The current status of intertidal waters in the wake of global change was assessed in a baseline study with a 36 month time series of water level, temperature, and salinity measurements from Bottsand lagoon, Baltic Sea, and from the mudflats off Schobüll, North Sea coast of Schleswig-Holstein, Germany. At Bottsand lagoon, annual average water temperatures varied from 12.1 to 12.6 °C, the air temperatures varied from 11.1 to 11.2 °C. The water temperatures followed the air temperatures in winter, and were higher than the air temperatures in spring and summer. The annual average salinities ranged from 14.7 to 16.9 units. The lagoon showed a different variability than the open Baltic surface waters, where the temperatures and salinities were lower in summer and higher in winter. The seasonal salinity differences were less developed in the mid 1960s, when the connectivity of the lagoon with the Baltic Sea was less restricted. In Husum Bight off Schobüll, annual average water temperatures ranged from 10.8 to 11.4 °C, and the air temperatures ranged from 9.9 to 10.2 °C. The water temperatures were lower than air temperatures in winter and higher in spring and summer, when the high waters were warmer during the day than at night-time. The annual average salinities off Schobüll ranged from 24.0 to 27.2 units. They were higher in summer and lower in winter, when the Elbe river runoff was generally enhanced. The same seasonal cycle was recorded in the Sylt Roads time series. Cross-correlations revealed that it takes seven weeks for an Elbe river freshwater pulse to reach Schobüll, and three weeks more to proceed to Sylt. The average salinities were 2.7 units lower off Schobüll than off Sylt. This pervasive gradient of landward decreasing salinities was induced by a local, low-salinity lens on top of tidal waters, fed by groundwater seepage or by freshwater runoff. A cross correlation with the precipitation record revealed salinity decreases about one week after high precipitation. The cumulative salt marsh submergence times per period of observation, i.e. inundation frequencies, were consistently higher at Bottsand than at Schobüll, where the same halophyte assemblages prevailed. As the average salinity was 10 units higher at Schobüll, the differences of inundation frequencies suggested that a certain salinity has to be maintained in the soils to sustain specific halophyte assemblages. A mass occurrence of small Austrominius modestus plates was observed before the vegetation boundary off Schobüll in spring 2024. The data suggested a wipe-out of juvenile barnacles during a short period of strong frost in late November 2023, when daily mean temperatures of up to −10.2 °C were recorded. This biotic response to environmental extremes highlighted the vulnerability of Wadden Sea ecosystems at times of Global Change.

Coastal ecosystems are affected by a multitude of anthropogenic stressors. As the Baltic Sea ecosystems rank among the most altered marine ecosystems worldwide, they represent ideal model regions to study ecosystem responses to anthropogenic pressures. Our statistical analysis of data including dissolved organic carbon and nitrogen, as well as bacterial abundance and -biomass production from the time-series station Boknis Eck in the southwestern Baltic Sea reveals that bacterial biomass production intensifies towards summer following the phytoplankton spring bloom. Moreover, warming, especially very high temperatures in summer, enhances stratification and bacterial biomass production despite long-term reduction in nutrient input. A strong decrease in oxygen in the bottom layer is possibly linked to this. We detect an increasing trend in bacterial biomass production along with intensifying warming and stratification, and more frequently occurring hypoxia despite ongoing restoration efforts. If this trend continues, the coastal Baltic Sea ecosystem is likely to be altered even further. Coastal ecosystems play pivotal roles in mitigating impacts of climate change but if destroyed, they may amplify climate change further calling for stronger ecosystem management strategies.

Particles afloat in the ocean are important components of marine element cycles. Most of this particulate matter is suspended in the sunlit surface layer and is mainly composed of microscopic living and dead organisms and fecal pellets. Aggregation of small, suspended particles into large, rapidly sinking aggregates can transport surface-​derived material to the deep ocean and the seafloor, contributing to the so-called biological pump. The comprehensive analysis of these sinking particles has increased our understanding of important biogeochemical ocean processes, such as the relationship between the rate of primary production and downward flux of particulate organic matter, the biological control of the removal of abiogenic particles from the surface ocean, and seasonal or inter­annual variations in downward particle fluxes. Sediment traps have been widely used since the late 1970s to capture the downward flux of particles for study (e.g., Staresinic et al., 1978; Knauer et al., 1979).

Short-lived bromo-, chloro- and iodocarbons from marine and anthropogenic sources contribute to the atmospheric halogen budgets and to ozone depletion in the troposphere and stratosphere. Their spatial variations are poorly known, given the sparse observations of marine and atmospheric concentrations. The distribution and air-sea fluxes of halocarbons need to be quantified in order to clarify the oceanic contributions to future tropospheric and stratospheric ozone chemistry.Here we present the first marine and atmospheric halocarbon dataset from the research cruise SO287-CONNECT (Pan-Atlantic connectivity of marine biogeochemical and ecological processes and the impact of anthropogenic pressures). The transit of RV SONNE from Las Palmas, Spain (departure: 11.12.2021) to Guayaquil, Ecuador (arrival: 11.01.2022) was conducted to decipher the coupling of biogeochemical and ecological processes and their influence on atmospheric chemistry along the transport pathway of water from the upwelling zones off Africa into the Sargasso Sea and further to the Caribbean and the equatorial Pacific. A comprehensive work program, which combined continuous underway and station work, marine and atmospheric measurements and sampling with incubation experiments was conducted.The distribution of short-lived halocarbons, e.g. bromoform (CHBr3), dibromomethane (CH2Br2), methyl iodide (CH3I), and trichloromethane (CHCl3) was highly dynamic in both ocean and atmosphere. We calculate the air-sea exchange of the compounds and relate physical and biological parameters to our observations. Among these are transport processes (e.g. long-range transport, eddies) and we show the varying composition of air and water masses and the potential sources of the compounds. For the first time, we estimate the contribution of the floating macroalgae Sargassum to halocarbon cycling around the North Atlantic gyre and in the Caribbean. The evaluation of the comprehensive data set collected during SO287-CONNECT improves our knowledge on the general role of the great Atlantic Sargassum belt and anthropogenic pollution in elemental biogeochemical cycles, as well as on trace gas exchange across the ocean-atmosphere interface.

Iron is an essential nutrient that limits primary productivity in up to 30% of the world’s ocean. Redox and complexation reactions control its solubility and therefore the fraction of dissolved and bioavailable iron. The iron (II) oxidation kinetic process was studied at 25 stations in coastal seawater of the Macaronesia region (around Cape Verde, the Canary Islands and Madeira). Laboratory experiments were carried out to study the pseudo-first-order oxidation rate constant (k’, min-1) over a range of pH (7.8-8.1) and temperature (T; 10-25ºC). Measured k’ varied from the calculated k’ (k'cal) at the same T, pH and salinity (S) at most stations. Measured iron (II) half-life times (t1/2=ln2/k’; min) at the 25 stations ranged from 1.8-3.5 min (mean 1.9±0.8 min) and for all but two stations were lower than the theoretically calculated t1/2 of 3.2±0.2 min. The biogeochemical context was considered by analysing nutrients and variables associated with the organic matter spectral properties (CDOM and FDOM). A multilinear regression model indicated that k’ can be described (R=0.921, SEE=0.064 for pH=8 and T=25ºC) from a linear combination of three organic variables. k’OM = k’cal -0.11* TDN + 29.9 * bDOM + 33.4 * C1humic where TDN is the total dissolved nitrogen, bDOM is the spectral peak obtained from coloured DOM analysis when protein-like or tyrosine-like components are present and C1humic is the component associated with humic-like compounds obtained from the parallel factor analysis (PARAFAC) of the fluorescent DOM. Experimentally, k’ and k’OM provide the net result between the compounds that accelerate the process and those that slow it down. Results show that compounds with nitrogen in their structures mainly explain the observed k’ increase for most of the samples, although other components could also present a relevant role.

The iron(II) oxidation kinetic process was studied at 25 stations in coastal seawater of the Macaronesia region (9 around Cape Verde, 11 around the Canary Islands, and 5 around Madeira). In a physicochemical context, experiments were carried out to study the pseudo-first-order oxidation rate constant (k', min-1) over a range of pH (7.8, 7.9, 8.0, and 8.1) and temperature (10, 15, 20, and 25 °C). Deviations from the calculated kcal' at the same T, pH, and S were observed for most of the stations. The measured t1/2 (ln 2/k', min) values at the 25 stations ranged from 1.82 to 3.47 min (mean 1.93 ± 0.76 min) and for all but two stations were lower than the calculated t1/2 of 3.21 ± 0.2 min. In a biogeochemical context, nutrients and variables associated with the organic matter spectral properties (CDOM and FDOM) were analyzed to explain the observed deviations. The application of a multilinear regression model indicated that k' can be described (R = 0.921 and SEE = 0.064 for pH = 8 and T = 25 °C) from a linear combination of three organic variables, k'OM = kcal' -0.11* TDN + 29.9*bDOM + 33.4*C1humic, where TDN is the total dissolved nitrogen, bDOM is the spectral peak obtained from colored dissolved organic matter (DOM) analysis when protein-like or tyrosine-like components are present, and C1humic is the component associated with humic-like compounds obtained from the parallel factor analysis of the fluorescent DOM. Results show that compounds with N in their structures mainly explain the observed k' increase for most of the samples, although other components could also play a relevant role. Experimentally, k' provides the net result between the compounds that accelerate the process and those that slow it down.

Anthropogenic activities are modifying the oceanic environment rapidly and are causing ocean warming and deoxygenation, affecting biodiversity, productivity, and biogeochemical cycling. In coastal sediments, anaerobic organic matter degradation essentially fuels the production of hydrogen sulfide and methane. The release of these compounds from sediments is detrimental for the (local) environment and entails socio-economic consequences. Therefore, it is vital to understand which microbes catalyze the re-oxidation of these compounds under environmental dynamics, thereby mitigating their release to the water column. Here we use the seasonally dynamic Boknis Eck study site (SW Baltic Sea), where bottom waters annually fall hypoxic or anoxic after the summer months, to extrapolate how the microbial community and its activity reflects rising temperatures and deoxygenation. During October 2018, hallmarked by warmer bottom water and following a hypoxic event, modeled sulfide and methane production and consumption rates are higher than in March at lower temperatures and under fully oxic bottom water conditions. The microbial populations catalyzing sulfide and methane metabolisms are found in shallower sediment zones in October 2018 than in March 2019. DNA-and RNA profiling of sediments indicate a shift from primarily organotrophic to (autotrophic) sulfide oxidizing Bacteria, respectively. Previous studies using data collected over decades demonstrate rising temperatures, decreasing eutrophication, lower primary production and thus less fresh organic matter transported to the Boknis Eck sediments. Elevated temperatures are known to stimulate methanogenesis, anaerobic oxidation of methane, sulfate reduction and essentially microbial sulfide consumption, likely explaining the shift to a phylogenetically more diverse sulfide oxidizing community based on RNA.

Iodine is a critical trace element involved in many diverse and important processes in the Earth system. The importance of iodine for human health has been known for over a century, with low iodine in the diet being linked to goitre, cretinism and neonatal death. Research over the last few decades has shown that iodine has significant impacts on tropospheric photochemistry, ultimately impacting climate by reducing the radiative forcing of ozone (O3) and air quality by reducing extreme O3 concentrations in polluted regions. Iodine is naturally present in the ocean, predominantly as aqueous iodide and iodate. The rapid reaction of sea-surface iodide with O3 is believed to be the largest single source of gaseous iodine to the atmosphere. Due to increased anthropogenic O3, this release of iodine is believed to have increased dramatically over the twentieth century, by as much as a factor of 3. Uncertainties in the marine iodine distribution and global cycle are, however, major constraints in the effective prediction of how the emissions of iodine and its biogeochemical cycle may change in the future or have changed in the past. Here, we present a synthesis of recent results by our team and others which bring a fresh perspective to understanding the global iodine biogeochemical cycle. In particular, we suggest that future climate-induced oceanographic changes could result in a significant change in aqueous iodide concentrations in the surface ocean, with implications for atmospheric air quality and climate.

Non-technical summary We summarize some of the past year's most important findings within climate change-related research. New research has improved our understanding about the remaining options to achieve the Paris Agreement goals, through overcoming political barriers to carbon pricing, taking into account non-CO 2 factors, a well-designed implementation of demand-side and nature-based solutions, resilience building of ecosystems and the recognition that climate change mitigation costs can be justified by benefits to the health of humans and nature alone. We consider new insights about what to expect if we fail to include a new dimension of fire extremes and the prospect of cascading climate tipping elements. Technical summary A synthesis is made of 10 topics within climate research, where there have been significant advances since January 2020. The insights are based on input from an international open call with broad disciplinary scope. Findings include: (1) the options to still keep global warming below 1.5 °C; (2) the impact of non-CO 2 factors in global warming; (3) a new dimension of fire extremes forced by climate change; (4) the increasing pressure on interconnected climate tipping elements; (5) the dimensions of climate justice; (6) political challenges impeding the effectiveness of carbon pricing; (7) demand-side solutions as vehicles of climate mitigation; (8) the potentials and caveats of nature-based solutions; (9) how building resilience of marine ecosystems is possible; and (10) that the costs of climate change mitigation policies can be more than justified by the benefits to the health of humans and nature. Social media summary How do we limit global warming to 1.5 °C and why is it crucial? See highlights of latest climate science.

Abstract Reaction with iodide (I − ) at the sea surface is an important sink for atmospheric ozone , and causes sea-air emission of reactive iodine which in turn drives further ozone destruction. To incorporate this process into chemical transport models, improved understanding of the factors controlling marine iodine speciation, and especially sea-surface iodide concentrations, is needed. The oxidation of I − to iodate (IO 3−) is the main sink for oceanic I− , but the mechanism for this remains unknown. We demonstrate for the first time that marine nitrifying bacteria mediate I − oxidation to IO3−. A significant increase in IO3− concentrations compared to media-only controls was observed in cultures of the ammonia-oxidising bacteria Nitrosomonas sp. (Nm51) and Nitrosoccocus oceani (Nc10) supplied with 9–10 mM I−, indicating I− oxidation to IO3−. Cell-normalised production rates were 15.69 (±4.71) fmol IO3− cell−1 d−1 for Nitrosomonas sp., and 11.96 (±6.96) fmol IO3− cell−1 d−1 for Nitrosococcus oceani, and molar ratios of iodate-to-nitrite production were 9.2 ± 4.1 and 1.88 ± 0.91 respectively. Preliminary experiments on nitrite-oxidising bacteria showed no evidence of I− to IO3− oxidation. If the link between ammonia and I− oxidation observed here is representative, our ocean iodine cycling model predicts that future changes in marine nitrification could alter global sea surface I− fields with potential implications for atmospheric chemistry and air quality.

An ocean iodine cycling model is presented, which predicts upper ocean iodine speciation. The model comprises a three‐layer advective and diffusive ocean circulation model of the upper ocean and an iodine cycling model embedded within this circulation. The two primary reservoirs of iodine are represented, iodide and iodate. Iodate is reduced to iodide in the mixed layer in association with primary production, linked by an iodine to carbon (I:C) ratio. A satisfactory model fit with observations cannot be obtained with a globally constant I:C ratio, and the best fit is obtained when the I:C ratio is dependent on sea surface temperature, increasing at low temperatures. Comparisons with observed iodide distributions show that the best model fit is obtained when oxidation of iodide back to iodate is associated with mixed layer nitrification. Sensitivity tests, where model parameters and processes are perturbed, reveal that primary productivity, mixed layer depth, oxidation, advection, surface freshwater flux, and the I:C ratio all have a role in determining surface iodide concentrations, and the timescale of iodide in the mixed layer is sufficiently long for nonlocal processes to be important. Comparisons of the modeled iodide surface field with parameterizations by other authors show good agreement in regions where observations exist but significant differences in regions without observations. This raises the question of whether the existing parameterizations are capturing the full range of processes involved in determining surface iodide and shows the urgent need for observations in regions where there are currently none.

An ocean iodine cycling model is presented, which predicts upper ocean iodine speciation. The model comprises a three-layer advective and diffusive ocean circulation model of the upper ocean, and an iodine cycling model embedded within this circulation. The two primary reservoirs of iodine are represented, iodide and iodate. Iodate is reduced to iodide in the mixed layer in association with primary production, linked by an iodine to carbon (I:C) ratio. A satisfactory model fit with observations cannot be obtained with a globally constant I:C ratio, and the best fit is obtained when the I:C ratio is dependent on sea surface temperature, increasing at low temperatures. Comparisons with observed iodide distributions show that the best model fit is obtained when oxidation of iodide back to iodate is associated with mixed layer nitrification. Sensitivity tests, where model parameters and processes are perturbed, reveal that primary productivity, mixed layer depth, oxidation, advection, surface fresh water flux and the I:C ratio all have a role in determining surface iodide concentrations, and the timescale of iodide in the mixed layer is sufficiently long for non-local processes to be important. Comparisons of the modelled iodide surface field with parameterisations by other authors shows good agreement in regions where observations exist, but significant differences in regions without observations. This raises the question of whether the existing parameterisations are capturing the full range of processes involved in determining surface iodide, and shows the urgent need for observations in regions where there are currently none.

The reaction between ozone and iodide at the sea surface is now known to be an important part of atmospheric ozone cycling, causing ozone deposition and the release of ozone-depleting reactive iodine to the atmosphere. The importance of this reaction is reflected by its inclusion in chemical transport models (CTMs). Such models depend on accurate sea surface iodide fields, but measurements are spatially and temporally limited. Hence, the ability to predict current and future sea surface iodide fields, i.e. sea surface iodide concentration on a narrow global grid, requires the development of process-based models. These models require a thorough understanding of the key processes that control sea surface iodide. The aim of this study was to explore if there are common features of iodate-to-iodide reduction amongst diverse marine phytoplankton in order to develop models that focus on sea surface iodine and iodine release to the troposphere. In order to achieve this, rates and patterns of changes in inorganic iodine speciation were determined in 10 phytoplankton cultures grown at ambient iodate concentrations. Where possible these data were analysed alongside results from previous studies. Iodate loss and some iodide production were observed in all cultures studied, confirming that this is a widespread feature amongst marine phytoplankton. We found no significant difference in log-phase, cell-normalised iodide production rates between key phytoplankton groups (diatoms, prymnesiophytes including coccolithophores and phaeocystales), suggesting that a phytoplankton functional type (PFT) approach would not be appropriate for building an ocean iodine cycling model. Iodate loss was greater than iodide formation in the majority of the cultures studied, indicating the presence of an as-yet-unidentified “missing iodine” fraction. Iodide yield at the end of the experiment was significantly greater in cultures that had reached a later senescence stage. This suggests that models should incorporate a lag between peak phytoplankton biomass and maximum iodide production and that cell mortality terms in biogeochemical models could be used to parameterise iodide production.

Reaction with iodide (I) at the sea surface is an important sink for atmospheric ozone, and causes sea-air emission of reactive iodine which in turn drives further ozone destruction. To incorporate...

The organic bromine compounds bromoform (CHBr3) and dibromomethane (CH2Br2) influence tropospheric chemistry and stratospheric ozone depletion. Their atmospheric abundance is generally related to a common marine source, which is not well characterized. A cruise between the three Macaroenesian Archipelagos of Cape Verde, the Canaries and Madeira revealed that anthropogenic sources increased oceanic CHBr3 emissions significantly close to some islands, especially at the Canaries, while heterotrophic processes in the ocean increased the flux of CH2Br2 from the sea to the atmosphere in the Cape Verde region. As anthropogenic disinfection processes, which release CHBr3 in coastal areas increase, and as more CH2Br2 may be produced from increased heterotrophy in a warming, deoxygenated ocean, both sources could supply higher fractions of stratospheric bromine in the future, with yet unknown consequences for stratospheric ozone.

Abstract. The reaction between ozone and iodide at the sea surface is now known to be an important part of atmospheric ozone cycling, causing ozone deposition and the release of ozone-depleting reactive iodine to the atmosphere. The importance of this reaction is reflected by its inclusion in chemical transport models (CTMs). Such models depend on accurate sea surface iodide fields but measurements are spatially and temporally limited. The ability to predict current and future sea surface iodide fields requires the development of process-based models which in turn require a thorough understanding of the key processes controlling inorganic iodine cycling. The aim of this study was to inform the development of ocean iodine cycling models by exploring if there are common features of iodate to iodide reduction amongst diverse marine phytoplankton. In order to achieve this, rates and patterns of changes in inorganic iodine speciation were determined in 10 phytoplankton cultures grown at ambient iodate concentrations. Where possible these data were analysed alongside results from previous studies. Iodate loss and some iodide production was observed in all cultures studied, confirming that this is a widespread feature amongst marine phytoplankton. We found no significant difference in log-phase, cell-normalised iodide production rates between key phytoplankton groups (diatoms, prymesiophytes including coccolithophores and phaeocystales) suggesting that a Phytoplankton Functional Type (PFT) approach would not be appropriate for building an ocean iodine cycling model. Iodate loss was greater than iodide formation in the majority of the cultures studied, indicating the presence of an as yet unidentified missing iodine fraction. Iodide yield at the end of the experiment was significantly greater in cultures that had reached a later senescence stage. This suggests that models should incorporate a lag between peak phytoplankton biomass and maximum iodide production, and that cell mortality terms in biogeochemical models could be used to parameterize iodide production.

Halogenated very short-lived substances (VSLSs) are naturally produced in the ocean and emitted to the atmosphere. When transported to the stratosphere, these compounds can have a significant influence on the ozone layer and climate. During a research cruise on RV Sonne in the subtropical and tropical west Indian Ocean in July and August 2014, we measured the VSLSs, methyl iodide (CH3I) and for the first time bromoform (CHBr3) and dibromomethane (CH2Br2), in surface seawater and the marine atmosphere to derive their emission strengths. Using the Lagrangian particle dispersion model FLEXPART with ERA-Interim meteorological fields, we calculated the direct contribution of observed VSLS emissions to the stratospheric halogen burden during the Asian summer monsoon. Furthermore, we compare the in situ calculations with the interannual variability of transport from a larger area of the west Indian Ocean surface to the stratosphere for July 2000–2015. We found that the west Indian Ocean is a strong source for CHBr3 (910 pmol m−2 h−1), very strong source for CH2Br2 (930 pmol m−2 h−1), and an average source for CH3I (460 pmol m−2 h−1). The atmospheric transport from the tropical west Indian Ocean surface to the stratosphere experiences two main pathways. On very short timescales, especially relevant for the shortest-lived compound CH3I (3.5 days lifetime), convection above the Indian Ocean lifts oceanic air masses and VSLSs towards the tropopause. On a longer timescale, the Asian summer monsoon circulation transports oceanic VSLSs towards India and the Bay of Bengal, where they are lifted with the monsoon convection and reach stratospheric levels in the southeastern part of the Asian monsoon anticyclone. This transport pathway is more important for the longer-lived brominated compounds (17 and 150 days lifetime for CHBr3 and CH2Br2). The entrainment of CHBr3 and CH3I from the west Indian Ocean to the stratosphere during the Asian summer monsoon is lower than from previous cruises in the tropical west Pacific Ocean during boreal autumn and early winter but higher than from the tropical Atlantic during boreal summer. In contrast, the projected CH2Br2 entrainment was very high because of the high emissions during the west Indian Ocean cruise. The 16-year July time series shows highest interannual variability for the shortest-lived CH3I and lowest for the longest-lived CH2Br2. During this time period, a small increase in VSLS entrainment from the west Indian Ocean through the Asian monsoon to the stratosphere is found. Overall, this study confirms that the subtropical and tropical west Indian Ocean is an important source region of halogenated VSLSs, especially CH2Br2, to the troposphere and stratosphere during the Asian summer monsoon.