Ambient Seawater Research Articles

Carbon Cycle and Circulation Change in the North Pacific Ocean at the Initiation of Northern Hemisphere Glaciation Constrained by Boron‐Based Proxies in Diatoms

AbstractThe intensification of Northern Hemisphere glaciation (iNHG) at 2.73 Ma is associated with a reorganization of the subarctic Pacific Ocean and abrupt drop in opal mass accumulation rates. Uncertainty, however, remains around the extent to which these changes altered carbon dynamics and contributed to a reduction in atmospheric pCO2 and global temperatures. These issues are addressed here using the boron isotope (δ11B) proxy in diatom frustules to reconstruct past changes in the pH and pCO2 of ambient seawater. Diatom δ11B and [B] indicate a subarctic Pacific surface water increase of 0.3–0.5 pH units over the iNHG. This confirms that delivery of carbon and nutrients into surface waters was reduced at this time, explaining the drop in opal productivity and limiting CO2 outgassing from the ocean interior. We consider two hypotheses to explain this based on potential changes in circulation from the late Pliocene to early Pleistocene: “ventilation to stratification” or “stratification to ventilation.” The ventilation to stratification hypothesis, which posits a switch from vigorous Pacific Meridional Overturning Circulation in the Pliocene to stratification over iNHG, has received more attention in the literature. The stratification to ventilation hypothesis, which posits a modest increase in ventilation, is more consistent with modern and late Pleistocene analogs, the majority of models and δ13C data. These late Pliocene changes in the subarctic Pacific, in conjunction with other external and internal processes including those in the Southern Ocean, would have contributed to a lowering of atmospheric pCO2 and the long‐term expansion of ice‐sheets across the Northern Hemisphere.

Effect of seasonal changes in temperature on capture efficiency in the blue mussel, Mytilus edulis, fed seston and microplastics

AbstractSuspension‐feeding bivalve molluscs are dominant benthic fauna in many near‐shore environments, with phytoplankton often being their main prey type. Feeding, which involves hydrosol filtration and mucociliary processes to capture particles and process them for digestion, occurs at low Reynolds numbers. Changes in water temperature have been shown to affect feeding processes of bivalves as a result of altered physiological processes or temperature‐dependent changes in kinematic viscosity of water. Most studies, however, have focused on feeding rates and have manipulated temperature under laboratory conditions. In this study, experiments were conducted using ambient seawater and acclimatized blue mussels, Mytilus edulis, to examine particle capture efficiency over a 1‐year period. During this period, water temperature decreased from ~18°C to 5°C with a concomitant increase in viscosity of ~41%. The capture of a wide variety of bacteria and phytoeukaryotes (0.8–8 μm), as well as two sizes of polystyrene microspheres (1 and 6 μm), was quantified to calculate capture efficiency. Data demonstrate that temperature, and the concomitant change in water viscosity, had no significant effect on capture efficiency. Cyanobacteria were captured at significantly higher rates than other bacteria of similar size. These results suggest that the laterofrontal cirri of blue mussels act as paddles rather than sieves because capture efficiency was constant over a range of viscosities and Reynolds numbers. Additionally, the efficient capture of some bacteria and smaller phytoeukaryotes by the mussels suggests that these plankters could play a larger role in the diet of M. edulis than previously considered. The ecological impact of these findings, especially regarding differences in capture efficiency of different bacterial cells, warrants further study.

Modeling the dispersion of wastewater pollutants in Gaza's coastal waters

The disposal of sewage water in Gaza City has emerged as a significant issue with extensive environmental repercussions. This study seeks to investigate the pollution plume resulting from the discharge of untreated or partially treated sewage water through Gaza City's main outlet into the Mediterranean Sea. Additionally, it aims to predict scenarios for various design configurations of submerged outfalls with either single-port or multi-port diffusers and compare these scenarios against the Environmental Protection Agency (EPA) recreational water quality criteria. According to the EPA, the concentration of Enterococci bacteria at the edge of the mixing zone should not exceed 35 CFU/100 ml to minimize the adverse environmental impact on the marine ecosystem.CORMIX software was utilized as a modeling tool to simulate the dispersion and attenuation behavior of pollutants resulting from this process and to conduct a sensitivity analysis to optimize the design configuration of the sewage disposal system. The simulation considered the influence of ambient conditions (ambient velocity, wind, and seawater density), effluent characteristics (density, flow rate, pollutant concentration, and pollutant decay rate), outfall configuration, and sea bathymetry.Simulation results indicate that a single-port diffuser is unsuitable according to EPA recreation standards. Multi-port unidirectional diffusers, extending 490 m from the shore into the water, meet the required standards. To a lesser extent, the multi-port staged distributor also meets the standards and is more recommended for counter-current situations.

Comprehensive Assessment of Deep-Water Vessel Implosion Mechanisms: OceanGate's Titan Submersible Failure Sequence Explained

This study outlines the physical mechanisms involved in a submersible implosion and analyzes the loss of the Titan submersible (‘sub’) that occurred on 18 June 2023 during a mission to visit the wreck of the Titanic. Titan’s collapse mechanisms at the moment of implosion are described in detail and outer hull fracturing rate, subsequent implosion rate, accompanying heat release and other key processes are quantified. Plausible causes of the hull’s leak leading up to critical loss of the sub's hermetic closure are reviewed using test results made publicly available by the U.S. Marine Board of Investigation. Their data indicated that the bond interfaces between the individual layers of the carbon fiber hull (Hull V2) were critically compromised due to manufacturing defects, voids, porosity, and inadequate adhesive integrity, resulting in significant delamination. Analysis of data from the real-time hull health monitoring system, revealed acoustic anomalies and strain shifts, pointing toward increasing structural fatigue, which went unaddressed prior to the fatal dive. The implosion process can be characterized by an instantaneous collapse of the air volume within the hull under extreme external pressure: even the tiniest leak would lead to destruction of the vessel's structural integrity. The destruction was the more devastating, because it was accompanied by a secondary explosion due to the heat exchange between the collapsing air volume and the ambient sea water. While the collapsing air was phase-changed into a supercritical state, the generated heat caused the adjacent seawater to evaporate and expand. Hull pieces fragmented by the initial implosion were strewn around during the secondary explosion phase, which ceased rapidly as the steam condensed back into seawater once again. The Titan incident underscores the urgent need for improved design standards, rigorous quality control in manufacturing, and enhanced real-time monitoring to prevent similar failures of future deep-sea exploration vehicles.

Transfer of rare earth elements from clay-sized fraction to phosphate in East South Pacific Ocean: Implication for REY-rich sediment related to hydrothermal influence

Pelagic sediment enriched in critical metals (e.g. rare earth elements and yttrium, REY) has attracted much attention in recent years. Extensive research has focused on identifying the specific host mineral of REY in bulk pelagic sediment, however, research on clay-sized fraction of REY-rich sediment has not been fully understood yet. In this study, we aimed to investigate the host phase and migration mechanism of REY in clay-sized fractions from two cores, GC23 and GC17, located on the western and eastern sides of the East Pacific Rise (EPR), respectively. To the best of our knowledge, this is the first comprehensive investigation of the clay-sized fraction of REY-rich sediment associated with hydrothermal activity. Results show that GC23 contains negligible clay minerals but well-crystallized Fe oxyhydroxides, while GC17 is rich in smectite and poor-crystallized Fe oxyhydroxides. REY are predominantly hosted in poorly crystallized Fe-Mn oxyhydroxides, with some phosphorus selectively scavenged by Fe oxyhydroxides from seawater. In addition, fluorapatite nanocrystals were first observed within the matrix of Fe oxyhydroxides using transmission electron microscopy (TEM), indicating the formation of fluorapatite. The post-Archean average shale (PAAS)-normalized REY patterns show similar seawater-like patterns in both the clay-sized and silt-sized fraction. The clay-sized fractions primarily derived from hydrothermal plumes plays an important role in scavenging REY from ambient seawater. This study represents a significant step towards understanding the formation of REY-rich sediment related to hydrothermal activity. A two-stage mineralization process is proposed for the formation of REY-rich sediment near the EPR fields. Firstly, REY are initially scavenged by hydrothermal Fe-Mn oxyhydroxide particles from seawater during their lateral dispersion with hydrothermal plumes under low sedimentation rate until they are buried by newly formed precipitates. With the process of early diagenesis, poor crystallized Fe oxyhydroxides will be experienced recrystallization. Subsequently, REY would be released into porewater with the process of recrystallization due to their tendency to remain in a poorly crystallized phase. Ultimately, they are captured by biogenic apatite and/or fluorapatite. The case study indicates that REY-rich sediments may primarily formed within the dispersion area of hydrothermal plumes. Simultaneously, the necessity of slow sedimentation rates, greater water depth, and deep currents all accountable for the formation of REY-rich layers.

Influence of altered freshwater discharge on the seasonality of nutrient distributions near La Grande River, northeastern James Bay, Québec

In subarctic marine environments, nutrient stocks are replenished through physical and biogeochemical processes in winter, largely setting an upper limit on new primary production for the next growing season. In spring, marine nutrient stocks are modified by freshwater-associated additions, especially in coastal areas. Hydroelectric development of the La Grande River (LGR) in northern Québec has shifted the timing of peak freshwater discharge from spring into winter, producing 10 times the natural winter discharge. Here, we considered salinity, oxygen isotope ratio (δ18O), and nutrient (nitrate, phosphate) data from coastal waters of northeast James Bay in different seasons of 2016 and 2017. We quantified two main freshwater sources, LGR and sea-ice melt, established by freshwater tracers, and their influence on coastal nutrient distributions. Our results show that LGR is the dominant source of freshwater to coastal waters throughout the year, especially during winter, and an important source of nitrate to nitrogen-limited coastal waters (winter concentrations of 4.53 μM versus 3.18 μM in ambient seawater). Despite being a poor phosphate source (0.11 μM versus 0.66 μM in ambient seawater), LGR provides the largest portion of the phosphate stock in surface waters near its mouth. LGR regulation has changed the pattern of natural fluvial nitrate inputs: what was observed in spring (pre-development) is now observed in winter (post-development). Thus, high winter surface nitrate stocks (22.5 mmol m−2) are available to support primary production, but are dispersed to offshore areas prior to the onset of the growing season, which begins only after the return of light. In northeast James Bay, the timing and magnitude of primary production, dependent on nutrients in the water column, is expected to have been impacted by altered freshwater input, reducing overall production in local areas and potentially increasing production further downstream with cascading effects on the marine ecosystem.

Genesis of Devonian volcanic-associated Lahn-Dill-type iron ores – part II: trace element fractionation evidences diffuse fluid venting

The iron (Fe)-oxide deposits of the Lahn-Dill-type are composed of haematite-quartz and rare siderite-haematite ores. These ores formed as marine chemical sediments on top of volcaniclastic rocks near the Middle to Late Devonian boundary (∼ 380 Ma). As such, their trace element fractionation patterns provide key information on venting style, ocean chemistry, particle-solution interaction, and depositional environment at the time of ore formation. This study combines WDXRF and ICP-MS/OES whole-rock geochemistry with complementary in-situ LA-ICP-MS analysis, and TEM element mapping of ore samples from the Fortuna Mine (Rhenish Massif, Germany). In-situ measurments were conducted on quartz-haematite, haematite, and siderite-haematite microdomains. Bulk major element contents of the ores indicate (volcani)clastic contamination and post-depositional hydrothermal alteration. Microdomain trace element distributions reveal four different trace element signatures, which are related to: (1) syngenetic apatite formation due to sorption of P and REY from seawater; (2) Fe-(oxyhydr)oxide-specific trace element scavenging and fractionation within the seawater column; (3) diagenetic Fe(III) reduction and trace element mobilisation in pore water; and (4) simultaneous deposition of (volcani)clastic material and Fe-(oxyhydr)oxides. These results show that Lahn-Dill-type iron ore formation resulted from mixing of a low-temperature vent fluid with ambient seawater at high seawater to vent fluid ratios. This likely was related to an environment in which diffuse venting dominated over focused venting, and in which quick Fe-particle precipitation led to formation of haematite-quartz ores. Local diagenetic Fe(III) reduction resulted in post-depositional siderite-haematite ore formation during which trace elements were partially remobilised in pore water.

Biochemical and molecular responses to long-term salinity challenges in northern quahogs Mercenaria mercenaria

Salinity is a key environmental factor for aquatic organisms for survival, development, distribution, and physiological performance. Salinity fluctuation occurs often in estuary and coastal zones due to weather, tide, and freshwater inflow and thus heavily affects coastal marine aquaculture. The northern quahog Mercenaria mercenaria is an important aquaculture species along the Atlantic coast in the US, but information on the effects of salinity stress on physiological, immunological, and molecular responses is still scarce. The goal of this study was to investigate cellular and molecular responses through challenges of long-term hypo- and hyper-salinities in northern quahogs. The objectives were to: 1) measure the survival of market-sized quahogs under a three-month salinity challenge at 15 (hyposalinity), 25 (control), and 35 ppt (hypersalinity); 2) determine cellular changes of hemocytes through analysis of immune functions; 3) determine changes of the total free amino acid concentration in gills, and 4) evaluate the molecular responses in gills using RNAseq technology with qPCR verification. After a three-month salinity challenge, no mortality was observed, and increases in body weight were identified with a significantly higher increase in the hypersalinity group. Northern quahogs equilibrated their hemolymph osmolality with the ambient seawater and were verified to be osmoconformers. Significant differences were observed in total hemocyte concentration, lysosomal presence, ROS production, and phagocytic rate, but no differences were found in cell viability. The total free amino acid concentration within gills was positively correlated to water salinity, indicating amino acids were critical organic osmolytes. The transcriptome of gills using RNAseq revealed differential expression genes (DEG) encoding amino acid transporters (SLC6A3, SLC6A6, SLC6A13, SLC25A38), ion channel proteins (T38B1, GluCl, ATP2C1), and water channel protein (AQP8) in hyposalinity or/and hypersalinity groups, indicating these genes play critical roles in intracellular isosmotic regulation. Overall, the findings in this study provided new insights into osmoregulation in northern quahogs.

Stable carbon isotope ratios of pristine carbohydrates preserved within nannofossil calcite

The geochemical characterization of phytoplankton-derived organic compounds found in marine sediments has been widely used to reconstruct atmospheric pCO2 throughout the Cenozoic. This is possible owing to a well-established relationship between the carbon isotope ratios of phytoplankton biomass and CO2 concentration in the ambient seawater. An ideal molecular target for such proxy reconstructions would be degradation resistant on geologic timescales and unambiguously associated with known, experimentally tractable, organisms, so that species-specific models can be developed, calibrated, and applied to appropriate material. However, existing organic matter targets do not meet these criteria, primarily owing to ambiguity in the source species of recalcitrant compounds in deep time. Here we explore the potential of a novel organic carbon target for isotopic analysis: acidic polysaccharides extracted from the calcite plates (coccoliths) that are produced by all calcifying haptophytes. Carbohydrates are usually rapidly remineralized in sediments, but coccolith-associated polysaccharides (CAPs) are mechanically protected from diagenesis within the coccolith calcite lattice. Coccoliths can be taxonomically separated by size and identified, often to species level, prior to CAP extraction, providing a species-specific record. Coccolith morphology and composition are important additional sources of information, which are then unambiguously associated with the extracted CAPs. We found that carbon isotope ratios of CAPs changed in response to the environmental changes associated with a glacial cycle, which we attribute to temperature-driven changes in average growth rate. Once the underlying biosynthetic processes and the associated isotope effects are better understood, this archive of pristine organic matter has the potential to provide insight into phytoplankton growth rates and atmospheric pCO2 far beyond the Cenozoic, to when the first coccolithophores inhabited the surface ocean over 200 million years ago.

Calcium-rich seawater affects the mechanical properties of echinoderm skeleton

Shifts in the magnesium to calcium ratio of seawater in the geological history are thought to have profoundly affected biomineralization of marine invertebrates, including some echinoderms, which changed their skeletal mineralogy from high-magnesium to low-magnesium calcite and vice versa. Here we report on experiments that aimed to investigate the effect of ambient seawater magnesium to calcium ratio on magnesium to calcium ratio and nanomechanical properties in the spines of two echinoid species (Arbacia lixula and Paracentrotus lividus). We found that echinoids cultured in seawater with a low magnesium to calcium ratio produced a skeleton with lower both magnesium content and nanohardness than those of the control specimens incubated under normal (high) magnesium to calcium ratio conditions. These results may suggest that at certain times in the geological past (during the so-called calcite seas) sea urchins with decreased skeletal magnesium contents were more susceptible to damage due to physical disturbances, predation and post-mortem taphonomic processes. Increased skeletal hardness of echinoids from the so-called aragonite seas is expected to enhance their taphonomic potential, thus, to some extent, mitigates the preservation bias related to increased solubility of high-magnesium calcite.

Temperature influences immune cell development and body length in purple sea urchin larvae

Anthropogenic climate change has increased the frequency and intensity of marine heatwaves that may broadly impact the health of marine invertebrates. Rising ocean temperatures lead to increases in disease prevalence in marine organisms; it is therefore critical to understand how marine heatwaves impact immune system development. The purple sea urchin (Strongylocentrotus purpuratus) is an ecologically important, broadcast-spawning, omnivore that primarily inhabits kelp forests in the northeastern Pacific Ocean. The S. purpuratus life cycle includes a relatively long-lived (∼2 months) planktotrophic larval stage. Larvae have a well-characterized cellular immune system that is mediated, in part, by a subset of mesenchymal cells known as pigment cells. To assess the role of environmental temperature on the development of larval immune cells, embryos were generated from adult sea urchins conditioned at 14 °C. Embryos were then cultured in either ambient (14 °C) or elevated (18 °C) seawater. Results indicate that larvae raised in an elevated temperature were slightly larger and had more pigment cells than those raised at ambient temperature. Further, the larval phenotypes varied significantly among genetic crosses, which highlights the importance of genotype in structuring how the immune system develops in the context of the environment. Overall, these results indicate that larvae are phenotypically plastic in modulating their immune cells and body length in response to adverse developmental conditions.

Study of the impact of low ambient air and seawater temperatures on the possibility of mining solid minerals in arctic waters

Study of the impact of low ambient air and seawater temperatures on the possibility of mining solid minerals in arctic waters

Stable carbon and oxygen isotope-based sclerochronology of bivalve mollusk shells from the Upper Cretaceous (upper Campanian) Coon Creek Formation in Tennessee, USA: Implications for paleoecology and paleoenvironment

Stable carbon and oxygen isotope-based sclerochronology was carried out on well-preserved fossil shells of bivalve mollusks Crassatella vadosa and Cucullaea vulgaris from the Upper Cretaceous (upper Campanian) Coon Creek Formation in Tennessee, USA, to reconstruct their life-history traits and sedimentary settings on an intra-annual time scale. The δ13CShell values along with the shell growth showed the cyclic variation with downward convex to lower value, likely reflecting the seasonal cycle of ambient seawater dissolved inorganic carbon (DIC) or metabolism of the bivalve or both. The δ13CShell cycles were largely associated with the strong growth disturbance lines, supporting hypothesis that the growth lines were formed annually. We estimated the ages of the shells based on both δ13C cycle or the growth cessation line. The modeled relationship between age and shell growth showed that Cr. vadosa and Cu. vulgaris grew up to about 40 mm at 7 years old and about 65 mm at 10 years old, respectively. The paleotemperatures estimated from the δ18OShell profiles of the examined specimens ranged from 16.2 °C to 24.2 °C and from 18.8 °C to 27.0 °C when assuming the bottom seawater δ18O to be −1.00‰ and −0.46‰ (VSMOW), respectively, and showed no clear seasonal cycle. The bottom seawater temperatures reconstructed from the specimens of the two bivalve species were lower than the near-surface seawater temperatures (29.0–30.6 °C) estimated from the planktic foraminiferal δ18O. We inferred that the benthic bivalve habitat of the Coon Creek Formation was at least 10 m or deeper below the thermocline.

Chemical composition and egg production capacity throughout bloom development of ctenophore Mnemiopsis leidyi in the northern Adriatic Sea.

High abundances of gelatinous zooplankton (GZ) can significantly impact marine ecosystem by acting as both sink and source of organic matter (OM) and nutrients. The decay of GZ bloom can introduce significant amount of OM to the ocean interior, with its variability influenced by GZ life traits and environmental factors, impacting microbial communities vital to marine biogeochemical cycles. The invasive ctenophores Mnemiopsis leidyi has formed massive blooms in the northern Adriatic Sea since 2016. However, the variability in the chemical composition and egg production of blooming populations, as well as the role of environmental factors in governing this variability, remains largely unknown. Our analysis of biometry, chemical composition, and fecundity of M. leidyi sampled in the Gulf of Trieste in 2021 revealed stable carbon and nitrogen content throughout bloom development, with no significant correlation with seawater temperature, salinity, oxygen, and chlorophyll a concentration. Although the studied population exhibited homogeneity in terms of biometry and chemical composition, the number of produced eggs varied substantially, showing no clear correlation with environmental variables and being somewhat lower than previously reported for the study area and other Mediterranean areas. We observed a positive correlation between the wet weight of individuals and the percentage of hatched eggs, as well as a significant positive correlation between the percentage of hatched eggs and ambient seawater temperature. Additionally, we noted that the speed of hatching decreased with decreasing seawater temperature in autumn, corresponding to the end of M. leidyi bloom.

Fluid-related manganese deposits at the incoming plate of the Southern Mariana subduction zone

Manganese is one of the most abundant transition metals in marine systems, controlling the behaviors of a variety of important elements in oceans worldwide. Deposited manganese minerals in marine systems mainly belong to poorly crystalline manganates or sometime tecto-manganates in the form of crusts and nodules origin from ambient bottom seawater, sediment porewater, or ascending hydrothermal fluids. Here, we report the discovery of two kinds of fluid-related manganese deposits on the incoming plate of the Southern Mariana subduction zone. Hausmannite that rich in copper and zinc with a strong negative Ce anomaly is identified in one sample. Notable features of this unusual manganese mineral indicate that reduced fluid derived from the nearby fluid discharge site reacted with original manganese oxides and then precipitated as hausmannite in the sub-oxic subsurface. However, another fluid derived sample located approximately 60 m away, shows normal hydrothermal mineral and elemental features. The atypical and diverse fluid-related manganese deposits more likely indicate multiple episodes of fluid activities at the subduction front. As bend-faults are proven to induce the fluid discharge at the outer-rise region of the incoming plate, manganese deposits in the present study suggest that an appreciable amount of manganese in the upper lithosphere of the incoming plate may be discharged into seawater through fluid activities before subduction. As such, the mass of manganese subducted into the subduction zone may be much lower than previously expected.

Boron isotopic compositions of middle Miocene to recent shallow-water carbonates from the South China Sea: Assessing diagenetic effects and implications for paleoclimate changes

The partial pressure of atmospheric CO2 (pCO2) significantly influences global climate change and biological evolution through geological history. Boron isotopic composition (δ11B) in carbonates has been used to reconstruct the paleo-pH of seawater, providing insight into atmospheric pCO2 levels. However, the fidelity of δ11B records in marine carbonates due to diagenesis remains uncertain. Here, to understand how diagenetic processes influence B isotopic records in marine carbonates, we examined B concentrations and δ11B values of modern corals (from Hainan Island), unconsolidated shallow pushcores (Jiuzhang A and Jiuzhang B from Nansha Islands, South China Sea), and a long drillcore (XK-1 from Xisha Islands, South China Sea) covering the Late Miocene to Holocene periods. Our coral samples show a uniform δ11B range (20.16 ± 0.85%), consistent with previously published values for modern corals in other open oceans. The δ11B values of the unlithified carbonate sediments from Jiuzhang A and B pushcores vary within a narrow range (18.70 ± 0.84%), yielding pH and CO2 concentrations consistent with the range between modern and pre-industrial values. Since we did not observe any statistically significant covariations between traditionally established diagenetic proxies (such as δ13C, δ18O, Mn/Sr, and Al/Ca) and δ11B values for non-dolomitized samples in the XK-1 drillcore, we suggest that δ11B values of bulk limestones are not significantly affected by typical diagenesis (meteoric, mixed-diagenetic, and marine diagenetic processes), likely due to limited post-depositional recrystallization of our study carbonates. In contrast, dolomitization significantly decreases δ11B values of bulk carbonates, rendering dolomites unsuitable as archives for reconstructing seawater δ11Bborate values. Our study supports that marine limestone with limited recrystallization or dolomitization have the potential to record ambient seawater pH values with high resolution. The established secular seawater pH variations based on XK-1 δ11B records provide information about ocean acidification over the past 5.1 million years.

Effects of experimental CO2 enrichment on the PSII photochemical efficiency of Symbiodinium sp. in Acropora millepora

Enrichment of seawater with CO2 decreases the concentration of the carbonate ion while increasing that of hydrogen and bicarbonate ions. We use pulse-amplitude-modulation (PAM) fluorometry to investigate whether, in the absence of warming, and in sub-saturating light, these changes affect the PSII photochemical efficiency of _Symbiodinium_ sp. in the reef-building coral _Acropora millepora_. We assessed this experimentally with 30-min-interval saturation pulse analyses at 25 °C, a daily peak in the intensity of the photosynthetically active radiation (PAR) at ~65 µmol quanta m–2 s–1, and a seawater _p_CO2 that we gradually increased over nine days from ~496 to ~1290 μatm by injection of CO2-enriched air. Nine 14-day time series, which, except one, were recorded at the growing apices of a coral branch, revealed diel oscillations in the PSII photochemical efficiency characterized by a steep nocturnal decrease followed by a steep increase and peak in the morning, a daily minimum at midday (∆F/Fm’,midday), and a daily maximum at the onset of darkness at 19:00 h (Fv/Fm,19:00 h). An inadvertent shift in the position of one of the PAM fluorometer measuring heads revealed differences between the basal part and the growing coral apices of a coral branch in ∆F/Fm’midday and Qm. In ambient seawater (Control) _Symbiodinium_ sp. exhibited a gradual decrease, over the course of the experiment, in ∆F/Fm’,midday, Fv/Fm,19:00 h, and the slope of the linear regression between the relative electron transport rate and the intensity of PAR (rETR/PAR). Although two of three successive experiments indicated that CO2 enrichment counteracted these trends, statistical analyses failed to confirm an influence of _p_CO2 on ∆F/Fm’,midday, Fv/Fm,19:00 h, and Qm, rendering this experiment inconclusive.

Ocean acidification in the tropical Indian Ocean over the past 37 years: Insights from [formula omitted]11B and B/Ca records in a Maldives coral

Boron isotopes (δ11B) in coral skeletons of Porites have been widely applied to reconstruct past seawater pH (pHSW) on decadal to centennial timescales. However, due to biological regulation within corals, an additional transfer function is required to estimate ambient seawater chemistry during the skeleton growth under the calcification site fluid pH. Temperature may also interfere with coral calcification fluid pH (pHCF) due to changes in kinetics of coral aragonite precipitation, or buffering capacity in coral calcification fluid. To decipher how coral Porites adjusts pHCF in response to pHSW from complex environmental controls, long-term records from sites with least fluctuations in environmental conditions other than pHSW are essential. Here we present a 37-year record of coral δ11B and B/Ca ratios derived from a coral core collected from southern Maldives, the tropical Indian Ocean. Our results show no clear seasonality in the coral δ11B and B/Ca ratios between monsoons, but a long-term decline in coral pHCF is evident across the entire record. When applying different existing transfer functions, we also observe discrepancies among the calculated pHCF values, model results and short-term instrumental data. Calculated calcification fluid dissolved inorganic carbon concentration ([DIC]CF) values are relatively low compared to literature, suggesting that coral calcification fluid carbonate chemistry may be under different levels of control, even within the same coral taxa. Thus, coral records from a wider geographic range are required to better quantify coral response to ocean acidification, and our results can serve as a baseline for future comparisons.

Single chamber Mg/Ca analyses of Globigerinoides ruber for paleo-proxy calibration using femtosecond LA-ICP-MS

Mg/Ca is an independent proxy in paleoceanography to reconstruct past seawater temperature. Femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry (fs-LA-ICP-MS) was employed to determine the Mg/Ca composition of tests (shells) of the planktic foraminifer species Globigerinoides ruber albus (white chromotype) and G. ruber ruber (red/pink chromotype) sampled alive from the temperate to subtropical eastern North Atlantic with the research sailing yacht Eugen Seibold. Mg/Ca data are compared to (i) the measured in-situ temperature of ambient seawater, (ii) average mixed layer temperature, and (iii) sea surface temperature (SST). The pooled mean chamber Mg/Ca from each plankton tow site exhibits a positive relationship with SST. Two chamber-specific calibrations are derived, which are consistent with previous calibration equations for comparable paleo-archives. The results confirm fs-LA-ICP-MS as reliable method for determining Mg/Ca in G. ruber, and both the penultimate and antepenultimate chambers of adult specimens may provide comprehensible Mg/Ca temperatures of the surface ocean.

Changes in holopelagic Sargassum spp. biomass composition across an unusual year

The year 2021 marked a decade of holopelagic sargassum (morphotypes Sargassum natans I and VIII, and Sargassum fluitans III) stranding on the Caribbean and West African coasts. Beaching of millions of tons of sargassum negatively impacts coastal ecosystems, economies, and human health. Additionally, the La Soufrière volcano erupted in St. Vincent in April 2021, at the start of the sargassum season. We investigated potential monthly variations in morphotype abundance and biomass composition of sargassum harvested in Jamaica and assessed the influence of processing methods (shade-drying vs. frozen samples) and of volcanic ash exposure on biochemical and elemental components. S. fluitans III was the most abundant morphotype across the year. Limited monthly variations were observed for key brown algal components (phlorotannins, fucoxanthin, and alginate). Shade-drying did not significantly alter the contents of proteins but affected levels of phlorotannins, fucoxanthin, mannitol, and alginate. Simulation of sargassum and volcanic ash drift combined with age statistics suggested that sargassum potentially shared the surface layer with ash for ~50 d, approximately 100 d before stranding in Jamaica. Integrated elemental analysis of volcanic ash, ambient seawater, and sargassum biomass showed that algae harvested from August had accumulated P, Al, Fe, Mn, Zn, and Ni, probably from the ash, and contained less As. This ash fingerprint confirmed the geographical origin and drift timescale of sargassum. Since environmental conditions and processing methods influence biomass composition, efforts should continue to improve understanding, forecasting, monitoring, and valorizing sargassum, particularly as strandings of sargassum show no sign of abating.