Ocean Acidification

Highlights

• Coastal freshening suppressed N removal via denitrification while accelerating net organic carbon mineralization.
• Salinity shaped N removal vs. N retention and carbonate vs. alkalinity balance through sulfate availability.
• Salinity changes had concurrent implications for coastal eutrophication and ocean acidification.

Abstract

Coastal wetlands naturally remediate nitrogen (N) pollution through microbial pathways that either remove reactive N via denitrification and anammox, or retain it via dissimilatory nitrate reduction to ammonium (DNRA). The balance among three processes is closely linked to the carbon (C) cycle, as both heterotrophic denitrification and DNRA consume organic C and release alkalinity. While salinity fluctuations can disrupt these processes through direct ionic stress or sulfur (S) cycling, their net impact on N removal and C preservation services remains unclear. Here, we deployed microcosm experiments using mangrove sediments under a large salinity gradient (0-30 psu). We quantified N transformation rates using ¹⁵N isotope tracing technique, combined with geochemical analysis, and functional genes quantification. Freshening from ambient 30 psu to 10 psμ decreased N removal efficiency by ∼20%. This decline was caused by reduced denitrification, whereas anammox and DNRA were unaffected. Meanwhile, lower salinity appears to have stimulated C decomposition via reduced ionic stress. The reduced sulfate input diminished total alkalinity (TA) generation relative to dissolved inorganic carbon (DIC). The stoichiometric shift of TA:DIC ratio could further contribute to acidification in adjacent coastal waters. Additionally, the S-mediated regulation of N partitioning appears to be nitrate-dependent: under nitrate limitation, higher sulfate favored N retention; conversely, with enriched nitrate, it potentially favored N removal. Integrating the coupling effect of salinity on interaction between N, C and S cycles, our study demonstrates that coastal water freshening may weaken wetlands’ ability to remove N and preserve C.

1. Social context is a critical yet underexplored determinant of behavioural resilience to climate change. Group living can buffer individuals against environmental stress through enhanced vigilance, reduced predation risk and improved foraging efficiency.
2. However, whether these behavioural expressions persist under chronic (warming, acidification) and acute (marine heatwaves) climate stressors remains unclear. Using natural climate analogues spanning present-day, ocean warming and combined warming–acidification reefs, we quantified how shoal size influences behavioural expression in a range-extending reef fish (Pomacentrus coelestis).
3. Across all climate conditions, fish in larger shoals consistently exhibited higher foraging and activity levels and reduced risk-avoidance behaviours, whereas direct effects of warming, acidification and heatwaves on behaviour were negligible.
4. In contrast, ocean acidification most likely constrained collective behaviour indirectly by simplifying benthic habitats, where fish densities were 84% lower than at the warming reef, resulting in shoals that were up to 79% smaller than the Warming and Control reefs.
5. Combined, our data suggest that shoal size mediates behavioural expression between foraging and predator avoidance and that acidification-driven habitat simplification can alter behavioural expression indirectly by reducing fish densities and the formation of large shoals.
6. We conclude that climate change can indirectly modify behavioural expression in shoal-forming fishes through habitat-driven erosion of social structure.

Ocean acidification and warming (OAW) are expected to alter physiology, growth and reproduction of marine ectotherms, yet their combined effects on life-history traits remain unresolved, particularly under poorly defined future food conditions. Using a Dynamic Energy Budget (DEB) model, we investigated how interacting changes in temperature, seawater pH, and food quality may shape somatic growth and reproductive phenology of the European abalone Haliotis tuberculata across four contrasting coastal environments and three Shared Socioeconomic Pathway (SSP) climate scenarios. OAW effects were modeled as increased metabolic maintenance costs, while reduced food quality, driven by OAW, lowered assimilation efficiency, aligning with experimentally-supported limited compensatory feeding.,Our results reveal that warming and food quality strongly drive somatic growth, whit ocean acidification playing a minor role within the modeled range. Food quality remained the primary determinant of maximum body size, while warming amplified growth across all locations, with the largest proportional increases in cooler northern bays. Individuals in the warmest areas remained the largest across scenarios within the model framework. Reproductive timing also shifted consistently, with first spawning occurring markedly earlier under end-of-century conditions, advancing consistently with scenario intensity. Food quality modulated reproductive investment but had weaker effects on the timing of first spawning., These findings highlight that food quality critically mediates organismal responses to OAW and can offset temperature-driven gains in growth and reproduction. By combining expected nutritional constraints with SSP scenarios, our DEB-based approach provide mechanistic insights into the future responses of benthic marine invertebrates to climate change, highlighting the value of these scenario-based projections for better management strategies.

Large-scale Euro-Atlantic variability, shaped by the polar jet stream, governs weather and climate in the Baltic Sea region, thereby impacting the physical and biogeochemical properties of the Baltic Sea ecosystem. This review synthesizes how key atmospheric circulation features and modes of climate variability, including the North Atlantic Oscillation, atmospheric blocking and the Atlantic Multidecadal Variability, influence the Baltic Sea region. By integrating evidence from the published literature, observational datasets, and both global and regional climate model simulations, we assess established as well as potential linkages to key climatic variables, including temperature, precipitation, and storm activity, across temporal scales ranging from synoptic events to multidecadal variability. We then evaluate how these climate controls cascade into ecosystem-relevant processes, namely oxygen dynamics, primary productivity and ocean acidification. Although physical links are already established, the pathways connecting large-scale atmospheric patterns to biogeochemistry are still poorly constrained, partly because dedicated field studies and targeted model experiments are limited. We outline priority research needs to enhance near-term predictability and reduce uncertainty in future projections for the Baltic Sea.

The rapidity of climate change in the polar regions underscores the need for improved understanding of its impacts on ocean circulation at both regional and global scales. Reconstructions of past polar ocean-cryosphere interactions can provide this context, but large uncertainties in existing proxies limit the utility of such studies. For instance, there are currently no low-temperature (<9 °C) culture-based Mg/Ca-calibrations for planktic foraminifera, a key tool for reconstructing past changes in ocean temperatures. There is also limited understanding of non-thermal influences on Mg/Ca in Neogloboquadrina pachyderma, the only modern polar planktic foraminifera. Moreover, this species exhibits considerable levels of heterogeneity in composition precipitating a thick lower-Mg/Ca outer crust over higher Mg/Ca inner lamellae calcite; specimens with predominantly, albeit variable crust–lamellae proportions, are thus thought to introduce substantial uncertainty into high-latitude palaeotemperature reconstructions. Here, we used N. pachyderma cultured across a range of temperatures, salinities, and carbonate chemistry conditions including experiments in which pH and [CO₃^2-] either covaried or were decoupled. By using a laser ablation approach to analyse crust and lamellae separately, we present new Mg/Ca-temperature calibrations for each component that extend culture-based calibrations in N. pachyderma down to the lower temperature-range (2–9 °C). The crust-specific calibration is of particular importance in high-latitude downcore records where N. pachyderma are commonly observed to preserve predominantly or only crust. Our results show significant carbonate chemistry influence on Mg/Ca with opposite influences from pH and carbonate ion concentration, when these variables changed in isolation. Additionally, we show that environmental conditions regulate crust-lamellar proportions, where increased salinity and temperature, and lower pH lead to less crust formation with implications for future ocean acidification and Arctic Atlantification, and for downcore reconstructions.

Highlights

• Seagrass-coral habitats act as CO₂ sources driven by intense nighttime respiration
• High-resolution data enable predictive modeling of DIC, DOC, and POC dynamics
• Metabolic cues govern DIC, while temperature and alkalinity regulate POC and DOC
• Episodic coral spawning and rainfall trigger rapid ocean acidification
• Short-term disturbances dramatically shift organic and inorganic nutrient loads

Abstract

Seagrass meadows and coral reefs are global hotspots for productivity, yet they are often studied in isolation despite their intense biogeochemical connectivity. Significant gaps remain in understanding how coupled inorganic and organic processes within the water column drive blue carbon services in such mixed habitats, particularly during rapid environmental disturbances. Here, we investigated a unique, intertwined ecosystem in the Dongsha Atoll, where massive Porites corals are distributed on seagrass meadows, creating a natural laboratory for studying water column carbon biogeochemistry. During a 10-day sampling period, we collected continuous hydrological and discrete biogeochemical data at two- to four-hour intervals. Our results reveal that the dissolved inorganic carbon (DIC) covaried with dissolved oxygen and pH in strong diurnal patterns, which were governed by photosynthesis and respiration. As an outcome, variable but mostly high pCO₂ values (141–2070 μatm) indicate the seagrass meadow was a source of CO₂ to the atmosphere due to strong night-time respiration. Particulate organic carbon (POC) increased with temperature but showed no diurnal pattern. Dissolved organic carbon (DOC) showed a weak diurnal pattern and was linked to variations in POC and total alkalinity, highlighting the tight coupling between the organic production of the meadow and the inorganic chemistry of the calcification framework. Additionally, coral spawning led to a surge in organic content and changed inorganic nutrient levels. Rainfall events significantly acidified the ocean and enhanced submarine groundwater discharge to the seagrass-coral habitat. The distinctive contributions of this study are the extremely high temporal resolution of discrete samples, allowing the simultaneous tracking of multiple organic and inorganic pools during natural disturbances. The high-resolution data provide fundamental information for parametrizing models that explain DIC, POC, and DOC, which yield insights into organic carbon cycling in seagrass meadow-coral habitats.

Climate change is increasingly disrupting freshwater ecosystems in sub-Saharan Africa, posing severe threats to the reproductive success and population viability of key fish species. This study investigated the mechanistic effects of elevated temperature across a gradient and the combined impact of elevated temperature, acidification and hypoxia under a simulated future climate scenario (IPCC SSP5-8.5) on the reproductive physiology and early life stages of Clarias gariepinus in the Cross River Estuary. Single-stressor trials examined the effect of temperature (28–38°C) on oestrogen synthesis, cortisol levels and gonadosomatic index (GSI). A combined-stressor scenario (35°C, pH 6.2, dissolved oxygen 2 mg/L) was used to simulate predicted climate conditions. Each treatment was replicated across triplicate tanks, with 10 broodstock per tank, over an 8-week period. Environmental parameters were tightly controlled using aquarium heaters, aerators and pH regulators. Combined stressors markedly disrupted reproductive function. Oestrogen synthesis ceased at 34°C, coinciding with a sharp decline in GSI (r² = 0.81, p < 0.001). Cortisol concentrations increased fourfold under concurrent heat and hypoxia. Cortisol concentrations increased fourfold under heat and hypoxia co-stress. Larval performance also declined sharply, with prey capture efficiency reduced by 33% at pH 6.0 and cumulative mortality reaching 82% by day 5 under combined-stressor conditions. Habitat suitability models projected a 71% reduction in spawning habitat availability in the estuary by 2070 under the SSP5-8.5 scenario. Genetic screening revealed a significant correlation (r² = 0.63, p = 0.004) between heat shock protein 70 (HSP70) allele frequency and larval survival, indicating potential for adaptive resilience. These findings suggest a compounded vulnerability of C. gariepinus to climate-related stressors and highlight the potential need for targeted conservation efforts. Recommended interventions include habitat restoration, enhancement of dissolved oxygen regimes and selective breeding programmes to support thermal and hypoxic tolerance in vulnerable populations.

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