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Subsurface chlorophyll maximum layers (SCMLs) are nearly ubiquitous in stratified water columns and exist at horizontal scales ranging from the submesoscale to the extent of oligotrophic gyres. These layers of heightened chlorophyll and/or phytoplankton concentrations are generally thought to be a consequence of a balance between light energy from above and a limiting nutrient flux from below, typically nitrate (NO3). Here we present multiple lines of evidence demonstrating that iron (Fe) limits or with light colimits phytoplankton communities in SCMLs along a primary productivity gradient from coastal to oligotrophic offshore waters in the southern California Current ecosystem. SCML phytoplankton responded markedly to added Fe or Fe/light in experimental incubations and transcripts of diatom and picoeukaryote Fe stress genes were strikingly abundant in SCML metatranscriptomes. Using a biogeochemical proxy with data from a 40-y time series, we find that diatoms growing in California Current SCMLs are persistently Fe deficient during the spring and summer growing season.We also find that the spatial extent of Fe deficiency within California Current SCMLs has significantly increased over the last 25 y in line with a regional climate index. Finally, we show that diatom Fe deficiency may be common in the subsurface of major upwelling zones worldwide. Our results have important implications for our understanding of the biogeochemical consequences of marine SCML formation and maintenance.

Global ocean primary production (PP) is a function of both light and nutrient availability. The vertical distribution of nutrients in the euphotic zone differs in both time and space. As a result, the vertical distribution of PP varies as well. Differences in the vertical distribution of PP have not, however, been systematically studied. Here, we focus on the open ocean and use nutricline depth, DNO3 (defined as the depth where [NO₃⁻] = 1 μmol kg−1), as a proxy for nutrient availability in the euphotic zone. Using our own and archived (WOD, HOT, BATS, CARIACO) data, we show universal relationships between DNO3 and (1) depth of the deep chlorophyll maximum (DCM), (2) total water column PP and (3) vertical distribution of PP. When DNO3 is located between ~20 and 90 m, the DCM and DNO3 are juxtaposed. However, the DCM is located above nutriclines found at > ~90 m. The observed relationships between DCM and DNO3 depths can be explained with a simple model including light and nutrient limitation. The global PP estimates indicate that ~25% of ocean PP occurs in the upper 10 m. Estimating total global ocean PP from surface optical characteristics and the relationship between vertical PP distribution and DNO3 indicates that oligotrophic regions of the ocean may be more productive than usually assumed. The relationship shown here between water column PP and DNO3 suggests that considering stratification characteristics in a future ocean is critical for predicting climate change effects on global PP.

ContextEcotones are boundary zones formed where overlap between neighboring ecosystems creates an intermediate ecosystem with unique ecological characteristics. Dynamic ecotones change position along a boundary over time and can be further categorized as either shifting, where the adjacent ecosystems alternatively drive movement of the ecotone but maintain the same relative location over time, or directional, where one system encroaches into the other and the ecotone moves laterally.ObjectivesThe purpose of this work was to examine how climate change alters movement dynamics of both directional and shifting ecotones.MethodsIn three ecosystem case studies, we examine the effects of climate change on landscape-scale ecotone movement across the marine, terrestrial, and interfacing environments.ResultsShifts in local and global climate drive changes in ecotone patterns, increasing directional ecotone movement at both shifting and directional ecotones. Specifically, unidirectional changes in climate patterns disrupt dynamic equilibria at shifting ecosystem boundaries, thereby facilitating unidirectional movement at the previously shifting boundaries. Climate changes additionally accelerate pre-existing directional migration of ecotones through changes to abiotic gradients.ConclusionDirectional climate change increases directional movement in multiple types of ecotone. Future work should consider the rate and feedback mechanisms of ecotone movement and function at additional ecotones.

The deep chlorophyll maximum (DCM) is a well-known feature of the global ocean. However, its description and the study of its formation are a challenge, especially in the peculiar environment that is the Black Sea. The retrieval of chlorophyll a (chl a) from fluorescence (Fluo) profiles recorded by Biogeochemical Argo (BGC-Argo) floats is not trivial in the Black Sea, due to the very high content of coloured dissolved organic matter (CDOM) which contributes to the fluorescence signal and produces an apparent increase in the chl a concentration with depth. Here, we revised Fluo correction protocols for the Black Sea context using co-located in situ high-performance liquid chromatography (HPLC) and BGC-Argo measurements. The processed set of chl a data (2014–2019) is then used to provide a systematic description of the seasonal DCM dynamics in the Black Sea and to explore different hypotheses concerning the mechanisms underlying its development. Our results show that the corrections applied to the chl a profiles are consistent with HPLC data. In the Black Sea, the DCM begins to form in March, throughout the basin, at a density level set by the previous winter mixed layer. During a first phase (April–May), the DCM remains attached to this particular layer. The spatial homogeneity of this feature suggests a hysteresis mechanism, i.e. that the DCM structure locally influences environmental conditions rather than adapting instantaneously to external factors. In a second phase (July–September), the DCM migrates upward, where there is higher irradiance, which suggests the interplay of biotic factors. Overall, the DCM concentrates around 45 % to 65 % of the total chlorophyll content within a 10 m layer centred around a depth of 30 to 40 m, which stresses the importance of considering DCM dynamics when evaluating phytoplankton productivity at basin scale.

The Malaspina-2010 circumnavigation expedition on board R/V Hesperides surveyed tropical and subtropical regions of the Atlantic, Indian and Pacific oceans between December 2010 and July 2011. This article examines the relationships between the distribution of chlorophyll a (Chl a), major inorganic nutrients and other hydrographic variables. A deep chlorophyll maximum (DCM) was found at most stations between 60 and 150 m depth; it occurred close to the level of 1% surface photosynthetically active radiation and was associated with the nitracline. There was a negative relationship between total Chl a at surface and the DCM depth, and between Chl a concentration at the DCM and DCM depth. In terms of Chl a concentration, picophytoplankton was the dominant size class at all sampled light intensities (surface, 20% of surface PAR and PAR at DCM), oceans and geoclimatic zones, except at some stations influenced by upwellings or divergences. Within the Chl a concentration ranges found in this study, the proportion of picophytoplankton increased with total Chl a, in contrast with some previous findings. Vertically integrated Chl a was positively correlated with surface Chl a, with similar slopes for the whole data set and for the different oceans and zones. In turn, surface Chl a and sea surface temperature showed a negative correlation for the Indian Ocean and the subtropical zone, a positive correlation for the Atlantic, and non-significant relationships for the remaining oceans and zones.

Background The photic zone of aquatic habitats is subjected to strong physicochemical gradients. To analyze the fine-scale variations in the marine microbiome, we collected seven samples from a single offshore location in the Mediterranean at 15 m depth intervals during a period of strong stratification, as well as two more samples during the winter when the photic water column was mixed. We were able to recover 94 new metagenome-assembled genomes (MAGs) from these metagenomes and examine the distribution of key marine microbes within the photic zone using metagenomic recruitment. Results Our results showed significant differences in the microbial composition of different layers within the stratified photic water column. The majority of microorganisms were confined to discreet horizontal layers of no more than 30 m (stenobathic). Only a few such as members of the SAR11 clade appeared at all depths (eurybathic). During the winter mixing period, only some groups of bloomers such as Pseudomonas were favored. Although most microbes appeared in both seasons, some groups like the SAR116 clade and some Bacteroidetes and Verrucomicrobia seemed to disappear during the mixing period. Furthermore, we found that some microbes previously considered seasonal (e.g., Archaea or Actinobacteria) were living in deeper layers within the photic zone during the stratification period. A strong depth-related specialization was detected, not only at the taxonomic level but also at the functional level, even within the different clades, for the manipulation and uptake of specific polysaccharides. Rhodopsin sequences (green or blue) also showed narrow depth distributions that correlated with the taxonomy of the microbe in which they were found but not with depth. Conclusions Although limited to a single location in the Mediterranean, this study has profound implications for our understanding of how marine microbial communities vary with depth within the photic zone when stratified. Our results highlight the importance of collecting samples at different depths in the water column when comparing seasonal variations and have important ramifications for global marine studies that most often take samples from only one single depth. Furthermore, our perspective and approaches (metagenomic assembly and recruitment) are broadly applicable to other metagenomic studies.

This study investigates the deep chlorophyll maximum and physico-chemical properties of the water column to delineate the underlying relation between vertical thermal structure and chlorophyll maxima. The biophysical variability in the Andaman Sea can be characterized as (i) enhanced Chl due to mixing and sediment resuspension in Northeastern Andaman Sea (ii) depressed Chl (0 < Chl < 0.5 mg/m3) in the offshore surface waters; (iii) deep Chl maxima (DCM; 0.5 < Chl < 3.3 mg/m3) observed below the surface mixed layer in the depth range 40–80 m; and (iv) well defined oxygen minima (DO < 3 mL/L) below the DCM. The empirical relation between subsurface temperature and Chl suggests a significant correlation (R2 = 0.78–0.96) indicating optimum light and nutrient conditions for deep accumulation of phytoplankton which linearly decreases down the water column. Future work has to be carried out to understand the underlying relationship between temperature and chlorophyll in coastal and shelf waters.

Stratified marine systems are often characterized by a deep chlorophyll maximum (DCM); however, the taxonomic and functional dynamics of protist assemblages within this layer remain poorly understood. We integrated microscopy, pigment-based CHEMTAX analysis, 18S rRNA metabarcoding, and metatranscriptomics to compare protist communities in the surface and DCM layers of the northeastern East China Sea. Microscopy and pigment data revealed higher cell abundances, increased chlorophyll- a levels, and distinct pigment signatures at the DCM, particularly for haptophytes, chlorophytes, and pelagophytes. Amplicon sequencing revealed increased representation of chlorophyte and Syndiniales at depth, whereas metatranscriptomic profiles showed elevated transcriptional activity in diatoms, dinoflagellates, and chlorophytes. Functional gene analyses revealed DCM-specific upregulation of photosystem I subunits, light-harvesting complex proteins, and nitrogen assimilation pathways, indicating photoacclimation and nutrient exploitation under low-light, nutrient-rich conditions. Syndiniales were abundant in DNA-based data but mostly transcriptionally inactive, suggesting dormancy or parasitic stages, while diatoms exhibited high transcriptional activity despite low DNA abundance. These findings indicate a clear decoupling between taxonomic presence and metabolic activity, emphasizing that ecological roles cannot be inferred from abundance alone. Our findings identify the DCM as a biogeochemical hotspot shaped by taxon-specific metabolic strategies and vertical niche partitioning, underscoring the key role of protists in sustaining productivity and carbon cycling in stratified ocean ecosystems.

Photosynthetic O2 production can be an important source of oxygen in sub-surface ocean waters especially in permanently stratified oligotrophic regions of the ocean where O2 produced in deep chlorophyll maxima (DCM) is not likely to be outgassed. Today, permanently stratified regions extend across approximately 40% of the global ocean and their extent is expected to increase in a warmer ocean. Thus, predicting future ocean oxygen conditions requires a better understanding of the potential response of photosynthetic oxygen production to a warmer ocean. Based on our own and published observations of water column processes in oligotrophic regions, we develop a one-dimensional water column model describing photosynthetic oxygen production in the Sargasso Sea to quantify the importance of photosynthesis for the downward flux of O2 and examine how it may be influenced in a warmer ocean. Photosynthesis is driven in the model by vertical mixing of nutrients (including eddy-induced mixing) and diazotrophy and is found to substantially increase the downward O2 flux relative to physical–chemical processes alone. Warming (2°C) surface waters does not significantly change oxygen production at the DCM. Nor does a 15% increase in re-mineralization rate (assuming Q10 = 2; 2°C warming) have significant effect on net sub-surface oxygen accumulation. However, changes in the relative production of particulate (POM) and dissolved organic material (DOM) generate relatively large changes in net sub-surface oxygen production. As POM/DOM production is a function of plankton community composition, this implies plankton biodiversity and food web structure may be important factors influencing O2 production in a warmer ocean. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.