Xosé Antón Álvarez-Salgado

Island‐induced eddies from the Canary Islands region propagate southwestward along the Canary Eddy Corridor, exporting biological and biogeochemical properties to the oligotrophic open ocean. However, their contribution to organic matter remineralization, and consequently to the biological carbon pump, throughout their life cycle remains poorly understood. Using respiratory electron transport system (ETS) activity as a proxy for microbial respiration, we investigated two cyclonic and two anticyclonic eddies at different stages of development and found distinct respiration patterns. Epipelagic respiration peaked in the most intense cyclonic eddy, whereas mesopelagic respiration showed localized maxima beneath most eddies and tended to be higher in cyclonic than in anticyclonic eddies. At submesoscale resolution, plume‐like ETS structures emerged within the mesopelagic layer, modulating respiration variability across eddies. Mesopelagic respiration variability was driven primarily by suspended particulate organic matter, whereas dissolved organic matter and its chromophoric and fluorescent protein‐like fractions contributing less. This contrasts with the greater contribution of dissolved organic matter to apparent oxygen utilization. Notably, fluorescent humic‐like substances were negatively correlated with respiration, indicating reduced microbial activity in older water masses. These findings demonstrate that mesoscale eddies can strongly regulate microbial organic matter remineralization and, consequently, influence ocean carbon cycling.

Dissolved organic carbon is the largest reservoir of reduced carbon in the ocean but its sensitivity to climate change and consequent impacts on ocean biogeochemistry remain poorly understood. Here, we implement four dissolved organic carbon pools with the empirically-derived varying reactivities and lifetimes in an Earth system model to investigate their global distribution, impacts, and responses to climate change. The refractory pool, which has a lifetime of several thousand years, improves the model representations of low oxygen, high nutrient conditions in aged water masses. Under a high warming scenario, substantial decreases are projected across all dissolved organic carbon pools. This leads to a net conversion of organic to remineralised nutrients, which could have an effect on future changes in primary production. Under both weak and strong future warming, the model projects a long-term loss in the global dissolved organic carbon pool that is irreversible over centennial time scale, highlighting the need to sustain observations and improve model representations.

Human activities are altering nutrient inputs into coastal waters, impacting the C:P and N:P ratios of dissolved matter. These changes may modify present nutrient limitations, especially P limitation, in microbial communities. The response of microbial communities from the Ría de Vigo (NW Iberian Peninsula) to both the type (inorganic versus inorganic and organic mixture) and the dissolved inorganic N (DIN) to dissolved inorganic P (DIP) ratio (above and below Redfield) of nutrient inputs was assessed in 8 microcosm experiments. Concomitantly, the response of phytoplankton to nutrient treatments in the presence or absence of bacterial activity was explored. The DIN:DIP ratio and dissolved organic C availability modulated the response of microbial communities to matter inputs and also the bacterial impact on phytoplankton growth, potentially playing a major role in regulating biotic interactions between these 2 microbial compartments. Our study strongly suggests that when labile C is available, bacteria seem to outcompete phytoplankton under P scarcity conditions, while when both C and P are available, the impact of bacteria on phytoplankton tends to be positive. In a future global change scenario, with increased organic C and N:P ratios of allochthonous matter inputs to coastal waters, our data suggest that bacteria could outcompete phytoplankton for inorganic nutrients, reducing coastal primary production and affecting higher trophic levels.

Subterranean estuaries play a key role in the land-ocean interface by modulating groundwater-borne and recycled solutes discharged to the coast. Despite their importance for coastal ecosystems, the sensitivity of these systems to human activities is still unknown. To address this gap, dissolved organic matter (DOM) and dissolved inorganic nutrients of a pristine site were compared with those from two nearby, semiurban sites characterized by contrasting oxygen conditions. The local aquifers surrounding the semiurban subterranean estuaries contained more nitrogen, silicate, and DOM of higher molecular weight than the aquifers surrounding the pristine site. Despite the different chemical composition of the arriving fresh groundwater, N/P ratios and the quantity of humic-like DOM compounds at the pristine site were intermediate between those of the two semiurban sites. This pattern reflected the intermediate permeability and oxygenation of the pristine beach, highlighting the role of the sediment matrix in modulating the exported solutes. Enhanced oxygenation at one semiurban site resulted from a human-derived gravel layer that increased sediment permeability and reduced internal residence times. The anthropogenic alteration of the sediment permeability had a greater influence on the nutrients and DOM found in subterranean estuaries than did the chemical composition of the inland aquifers.

Respiration in the mesopelagic layer of the oceans (200–1,000 dbar) is a key factor controlling the biological carbon pump, as it attenuates the downward flux of organic carbon to the ocean interior. Using a global data set of dissolved oxygen and transient tracer gas measurements, we report spatially resolved oxygen utilization rates (OUR) across 34 water masses of the global mesopelagic ocean. We find that OURs ranged between 0.6 and 27.3 μmol kg ⁻¹ yr ⁻¹ , varying both within and across water masses. Two main patterns emerge from our analysis: (a) OURs in central water masses (150–500 dbar) shared patterns with downward organic carbon flux proxies, and (b) OURs in intermediate and deep water masses (500–1,500 dbar) were overall consistent with basin‐scale circulation and aging patterns. These findings will help to constrain and parameterize mesopelagic respiration rates, a key component of global biogeochemical models.

Marine macrophytes play a significant role in the marine carbon cycle by releasing dissolved organic carbon (DOC), including a recalcitrant fraction with potential for long-term carbon sequestration. Here, we investigated how warming and the presence of an invasive species affect DOC dynamics in different native temperate macrophyte communities (Zostera noltei, Cymodocea nodosa and Caulerpa prolifera) from the south of the Iberian Peninsula, a transitional habitat between Atlantic and Mediterranean marine regimes. Additionally, we introduced a standardized framework to link DOC release to internal carbon content, facilitating comparisons of blue carbon pathways among macrophyte communities across diverse ecosystems. Controlled mesocosm experiments across three temperatures (24, 26 and 28 °C) revealed that the presence of the invasive seagrass Halophila stipulacea did not significantly alter the carbon metabolism or DOC fluxes of native macrophytes. However, temperature significantly affected both the quantity and composition of the released DOC. In particular, recalcitrant DOC decreased by 28%, while labile DOC increased by a similar proportion as temperature rose, and bioavailable DOC decay rates also declined significantly at higher incubation temperatures of the tested macrophytes. These results suggest that warming may enhance both net and labile DOC production, while the remaining DOC is less bioavailable than that produced at lower temperatures. This clearly indicates that warming restructures DOC composition, potentially reducing coastal carbon storage capacity and the role of recalcitrant DOC. By applying our proposed standardization, we estimate that the recalcitrant fraction produced in the tested macrophyte communities was comparable in magnitude, although 1.41 higher, to the carbon burial rates in the sediment measured in the same communities, which underscores the potential contribution of recalcitrant DOC produced by macrophyte communities to the long-term carbon storage. This standardized approach positions recalcitrant DOC as a crucial climate-sensitive blue carbon pathway that should be integrated into global carbon budget estimates.

Subterranean estuaries (i.e., seawater‐fresh groundwater mixing zones at coastal aquifers) are highly reactive boundaries between continental groundwater and coastal surface seawater. Because particulate organic matter is retained in shallow sediments, internal microbial transformations rely on dissolved organic matter (DOM) supply and bioavailability. Here, we investigated DOM carbon content and optical characteristics in two nearby subterranean estuaries with contrasting oxygenation. Coastal organic carbon processing in the anoxic subterranean estuary resulted in the export of DOM enriched in recalcitrant compounds compared to the oxygenated one, which was a net sink of DOM. This contrasting behavior was not driven by opposite redox conditions but from the fast transfer of labile DOM and oxygen to the beach interior of the oxygenated subterranean estuary. There, heterotrophic processes, which rely almost exclusively on DOM, are enhanced, resulting in net DOM consumption prior discharge to surface waters.