The Belait River is a typical blackwater river in Brunei, characterized by deep tea-brown water rich in dissolved organic matter (DOM) derived from surrounding peat swamps and rainforests. This study provides a systematic assessment of the chemical composition, transformation, and transport processes of dissolved nitrogen (N) in the Belait River during alternating wet and dry seasons by integrating nitrate (NO3-) stable isotope techniques with microbial analyses. Results indicate that dissolved organic nitrogen (DON) constitutes the primary component of total dissolved nitrogen (TDN), with its seasonal dynamics significantly controlled by hydrological climate and microbial activity. Along the salinity gradient, DON decreases due to dilution and degradation, while ammonium (NH4+) and NO3- accumulate significantly in densely populated areas owing to anthropogenic inputs. The extremely low dissolved inorganic phosphorus (DIP) concentrations in the Belait River highlight the crucial role of P limitation in maintaining the system’s oligotrophic state. Through the investigation in this representative blackwater system, this study highlights complex N cycling mechanisms in blackwater and provides a critical gauge in land-ocean material fluxes at the Indo-Pacific Convergence Area.

Abstract. The Oxygen Minimum Zones (OMZs) in the northern Indian Ocean (i.e., the Arabian Sea and the Bay of Bengal) are among the most intense OMZs in the world’s oceans. While there is no clear evidence of a significant change in the Bay of Bengal (BoB) OMZ, the Arabian Sea (AS) OMZ followed the global trend and expanded in the last decades until 2013. Since then, however, this trend has reversed, and the AS OMZ seems to have shrunk. The stability of the BoB OMZ as well as the expansion and shrinkage of the AS OMZ in response to global warming is poorly understood. In this study we redefined the water masses and employed an extended Optimum Multiparameter (eOMP) Analysis to investigate changes in the oxygen supply due to mixing and biological oxygen consumption dynamics in these OMZs based on empirical field data from the Global Ocean Data Analysis Project version 2 (GLODAPv2) and a research cruise conducted with a German research vessel Sonne in 2024. Our findings reveal in line with previous studies a reversal in the expansion trend of the AS OMZ but also a shrinkage of the BoB OMZ between 1995 and 2016. In both regions this is due to an increased northward influx of oxygen-rich water masses from southern Indian Ocean, combined with a reduced contribution from relatively oxygen-poor local and equatorial water masses. However, we also observed that increased physical oxygen supply was accompanied by an increased biological oxygen consumption. These changes are likely linked to the slowdown of the global thermohaline circulation in the Indian Ocean. The slowdown is accompanied by a reduced inflow of the Indonesian Throughflow Water into the Indian Ocean and a lower output of Indian Ocean waters via the Agulhas Current/Leakage (at 32° S) into the Atlantic Ocean. A resulting increase in the residence time of water masses in the Indian Ocean is consistent with the detected biological oxygen consumption while the weaker zonal circulation seems have favored the meridional circulation which carried water from the southern Indian Ocean northwards. This implies a coupling between the OMZ in the Indian Ocean and climate change via the effect of the latter on the global thermohaline circulation as also seen in palaeoceanographic archives, whereas the drivers in past and today differs.

The Benguela Upwelling System, with its high variability and productivity, is challenging to monitor owing to limited in situ ocean observations. Researchers therefore often use ocean reanalysis products to simulate oceanographic conditions. We evaluated the performance of the GLORYS and BRAN reanalyses against in situ hydrographic data obtained from four research cruises in 2019 and from multi-year moored observations. The models reproduced cross-shelf temperature and salinity distributions reasonably well (r = 0.66-0.98, p < 0.001) for most stations. At the time of the cruises, GLORYS was 3-6 degrees C higher and had salinities 0.2-0.4 higher than in situ data in summer, but temperatures of 1-2 degrees C and salinities of 0.1-0.3 lower in winter. BRAN consistently underestimated temperature by 2-5 degrees C and salinity by 0.2-0.7. Both models performed better at depth than in the upper layers of the water column, reproducing temperature better than salinity. At longer time-scales, both models simulated current magnitude better than temperature (r = 0.77-0.79, p < 0.001). Model performance varied with latitude, with stronger correlations in the south than in the north, particularly for salinity. Our results suggest that these reanalyses need improved representation of regional ocean-atmosphere interactions and bathymetry. Thus, caution is required when using the model outputs for monitoring and operational studies.

Coral reef development is influenced by a wide variety of factors, including temperature, salinity, nutrient concentrations, and carbonate chemistry. Studies focusing on physicochemical drivers of coral reef distribution and composition in the Eastern Tropical Pacific (ETP) are scarce, and carbonate chemistry and nutrient data for this region are limited. This study measured coral reef composition and physicochemical parameters along the Pacific coast of Costa Rica, over a one-year period at three locations: Santa Elena and Matapalo in the north, and Parque Nacional Marino Ballena in the south. Our results show high seasonal and spatial variability of physicochemical conditions with significant differences mainly explained by inorganic nutrient concentrations, with driving processes also having a strong influence on the variability of carbonate chemistry parameters. Coastal upwelling is the main driver of the seasonal variability in Santa Elena. Comparison of seasonal dissimilarity within locations confirms the presence of a geographical gradient, with stronger influence of the upwelling in Santa Elena relative to Matapalo, where several parameters displayed a lower seasonality and a carbonate system that supports reef development throughout the year. Conversely, in Marino Ballena the river discharges during rainy season exerted a strong control on the seasonal variability. The integrated analysis of coral reef composition and physicochemical parameters suggests that in addition to inorganic nutrients carbonate chemistry also plays a key role in coral distribution. Analyzing the spatial distribution of the main reef builders provides insights into the species-specific tolerance to varying conditions. Pavona clavus is widely distributed in both the northern and southern locations, suggesting that this massive coral is very tolerant to the high variability of physicochemical conditions. The dominant corals in the north (Pavona gigantea and Pocillopora spp.) are highly tolerant to nutrient-enriched cold waters with low aragonite saturation, while one of the main reef-builders in southern locations (Porites cf. lobata) cope better with low salinity, low aragonite saturation and low light intensity caused by river discharges. Understanding the preferences of individual coral species at our study locations can shed light on the environmental factors driving coral reef distribution in other locations of the ETP.

AbstractQuantifying greenhouse gas (GHG) emissions is essential for mitigating global warming, and has become the task of individual countries assigned to the Paris agreement in the form of National Greenhouse Gas Inventory Reports (NIR). The NIR informs on GHG emissions and removals over national territory encompassing the 200-mile Exclusive Economic Zone (EEZ). However, apart from only a few countries, who have begun to report on coastal ecosystems, mostly mangroves, salt marshes, and seagrass meadows, the NIR does not cover or report on GHG sources and sinks of the 200-mile exclusive economic zone which, for Namibia and South Africa includes the Benguela Upwelling System (BUS). Based on our results, we estimated a CO2 uptake by the biological carbon pump of 18.5 ± 3.3 Tg C year−1 and 6.0 ± 5.0 Tg C year−1 for the Namibian and South African parts of the BUS, respectively. Even though it is assumed that the biological carbon pump already responds to global change and fisheries, uncertainties associated with estimates of the CO2 uptake by the biological carbon pump are still large and hamper a thorough quantification of human impacts on the biological carbon pump. Despite these uncertainties, it is suggested to include parameters such as preformed nutrient supply, carbon export rates, Redfield ratios, and CO2 concentrations measured at specific key sites into the NIR to stay focussed on the biological carbon pump and to support research addressing open questions, as well as to improve methods and observing concepts.

Satellite gravimetry is used to study the global hydrological cycle. It is a key component in the investigation of groundwater depletion on the Indian subcontinent. Terrestrial mass loss caused by river sediment transport is assumed to be below the detection limit in current gravimetric satellites of the Gravity Recovery and Climate Experiment Follow-On mission. Thus, it is not considered in the calculation of terrestrial water storage (TWS) from such satellite data. However, the Ganges and Brahmaputra rivers, which drain the Indian subcontinent, constitute one of the world's most sediment-rich river systems. In this study, we estimate the impact of sediment mass loss within their catchments on local trends in gravity and consequential estimates of TWS trends. We find that for the Ganges–Brahmaputra–Meghna catchment sediment transport accounts for (4 ± 2) % of the gravity decrease currently attributed to groundwater depletion. The sediment is mainly eroded from the Himalayas, where correction for sediment mass loss reduces the decrease in TWS by 0.22 cm of equivalent water height per year (14 %). However, sediment mass loss in the Brahmaputra catchment is more than twice that in the Ganges catchment, and sediment is mainly eroded from mountain regions. Thus, the impact on gravimetric TWS trends within the Indo–Gangetic Plain – the main region identified for groundwater depletion – is found to be comparatively small (< 2 %).

AbstractThe sustainability of southern Africa’s natural and managed marine and terrestrial ecosystems is threatened by overuse, mismanagement, population pressures, degradation, and climate change. Counteracting unsustainable development requires a deep understanding of earth system processes and how these are affected by ongoing and anticipated global changes. This information must be translated into practical policy and management interventions. Climate models project that the rate of terrestrial warming in southern Africa is above the global terrestrial average. Moreover, most of the region will become drier. Already there is evidence that climate change is disrupting ecosystem functioning and the provision of ecosystem services. This is likely to continue in the foreseeable future, but impacts can be partly mitigated through urgent implementation of appropriate policy and management interventions to enhance resilience and sustainability of the ecosystems. The recommendations presented in the previous chapters are informed by a deepened scientific understanding of the relevant earth system processes, but also identify research and knowledge gaps. Ongoing disciplinary research remains critical, but needs to be complemented with cross-disciplinary and transdisciplinary research that can integrate across temporal and spatial scales to give a fuller understanding of not only individual components of the complex earth-system, but how they interact.

We conducted sediment trap experiments in the Benguela Upwelling System (BUS) in the southeastern Atlantic Ocean to study the influence of zooplankton on the flux of particulate organic carbon (POC) through the water column and its sedimentation. A total of 2 long-term moored and 16 short-term free-floating sediment trap systems (drifter systems) were deployed. The mooring experiments were conducted over more than a decade (2009–2022), and the 16 drifters were deployed on three different research cruises between 2019 and 2021. Zooplankton was separated from the trapped material and divided into eight different zooplankton groups. In contrast to zooplankton which actively carries POC into the traps in the form of biomass (active POC flux), the remaining fraction of the trapped material was assumed to fall passively into the traps along with sinking particles (passive POC flux). Our results show, in line with other studies, that copepods dominate the active POC flux, with the active POC flux in the southern BUS (sBUS) being about 3 times higher than in the northern BUS (nBUS). In contrast, the differences between the passive POC fluxes in the nBUS and sBUS were small. Despite large variations, which reflected the variability within the two subsystems, the mean passive POC fluxes from the drifters and the moored traps could be described using a common POC flux attenuation equation. However, the almost equal passive POC flux, on the one hand, and the high POC concentration in the surface sediments of the nBUS in comparison to the sBUS, on the other hand, imply that the intensity of the near-bottom oxygen minimum zone (OMZ), which is more pronounced in the nBUS than in the sBUS, controls the preservation of POC in sediments significantly. This highlights the contrasting effects of the globally observed expansion of OMZs, which on the one hand mitigates the accumulation of CO2 in the atmosphere and the ocean by increasing POC storage in sediments and on the other hand poses a threat to established ecosystems and fisheries.

Bottom-trawl fishery is known to cause major disturbances to marine sediments as the dragging of trawl gears across the seabed fosters sediment resuspension, which can lead to organic particle remineralization and release of benthic CO2 and nutrients into bottom waters. However, its effects on carbon cycling and biological productivity, especially in highly productive regions like the Benguela Upwelling System (BUS), are less well studied. Here, we simulated carbon (C) and nutrient pathways from the trawled coastal seabed to overlying water masses that are being upwelled into the sunlit surface within the BUS, using shipboard data on sea surface and water column characteristics and published benthic CO2 emission estimates from bottom-trawled sediments. The latter reports 4.35 and 0.64 Tg C year-1 to be released from the seabed into upwelling source waters after bottom trawling in the northern (NBUS) and southern (SBUS) subsystems, respectively. Based on these values, we estimated a corresponding nitrate (N) input of 1.39 and 0.47 µmol kg-1 year-1, enhancing source water nitrate concentrations by ~5% and ~2%. Trawl-induced nitrate input into the sunlit surface could support a new production of 3.14 and 0.47 Tg C year-1 in the NBUS and SBUS, respectively, recapturing only 2/3 of CO2 released after bottom trawling into biomass, mainly due to differences in stoichiometric C:N ratios between the sediment (~9) and surface biomass (Redfield, 6.6). The remaining benthic CO2 can thereby lead to an increase in surface CO2 concentration and its partial pressure (pCO2), impeding CO2 uptake of the biological carbon pump in the BUS by 1.3 Tg C year-1, of which 1 Tg C year-1 is emitted to the atmosphere across the northern subsystem. Our results demonstrate the extent to which bottom trawling may affect the CO2 storage potential of coastal sediments on a basin-wide level, highlighting the need to better resolve small-scale sediment characteristics and C:N ratios to refine trawl-induced benthic carbon and nutrient effluxes within the BUS.

AbstractThe southern African subcontinent and its surrounding oceans accommodate globally unique ecoregions, characterized by exceptional biodiversity and endemism. This diversity is shaped by extended and steep physical gradients or environmental discontinuities found in both ocean and terrestrial biomes. The region’s biodiversity has historically been the basis of life for indigenous cultures and continues to support countless economic activities, many of them unsustainable, ranging from natural resource exploitation, an extensive fisheries industry and various forms of land use to nature-based tourism.Being at the continent’s southern tip, terrestrial species have limited opportunities for adaptive range shifts under climate change, while warming is occurring at an unprecedented rate. Marine climate change effects are complex, as warming may strengthen thermal stratification, while shifts in regional wind regimes influence ocean currents and the intensity of nutrient-enriching upwelling.The flora and fauna of marine and terrestrial southern African biomes are of vital importance for global biodiversity conservation and carbon sequestration. They thus deserve special attention in further research on the impacts of anthropogenic pressures including climate change. Excellent preconditions exist in the form of long-term data sets of high quality to support scientific advice for future sustainable management of these vulnerable biomes.

Eastern Boundary Upwelling Systems (EBUS) are highly productive ecosystems. However, being poorly sampled and represented in global models, their role as atmospheric CO2 sources and sinks remains elusive. In this work, we present a compilation of shipboard measurements over the past two decades from the Benguela Upwelling System (BUS) in the southeast Atlantic Ocean. Here, the warming effect of upwelled waters increases CO2 partial pressure (pCO2) and outgassing in the entire system, but is exceeded in the south through biologically-mediated CO2 uptake through biologically unused, so-called preformed nutrients supplied from the Southern Ocean. Vice versa, inefficient nutrient utilization leads to preformed nutrient formation, increasing pCO2 and counteracting human-induced CO2 invasion in the Southern Ocean. However, preformed nutrient utilization in the BUS compensates with ~22-75 Tg C year-1 for 20-68% of estimated natural CO2 outgassing in the Southern Ocean's Atlantic sector (~ 110 Tg C year-1), implying the need to better resolve global change impacts on the BUS to understand the ocean's role as future sink for anthropogenic CO2.

Anthropogenic pressures are restructuring coral reefs globally. Sound predictions of the expected changes in key reef functions require adequate knowledge of their drivers. Here we investigate the determinants of a poorly-studied yet relevant biogeochemical function sustained by marine bony fishes: the excretion of intestinal carbonates. Compiling carbonate excretion rates and mineralogical composition from 382 individual coral reef fishes (85 species and 35 families), we identify the environmental factors and fish traits that predict them. We find that body mass and relative intestinal length (RIL) are the strongest predictors of carbonate excretion. Larger fishes and those with longer intestines excrete disproportionately less carbonate per unit mass than smaller fishes and those with shorter intestines. The mineralogical composition of excreted carbonates is highly conserved within families, but also controlled by RIL and temperature. These results fundamentally advance our understanding of the role of fishes in inorganic carbon cycling and how this contribution will change as community composition shifts under increasing anthropogenic pressures.

Greenhouse gas (GHG) emissions caused by food production contributed on average 23% (12 & PLUSMN; 2.9 P(=1015)gCO2e yr -1) to the total anthropogenic GHG emissions between 2007 and 2017. Globally, China has the largest aquatic food production and the South American white shrimp Litopenaeus vannamei (L. vannamei) is gaining more and more economic importance. Here, we set-up an GHG monitoring system within a plastic-lined L. vannamei pond on the Chinese island of Hainan and developed a simple box model to better understand processes controlling the aquatic GHG production and emission within and from the pond. Measured data in line with model results showed that an increase of the pond water pH due to liming could have turned the studied pond from a CO2 source into a CO2 sink. Nevertheless, it remained elusive whether liming would finally act as a net CO2 sink to the atmosphere, considering CO2 emissions associated with its supply and the treatment of alkaline pond waters. The transformation of mangroves into aquacultures can, in turn, be seen as a CO2 source for the atmosphere, which enhances the residence of CO2 in the climate system by reducing its burial in sediments. However, in comparison to shrimp cultures in estuaries as well as mangroves and mangroves converted into paddy fields, crab ponds as well as earthen shrimp ponds, methane (CH4) and nitrous oxide (N2O) emission were low in the pond we studied. While N2O emissions from ponds were in general low, CH4 emission in the studied pond was up to orders of magnitude lower than at the other sites. Shielding pond-waters from impacts of ambient environments such as mangrove soils and ocean waters could be one reason for the low CH4 and N2O emission. However, the still poorly un-derstood processes controlling fluxes of N2O and CH4 in aquacultures and mangroves impede the discussion on impacts of aquacultures on the greenhouse effect and hampers the development of further, more specific strategies to lower GHG emissions from shrimp cultures.

Southeast Asian peatlands represent a globally significant carbon store that is destabilized by land-use changes like deforestation and the conversion into plantations, causing high carbon dioxide (CO2) emissions from peat soils and increased leaching of peat carbon into rivers. While this high carbon leaching and consequentially high DOC concentrations suggest that CO2 emissions from peat-draining rivers would be high, estimates based on field data suggest they are only moderate. In this study, we offer an explanation for this phenomenon by showing that carbon decomposition is hampered by the low pH in peat-draining rivers. This limits CO2 production in and emissions from these rivers. We find an exponential pH limitation that shows good agreement with laboratory measurements from high-latitude peat soils. Additionally, our results suggest that enhanced input of carbonate minerals increases CO2 emissions from peat-draining rivers by counteracting the pH limitation. As such inputs of carbonate minerals can occur due to human activities like deforestation of river catchments, liming in plantations, and enhanced weathering application, our study points out an important feedback mechanism of those practices.

Indonesia, with its more than 17,000 islands, is an extraordinary place on this planet. It is among the countries with the highest river fluxes of dissolved and particulate substances into the ocean, the most abundant mangrove forests, seagrass meadows and coral reefs, and the highest marine biodiversity. However, Indonesia is also vulnerable to man-made environmental change and the outcomes of climate change. In order to face these challenges, the Indonesian – German inter- and transdisciplinary research and capacity building program "Science for the Protection of Indonesian Coastal Ecosystems" (SPICE) was set up. It addressed the scientific, social and economic issues related to the management of the Indonesian coastal ecosystems and their resources in three phases over a period of 12 years. This chapter outlines the major goals and themes of the program, its organizational structure, and it briefly synthesizes its major findings.