Description: Connectivity is a fundamental process driving the persistence of marine populations and their adaptation potential in response to environmental change. In this study, we analysed the population genetics of two morphologically highly similar deep-sea sponge clades (Phakellia hirondellei and the 'Topsentia-and-Petromica (TaP)' clade) at three locations in the Cantabrian Sea. Sponge taxonomy was assessed by spicule analyses, as well as by 18S sequencing and COI sequencing. The corresponding host microbiome was analysed by 16S rRNA gene sequencing. In addition we set up an oceanographic modelling framework, for which we used seawater flow cytometry data (derived from bottom depths of CTD casts) as ground-truthing data.

Description: This data set presents the results of an automated cluster analysis using Gaussian mixture models of the entire Atlantic seafloor environment. The analysis was based on eight global datasets and their derivatives: Bathymetry, slope, terrain ruggedness index, topographic position index, sediment thickness, POC flux, salinity, dissolved oxygen, temperature, current velocity, and phytoplankton abundance in surface waters along with seasonal variabilities (see Source data set). We obtained nine seabed areas (SBAs) that portray the Atlantic seafloor that are shown as polygons in the data set. The attribute table holds short descriptions of each SBA as well as about the colours used in the accompanying paper publication. Data sets like this can be used for further analysis like e.g. for landscape ecology metrics to identify regions of interest. The compressed file further contains a style file that can be used to directly load the correct style in the QGIS software package.

Description: A projection of larval dispersal patterns of Atlantic cold seep mussels Gigantidas childressi, G. mauritanicus, Bathymodiolus heckerae and B. boomerang was carried out for the next 50 years under the constraint of global warming predicted by the IPCC for the most pessismistic scenario. Simulations were run at +00 years, +25 years and +50 years from initial years of 2014 to 2019 (+00Y) at 21 locations on the US, European and African coasts using the VIKING20X model, in which the Atlantic water temperatures predicted by the FOCI model were forced to the future dates. The dataset consists of a number of 5775 simulations carried out over 5 years X 5 spawning dates per prediction period (+00Y, +25Y, +50Y) with, for predictions at +25Y and +50Y, a repetition of simulations per quantile (0.025, 0.16, 0.5, 0.67 and 0.975) to take into account for the most extreme variations in water mass temperatures predicted by the FOCI model for a given date.

Description: These data aim at evaluating the hypothesis of long-distance dispersal across the North Atlantic and the Equatorial Atlantic belt for the cold seep mussels Gigantidas childressi, G. mauritanicus, Bathymodiolus heckerae and B. boomerang. We combined mitochondrial Cox1 barcoding of some mussel specimens from both sides of the Atlantic (American vs European/African margins) with larval dispersal trajectories simulated from the VIKING20X model of the Atlantic circulation at a spatial scale not yet investigated. Larval dispersal modelling data correspond to transports of larvae over one year in surface waters from 21 geographic localities over 5 consecutive years (2015, 2016, 2017, 2018 and 2019) and 5 spawning dates (November, December, January, February and March) per year. Genetic data correspond to the geo-referenced sequences obtained for the 4 mussel species from some of the localities where larvae have been released during the modelling approach.

Description: The skill of numerical Lagrangian drifter trajectories in three numerical models is assessed by comparing these numerically obtained paths to the trajectories of drifting buoys in the real ocean. The skill assessment is performed using the two-sample Kolmogorov-Smirnov statistical test. To demonstrate the assessment procedure, it is applied to three different models of the Agulhas region. The test can either be performed using crossing positions of one-dimensional sections in order to test model performance in specific locations, or using the total two-dimensional data set of trajectories. The test yields four quantities: a binary decision of model skill, a confidence level which can be used as a measure of goodness-of-fit of the model, a test statistic which can be used to determine the sensitivity of the confidence level, and cumulative distribution functions that aid in the qualitative analysis. The ordering of models by their confidence levels is the same as the ordering based on the qualitative analysis, which suggests that the method is suited for model validation. Only one of the three models, a 1/10 degree two-way nested regional ocean model, might have skill in the Agulhas region. The other two models, a 1/2 degree global model and a 1/8 degree assimilative model, might have skill only on some sections in the region.

Description: Coordinated Ocean-ice Reference Experiments (COREs) are presented as a tool to explore the behaviour of global ocean-ice models under forcing from a common atmospheric dataset. We highlight issues arising when designing coupled global ocean and sea ice experiments, such as difficulties formulating a consistent forcing methodology and experimental protocol. Particular focus is given to the hydrological forcing, the details of which are key to realizing simulations with stable meridional overturning circulations. The atmospheric forcing from [Large, W., Yeager, S., 2004. Diurnal to decadal global forcing for ocean and sea-ice models: the data sets and flux climatologies. NCAR Technical Note: NCAR/TN-460+STR. CGD Division of the National Center for Atmospheric Research] was developed for coupled-ocean and sea ice models. We found it to be suitable for our purposes, even though its evaluation originally focussed more on the ocean than on the sea-ice. Simulations with this atmospheric forcing are presented from seven global ocean-ice models using the CORE-I design (repeating annual cycle of atmospheric forcing for 500 years). These simulations test the hypothesis that global ocean-ice models run under the same atmospheric state produce qualitatively similar simulations. The validity of this hypothesis is shown to depend on the chosen diagnostic. The CORE simulations provide feedback to the fidelity of the atmospheric forcing and model configuration, with identification of biases promoting avenues for forcing dataset and/or model development.

Description: The greater Agulhas Current system has several components with high mesoscale turbulence. The phytoplankton distribution in the southwest Indian Ocean reflects this activity. We have used a regional eddy-permitting, coupled physical–biological model to study the physical–biological interactions and to address the main processes responsible for phytoplankton distribution in three different biogeochemical provinces: the southwest Subtropical Indian Gyre (SWSIG), the subtropical convergence zone (SCZ) and the subantarctic waters (SAW) south of South Africa. The biological model with four compartments (Nitrate–Phytoplankton–Zooplankton–Detritus) adequately reproduces the observed field of chlorophyll a. The phase of the strong modelled seasonality in the SWSIG is opposite to that of the SCZ that forms the southern boundary of the subtropical gyre. Phytoplankton concentrations are governed by the source-minus-sink terms, which are one order of magnitude greater than the dynamical diffusion and advection terms. North of 35°S, in the SWSIG, phytoplankton growth is limited by nutrients supply throughout the year. However, deeper stratification, enhanced cross-frontal transport and higher detritus remineralization explain the simulated higher concentrations of phytoplankton found in winter in the SWSIG. The region between 35° and 40°S constitutes a transition zone between the SCZ and the oligotrophic subtropical province. Horizontal advection is the main process bringing nutrients for phytoplankton growth. The front at 34°S represents a dynamical barrier to an extension further to the north of this advection of nutrients. Within the SCZ, primary production is high during spring and summer. This high productivity depletes the nutrient standing stock built up during winter time. In winter, nutrients supply in the convergence zone is indeed large, but the deep mixing removes phytoplankton from the euphotic zone and inhibits photosynthesis, yielding lower surface chlorophyll a concentrations. Waters south of the Subantarctic Front have a summer biomass close to that of frontal waters and higher than for subtropical waters. However, these simulated concentrations are slightly higher than the observed ones suggesting that limitation by iron and/or silica may play a role

Description: Highlights: • Fan-shaped sponges display panmixia at three locations in the Cantabrian Sea. • Subtle sponge population genetic and pronounced microbial differences were observed between a canyon and bank (〈100km apart). • Lagrangian modelling reveals variable inter-annual connectivity via ocean currents between the sampling regions. • Interdisciplinary approaches can help to improve understanding about connectivity in the deep-sea. Abstract: Connectivity is a fundamental process driving the persistence of marine populations and their adaptation potential in response to environmental change. In this study, we analysed the population genetics of two morphologically highly similar deep-sea sponge clades (Phakellia hirondellei and the ‘Topsentia-and-Petromica’ clade, (hereafter referred to as ‘TaP clade’)) at three locations in the Cantabrian Sea and simultaneously assessed the corresponding host microbiome by 16S rRNA gene sequencing. A virtual particle tracking approach (Lagrangian modelling) was applied to assess oceanographic connectivity in the study area. We observed overall genetic uniformity for both sponge clades. Notably, subtle genetic differences were observed for sponges of the TaP clade and also their microbiomes between a canyon and bank location, 〈 100 km apart and with the same depth range. The Lagrangian model output suggests a strong retention of larvae in the study area with variable inter-annual connectivity via currents between the three sampling regions. We conclude that geologic features (canyons) and the prevailing ocean currents may dictate sponge holobiont connectivity and that differentiation can emerge even on small spatial scales.

Description: The Indian Ocean is an important conduit for the exchange of physical and biogeochemical properties through many distinct interbasin oceanic connections. The Indonesian archipelago provides a gappy pathway for warm tropical waters to enter the Indian Ocean from the Pacific. South of Australia, a complex circulation transports cooler subtropical waters from the Pacific while Indian Ocean waters from within the Leeuwin Current feed a series of currents along the southern Australian continental margin. Southern Ocean waters source both the deep and shallow overturning circulations into the Indian Ocean. The westward leakage of eddies spawned from the Agulhas Current off South Africa returns warm and salty Indian Ocean waters into the Atlantic and plays a significant role in the upper branch of the global meridional overturning circulation. This chapter discusses these pathways and highlights how they change with time and influence the circulation and properties of the Indian and global oceans.