Iron co-limitation of Sargassum fluitans

In recent years, global distribution of holopelagic Sargassum spp. (sargassum) has extended from the subtropical Sargasso Sea and Gulf of Mexico into the tropical Atlantic. Climate and current patterns drive seasonal and year-to-year fluctuations of biomass in the ocean, but the underlying drivers of sargassum growth are poorly understood. Previous experimental studies showed that nitrogen (N) and phosphorus (P) can be limiting to sargassum. However, iron (Fe) also limits primary production in large parts of the ocean. We therefore (1) conducted a mesocosm experiment studying the effects of N+P and Fe addition on the growth rate and nutrient content of Sargassum fluitans, and (2) compiled literature on Fe tissue levels in sargassum throughout its distribution area. The Fe levels in collected experimental specimens (Mexican Caribbean) were like those previously reported near coastlines with low terrestrial nutrient runoff, and in the open ocean. The addition of Fe greatly boosted growth, averaging 0.13 doublings day−1, 40 % faster than our controls, and maximum growth rate (doubling biomass in 5�\u27d) was 46 % above previously reported maximal value. While oceanic Fe is relatively abundant in the tropical North Atlantic during rain episodes in the summers due to Saharan dust deposition, its availability is likely more limiting during other parts of the year, particularly in the western Caribbean. However, the true limiting potential of Fe depends on many factors. Our study suggests Fe co-limitation might occur widely and urges to include Fe availability in future sargassum forecasting models.</span

Effect of temperature on growth and nitrate and phosphate uptake kinetics of juvenile Saccharina latissima sporophytes (Phaeophyceae)

Kelp forests play a vital role in marine ecosystems by contributing to nutrient cycling and providing habitat for marine organisms. However, the impacts of rising ocean temperatures threaten the survival and growth of kelp species, with implications for ecosystem resilience. The aim of this study was to assess the effect of temperature on growth and nutrient uptake kinetics of young Saccharina latissima sporophytes. Growth and uptake rates of nitrate (NO₃-) and phosphate (PO₄3-) were examined under 5 temperature treatments ranging from 7.6 °C to 24.5 °C. Our findings revealed that NO3- uptake significantly decreased when temperature was at or above 15.7 °C, while high temperatures had no effect on PO43- uptake rates. Nitrate uptake significantly correlated with growth only at lower temperatures of 7.6 °C and 12.6 °C. In contrast, PO43- uptake was significantly correlated with growth across all temperature treatments except the highest (24.5 °C). Interestingly, at high temperatures (20.9 °C and 24.5 °C), we observed NO3- release, while PO43- uptake consistently showed positive values, suggesting distinct regulatory mechanisms for N (nitrogen) and P (phosphorus). These findings highlight potential disruptions in nutrient cycling under climate change and underscore the importance of optimizing nutrient availability in kelp aquaculture.</span

Vertical transport of anthropogenic lead by reversible scavenging in the South Atlantic Ocean

Stable lead (Pb) isotopes have been regarded as tracers of ocean circulation, both in the present time and geological past. Here we present a new dataset of seawater Pb concentrations and isotope compositions for ten depth profiles from the South Atlantic Ocean (GEOTRACES cruises GA02 and GA10). By comparing Pb isotope data collected on the two cruises, and by modelling the distribution of Pb with an extended optimum multiparameter analysis, we find evidence of vertical transport of anthropogenic Pb pollution due to reversible scavenging. Surface to depth transfer of polluted Pb is aided by high suspended particulate matter loads at the Brazil – Malvinas Confluence and along ∼40°S in the South Atlantic. Overall, our findings caution the use of Pb isotope ratios as ventilation tracers in the South Atlantic and emphasize the importance of particle-seawater interaction for biogeochemical cycles.</span

Pathway for personality development: juvenile red knots vary more in diet and exploratory behaviour than adults

Evidence is accumulating that foraging behaviour and diet link to personality traits, yet little is known about how these associations emerge during development. Behaviour is expected to become more consistent with age and with foraging experience. We compared exploratory behaviour and diet variances of juvenile and adult red knots, Calidris canutus islandica, shortly after migration to marine intertidal mudflats from terrestrial tundra breeding grounds. By identifying the timing of the switch from terrestrial to marine isotopic signatures, we were also able to ask whether juveniles that arrived earlier, and thus had longer experiences in a particular environment (nonbreeding grounds), were more consistent in exploration behaviour. We found that juveniles had a more diverse diet and were less repeatable in their exploratory behaviour than adults. This change in repeatability was largely driven by greater within-individual behavioural variance in the juveniles compared to the adults. The amount of time juveniles experienced in a marine environment did not affect the variation in their exploratory behaviour, suggesting that consistency in exploration was developed over a longer period than the 4 weeks of our study. Our findings suggest that after initial exploration of a novel habitat, juveniles likely try out foraging techniques which later develop into consistent behaviours that differ between individuals. This study illuminates how personality can develop with experience in a free-living animal

Tracking Agulhas leakage in the South Atlantic using modern planktic foraminifera nitrogen isotopes

Seawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial-interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the 15N/14N ratio (δ15N) of mixed-layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ15N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ15N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N2 fixation that occurs in the Indian Ocean. Foraminifera δ15N may have been lowered further during eddy migration by in situ N2 fixation and/or recycling of low-δ15N ammonium. The absence of low-δ15N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ15N. The low δ15N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer-bound δ15N from the region could record variations in past Agulhas leakage.</span

Using sporeless sporophytes as a next step towards upscaling offshore kelp cultivation

The feasibility, benefits, risks, and consequences of using sporeless sporophytes to aid the upscaling of kelp cultivation in Europe and North America, as well as the barriers that currently exist and how these may be overcome, are reviewed here. Taking environmental, industrial, and regulatory factors into account, we will discuss the use of domesticated sporeless sporophytes as an asset to facilitate upscaling kelp cultivation without potentially impacting wild genetic diversity.</span

Heterogenous distribution and burial flux of black carbon in Chinese lakes and its global implication

Black carbon (BC) plays a crucial role in global carbon cycle and climate change. However, its source and burial flux in environments are not well constrained. Here, we investigated surface sediments from 22 Chinese lakes across wide geographical areas and different socioeconomic status. The BC content accounts for 0.09–10.5 % of total organic carbon (TOC), and its average 14C age is older than that of TOC by 1640 years. The application of δ13C-based MixSIAR model shows that the contribution of fossil fuel combustion is highest in the most developed Eastern China (85.7 %) and lowest in the rural Qinghai-Tibetan Plateau (51.4 %), which is corroborated by the results from 14C-based two endmember mixing model. The BC data from this study and literatures suggest that the current BC burial flux is 126.4 ± 15.8 Gg year −1 in Chinese lakes, and approximately 2987 ± 1022 Gg year −1 in global lakes. Globally, lakes accumulate 1.2 %–6.4 % of the total BC production and thus are an important and heterogenous BC sink

The emerging picture of a diverse deep Arctic Ocean seafloor: From habitats to ecosystems

Interest in the deep Arctic Ocean is rapidly increasing from governments, policy makers, industry, researchers, and conservation groups, accentuated by the growing accessibility of this remote region by surface vessel traffic. In this review, our goal is to provide an updated taxonomic inventory of benthic taxa known to occur in the deep Arctic Ocean and relate this inventory to habitat diversity. To achieve this goal, we collected data for Arctic metazoan deep-sea taxa from open-access databases, information facilities, and non-digitised scientific literature, limiting the collection to the area north of 66°N and below 500 m depth (excluding all shelf seas). Although notable progress has been made in understanding the deep Arctic using novel technologies and infrastructure, this data gathering shows that knowledge of deep-sea benthic Arctic communities remains very limited. Yet, through our compilation of habitat maps, we show that the Arctic contains a high diversity of geomorphological features, including slopes, deep basins, submarine canyons, ridges, and seamounts, as well as chemosynthesis-based and biogenic (biologically engineered) ecosystems. To analyse taxon richness and density, using both morphological and molecular data, we compiled 75,404 faunal records with 2,637 taxa. Phyla with the most records were the Arthropoda (21,405), Annelida (13,763) and Porifera (12,591); phyla with the most documented taxa were the Arthropoda (956), Annelida (566) and Mollusca (351). An overview of the dominant groups inhabiting the different geomorphological features highlights regions in the deep Arctic where data are particularly scarce and increased research efforts are needed, particularly the deep basins of the central Arctic Ocean. This scarcity of deep benthic Arctic biodiversity data creates a bottleneck for developing robust management and conservation measures in a rapidly changing region, leading to a call for international collaboration and shared data to ensure understanding and preservation of these fragile Arctic ecosystems

Hot spots drive uptake and short‐term processing of organic and inorganic carbon and nitrogen in intertidal sediments

This study uses dual-labeled (13C and 15N) stable isotope applications to examine microbial uptake and short-term processing of carbon (C) and nitrogen (N) from organic and inorganic compounds in subtropical intertidal sediment. Four treatment applications were applied: (1) algal dissolved organic matter (DOM), (2) amino acid mixture, (3) glucose and NHþ 4 , and (4) NaHCO3 and NHþ 4 to assess bioavailability effects on processing (1 vs. 2) and short-term processing for OM fixed via microphytobenthos only (pennate diatom domi?nated) (4) vs. material taken up by the entire microbial community (3) across 24 h. 13C from algal-DOM was preferentially used by the microbial community vs. 15N. At 24 h more 13C from algal-DOM remained in micro?bial biomass indicating use of labeled precursor molecules to form biomass. Conversely, 13C from the amino acid treatment was not incorporated into biomass and was either rapidly respired to DIC or discarded as the in situ microbial community preferentially used and retained 15N from amino acids. Short-term export of 13C as CO2 from glucose was lower than from microphytobenthos-C, while retention of 15N from NHþ 4 was similar between treatments (3 and 4) despite doubling the application N concentration, suggesting potentially higher glucose-stimulated 15N export via nitrification–denitrification that was not confirmed via flux measurements in this study. Despite careful site selection for similar tidal exposure and sediment types among the three estuaries, the uptake and processing of labeled substrate varied substantially between replicates and sites which chal?lenged traditional statistical analysis. Disproportionate processing of substrates occurring in sediment hotspots of microbial activity can cause variability spanning orders of magnitude which was found to be widespread through comparison of our results against 19 previous studies in intertidal settings. Development of robust ana?lytical techniques to handle variability from abiotic and biotic factors will allow greater clarity surrounding in situ biogeochemical processing in intertidal environments

Habitat use across the tidal cycle by black-headed gulls breeding in the Wadden Sea

Movements of birds foraging in intertidal areas are often strongly linked to the tidal cycle, as water levels determine where and when birds can forage. The strength of this link likely depends on the ability to forage in habitats other than intertidal areas and on constraints imposed by breeding duties. Few studies have focused on the use of intertidal areas by generalists, such as the black-headed gull Chroicocephalus ridibundus , that occupy a wide variety of habitats within and beside intertidal mudflats. We investigated to what degree black-headed gulls (1) use intertidal mudflats versus terrestrial habitats during different phases of the tidal cycle, (2) follow the tidal wave to exploit recently exposed mudflats and (3) whether these behaviours are influenced by central place foraging. For this, 11 black-headed gulls breeding on the Wadden Sea island Griend were tracked during two years using GPS loggers. When commuting to and from Griend, up to 75% of their time was spent in intertidal areas during low and incoming tide, which increased to 92% when not behaving as central place foragers. While their movements were strongly linked to the tidal cycle, they did not follow the tidal wave across the tidal basin during either period. Rather, individuals foraged either predominantly west or east of Griend during low and incoming tide and mostly remained visiting these areas when not behaving as central place foragers. As one of the most abundant species in the Wadden Sea, the extensive use of intertidal mudflats highlights the importance of black-headed gulls within the intertidal food web of the Wadden Sea