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The offshore Gulf of Alaska—in the north-eastern subarctic Pacific—is a high nitrate, low chlorophyll (HNLC) oceanic region where concentrations of dissolved iron (DFe) and zinc (DZn) are of the order ~0.1 nmol kg-1 in surface waters. At the average winter mixed layer depth of ~120 m, concentrations of DFe and DZn are of the orders ~0.1–0.2 nmol kg-1 and ~1–2 nmol kg-1 respectively. Vertical supply of Fe to surface waters is therefore limited, with phytoplankton blooms driven by episodic inputs of Fe. In contrast, DZn is presumably replenished through seasonal vertical mixing, though maintained at low levels in surface waters, even in winter, implying that removal processes are in operation.Previous supplementation experiments have demonstrated that phytoplankton biomass (chl-a) and growth, and drawdown of nitrate (NO3-), are stimulated strongly by Fe but only minimally, if at all, by Zn alone. Using on-deck bioassay incubations of surface waters, we confirm that addition of Fe stimulates significant increases in chl-a and large diatoms, and drawdown of NO3-and silicic acid (Si(OH)4). Associated drawdown of DZn and dissolved cadmium (DCd) indicated Zn stress in control and +Fe treatments. Supplementation with Zn alone had no significant impact on dissolved NO3- and commonly monitored pigments such as chl-a and fucoxanthin. However, in +Zn treatments, DFe and coccolithophore abundance were significantly lower, and the concentration of particulate organic carbon (POC) and the pigments alloxanthin and chlorophyll c1+2 were significantly higher than in control incubations. Our experiment corroborates previously observed relationships between DZn and alloxanthin and chlorophyll c across the subarctic north Pacific and Bering Sea and could help to explain the low or undetectable concentrations of alloxanthin frequently observed in this region. Alloxanthin is a pigment specific to the cryptophytes, either within free-living cells, or within functional plastids retained by mixotrophic protists. Both cryptophytes and mixotrophic ciliates, such as Laboea sp. and Stombidium spp., make a variable but significant contribution to phytoplankton biomass in this area. In the absence of Fe supply to surface waters, Zn could play a hitherto unsuspected role in shaping plankton communities in the HNLC Gulf of Alaska.

Este artículo contiene 18 páginas, 6 figuras, 1 tabla., Our knowledge of the factors that can influence the stock of organic carbon (OC) that is stored in the soil of seagrass meadows is evolving, and several causal effects have been used to explain the variation of stocks observed at local to national scales. To gain a global-scale appreciation of the drivers that cause variation in soil OC stocks, we compiled data on published species-specific traits and OC stocks from monospecific and mixed meadows at multiple geomorphological settings. Species identity was recognized as an influential driver of soil OC stocks, despite their large intraspecific variation. The most important seagrass species traits associated with OC stocks were the number of leaves per seagrass shoot, belowground biomass, leaf lifespan, aboveground biomass, leaf lignin, leaf breaking force and leaf OC plus the coastal geomorphology of the area, particularly for lagoon environments. A revised estimate of the global average soil OC stock to 20 cm depth of 15.4 Mg C ha −1 is lower than previously reported. The largest stocks were still recorded in Mediterranean seagrass meadows. Our results specifically identify Posidonia oceanica from the Mediterranean and, more generally, large and persistent species as key in providing climate regulation services, and as priority species for conservation for this specific ecosystem service., HK was supported by the Ecosystem Services for Poverty Alleviation program Coastal Ecosystem Services in East Africa (NE/L001535/1). JFP acknowledges financial support from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant 795315. JWF and JLH were supported by the Florida Coastal Everglades Long-Term Ecological Research program under the U.S. National Science Foundation Grant DEB-2025954, and this paper is contribution #1494 from the Institute of Environment at Florida International University. TK was supported in part by Grants-in-Aid for Scientific Research (KAKENHI) Grant 18H04156 from the Japan Society for the Promotion of Science. DKJ was funded by European Union H2020 (FutureMARES, contract #869300). OS was supported by I+D+i projects RYC2019-027073-I and PIE HOLOCENO 20213AT014 funded by MCIN/AEI/10.13039/501100011033 and FEDER. PIM was supported by an Australian Research Council Discovery Grant (DP200100575). PM This work is contributing to the ICTA ‘‘Unit of Excellence’’ (MinECo, MDM2015-0552). The IAEA is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco. NM was supported by the project RTI2018-095441-B-C21 funded by MCIN/AEI/10.13039/501100011033 and by FEDER. IM was supported by a Juan de la Cierva Incorporación postdoctoral fellowship (IJC2020-045917-I I) from the Ministry of Science and Innovation (Spanish Government).

Global inventories that show mangrove forests have rich carbon stores currently lack data from arid areas where carbon stocks may be functionally impoverished relative to humid regions. We quantified total carbon stocks (C) of three arid Avicennia marina stands in Qatar and report an aboveground biomass allometric equation and the first below ground biomass allometric equation in the region. The allometric relationships indicate that below ground mangrove C stocks in arid locations are more important than previously reported. Comparison of previously published and our locally developed allometric equations show that A. marina in Qatar allocate comparatively more biomass to below ground components than the same species in tropical humid settings, which is consistent with plant adaptations to living in stressed conditions. Total C stocks were 45.70 ± 3.70 Mg C ha−1, of which tree and soil C stocks to 50 cm depth represented 10.18 ± 0.82 Mg C ha−1 and 35.52 ± 2.88 Mg ha−1 respectively. Soil C stocks to 1 m depth were 50.17 ± 6.27 Mg C ha−1. Overall, mangroves sustain relatively small C stocks in the arid, hypersaline environment of Qatar, which may be due to both relatively low tree productivity and growth, as well as limited rainfall-driven transport of terrigenous sediment inputs. By providing further estimates of mangrove carbon at their climatic extremes, these results can contribute to a better quantification of global mangrove carbon, reduce uncertainty in below ground tree C estimates from arid mangroves and have implications for mangrove carbon stocks in the face of climate change. Qatar National Research Fund, National Priorities Research Program (NPRP) [grant number 7–1302 – 1–242

Seagrass ecosystems provide a wide variety of services that support human well-being around the world (Barbier et al. 2011). It is estimated that more than 1 billion people live within 100 km of a coast with seagrass meadows, thus potentially benefiting from their provisioning, regulating and cultural services. Seagrasses play a significant global role in supporting food security, mitigating climate change, enriching biodiversity, purifying water, protecting the coastline and controlling diseases (Figure 2). The integrity and provision of services by seagrass meadows are enhanced by their proximity and connectivity to other coastal ecosystems such as tidal marshes, coral reefs, mangrove and kelp forests, and oyster and mussel beds. The maintenance and regulation of these services is therefore essential to support human well-being and promote development in the future. info:eu-repo/semantics/publishedVersion

The sea ice microstructure is permeated by brine channels and pockets that contain concentrated seawater-derived brine. Cooling the sea ice results in further formation of pure ice within these pockets as thermal equilibrium is attained, resulting in a smaller volume of increasingly concentrated residual brine. The coupled changes in temperature and ionic composition result in supersaturation of the brine with respect to mirabilite (Na2SO4·10H2O) at temperatures below −6.38 °C, which consequently precipitates within the sea ice microstructure. Here, mirabilite solubility in natural and synthetic seawater derived brines, representative of sea ice at thermal equilibrium, has been measured in laboratory experiments between 0.2 and −20.6 °C, and hence we present a detailed examination of mirabilite dynamics within the sea ice system. Below −6.38 °C mirabilite displays particularly large changes in solubility as the temperature decreases, and by −20.6 °C its precipitation results in 12.90% and 91.97% reductions in the total dissolved Na+ and SO42− concentrations respectively, compared to that of conservative seawater concentration. Such large non-conservative changes in brine composition could potentially impact upon the measurement of sea ice brine salinity and pH, whilst the altered osmotic conditions may create additional challenges for the sympagic organisms that inhabit the sea ice system. At temperatures above −6.38 °C, mirabilite again displays large changes in solubility that likely aid in impeding its identification in field samples of sea ice. Our solubility measurements display excellent agreement with that of the FREZCHEM model, which was therefore used to supplement our measurements to colder temperatures. Measured and modelled solubility data were incorporated into a 1D model for the growth of first-year Arctic sea ice. Model results ultimately suggest that mirabilite has a near ubiquitous presence in much of the sea ice on Earth, and illustrate the spatial and temporal evolution of mirabilite within sea ice as it grows throughout an Arctic winter, reaching maximum concentrations of 2.3 g kg−1.