Key Expoitable Results (KERs)

The educational animation series “Max and his friends Discovering the Sea” consists of three episodes: 1. The Sea we breathe, 2. Plastic on its way to the sea, and 3. Plastic on its way to the sea.

While a rich history of patchiness research has explored spatial structure in the ocean, there is no consensus over the controls on biological patchiness and how physical-ecological-biogeochemical processes and patchiness relate. The prevailing thought is that physics structures biology, but this has not been tested at basin scale with consistent in situ measurements. Here we use the slope of the relationship between variance vs spatial scale to quantify patchiness and ~650,000 nearly continuous (dx ~ 200 m) measurements - representing the Atlantic, Pacific, and Southern Oceans - and find that patchiness of biological parameters and physical parameters are uncorrelated. We show variance slope is an emergent property with unique patterns in biogeochemical properties distinct from physical tracers, yet correlated with other biological tracers. These results provide context for decades of observations with different interpretations, suggest the use of spatial tests of biogeochemical model parameterizations, and open the way for studies into processes regulating the observed patterns.

Copepods are a major group of the mesozooplankton and thus a key part of marine ecosystems worldwide. Their fitness and life strategies are determined by their functional traits which allow different species to exploit various ecological niches. The range of functional traits expressed in a community defines its functional diversity (FD), which can be used to investigate how communities utilize resources and shape ecosystem processes. However, the spatial patterns of copepod FD and their relation to ecosystem functioning remain poorly understood on a global scale. Here, we use estimates of copepod community composition derived from species distribution models in combination with functional traits and indicators of ecosystem functioning to investigate the distribution of multiple facets of copepod FD, their relationships with species richness and ecosystem processes. We also project how anthropogenic climate change will impact the facets of copepod FD. We find that the facets of FD respond to species richness with variable strength and directions: functional richness, divergence, and dispersion increase with species richness whereas functional evenness and trait dissimilarity decrease. We find that primary production, mesozooplankton biomass and carbon export efficiency decrease with species richness, functional richness, divergence and dispersion. This suggests that ecosystem functioning may be disproportionally influenced by the traits of a few dominant species in line with the mass ratio hypothesis. Furthermore, climate change is projected to promote trait homogenization globally, which may decrease mesozooplankton biomass and carbon export efficiency globally. The emergent covariance patterns between copepod FD and ecosystem functions we find here strongly call for better integrating FD measurements into field studies and across scales to understand the effects of changing zooplankton biodiversity on marine ecosystem functioning.

The Southern Ocean hosts a high degree of endemic plants and animals, yet the genetic diversity, function and evolutionary relationships of microbial communities remains unexplored, particularly in the aphotic “dark ocean,” where microbes play critical roles in local and global food webs. Here, we performed a metagenomic analysis of 44 aphotic seawater samples collected from multiple depths across the Southern Ocean to characterize the functional gene repertoire of these microbial communities. Of the 11,896,546 species-level unigenes1 identified, ~ 87% appear specific to the Southern Ocean and are distinct from other major ocean datasets. We reconstructed 502 bacterial and 108 archaeal metagenome-assembled genomes (MAGs), revealing widespread capacities for inorganic carbon fixation via the Calvin cycle, the hydroxypropionate-hydroxybutyrate cycle, and the 3-hydroxypropionate bi-cycle. Metapangenomic analyses indicated that several genes involved in the oxidation of reduced nutrients including ammonia, nitrite, and thiosulfate, are shared across the aphotic water column through horizontal gene transfer. MAGs belonging to Acidimicrobia, Gammaproteobacteria, and SAR324 were abundant throughout the dark Southern Ocean and showed potential for both chemolithoautotrophy and carbohydrate degradation, suggesting mixotrophy as a key metabolic strategy. Together, these findings reveal the unique functional and genomic diversity of deep Southern Ocean microbiomes and provide insights into their roles in carbon cycling within one of Earth’s most important marine carbon sinks.

Understanding how climate change affects marine primary productivity requires examining the evolving causal relationships between physical and biogeochemical processes. We applied the PCMCI+ causal discovery algorithm to investigate how the mechanisms controlling Net Primary Productivity (NPP) in the North Atlantic Subpolar Gyre evolve under different climate scenarios across five Earth System Models. Using 100-year sliding windows, we compare causal relationships in future scenarios against pre-industrial conditions, focusing on the roles of mixed layer nutrients, vertical mixing and horizontal transport. Our analysis reveals three main categories of relationship evolution: the emergence of links, the disappearance of links, and changes in link strengths. For example, while the link between stratification and NPP emerges under climate change in CanESM5-CanOE, it strengthens in CMCC-ESM2 and remains stable with moderate to high intensities in other models. At the end of the 21st century, the spread between models regarding the effect of stratification on NPP is reduced compared to pre- industrial conditions, suggesting a reduction in inter-model uncertainty. However, the transport and vertical mixing controls on the supply of nutrients to the mixed layer exhibit a more diverse evolution among the ESMs studied. The CMCC-ESM2 model has a strengthening of the relationships between winter vertical mixing and nutrients, while IPSL-CM6A-LR and CanESM5CanOE show weakening of these relationships. Furthermore, the evolution of the link between nutrient supply to the mixed layer for NPP exhibits a large variability between models. These divergent pathways reveal that the dynamics of nutrients has uncertain evolution between models. Lastly, model-specific dynamics are also observed, such as the strengthening of the link between horizontal transport and the nutrient content of the mixed layer in IPSL-CM6A-LR. Together with the decreasing strength of the vertical mixing/nutrients link, this suggests the presence of compensation mechanisms and a shift from vertical mixing dominance to enhanced horizontal transport control over the course of the scenario. These findings offer mechanistic insights into the dynamics of ESMs, specifically in the evolving relationships between physical and biogeochemical processes that shape the projections of NPP and nutrients. The causality-based approach identifies mechanisms that traditional analyses miss, offering a novel framework for model intercomparison and understanding ecosystem responses to climate change.

Interactions between mesoscale eddies and seamounts can strongly modify local circulation, influencing nutrient transport and planktonic communities. This study investigates the coupling between hydrodynamics features and zooplankton dynamics along the Vitória–Trindade Seamount Chain (VTC) in the southwestern Atlantic, using hydrographic (CTD) measurements and Underwater Vision Profiler (UVP5) observations collected along two transects: one crossing the VTC and a second located slightly south of the chain. Flow–topography interactions involving the Vitória Eddy (VE) and regional seamounts were found to modulate chlorophyll-a distributions and zooplankton abundance. Although classical seamount-induced upwelling was not detected, elevated chlorophyll-a concentrations within thermocline depths and downstream of Davis Bank indicate enhanced vertical mixing associated with flow–seamount interactions and eddy deformation. Rhizaria numerically dominated the zooplankton community, particularly within the euphotic zone and thermocline layers. Analysis of the normalized biovolume size spectrum (NBSS) revealed flatter slopes and higher intercepts near the shelf and in VE-influenced regions, indicating a greater contribution of large-bodied organisms and higher trophic transfer efficiency. In contrast, offshore and seamount-only stations exhibited steeper size spectra consistent with oligotrophic, energy-limited conditions. Particulate organic carbon (POC) fluxes were highest at stations influenced by the VE, reflecting the combined effects of lateral shelf inputs and particle retention driven by flow–topography interactions. Enhanced subsurface and seamount-associated POC fluxes further highlight the joint role of mesoscale eddies and seamounts in regulating carbon export along the VTC.