A total of fourteen hydrographic cruises from 2000 to 2008 were conducted during the spring and autumn seasons between Spain and the Southern Ocean under the framework of the Spanish research project FICARAM. The underway measurements were processed and analysed to describe the meridional air-sea CO₂ fluxes (FCO₂) in the covered sector of the Atlantic Ocean. The data has been grouped into different biogeochemical oceanographic provinces based on thermohaline characteristics. The spatial and temporal distributions of FCO₂ followed expected distributions and annual trends reproducing the recent climatological ΔfCO₂ estimations with a mean difference of −3 ± 18 μatm (Takahashi et al., 2009). The reduction in the CO₂ saturation along the meridional FICARAM cruises represented an increase of 0.02 ± 0.14 mol m⁻² yr⁻¹ in the ocean uptake of atmospheric CO₂. The subtropical waters in both Hemispheres acted as a sink of atmospheric CO₂ during the successive spring seasons and as a source in autumn. The coarse reduction of the ocean uptake of atmospheric CO₂ observed in the North Atlantic Ocean was linked to conditions of negative phase of the North Atlantic Oscillation that prevailed during the FICARAM period. Surface waters in the North Equatorial Counter Current revealed a significant long-term decrease of sea surface salinity of −0.16 ± 0.01 yr⁻¹ coinciding with a declination of −3.5 ± 0.9 μatm yr⁻¹ in the air–sea disequilibrium of CO₂ fugacity and a rise of oceanic CO₂ uptake of −0.09 ± 0.03 mol m⁻² yr⁻¹. The largest CO₂ source was located in the equatorial upwelling system. These tropical waters that reached emissions of 0.7 ± 0.5 and 1.0 ± 0.7 mol m⁻² y⁻¹ in spring and autumn, respectively, showed an interannual warming of 0.11 ± 0.03 °C yr⁻¹ and a wind speed decrease of −0.58 ± 0.14 m s⁻¹ yr⁻¹ in spring cruises which suggest the weakening of upwelling events associated with warm El Niño – Southern Oscillation episodes. Contrary the surface waters of the Patagonian Sea behaved as an intense sink of CO₂ in March and November. The oceanic waters of the convergence of Falkland and Brazil Currents showed the strongest CO₂ absorption with a rate of −5.4 ± 3.6 mol m⁻² yr⁻¹ in November. The Southern Oceans sampled in the Drake Passage behave as an average uptake rate of −1.1 ± 0.9 mol m⁻² yr⁻¹ while the distal shelf of the Livingston Island acted as a slight source of CO₂ to the atmosphere.

A data product of 17 653 discrete samples from 3343 oceanographic stations combining measurements of pH, alkalinity and other biogeochemical parameters off the northwestern Iberian Peninsula from June 1976 to September 2018 is presented in this study. The oceanography cruises funded by 24 projects were primarily carried out in the Ría de Vigo coastal inlet but also in an area ranging from the Bay of Biscay to the Portuguese coast. The robust seasonal cycles and long-term trends were only calculated along a longitudinal section, gathering data from the coastal and oceanic zone of the Iberian upwelling system. The pH in the surface waters of these separated regions, which were highly variable due to intense photosynthesis and the remineralization of organic matter, showed an interannual acidification ranging from −0.0012 to −0.0039 yr⁻¹ that grew towards the coastline. This result is obtained despite the buffering capacity increasing in the coastal waters further inland as shown by the increase in alkalinity by 1.1±0.7 and 2.6±1.0 µmol kg⁻¹ yr⁻¹ in the inner and outer Ría de Vigo respectively, driven by interannual changes in the surface salinity of 0.0193±0.0056 and 0.0426±0.016 psu yr⁻¹ respectively. The loss of the vertical salinity gradient in the long-term trend in the inner ria was consistent with other significant biogeochemical changes such as a lower oxygen concentration and fertilization of the surface waters. These findings seem to be related to a growing footprint of sediment remineralization of organic matter in the surface layer of a more homogeneous water column. Data are available at https://doi.org/10.20350/digitalCSIC/12498 (Pérez et al., 2020).

The impact of simulated seawater acidification and warming conditions on specimens of the mussel Mytilus galloprovincialis locally adapted to very distinct, widely separated sites in the Mediterranean Sea (Tunisia) and Atlantic Sea (Galicia, NW Spain) was evaluated in relation to key behavioural and eco-physiological parameters. Over the 2-month exposure to the experimental conditions, mussels were fed optimally to ensure that there are no synergistic interactions between climate change drivers and energetic status of the individuals. In general, regardless of origin (Atlantic or Mediterranean), the mussels were rather resilient to acidification for most of the parameters considered and they were able to grow in strongly acidified seawater through an increased feeding activity. However, shell strength decreased (40%) consistently in both mussel populations held in moderately and highly acidified seawater. The observed reduction in shell strength was not explained by slight alterations in organic matter, shell thickness or aragonite:calcite ratio. The combined effects of high acidification and warming on the key response of byssus strength caused a strong decline in mussel performance, although only in Galician mussels, in which the valve opening time decreased sharply as well as condition index (soft tissue state) and shell growth. By contrast, the observed negative effect of highly acidified scenario on the strength of Tunisian mussel shells was (partly but not totally) counterbalanced by the higher seawater temperature. Eco-physiological and behavioural interactions in mussels in relation to climate change are complex, and future scenarios for the ecology of the species and also the feasibility of cultivating them in Atlantic and Mediterranean zones are discussed.

A general decline in foundation species at the rocky intertidal has been observed during the last decades all around the world and primarily related to climate change. In agreement with that trend, the mussel aquaculture sector in Galicia (NW Spain), the main production area in Europe, has warned over the last years about a decline in the availability of wild mussel seed from the rocky coast. Here we compile for the first time, mussel seed collection reports by mussel farmers in Galicia for the period 2006–2021. We employed that dataset as a proxy of mussel recruitment evolution in the rocky shore for the last 16 years. Temporal analysis of our data confirmed the reported decline (-148 t yr-1), particularly pronounced from 2012 onwards. The data base also allowed us to analyze inter-annual variability according to both, climatic variations and management scenarios. Since cultivated mussels conform a meta-population with wild mussels from the rocky shore, alterations on the market preferences towards smaller individuals at harvest, could also contribute to a reduction in reproductive output. Our results show a decrease in life-time egg production under certain scenarios. Nonetheless, coastal upwelling seems to be the largest factor conditioning recruitment abundance, explaining as much as 60% of the variability observed. Decline on recruitment abundance was highly modulated by the observed increment in frequency of intense upwelling events, exceeding 500 m3 km-1 s-1 between July and November. Meridional winds also determined the spatial recruitment patterns, pointing to the large role of wind forcing on mussel larval dispersal. Our results highlights how alterations on upwelling regimes related to climate change can interact with mussel population dynamics and also condition aquaculture sustainability and food security.

Local environmental conditions can have strong effects on ecophysiological responses in bivalve molluscs. Molluscs must allocate the energetic resources acquired in their habitats to survival and fitness. In this study, we investigated the spatial and temporal patterns of response in relation to byssus strength and soft tissue weight during the maximum growth stage in the subtidal mussel Mytilus galloprovincialis between March and July 2015 in the Ría de Arousa (NW Spain). Mussels growing in high‐energy environments secreted stronger byssus than those inhabiting low‐energy sites. The integrated values of wave height and food availability for the previous 15 days showed a positive relationship explaining 45% and 9% of total variability in tenacity of the byssus threads. Differences in condition index were greatest between the northern (highest values) and southern sites at the mouth of the ria owing to differences in food availability. Despite similar hydrodynamic energy values, the high food availability in the exposed northern zones supplied sufficient energetic resources for both strong byssus secretion and high CI. Despite lower food availability, high CI values were also reached in mussels in the innermost, sheltered sites, because of the low need for survival with strong attachment. The condition index was positively correlated with temperature and food availability integrated during 45 previous days prior explained a 57% and 13% of total variance, respectively, while wave height explained 2% but with an inverse relationship. Environmental differences between the geographical zones of the estuary modulated byssus secretion functionality affecting the soft tissues status of mussels.

Marine mussel aggregations act as a substratum and refuge for many fouling species. Mussel cultivation in Galicia, Spain, is carried out on hanging ropes in subtidal systems. The fauna associated with this cultivation includes a large number of invertebrates that compete for space or food with the mussels, or use their clusters as a refuge from predators or water turbulence. Outbreaks of the epibiont anemone Actinothoe sphyrodeta have been reported in cultivated Galician mussels since 2013, but their impact has not been investigated rigorously. Here, the temporal and spatial variability of Actinothoe sphyrodeta on mussel shells throughout one year is presented. Sampling of mussel size, weight and byssus attachment strength allowed mussel tenacity (attachment strength relative to size) to be calculated. A higher presence of Actinothoe sphyrodeta correlated with lower mussel tenacity and greater biomass losses, suggesting that this species could be an economically important biofouling component.