Global Biogeochemical Cycles最新文献

The biological carbon pump sequesters carbon through passive fluxes of biologically derived carbon, and by active vertical movement of marine organisms. Trophic coupling between pelagic and benthic communities increases the efficiency of the biological carbon pump as less carbon is lost to remineralization. Such fish-mediated benthic-pelagic coupling, which can be described as the sum of carbon fluxes that is “passed on” when predators eat prey that occupy different vertical habitats in the water column, remains highly uncertain. Here, we applied a size- and trait-based food web model to estimate the amount of carbon that fish actively transport through benthic-pelagic coupling to the seafloor across the shelf-slope-abyssal continuum in different systems of the North Atlantic. The model estimates that benthic-pelagic coupling transports on average 813 kg C km⁻² yr⁻¹ to the demersal fish communities in North Atlantic shelf-slope-abyssal systems, which is equivalent to 5% of the modeled detritus flux reaching the sea floor. In some slopes, midwater fishes mediate up to 50% of the carbon transported downwards via benthic-pelagic fish coupling. We validated model-estimated biomasses of demersal fishes with biomass estimates of bottom trawl-surveys in the same area. Both modeling and survey approaches show that demersal fish biomass estimates are at the same order of magnitude and decrease with bottom depth following a similar trend. Our study shows that benthic-pelagic coupling is an important mechanism transporting carbon to demersal communities, supplying energy to sustain abundant seafloor fish fauna and fueling commercially valuable fisheries.

Northern peatlands (>50°N) account for approximately 70% of the global peatland area and play a key role in the global carbon cycle. However, their role as long-term carbon sinks is vulnerable to modern climate warming at high latitudes. Future climate predictions require data on how peatlands respond to observed changes in global environmental parameters, particularly in the Northern Hemisphere taiga zones. Here, we compared CO₂ net ecosystem exchange (NEE), ecosystem respiration (R_eco), gross primary production (GPP), and their responses to changes in environmental parameters using direct Eddy covariance measurements in four representative peatlands: two southern and two middle taiga sites located in European Russia and West Siberia, respectively. Three sites were ombrotrophic bogs, and one was a transitional (between bog and fen) mesotrophic peatland. All studied peatlands functioned as CO₂ sinks either annually or during the growing season. The largest net CO₂ uptake was detected in the mesotrophic peatland in the middle taiga of European Russia. In the ombrotrophic bogs, elevated air temperatures (>25°C) and vapor pressure deficits (>2 kPa) negatively impacted GPP in the summer; however, at the mesotrophic peatland, these conditions corresponded to the highest GPP observed during the measurement period. Additionally, a reduction in net CO₂ uptake was detected at the mesotrophic site during the anomalously wet summer. The study findings suggest that the differences in CO₂ exchange processes and their responses to ambient conditions in ombrotrophic bogs and transitional peatland types are important to consider in modelling studies and flux predictions.

Tropical peatlands contain around one-sixth of the global peat carbon stock. Decomposition is a key determinant of tropical peat persistence, but there is a scarcity of data on decomposition in tropical peatlands. To further understand decomposition in tropical peatlands, we conducted an 8-year field experiment in a primary peat swamp forest in Brunei. We tracked mass loss and the organic matter composition of Shorea albida wood buried at multiple depths over 8 years, including blocks buried with and without termite exclusion mesh. The proportion of time wood blocks spent above the water table explained the majority of the variation in wood decomposition over time. Carbon loss from wood that spent <1% of the time under the water table was 32.1%–86.5% higher on average than from wood that spent 30%–100% of the time under the water table. We estimate that termites enhanced wood decomposition by ∼2% per year. Despite significant decomposition, we did not observe a strong shift in wood organic matter composition. To contextualize our results, we synthesized past work on wood decomposition across tropical peatlands. We found that burial in waterlogged peat soils slows decomposition across tropical peatlands and that decomposition is also strongly influenced by peatland trophic status. Overall, our results affirm that waterlogging is the key to tropical peat persistence. Our study highlights the vulnerability of tropical peat carbon stocks to lowered water tables by either drainage or prolonged dry spells, as well as the promise of peatland rewetting to mitigate carbon losses from disturbed peatlands.

Marine primary production in the subtropical oceans is strongly regulated by (micro)nutrient availability, yet the types of nutrient stress in the South Pacific remain poorly resolved. Here we assessed this along a >10,000-km transect of the subtropical South Pacific Ocean (GEOTRACES Section GP21). The transect was separated into three regimes in terms of phytoplankton photophysiology: a heterogeneous coastal margin, an eastern gyre boundary transition zone, and the oligotrophic subtropical gyre. The transition zone exhibited the lowest surface apparent photochemical efficiency (F_v/F_m; 0.09–0.26) and significant responses to experimental supply of both iron (Fe) and combined nitrogen (N) and Fe (with differences relative to controls, ΔF_v/F_m, of 0.07–0.11) but depressions to N supply alone (ΔF_v/F_m of −0.03 to −0.07), diagnosing this zone as Fe stressed with low N availability. The offshore coastal margin showed intermediate surface F_v/F_m (0.21–0.41) that increased after N addition but not Fe alone, suggesting prevalent N limitation and no Fe stress. In the nutrient-depleted gyre, surface F_v/F_m was elevated (mean ± SD; 0.42 ± 0.07, n = 41), remained unchanged following any nutrient addition, and showed dawn/dusk peaks with relatively small nocturnal declines (∼33%), consistent with the absence of Fe stress and steady-state N limitation. Basin-wide, nitrate to dissolved Fe ratios best predicted surface F_v/F_m and thereby Fe stress status (R² = 0.54). Additional observations and experiments suggested a basin-wide absence of Fe stress at the deep chlorophyll maximum. Such findings are important for predicting ecosystem responses to climate-driven shifts in nutrient supply.

Methylated mercury (MeHg), including dimethylmercury and monomethylmercury (MMHg), is a pollutant of concern because it biomagnifies in marine biota. The formation of MeHg in the oceans, specifically at highly productive regions and at high oxygen levels, remains elusive. We investigated dissolved and particulate total (THg) and MeHg in the water column and sediments at six stations in a highly productive area of the Southern Atlantic Ocean. Total MeHg concentrations and proportions of THg in seawater were higher (50%–73%) at eutrophic stations. We found that the distribution of MeHg in the mixed layer is strongly controlled by the biological pump. Concentrations of THg and MMHg were highest in particles within the chlorophyll maximum, suggesting THg and MMHg scavenging or assimilation by phytoplankton and an oxic pathway of MMHg formation. Particle breakdown and respiration appeared to increase dissolved MeHg concentrations and MMHg concentrations in small particles (2–51 μm) at greater depths. MMHg concentrations in the sediments were consistently lower than in particles from the mixed layer, indicating MMHg release during particle sinking and that deep-sea sediments are unlikely to be an important source of MeHg in the water phase. Our study identifies productive marine areas as hotspots of MeHg formation and suggests increasing Hg methylation with increasing ocean eutrophication and may amplify biomagnification in marine food webs.

Highly productive summer phytoplankton blooms in the central North Pacific Subtropical Gyre (NPSG) are an annual occurrence that leads to the export of considerable amounts of surface particulate carbon to depth. The mechanisms that control the formation of these blooms remain unresolved, but iron (Fe) availability may be an important factor. From July to October 2022, a large, persistent phytoplankton bloom was detected near 23.3°N, 154.6°W in satellite imagery and in situ measurements. Elevated Fe concentrations and nitrogen (N₂) fixation activity measured within the bloom suggest that high Fe may have supported enhanced diazotrophic activity. To evaluate whether aerosol deposition created favorable conditions for bloom formation, we reconstructed the Fe deposition history of the bloom's source waters by integrating surface water back trajectory analyses with aerosol Fe flux simulations. Our results show that waters that hosted the diatom-diazotroph assemblage bloom received up to 20% more soluble Fe through aerosol deposition than its surrounding waters, primarily from a strong wet deposition event that occurred approximately 1 month before the bloom. The observed lag between deposition and bloom suggests a delayed biological response to atmospheric Fe inputs. Although this moderate increase does not represent incontrovertible evidence that the bloom was stimulated by aerosol Fe deposition, our findings establish the potential for episodic delivery of atmospheric Fe to stimulate diazotrophic activity and phytoplankton growth over month-long timescales in the NPSG.

The Southern Ocean south of 35°S represents a small source of natural inorganic carbon for the atmosphere but a major sink of anthropogenic carbon. The magnitude of the inorganic carbon sink, and the sequestration of inorganic and organic carbon strongly depend on the rate at which they are subducted below the mixed layer. We use a global ocean model at 0.25° resolution to quantify the drivers of the pathways of total and anthropogenic dissolved inorganic carbon (DIC) and organic carbon (OC) across and within the time-varying mixed layer of five physically consistent regions of the Southern Ocean over the period 1995–2014. Total DIC is brought into the mixed layer through obduction south of the Antarctic Circumpolar Current (ACC) and subducted north of the ACC, resulting in a net obduction of 11.2 PgC/year, with advective processes being responsible for about two-thirds of the total transfer. Anthropogenic carbon is brought to the mixed layer through the ocean surface in all regions but mainly subducted north of the ACC, with the subduction (1.05 PgC/year) being achieved through both advection and diffusion, each dominating respectively north and south of the Subantarctic Front. Two thirds of the organic carbon are subducted through the gravitational pump (1.9 PgC/year) and one-third through physical transfer (0.9 PgC/year), with an equivalent contribution from advection and diffusion. At the local scale, advective fluxes largely dominate other physical processes in transferring carbon across the base of the mixed layer, and are found to be increased near topographic features and boundary currents.

Anthropogenic nitrogen (N) deposition is increasing globally and has been documented to enhance soil carbon (C) storage; however, its concurrent effects on ecosystem phosphorus (P) limitation remain unclear. By conducting a meta-analysis of 360 observations from 63 field N addition experiments in forest, grassland, and cropland ecosystems, we systematically assessed the consequence of terrestrial ecosystem P limitation under increasing N deposition. Our results demonstrate that N deposition significantly increased soil N:P, plant C:P and N:P ratios by 12%–29%, suggesting intensified P limitation across terrestrial ecosystems. Critically, N deposition induced differential responses between plants and soil microorganisms, with plants experiencing more severe P limitation. Notably, ectomycorrhizal (ECM) symbiosis is useful for alleviating P limitation in plants, whereas arbuscular mycorrhizal (AM) symbiosis is more useful for microorganisms in this context than for plants. Furthermore, the magnitude of N-induced P limitation varied substantially across ecosystems, with particularly strong effects observed in croplands compared with forests and grasslands. This discrepancy may be attributed to the higher dependence of cultivated crops on P for achieving rapid growth under intensive breeding conditions. The response of C:P and N:P ratios in soils and plants negatively correlated with soil pH changes but was significant only in AM-dominated ecosystems. This suggests that the former is more sensitive to N-induced pH shifts than ECM-associated ecosystems. Our findings demonstrate that mycorrhizal types mediate the ecosystem trajectory of N-induced P limitation, highlighting the critical role of plant-microbial interactions in mitigating the impacts of increasing N deposition and climate change.

The stability of dissolved organic matter (DOM) significantly influences the regional and global carbon budget balances. However, current studies on DOM have largely overlooked the winter–spring season in temperate inland waters of the Northern Hemisphere, a critical transitional period characterized by low biological productivity, low temperatures. Here, we selected three representative inland aquatic ecosystems in China, rivers, lakes and ponds, to elucidate the driving mechanisms of environmental and intrinsic properties on the biodegradability (BDOC) of DOM. Results indicated that dissolved organic carbon (DOC) concentrations in rivers, lakes and ponds did not differ significantly with means of 7.52, 8.21 and 10.71 mg L⁻¹, respectively. BDOC was highest in rivers (44.53%), followed by lakes (37.58%), and lowest in ponds (33.71%). We first observed that the physicochemical and DOM properties of rivers, lakes and ponds exhibited homogeneity during winter–spring season. Humic-like substances were identified as the primary components of DOM in these aquatic ecosystems during this period. DOC and BDOC were strongly influenced by oxidation reduction potential, electrical conductivity, total nitrogen, total phosphorus, ammonium nitrogen, chlorophyll a, spectral slope, specific ultraviolet absorbance at 254 nm, and humification index. Although geographical (longitude and latitude), climatic (temperature and precipitation), and anthropogenic factors (population and gross domestic product) also exerted effects on DOC and BDOC, their influence was relatively weak. Environmental and intrinsic properties jointly determined the homogenization of DOM in inland waters during winter–spring season. These findings have important implications for understanding the effects of both environmental and intrinsic properties on DOM at a geoclimatic scale.

Predicting dissolved organic carbon (DOC) mineralization and removal rates across the aquatic continuum is vital for addressing questions relating to carbon cycling, ecosystem functioning, contaminant transport and drinking water safety. Previous research has shown a decline in DOC reactivity with increasing water retention time (WRT), implying reduced processing rates from headwaters to the coast. However, these findings were largely based on bioassays and lake data, which may not reflect real-world conditions across the full aquatic continuum. Using an expanded field-based data set and a statistical model comparison exercise, we found evidence for a more rapid decline in DOC reactivity with WRT than previously reported. Headwaters may therefore act as even stronger DOC processing hotspots than previously recognized. We present updated equations for predicting DOC removal as a function of WRT, which should replace existing formulations in modeling studies to avoid underestimating removal, particularly in headwaters. In a boreal case study, for example, updated equations predict nearly 40% higher DOC mineralization across the aquatic continuum than previous formulations. In addition, we recommend a steady-state Vollenweider approach for simulating DOC transmission in open systems such as lakes, rather than the commonly used exponential decay model, which assumes closed-system dynamics. Nonetheless, large residual variance highlights the limitations of these simple models. Future efforts should focus on developing more nuanced approaches that better capture the complexity of DOC dynamics across diverse aquatic environments.