg variations between these populations of tuna. Environ Toxicol Chem 2022;41:2732-2744. © 2022 SETAC.

Glacial expansion of oxygen-depleted seawater in the eastern tropical Pacific

Increased storage of carbon in the oceans has been proposed as a mechanism to explain lower concentrations of atmospheric carbon dioxide during ice ages; however, unequivocal signatures of this storage have not been found¹. In seawater, the dissolved gases oxygen and carbon dioxide are linked via the production and decay of organic material, with reconstructions of low oxygen concentrations in the past indicating an increase in biologically mediated carbon storage. Marine sediment proxy records have suggested that oxygen concentrations in the deep ocean were indeed lower during the last ice age, but that near-surface and intermediate waters of the Pacific Ocean-a large fraction of which are poorly oxygenated at present-were generally better oxygenated during the glacial^1-3. This vertical opposition could suggest a minimal net basin-integrated change in carbon storage. Here we apply a dual-proxy approach, incorporating qualitative upper-water-column and quantitative bottom-water oxygen reconstructions^4,5, to constrain changes in the vertical extent of low-oxygen waters in the eastern tropical Pacific since the last ice age. Our tandem proxy reconstructions provide evidence of a downward expansion of oxygen depletion in the eastern Pacific during the last glacial, with no indication of greater oxygenation in the upper reaches of the water column. We extrapolate our quantitative deep-water oxygen reconstructions to show that the respired carbon reservoir of the glacial Pacific was substantially increased, establishing it as an important component of the coupled mechanism that led to low levels of atmospheric carbon dioxide during the glacial.

Widespread Anthropogenic Nitrogen in Northwestern Pacific Ocean Sediment

Sediment samples from the East China and Yellow seas collected adjacent to continental China were found to have lower δ¹⁵N values (expressed as δ¹⁵N = [¹⁵N:¹⁴N_sample/¹⁵N:¹⁴N_air - 1] × 1000‰; the sediment ¹⁵N:¹⁴N ratio relative to the air nitrogen ¹⁵N:¹⁴N ratio). In contrast, the Arctic sediments from the Chukchi Sea, the sampling region furthest from China, showed higher δ¹⁵N values (2-3‰ higher than those representing the East China and the Yellow sea sediments). Across the sites sampled, the levels of sediment δ¹⁵N increased with increasing distance from China, which is broadly consistent with the decreasing influence of anthropogenic nitrogen (N^ANTH) resulting from fossil fuel combustion and fertilizer use. We concluded that, of several processes, the input of N^ANTH appears to be emerging as a new driver of change in the sediment δ¹⁵N value in marginal seas adjacent to China. The present results indicate that the effect of N^ANTH has extended beyond the ocean water column into the deep sedimentary environment, presumably via biological assimilation of N^ANTH followed by deposition. Further, the findings indicate that N^ANTH is taking over from the conventional paradigm of nitrate flux from nitrate-rich deep water as the primary driver of biological export production in this region of the Pacific Ocean.

Stable Isotopes Reveal Long-Term Fidelity to Foraging Grounds in the Galapagos Sea Lion (Zalophus wollebaeki)

Most otariids have colony-specific foraging areas during the breeding season, when they behave as central place foragers. However, they may disperse over broad areas after the breeding season and individuals from different colonies may share foraging grounds at that time. Here, stable isotope ratios in the skull bone of adult Galapagos sea lions (Zalophus wollebaeki) were used to assess the long-term fidelity of both sexes to foraging grounds across the different regions of the Galapagos archipelago. Results indicated that the stable isotope ratios (δ¹³C and δ¹⁵N) of sea lion bone significantly differed among regions of the archipelago, without any significant difference between sexes and with a non significant interaction between sex and region. Moreover, standard ellipses, estimated by Bayesian inference and used as a measure of the isotopic resource use area at the population level, overlapped widely for the sea lions from the southern and central regions, whereas the overlap of the ellipses for sea lions from the central and western regions was small and non-existing for those from the western and southern regions. These results suggest that males and females from the same region within the archipelago use similar foraging grounds and have similar diets. Furthermore, they indicate that the exchange of adults between regions is limited, thus revealing a certain degree of foraging philopatry at a regional scale within the archipelago. The constraints imposed on males by an expanded reproductive season (~ 6 months), resulting from the weak reproductive synchrony among females, and those imposed on females by a very long lactation period (at least one year but up to three years), may explain the limited mobility of adult Galapagos sea lions of both sexes across the archipelago.

Nitrogen and Oxygen Isotopic Studies of the Marine Nitrogen Cycle

The marine nitrogen cycle is a complex web of microbially mediated reactions that control the inventory, distribution, and speciation of nitrogen in the marine environment. Because nitrogen is a major nutrient that is required by all life, its availability can control biological productivity and ecosystem structure in both surface and deep-ocean communities. Stable isotopes of nitrogen and oxygen in nitrate and nitrite have provided new insights into the rates and distributions of marine nitrogen cycle processes, especially when analyzed in combination with numerical simulations of ocean circulation and biogeochemistry. This review highlights the insights gained from dual-isotope studies applied at regional to global scales and their incorporation into oceanic biogeochemical models. These studies represent significant new advances in the use of isotopic measurements to understand the modern nitrogen cycle, with implications for the study of past ocean productivity, oxygenation, and nutrient status.

Historical accumulation of N and P and sources of organic matter and N in sediment in an agricultural reservoir in Northern China

Agriculture has significantly intensified in Northern China since the 1980s. This intensification has caused a series of simultaneous lake ecological environment problems in this area. However, little is known about the role of agricultural intensification in historical nutrient dynamics and lake eutrophication processes. The Yanghe reservoir, a typical artificial reservoir characterized by high-yield grain production in Northern China, has been suffering from serious eutrophication and water quality deterioration. This study evaluates the effect of agricultural intensification on nutrient retention and source in the sediments using (210)Pb and (137)Cs dating techniques combined with stable C and N isotopes (δ(13)C, δ(15)N) and total organic carbon/total nitrogen, as well as total nitrogen (TN), total phosphorus (TP), and P fractions. Results suggested that agricultural intensification was keys to the accumulation of nutrients and was a source of organic matter (OM) and N in sediment for the past three decades. N and P pollution started in the 1980s and worsened from the 1990s. Good water quality status and steady sedimentary environment with low nutrient content (mean concentrations of TN and TP were 815 and 387 mg kg(-1), respectively) were observed before the 1980s. Sediment OM was primarily derived from aquatic plants, whereas N was primarily derived from soil erosion and aquatic plants. However, water quality began to deteriorate while sediment nutrient content began to increase after the 1980s, with values of 1186 mg kg(-1) for TN and 434 mg kg(-1) for TP in 1989. Sediment OM was primarily derived from C3 (sweet potato) and aquatic plants, and the major sources of N were soil erosion, fertilizer, and sewage, which accompany the rapid development of agriculture in the watershed. Following the further growth of grain production and fertilizers, excessive external nutrient loading has resulted in dramatic water quality and ecosystem deterioration since 1990. The increasing rate of TN and TP contents was also augmented during these periods, reaching as high as 2624 and 846 mg kg(-1) in surface sediment, respectively. In addition, sources of OM and N in sediment were similar to those in the 1980s, but the contribution of aquatic organic N in sediment has continued to increase (aquatic organic N that accounts for TN increased from 14.5% before the 1980s to 48% in 2007). This condition could be attributed to the impact of frequent "water bloom" and recession of aquatic plant due to worsening water pollution.

Carbon and nitrogen isotopes from top predator amino acids reveal rapidly shifting ocean biochemistry in the outer California Current

Climatic variation alters biochemical and ecological processes, but it is difficult both to quantify the magnitude of such changes, and to differentiate long-term shifts from inter-annual variability. Here, we simultaneously quantify decade-scale isotopic variability at the lowest and highest trophic positions in the offshore California Current System (CCS) by measuring δ¹⁵N and δ¹³C values of amino acids in a top predator, the sperm whale (Physeter macrocephalus). Using a time series of skin tissue samples as a biological archive, isotopic records from individual amino acids (AAs) can reveal the proximate factors driving a temporal decline we observed in bulk isotope values (a decline of ≥1 ‰) by decoupling changes in primary producer isotope values from those linked to the trophic position of this toothed whale. A continuous decline in baseline (i.e., primary producer) δ¹⁵N and δ¹³C values was observed from 1993 to 2005 (a decrease of ∼4‰ for δ¹⁵N source-AAs and 3‰ for δ¹³C essential-AAs), while the trophic position of whales was variable over time and it did not exhibit directional trends. The baseline δ¹⁵N and δ¹³C shifts suggest rapid ongoing changes in the carbon and nitrogen biogeochemical cycling in the offshore CCS, potentially occurring at faster rates than long-term shifts observed elsewhere in the Pacific. While the mechanisms forcing these biogeochemical shifts remain to be determined, our data suggest possible links to natural climate variability, and also corresponding shifts in surface nutrient availability. Our study demonstrates that isotopic analysis of individual amino acids from a top marine mammal predator can be a powerful new approach to reconstructing temporal variation in both biochemical cycling and trophic structure.

Foraging segregation and genetic divergence between geographically proximate colonies of a highly mobile seabird

Foraging segregation may play an important role in the maintenance of animal diversity, and is a proposed mechanism for promoting genetic divergence within seabird species. However, little information exists regarding its presence among seabird populations. We investigated genetic and foraging divergence between two colonies of endangered Hawaiian petrels (Pterodroma sandwichensis) nesting on the islands of Hawaii and Kauai using the mitochondrial Cytochrome b gene and carbon, nitrogen and hydrogen isotope values (δ(13)C, δ(15)N and δD, respectively) of feathers. Genetic analyses revealed strong differentiation between colonies on Hawaii and Kauai, with Φ(ST) = 0.50 (p < 0.0001). Coalescent-based analyses gave estimates of <1 migration event per 1,000 generations. Hatch-year birds from Kauai had significantly lower δ(13)C and δ(15)N values than those from Hawaii. This is consistent with Kauai birds provisioning chicks with prey derived from near or north of the Hawaiian Islands, and Hawaii birds provisioning young with prey from regions of the equatorial Pacific characterized by elevated δ(15)N values at the food web base. δ(15)N values of Kauai and Hawaii adults differed significantly, indicating additional foraging segregation during molt. Feather δD varied from -69 to 53‰. This variation cannot be related solely to an isotopically homogeneous ocean water source or evaporative water loss. Instead, we propose the involvement of salt gland excretion. Our data demonstrate the presence of foraging segregation between proximately nesting seabird populations, despite high species mobility. This ecological diversity may facilitate population coexistence, and its preservation should be a focus of conservation strategies.

Enhanced carbon pump inferred from relaxation of nutrient limitation in the glacial ocean

The modern Eastern Equatorial Pacific (EEP) Ocean is a large oceanic source of carbon to the atmosphere. Primary productivity over large areas of the EEP is limited by silicic acid and iron availability, and because of this constraint the organic carbon export to the deep ocean is unable to compensate for the outgassing of carbon dioxide that occurs through upwelling of deep waters. It has been suggested that the delivery of dust-borne iron to the glacial ocean could have increased primary productivity and enhanced deep-sea carbon export in this region, lowering atmospheric carbon dioxide concentrations during glacial periods. Such a role for the EEP is supported by higher organic carbon burial rates documented in underlying glacial sediments, but lower opal accumulation rates cast doubts on the importance of the EEP as an oceanic region for significant glacial carbon dioxide drawdown. Here we present a new silicon isotope record that suggests the paradoxical decline in opal accumulation rate in the glacial EEP results from a decrease in the silicon to carbon uptake ratio of diatoms under conditions of increased iron availability from enhanced dust input. Consequently, our study supports the idea of an invigorated biological pump in this region during the last glacial period that could have contributed to glacial carbon dioxide drawdown. Additionally, using evidence from silicon and nitrogen isotope changes, we infer that, in contrast to the modern situation, the biological productivity in this region is not constrained by the availability of iron, silicon and nitrogen during the glacial period. We hypothesize that an invigorated biological carbon dioxide pump constrained perhaps only by phosphorus limitation was a more common occurrence in low-latitude areas of the glacial ocean.

Monsoon-driven export fluxes and early diagenesis of particulate nitrogen and its δ15N across the Somalia margin

Settling nitrogen fluxes intercepted by sediment traps on the mid-slope and in the deep basin off Somalia show a consistent annual range of 3.4 ± 0.2‰ in their stable isotope composition. Seasonal minima in δ15N of 3.7‰ are associated with the moderate N fluxes derived from coastally upwelled water, which is rapidly carried offshore along eddy margins passing over the mooring sites during the SW monsoon (June–September). Coastal upwelling, offshore transport and deep wind mixing cease at the end of the SW monsoon, leading to enhanced utilization of the up to 20 μM of NO3− in the photic layer, maxima in the N export flux, and an increasing δ15N by Rayleigh distillation. Yet as stratification develops, nutrient exhaustion follows and export production collapses as the δ15N increases to over 7‰. Cyanobacterial N2 fixation probably diminishes the δ15N by 0.4–1.6‰ during the autumn intermonsoon (November–December) when settling N fluxes are lowest. Nutrient utilization remains high during the NE monsoon (January–March), when nutrient entrainment by deep wind mixing results in enhanced N export with maxima in δ15N of up to 7.4‰. Annual N fluxes have virtually the same δ15N of 6.0‰ in all traps despite considerable differences in both N flux and δ15N between the traps during the year and at different depths. In comparison with the annual δ15N of 6.0‰ arriving on the sea floor, core-top sediments are enriched by +0.6‰ on the upper slope (at 487 m) increasing to +2.9‰ in the deep basin (at 4040 m), whereas the N sediment burial efficiency declines from about 17% to 3%. It appears that the extent of oxic decomposition at the sediment-water interface is the most likely cause of such isotope enrichment. Similar positive gradients in δ15N with bottom depth have been reported from other continental margin transects and are generally attributed to increased nutrient utilization in the photic ocean with distance offshore. As for Somalia, nitrogen isotope fractionation as a result of oxic decomposition on the bottom rather than nutrient utilization at the ocean surface may account for the observed increase of sedimentary δ15N down continental margins in general.

Glacial/interglacial variations in atmospheric carbon dioxide

Twenty years ago, measurements on ice cores showed that the concentration of carbon dioxide in the atmosphere was lower during ice ages than it is today. As yet, there is no broadly accepted explanation for this difference. Current investigations focus on the ocean's 'biological pump', the sequestration of carbon in the ocean interior by the rain of organic carbon out of the surface ocean, and its effect on the burial of calcium carbonate in marine sediments. Some researchers surmise that the whole-ocean reservoir of algal nutrients was larger during glacial times, strengthening the biological pump at low latitudes, where these nutrients are currently limiting. Others propose that the biological pump was more efficient during glacial times because of more complete utilization of nutrients at high latitudes, where much of the nutrient supply currently goes unused. We present a version of the latter hypothesis that focuses on the open ocean surrounding Antarctica, involving both the biology and physics of that region.

Marine control of biological production in the eastern equatorial Pacific Ocean

The eastern equatorial Pacific Ocean is the site of approximately 20-50% of new biological production in the global oceans. This region is also responsible for the greatest efflux of CO2 from oceans to the atmosphere. New production, which fixes carbon in response to external inputs of nutrients as opposed to supply from local nutrient recycling, is thought to modulate the CO2 release. But what controls new production in this region is less clear. Here we present a quantitative reconstruction of biological production in the surface ocean for this region over the past 130,000 years, which shows that the equatorial Pacific Ocean exhibits higher-frequency variations than the South Equatorial Current. Comparison of these records with palaeotemperature reconstructions indicates that atmospherically driven mechanisms--such as aeolian flux of iron or wind-driven changes in upwelling rate of nutrient-rich waters--are unlikely to have influenced longer-term rates of production in this region. Instead, biological production appears to be governed by changes in ocean circulation and the chemical composition of upwelled water.