Annual Primary Production Research Articles

Inherent carbon burial potential of lakes across China

Eutrophication is one of the most important factors for the increasing production of organic carbon in inland waters. Variations in primary productivity across lakes due to eutrophication complicate predictions regarding future carbon burial and management of carbon sequestration in lakes. Phosphorus, an essential nutrient for plants and notorious for eutrophication, often limits primary productivity in lakes. It is crucial to normalize organic carbon burial to the content of total phosphorus (TP) in the water column to compare the inherent carbon burial capacity across lakes. However, the absence of long-term observational data of TP in lake water columns has hindered the ability to do so. In this study, we first verified that TP in sediments of Chinese lakes could substitute for that in the water column to reflect the annual primary productivity of lakes. Then we defined the inherent carbon burial potential of lakes as the total organic carbon to TP atomic ratio (TOC/TP) in the sediments. We quantified the inherent carbon burial potential of 90 lakes across China, exploring its spatial distribution, temporal trends, and potential controlling factors. The inherent carbon burial potential, based on surface sediments, decreased from western to eastern China, ranging from 466.1 to 24.3 with a mean of 137.0 ± 87.4. Mean annual precipitation was negatively related, while mean lake depth was positively related to the inherent carbon burial potential of the lakes across China. Analysis of sediment cores revealed that the mean inherent carbon burial potential of the lakes across China increased from 75.1 to 122.9 since the 1950s. Inherent carbon burial potential of the lakes in southwestern and eastern China, however, decreased since the 1990s. Changes in the extent of cultivated land in the watersheds were positively related to variations in the inherent carbon burial potential of Chinese lakes in the past few decades.

Gross Primary Production of Antarctic Landfast Sea Ice: A Model‐Based Estimate

AbstractMuch of the Antarctic coast is covered by seasonal landfast sea ice (fast ice), which serves as an important habitat for ice algae. Fast‐ice algae provide a key early season food source for pelagic and benthic food webs, and contribute to biogeochemical cycling in Antarctic coastal ecosystems. Summertime fast ice is undergoing a decline, leading to more seasonal fast ice with unknown impacts on interconnected Earth system processes. Our understanding of the spatiotemporal variability of Antarctic fast ice, and its impact on polar ecosystems is currently limited. Evaluating the overall productivity of fast‐ice algae has historically been hampered by limitations in observations and models. By linking new fast‐ice extent maps with a one‐dimensional sea‐ice biogeochemical model, we provide the first estimate of the spatio‐seasonal variability of Antarctic fast‐ice algal gross primary production (GPP) and its annual primary production on a circum‐Antarctic scale. Experiments conducted for the 2005–2006 season provide a mean fast ice‐algal production estimate of 2.8 Tg C/y. This estimate represents about 12% of overall Southern Ocean sea‐ice algae production (estimated in a previous study), with the mean fast‐ice algal production per area being 3.3 times higher than that of pack ice. Our Antarctic fast‐ice GPP estimates are probably underestimated in the Ross Sea and Weddell Sea sectors because the sub‐ice platelet layer habitats and their high biomass are not considered.

Anomalous 2022 deep-water formation and intense phytoplankton bloom in the Cretan area

Abstract. The Mediterranean Sea is a quasi-permanently stratified and oligotrophic basin with intense late-winter and early-spring phytoplankton blooms typically limited to few regions (i.e. northwestern Mediterranean Sea, the southern Adriatic Sea, and the Rhodes Gyre). In these areas, blooms are sustained by nutrient injection to surface layers by winter vertical mixing and convective processes. A markedly intense bloom was predicted in spring 2022 in an unusual area of the southeastern Mediterranean Sea (i.e. southeast of Crete) by the Mediterranean Sea Copernicus Marine Forecasting Centre (MED MFC) system. Combining Copernicus modelling and observation products, the 2022 event and a number of driving and concurrent features have been investigated in a multidisciplinary way. A noticeable cold spell that occurred in Eastern Europe at the beginning of 2022 has been identified as the main driver of an intense deep-water formation event, with associated high nutrient concentrations in the surface layers. Consequently, an extreme phytoplankton bloom that was 50 % more intense than usual occurred in the area southeast of Crete, starting nearly 1 month later than usual and lasting for 3–4 weeks. Impacts on primary production were also relevant in the 2022 event area and were 35 % higher than the climatological annual primary production. Furthermore, the documented link between primary productivity and fishery catches suggests possible consequences along the whole food chain up to the marine ecosystem in the eastern Mediterranean Sea.

Quantification of biomass availability for wood harvesting and storage in the continental United States with a carbon cycle model

BackgroundWood Harvesting and Storage (WHS) is a form of Biomass Carbon Removal and Storage (BiCRS) that utilizes a combined natural and engineered process to harvest woody biomass and put it into long term storage, most frequently in the form of subterranean burial. This paper aims to quantify the availability of woody biomass for the purposes of WHS in the continental United States using a carbon cycle modeling approach. Using a regional version of the VEGAS terrestrial carbon cycle model at 10 km resolution, this paper calculates the annual woody net primary production in the continental United States. It then applies a series of constraints to exclude woody biomass that is unavailable for WHS. These constraints include fine woody biomass, current land use, current wood utilization, land conservation, and topographical limitations. These results were then split into state by state and regional totals.ResultsIn total, the model projects the continental United States could produce 1,274 MtCO2e (CO2 equivalent) worth of coarse woody biomass annually in a scenario with no anthropogenic land use or constraints. In a scenario with anthropogenic land use and constraints on wood availability, the model projects that 415 MtCO2e of coarse woody biomass is available for WHS annually. This is enough to offset 8.5% of the United States’ 2020 greenhouse gas emissions. Of this potential, 20 MtCO2e is from the Pacific region, 77 MtCO2e is from the Western Interior, 91 MtCO2e is from the Northeast region, and 228 MtCO2e is from the Southeast region.ConclusionThere is enough coarse woody biomass available in the continental United States to make WHS a viable form of carbon removal and storage in the country. There is coarse woody biomass available across the continental United States. All four primary regions analyzed have enough coarse woody biomass available to justify investment in WHS projects.

Reduced productivity and carbon drawdown of tropical forests from ground-level ozone exposure

AbstractElevated ground-level ozone, a result of human activity, is known to reduce plant productivity, but its influence on tropical forests remains unclear. Here we estimate how increased ozone exposure has affected tropical-forest productivity and the global carbon cycle. We experimentally measure the ozone susceptibility of various tropical tree species, and then incorporate these data into a dynamic global vegetation model. We find that current anthropogenic-derived ozone results in a substantial decline in annual net primary productivity (NPP) across all tropical forests, with some areas being particularly impacted. For example, Asia sees losses of 10.9% (7.2–19.7%) NPP. We calculate that this productivity decline has resulted in a cumulative loss in carbon drawdown of 0.29 PgC per year since 2000, equating to ~17% of the tropical contemporary annual land carbon sink in the twenty-first century. We also find that areas of current and future forest restoration are disproportionately affected by elevated ozone. Future socioeconomic pathways that reduce ozone formation in the tropics will incur benefits to the global carbon budget by relieving the current ozone impacts seen across both intact forest and areas of forest restoration, which are critical terrestrial regions for mitigation of rising atmospheric carbon dioxide.

Increasing drought sensitivity of plant photosynthetic phenology and physiology

Understanding the impacts of drought on plant photosynthetic phenology and physiology is essential for accurately predicting annual gross primary productivity (GPP) and global carbon cycles. While droughts in different seasons can have different effects on plant growth and photosynthesis, a comprehensive understanding of how plant photosynthetic phenology and physiology respond to different seasons’ droughts and their temporal changes is lacking. Here, we comprehensively evaluated the drought sensitivities of plant photosynthetic phenology and physiology and their temporal trends in the past two decades, using the start/end of the photosynthetic seasons (SOS/EOS) and peak GPP (GPPmax) derived from FLUXNET ground observations since the 1990s and remote-sensing data (2001–2020) over the globe. Our results show that spring droughts leads to delayed SOS in arid ecosystems and advanced SOS in humid ecosystems. In contrast, droughts in summer and autumn generally advance EOS. In addition, spring droughts decrease GPPmax in arid regions but increase it in humid regions, whereas summer droughts lead to significant GPPmax reductions. More importantly, we observed significant increases in the drought sensitivity of SOS, EOS, and GPPmax over the last two decades, which is driven by long-term decreases in background soil moisture and increases in atmospheric water deficit under climate change. Furthermore, these increasing trends were projected to persist in the 21st century under Shared Socioeconomic Pathways 8.5 and 4.0. Overall, our findings underscore the critical role of drought timing in shaping ecosystem responses and highlight the growing vulnerability of ecosystem phenology and physiology to drought, which may restrict the increases in terrestrial carbon uptake under warming.

Better representation of vegetation phenology improves estimations of annual gross primary productivity

Carbon uptake by vegetation plays a vital role in the global carbon cycle. Annual gross primary productivity (AGPP) represents the total amount of carbon compounds produced by vegetation photosynthesis over a year and is a crucial metric for quantifying vegetation carbon uptake. Although some theories have been developed to explain the spatiotemporal variation in AGPP, they often overlook the seasonal differences in photosynthesis across vegetation phenological periods. This gap highlights the need for a more reasonable representation of vegetation phenology in AGPP modelling. Therefore, we developed a novel theoretical AGPP model that decomposes AGPP into detailed vegetation phenological periods (green-up, maturation, and senescence) and investigated the effects of climatic factors on the seasonal components of AGPP. Compared with existing models, our model considers the length of the multiple vegetation phenological periods, rather than just the length of the carbon uptake period. When evaluated against flux tower data, our model outperformed the comparative model, with a higher determination coefficient and lower root mean square error. The analysis also demonstrated that the developed model can reproduce spatial and temporal variations in satellite-based global AGPP. In addition, we identified distinct differences in the responses of AGPP's seasonal components to climatic factors: shortwave radiation predominantly affected the seasonal component of AGPP during senescence, air temperature during green-up, and vapor pressure deficit during maturation. Our study proposes a potential mechanism for AGPP estimation and highlights the importance of accurately representing vegetation phenology in AGPP modelling.

Photoperiodic dependent regulation of photosynthesis in the polar diatom Fragilariopsis cylindrus

Introduction: Polar microalgae are exposed to dramatic seasonal changes in light availability, from continuous summer days to winter nights with rapid changes of the daylength in spring and fall. Under this challenging light climate, large diatoms spring blooms occur at the bottom sea-ice and underneath the icepack, accounting for a significant proportion of the annual marine primary production in the Arctic Ocean. The on-going earlier melt down of the snow and ice covers result in a stronger light penetration and consequent increase in irradiance at the bottom of the sea ice leading to earlier seasonal sea-ice diatom blooms under shorter daylengths. Therefore, elucidating the response of polar diatoms to different photoperiods will help to better understand the consequences of the changing arctic climate on their photosynthetic productivity.Methods: In this study, we characterized the response of F. cylindrus, a model polar diatom, across five different photoperiods with similar light and temperature conditions (30 μmol photons m-2 s-1 and 0°C respectively).Results: We report different photoacclimative strategies under shorter and longer daylengths, with the special case of prolonged darkness (mimicking winter polar night). We also observed a repeated daily regulation of the photochemistry and photoprotection parameters when cells were exposed to a light:darkness alternation, despite the constant and optimal light intensity during the light periods.Discussion: Our results highlight the ability of F. cylindrus to grow efficiently under a wide range of daylengths, finely adjusting the balance between photochemistry and photoprotection to make the best use of the available light, supporting sustained production and growth despite low light and temperature.

Satellite Long-Term Monitoring of Wetland Ecosystem Functioning in Ramsar Sites for Their Sustainable Management

The long-term monitoring of wetland ecosystem functioning is critical because wetlands, which provide multiple services, can be affected by human activities and climate change. The aim of this study was to monitor wetland ecosystem functioning in the long term using the Landsat archive. Four contrasting, Ramsar wetlands were selected in boreal, temperate, arid, and tropical areas. First, the annual sum of the normalized difference vegetation index (NDVI-I) was calculated as an indicator of annual net primary productivity for the period 1984–2021 using the continuous change detection and classification (CCDC) algorithm. Next, the influence of the number of Landsat images and class of land use and land cover (LULC) on the accuracy of the CCDC was investigated. Finally, correlations between annual NDVI-I and climate were analyzed. The results revealed that NDVI-I accuracy was influenced mainly by the LULC class and to a lesser extent by the number of cloud-free Landsat observations. Infra- and inter-site variations in NDVI-I were high and showed an overall increasing trend. NDVI-I was positively correlated with the mean temperature. This study shows that this approach applied in contrasting sites is robust for the long-term monitoring of wetland ecosystem functioning and can be used to improve the implementation of international biodiversity conservation policies.

Estimating the global root exudate carbon flux

Root exudation, the export of low-molecular weight organic carbon (C) from living plant roots to soil, influences microbial activity, nutrient availability, and ecosystem feedbacks to climate change, but the magnitude of this C flux at ecosystem and global scales is largely unknown. Here, we synthesize in situ measurements of root exudation rates and couple those to estimates of fine root biomass to estimate global and biome-level root exudate C fluxes. We estimate a global root exudate flux of 13.4 (10.1–20.2) Pg C y−1, or about 9% (7–14%) of global annual gross primary productivity. We did not find differences in root mass-specific exudation rates among biomes, though total exudate fluxes are estimated to be greatest in grasslands owing to their high density of absorptive root biomass. Our synthesis highlights the global importance of root exudates in the terrestrial C cycle and identifies regions where more in situ measurements are needed to improve future estimates of root exudate C fluxes.

An improved canopy interception scheme into biogeochemical model for precise simulation of carbon and water fluxes in subtropical coniferous forest

The process-based Biome-BGC (Biome Biogeochemical Cycles) model is widely used to simulate the fluxes of carbon and water of terrestrial ecosystems. While exhibiting excellent performance in simulating carbon cycle processes, this model provides a relatively simple simulation of the water cycle processes, particularly in canopy interception, which may result in significant errors in evapotranspiration (ET) and soil moisture estimations. This study initially refined the temporal scale of the original Biome-BGC model from a daily scale to an hourly scale, and then incorporated a theoretical interception module into the hourly-scale model for correcting canopy interception algorithm, to enhance the accuracy of hydrological processes simulations. The modified model was tested using the half-hourly observed meteorological and eddy covariance flux data at the Qianyanzhou subtropical coniferous forest site. In comparison with the original daily-scale model, simulations from the hourly-scale model showed better agreement with measurements at Qianyanzhou forest site, with 30.61 % and 40.92 % lower annual average gross primary productivity (GPP) and ET simulations. Although the accuracy of GPP and ET simulations did not show a significant improvement after canopy interception correction, it notably enhanced the accuracy of canopy interception and soil moisture simulations. The canopy interception rates were 55.8 % and 44.1 % for the original daily-scale and hourly-scale model, obviously overstimated compared with canopy correction simulation (31.1 %), leading to a significant underestimation of runoff. The canopy interception and runoff simulations after interception correction were proven relatively more reasonable. Additionally, the coefficient of determination (R2) for measured vs simulated soil water content (SWC) were 0.38, 0.51 and 0.60 for the original daily-scale, hourly-scale, and canopy interception correction simulations respectively, with a notable improvement in accuracy. This study suggested that improvements in temporal scale and canopy interception for process-based biogeochemical model can provide a better understanding of the carbon and water exchanges in response to instantaneous meteorological conditions and support improved water resource management.

Dissecting the Characteristics and Driver Factors on Global Water Use Efficiency Using GLASS Data Sets

AbstractEcosystem water use efficiency (WUE) is a crucial parameter for understanding the interaction between carbon and water cycles. However, the spatio–temporal evolution and drivers of WUE remain unclear. This study utilized global annual scale global land surface satellite gross primary productivity and evapotranspiration data from 1982 to 2018 to estimate WUE and analyze its spatio–temporal characteristics. Additionally, the study investigated the response of WUE changes to five environmental factors (precipitation [PRE], soil moisture, temperature [TEM], palmer drought severity index, and vapor pressure deficit [VPD]) on WUE changes using partial correlation and structural equation modeling. The results suggested that the global annual WUE increased markedly over the study period, at an average rate of 0.0016 gC m−2 mm−1 H2O year−1. In contrast to the existing knowledge on the drivers of WUE change, climate change was found to have a larger contribution to WUE changes at the global and regional scales, especially in terms of TEM and VPD. A positive correlation between TEM and WUE was observed, but extreme TEM could lead to a decrease in WUE. VPD had the most significant direct effect on WUE, and its negative effect offset the positive influence of TEM especially in hyper‐arid, semi‐arid, and arid regions. These findings offer new insights into the impact of VPD and global warming on WUE.

Plankton Community Metabolism Variations in Two Temperate Coastal Waters of Contrasting Nutrient Richness

AbstractEstuarine ecosystems play a crucial role in global carbon cycling. Understanding the factors controlling plankton metabolism in these regions is critical. This study investigates how contrasting nutrient conditions influence plankton metabolism and carbon flow in two Danish estuaries, Roskilde Fjord (RF) (eutrophic) and the Great Belt (GB) (less eutrophic). Despite higher nutrient concentrations in RF, chlorophyll a and biomass only showed a moderate increase compared to the GB. Interestingly, metabolic rates (photosynthesis and respiration) in RF displayed greater temperature sensitivity, suggesting potential nutrient limitation effects in the GB. While both stations exhibited similar annual net primary production, RF's higher net community production highlights the importance of nutrient availability for carbon accumulation within the system. Additionally, the study observed significant seasonal variations in plankton metabolism and its impact on the carbon cycle. Notably, the more dynamic hydrography in the GB weakened correlations between biological and environmental factors.

Effects of Sheep Grazing and Nitrogen Addition on Dicotyledonous Seedling Abundance and Diversity in Alpine Meadows

Seedling is a crucial stage in the growth and development of plants, and the expansion and persistence of plant populations can be achieved through seed regeneration. Sheep grazing, fertilization, light, soil moisture, vegetation diversity and biomass, and litter all have potential impacts on species regeneration. We measured vegetation diversity, annual net primary productivity (ANPP), litter, ground photosynthetically active radiation (PAR), and soil moisture of alpine meadows under sheep grazing and nitrogen addition treatments, and studied their effects on the dicotyledonous seedling abundance and diversity using linear regression models (LMs) and structural equation models (SEMs). We found that sheep grazing reduced ANPP, increased vegetation diversity and PAR, and decreased soil moisture. Fertilization increased ANPP and litter, decreased vegetation diversity and PAR, but had no effect on soil moisture. Sheep grazing and fertilization both reduced the abundance of dicotyledonous seedlings, and simultaneously fertilization can reduce the diversity of dicotyledonous seedlings, while sheep grazing had no effect on the diversity of dicotyledonous seedlings. LMs showed that vegetation diversity, ANPP, and litter, rather than light and soil moisture, affected dicotyledonous seedling abundance and diversity. SEMs revealed that sheep grazing and fertilization indirectly influenced seedling regeneration through vegetation diversity rather than ANPP and litter. Our research will increase our understanding of the dicotyledonous plant regeneration process in alpine grasslands and facilitate the development of strategies for management and protection of alpine grassland.

Drivers of dune formation control ecosystem function and response to disturbance in a barrier island system.

Barrier islands are landscape features that protect coastlines by reducing wave energy and erosion. Quantifying vegetation-topographic interactions between adjacent habitats are essential for predicting long-term island response and resilience to sea-level rise and disturbance. To understand the effects of dune dynamics on adjacent interior island ecosystem processes, we quantified how sediment availability and previous disturbance regime interact with vegetation to influence dune building and ease of seawater and sediment movement into the island interior on two US mid-Atlantic coast barrier islands. We conducted field surveys of sediment accretion, vegetative cover, and soil characteristics in dune and swale habitats. Digital elevation models provided assessment of water flow resistance from the mean high water mark into the island interior. We found that geographic location impacted sediment accretion rates and Panicum amarum (a species increasing in abundance over time in the Virginia barrier islands) accreted sediment at a significantly lower rate compared to other dune grasses. Dune elevation impacted the ease of seawater flow into the island interior, altering soil chlorides, annual net primary productivity, and soil carbon and nitrogen. Our work demonstrates the importance of incorporating biological processes and cross-island connectivity into future scenario modeling and predictions of rising sea-levels and increased disturbance.

Recent changes of the dissolved oxygen distribution in the deep convection cell of the southern Adriatic Sea

The dynamics of dissolved oxygen in the ocean are of crucial importance for understanding marine ecosystems, with influences ranging from exchange with the atmosphere to biological processes and ocean circulation. In this study, we focus on the southern Adriatic Sea, an essential component of the Eastern Mediterranean “conveyor belt”, to investigate long-term oxygen dynamics and its driving factors. We use cross-platform datasets from 2013 to 2020, including remote sensing data, model reanalysis and in-situ observations from Argo floats, ocean gliders and ship-based measurements. Our analysis investigate the interplay of physical, biological and atmospheric forcing that drive oxygen variability. The distribution of dissolved oxygen in the southern Adriatic Sea is influenced by vertical mixing, advection of water masses and ecosystem dynamics. In the surface layer, the variability of dissolved oxygen is triggered by annual primary production and deep convection events. The dynamics in the intermediate and the deep layers are instead primarily influenced by physical processes, such as the vertical mixing and the water masses inflow from the adjacent sub-basins, which is driven by the periodic reversals of northern Ionian Gyre circulation. In particular our study reveals that the water masses advective dynamics driving the increase and decrease of dissolved oxygen have drastically changed in recent years. The highest dissolved oxygen concentrations are currently observed during the northern Ionian Gyre anticyclonic phase, while they have been previously documented during the cyclonic phase. This change appears to be connected with the significant increase in salinity observed in the southern Adriatic Sea in the same period and contributes to a better understanding of the processes that determine oxygen distribution in the Eastern Mediterranean basin.

Estimating global annual gross primary production based on satellite-derived phenology and maximal carbon uptake capacity

The high uncertainty regarding global gross primary production (GPP) remains unresolved. This study explored the relationships between phenology, physiology, and annual GPP to provide viable alternatives for accurate estimation. A statistical model of integrated phenology and physiology (SMIPP) was developed using GPP data from 145 FLUXNET sites to estimate the annual GPP for various vegetation types. By employing the SMIPP model driven by satellite-derived datasets of the global carbon uptake period (CUP) and maximal carbon uptake capacity (GPPmax), the global annual GPP was estimated for the period from 2001 to 2018. The results demonstrated that the SMIPP model accurately predicted annual GPP, with relative root mean square error values ranging from 11.20 to 19.29% for forest types and 20.49–35.71% for non-forest types. However, wetlands, shrublands, and evergreen forests exhibited relatively low accuracies. The average, trend, and interannual variation of global GPP during 2001–2018 were 132.6 Pg C yr−1, 0.25 Pg C yr−2, and 1.57 Pg C yr−1, respectively. They were within the ranges estimated in other global GPP products. Sensitivity analysis revealed that GPPmax had comparable effects to CUP in high-latitude regions but significantly greater impacts at the global scale, with sensitivity coefficients of 0.85 ± 0.23 for GPPmax and 0.46 ± 0.28 for CUP. This study provides a simple and practical method for estimating global annual GPP and highlights the influence of GPPmax and CUP on global-scale annual GPP.

Elevated atmospheric CO2 concentration and vegetation structural changes contributed to gross primary productivity increase more than climate and forest cover changes in subtropical forests of China

Abstract. The subtropical forests of China play a pivotal role in the global carbon cycle and in regulating the global climate. Quantifying the individual and combined effects of forest cover change (FCC), vegetation structural change (e.g. leaf area index (LAI)), CO2 fertilisation, and climate change (CC) on the annual gross primary productivity (GPP) dynamics of different subtropical forest types are essential for mitigating carbon emissions and predicting future climate changes, but these impacts remain unclear. In this study, we used a processed-based model to comprehensively investigate the impacts of these factors on GPP variations with a series of model experiments in China's subtropical forests from 2001 to 2018. Simulated GPP showed a significant increasing trend (20.67 gCm-2yr-1, p<0.001) under the interaction effects of FCC, LAI change, rising CO2, and CC. The CO2 fertilisation (6.84 gCm-2yr-1, p<0.001) and LAI change (3.79 gCm-2yr-1, p=0.004) were the two dominant drivers of total subtropical forest GPP increase, followed by the effects of FCC (0.52 gCm-2yr-1, p<0.001) and CC (0.92 gCm-2yr-1, p=0.080). We observed different responses to drivers depending on forest types. The evergreen broad-leaved forests showed the maximum carbon sequestration rate due to the positive effects of all drivers. Both the FCC (0.19 gCm-2yr-1, p<0.05) and CC (1.22 gCm-2yr-1, p<0.05) significantly decreased evergreen needle-leaved forest GPP, while their negative effects were almost offset by the positive impact of LAI changes. Our results indicated that LAI outweighed FCC in promoting GPP, which is an essential driver that needs to be accounted for in studies and ecological and management programmes. Overall, our study offers a novel perspective on different drivers of subtropical forest GPP changes and provides valuable information for policy makers to better manage subtropical forests to mitigate climate change risks.

Assessing the responses of ecosystem patterns, structures and functions to drought under climate change in the Yellow River Basin, China

Understanding how ecosystems respond and adapt to drought has become an urgent issue as drought stress intensifies under climate change, yet this topic is not fully understood. Currently, conclusions on the response of ecosystems in different regions to drought disturbance are inconsistent. Based on long MODIS data and observed data, this study systematically explored the relationships between ecosystem patterns, structures and functions and drought, taking a typical climate change-sensitive area and an ecologically fragile area—the Yellow River Basin—as a case study. Drought assessment results revealed that the Yellow River Basin has experienced meteorological and hydrological drought during most of the last two decades, predominantly characterized by medium and slight droughts. The ecosystem patterns and structures changed dramatically as the grassland decreased and the landscape fragmentation index (F) increased with increasing wetness. The annual gross primary productivity (GPP) increased, the water use efficiency (WUE) declined and ecosystem service value (ESV) exhibited a W-shaped increase at the watershed scale, but there were significant regional differences. There were positive correlations between F, GPP, ESV and drought indices, while there was a negative correlation between WUE and drought indices at the watershed scale. Under drought stress, the ecosystem structure in the basin was disrupted, the GPP and ESV decreased, but the WUE increased. Notably, approximately 106 %, 20 %, and 1 % of the maximum reductions in F, GPP, and ESV, respectively, were caused by drought, while the maximum 4 % of WUE increased. Responses of some functions in the wetland and grassland to drought vary from those in other ecosystems. The mechanisms underlying ecosystem responses to drought were further investigated. This study enhances the understanding of these responses and will help stakeholders formulate drought mitigation policies and protect ecosystem health.

Significant dark inorganic carbon fixation in the euphotic zone of an oligotrophic sea

AbstractEstimates of primary productivity have traditionally disregarded dark inorganic carbon fixation by marine microorganisms. Currently, only limited data are available from different systems on this potentially ecologically important process. We present monthly dark inorganic carbon fixation and photosynthetic rates from the euphotic layer of the northern Gulf of Aqaba collected over a decade between 2010 and 2020. Averaged dark inorganic carbon fixation rates from surface to 100 m depth, ranged from 99 to 173 mg C m−2 d−1, which corresponds to ~ 43% of the annual primary productivity at this location. The lowest dark inorganic carbon fixation rates were found during winter, contributing ~ 7.5% of the integrated primary productivity. During the oligotrophic summer, dark inorganic carbon fixation comprised a larger fraction of the integrated primary productivity estimated as ~ 12%. In accordance, dark inorganic carbon fixation contributed ~ 6% to the particulate organic carbon flux during the winter and ~ 30% during summertime. Complimentary nutrient‐enrichment bioassays of seawater from 5 m show that dissolved organic nutrient enrichment (P and C based) significantly elevates dark inorganic carbon fixation, whereas addition of dissolved inorganic nutrients (, , or both) significantly increased photosynthesis but to a lesser extent dark inorganic carbon fixation. These results suggest that dark inorganic carbon fixation may be an important biochemical process throughout the euphotic zone of oligotrophic seas, and thus should be incorporated into oceanic carbon production estimates.