The divergence of total alkalinity (TA) from conservation with salinity (S) and relatively acidic conditions (pH) in surface seawater was suggested to explain the high prevalence of lightning ...superbolts in the Mediterranean sea, North sea and upwelling regions of the oceans. In this study we tested the combined effects of changes in S, TA and pH of Mediterranean sea surface water on the intensity of laboratory generated electrical sparks, which are considered to be analogous to cloud to sea-surface intensity of lightning discharges. The experimental results were used to develop a multivariate linear equation (MLE) of Lightning Flash Intensity (LFI) as a function of S, TA/S and pH. This relation was validated with wintertime (DJF) LFI measurements along a Mediterranean sea zonal profile during the period 2009-2020 compared to corresponding climate model outputs of S, TA and pH. Based on the resulting MLE, the combined effects of climate change, ocean acidification and the damming of the Nile, may have increased LFI in the Levantine Sea by 16 ± 14% until now relative to the pre-Aswan Dam period. Furthermore, assuming that salinization and acidification of the Levantine Sea will continue at current trends, the LFI is predicted to increase by 25 ± 13% by the year 2050.
Global warming mediates and maintains the tropicalization of temperate marine ecosystems. Recent studies have demonstrated that this process causes shifts from algal forests to denuded non‐canopy ...states in temperate reefs. It has been suggested that these changes would incur significant consequences to ecosystem functioning.
In this study, we tested how tropicalization affects habitat‐provisioning functions and carbon turnover of a shallow reef in the fast‐warming and highly invaded southeastern Mediterranean Sea, in‐situ. On a single shallow reef, we conducted measurements of these functions in three habitats: dwindling native brown algal (Cystoseira) forest, dominant turf (formed by overgrazing of tropical rabbitfish) and expanding tropical shrubs dominated by red calcifying algae (Galaxaura rugosa).
Algal forest was an autotrophic net carbon sink and provided habitat for high species diversity and the largest community biomass. The denuded turf was heterotrophic and provided habitat for the lowest species diversity and community biomass. While diversity was as high in tropical shrubs as in the algal forest, it had lower biomass and functioned as a heterotrophic net carbon source.
Synthesis. Our study exemplifies possible functional consequences of tropicalization‐driven regime shifts on shallow temperate rocky reefs, and how these can invert the net trophic state and carbon balance.
Our study exemplifies possible functional consequences of tropicalization‐driven regime shifts, and how these can invert the net trophic state and carbon balance of shallow temperate rocky reefs. This figure shows the O2 and DIC fluxes driven by primary production, respiration, calcification and CaCO3 dissolution of a native algal forest and tropicalized turf and shrubs. Only the algal forest was in an autotrophic carbon sink balance, while the turf and tropical shrubs were in a heterotrophic carbon neutral and source balances, respectively.
Calcification rates in stony corals are expected to decline significantly in the near future due to ocean acidification. In this study we provide a global estimate of the decline in calcification of ...coral reefs as a result of increase in sea surface temperature and partial pressure of CO2. This estimate, unlike previously reported estimates, is based on an empirical rate law developed from field observations for gross community calcification as a function of aragonite degree of saturation (Ωarag), sea surface temperature and live coral cover. Calcification rates were calculated for more than 9,000 reef locations using model values of Ωarag and sea surface temperature at different levels of atmospheric CO2. The maps we produced show that by the time atmospheric partial pressure of CO2 will reach 560 ppm all coral reefs will cease to grow and start to dissolve.
The anthropogenic increase in atmospheric CO
is not only considered to drive global warming, but also ocean acidification. Previous studies have shown that acidification will affect many aspects of ...biogenic carbon uptake and release in the surface water of the oceans. In this report we present a potential novel impact of acidification on the flash intensity of lightning discharged into the oceans. Our experimental results show that a decrease in ocean pH corresponding to the predicted increase in atmospheric CO
according to the IPCC RCP 8.5 worst case emission scenario, may increase the intensity of lightning discharged into seawater by approximately 30 ± 7% by the end of the twenty-first century relative to 2000.
Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion ...concentration (CO3(2-)), and saturation state of carbonate minerals (Ω). This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and corals. Laboratory and field studies have shown that calcification rates of many organisms decrease with declining pH, CO3(2-), and Ω. Coral reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms. Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century. While retrospective studies show large-scale declines in coral, and community, calcification over recent decades, determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a coral reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.
Experimental and field evidence support the assumption that global warming and ocean acidification is decreasing rates of calcification in the oceans. Local measurements of coral growth rates in ...reefs from various locations have suggested a decline of ~6-10% per decade since the late 1990's. Here, by measuring open water strontium-to-alkalinity ratios along the Red Sea, we show that the net contribution of hermatypic corals to the CaCO
budget of the southern and central Red Sea declined by ~100% between 1998 and 2015 and remained low between 2015 and 2018. Measured differences in total alkalinity of the Red Sea surface water indicate a 26 ± 16% decline in total CaCO
deposition rates along the basin. These findings suggest that coral reefs of the southern Red Sea are under severe stress and demonstrate the strength of geochemical measurements as cost-effective indicators for calcification trends on regional scales.
In this study we investigated the relations between community calcification of an entire coral reef in the northern Red Sea and annual changes in temperature, aragonite saturation and nutrient ...loading over a two year period. Summer (April–October) and winter (November–March) average calcification rates varied between 60 ± 20 and 30 ± 20 mmol·m−2·d−1, respectively. In general, calcification increased with temperature and aragonite saturation state of reef water with an apparent effect of nutrients, which is in agreement with most laboratory studies and in situ measurements of single coral growth rates. The calcification rates we measured in the reef correlated remarkably well with precipitation rates of inorganic aragonite calculated for the same temperature and degree of saturation ranges using empirical equations from the literature. This is a very significant finding considering that only a minute portion of reef calcification is inorganic. Hence, these relations could be used to predict the response of coral reefs to ocean acidification and warming.
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•Long-term (∼40 y) record of deep-sea sharks denotes mercury accumulation.•Anthropogenic Hg accumulation is enhanced in oligotrophic deep-sea sharks.•Long-living deep-sea species show ...a temporal increase in THg levels.•THg is expected to further increase in deep marginal seas sharks.•Consumption of deep-sea sharks is potentially a high risk to human health.
Deep-sea habitats are currently recognized as a hot spot for mercury (Hg) accumulation from anthropogenic sources, resulting in elevated concentrations of total mercury (THg) in deep-sea megafauna. Among them, deep-sea sharks (Class Chondrichthyes) are characterized by high trophic position and extended longevity and are, therefore, at high risk for mercury contamination. Despite this, sharks are overexploited by fishing activity in increasingly deeper water, worldwide, imposing health risks to human consumption. While it is imperative to better understand long-term mercury contamination in deep-sea megafauna, few historical data sets exist to capture this process. Here we explore four decades (1985–2022) of THg accumulation in five species of deep-sea sharks (G. melastomus, E. spinax, S. rostratus, C. granulosus, and D. licha) of the ultra-oligotrophic Southeastern Mediterranean Sea (SEMS) sampled during 19 research cruises. We exhibited exceptionally high THg levels (per length/weight), the highest as 16.6 μg g−1 (wet wt.), almost entirely (98.9 %; n = 298 specimens) exceeding the limit for safe consumption (0.3–0.5 μg THg g−1 wet wt.). The maximal THg levels of the long-lived species D. licha and C. granulosus in the SEMS were enriched by a factor of ∼ 7 and >10 compared to counterpart species from other oceanic areas, respectively. We attribute this to the ultra-oligotrophic conditions of the SEMS, which cause slower growth rates and dwarfism in deep-sea sharks, resulting in an extended exposure time to mercury contamination. In the long-lived species, C. granulosus and D. licha, a temporal increase of average THg levels of ∼ 80 % was recorded between 1987–1999 and 2021–2022. This likely reflects the long-term accumulation of historical anthropogenic Hg in deep-sea environments, which is further amplified in marginal seas such as the Mediterranean, impacted by global air pollution crossroads and surrounded by land-based pollution sources. Future consumption of products from deep-sea sharks is potentially high risk to human health.
Abstract
The oceans play a major role in the earth’s climate by regulating atmospheric CO
2
. While oceanic primary productivity and organic carbon burial sequesters CO
2
from the atmosphere, ...precipitation of CaCO
3
in the sea returns CO
2
to the atmosphere. Abiotic CaCO
3
precipitation in the form of aragonite is potentially an important feedback mechanism for the global carbon cycle, but this process has not been fully quantified. In a sediment-trap study conducted in the southeastern Mediterranean Sea, one of the fastest warming and most oligotrophic regions in the ocean, we quantify for the first time the flux of inorganic aragonite in the water column. We show that this process is kinetically induced by the warming of surface water and prolonged stratification resulting in a high aragonite saturation state (Ω
Ar
≥ 4). Based on these relations, we estimate that abiotic aragonite calcification may account for 15 ± 3% of the previously reported CO
2
efflux from the sea surface to the atmosphere in the southeastern Mediterranean. Modelled predictions of sea surface temperature and Ω
Ar
suggest that this process may weaken in the future ocean, resulting in increased alkalinity and buffering capacity of atmospheric CO
2
.
Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, ...and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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