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The browning of coastal waters has emerged as one of the more visually striking signatures of global change in the marine realm. Across large stretches of the Northern Hemisphere, increasing runoff from land delivers terrigenous dissolved organic matter to the sea, staining coastal waters a tea-like amber colour and fundamentally altering the physics and chemistry of the underwater environment. This material, collectively referred to as coloured dissolved organic matter, or CDOM, originates largely from the decomposition of plant material in soils, wetlands, and forests. As precipitation patterns intensify and permafrost thaws, the flux of this terrestrial organic carbon into rivers and ultimately the coastal ocean is expected to rise substantially. While limnologists have studied lake browning for decades, its consequences for marine plankton communities have received comparatively less attention, partly because the open ocean has traditionally been viewed as too distant from terrestrial influences to be affected. The new mesocosm findings underscore that this assumption no longer holds for many coastal seas, where the interplay between land-derived organic matter and trace metal chemistry can reorganise the base of the food web.
The experimental approach behind these findings deserves particular attention because of the realism it offers. Mesocosm experiments occupy a valuable middle ground between laboratory cultures and field observations, allowing researchers to manipulate single variables while keeping an otherwise natural plankton community intact. In this study, large enclosed volumes of coastal seawater received additions of a humic substance as a source of CDOM and deferoxamine B, a strong iron-binding ligand, as a means of manipulating the speciation of dissolved iron. Deferoxamine B is a siderophore produced naturally by bacteria to acquire iron, and its use in experiments allows researchers to control how much iron remains biologically available. By combining these two treatments in a fully crossed design over 22 days, the team could separate the effects of light attenuation caused by browning from the effects of altered iron chemistry, and crucially, examine their interactions. This is important because in the real ocean these two stressors do not arrive independently; humic substances are themselves complexing agents that bind trace metals, meaning that increased terrigenous organic matter input changes not only the light field but also the chemical form and availability of iron.
The optical consequences of browning are more nuanced than simple darkening. CDOM absorbs strongly in the ultraviolet and blue portions of the spectrum, the very wavelengths that penetrate deepest in clear water and that many phytoplankton pigments exploit most efficiently. As terrigenous CDOM accumulates, the euphotic zone shoals, the spectral quality of available light shifts toward the green, and total photons reaching photosynthetic organisms decline. For large, slowly sinking phytoplankton species that depend on well-lit surface layers, these changes can be directly detrimental. Yet the experiment revealed a paradox: during the first eight days, small-sized phytoplankton actually increased their biomass under browning despite these unfavourable light conditions. The explanation lies in the dual nature of dissolved organic matter as both a screen and a substrate. Small phytoplankton and mixotrophic organisms can exploit dissolved organic compounds directly as a supplementary carbon and nutrient source, a capacity that compensates for reduced photosynthetic income. At the same time, the biomass gains suggest that growth under these conditions outpaced the grazing pressure exerted by microzooplankton, indicating that the net effect of browning on the smallest primary producers was initially positive rather than negative.
Iron chemistry sits at the heart of why this matters for ocean biogeochemistry. Iron is an essential micronutrient for phytoplankton, required for photosynthetic electron transport chains, nitrogen assimilation enzymes, and respiratory proteins. In vast regions of the ocean, iron availability limits primary production, and even in coastal waters where total iron concentrations are higher, the fraction that is truly bioavailable is governed by complexation with organic ligands. More than 99 percent of dissolved iron in seawater is typically bound to organic ligands, and the identity and origin of these ligands strongly influence whether microorganisms can access the metal. Terrestrial humic substances are increasingly recognised as important coastal ligands for iron. The experimental use of deferoxamine B to manipulate dissolved iron speciation therefore mimics a natural process: as browning intensifies, the organic ligand pool in coastal waters is augmented with terrigenous compounds that can hold iron in solution and potentially deliver it across microbial cell membranes. The finding that both phytoplankton succession and microzooplankton dynamics responded to altered iron speciation demonstrates that climate-driven changes in organic matter inputs propagate through the food web partly via trace metal chemistry, a pathway that is often overlooked in favour of purely optical or carbon-centric framing.
The temporal structure of the observed responses carries its own lessons. The benefits of browning for small phytoplankton were confined to roughly the first half of the experiment; during the second half, no changes attributable to browning were detected. This transient response pattern likely reflects the exhaustion of labile components of the added organic matter, adaptive adjustments by grazers, or the reorganisation of the community toward a new quasi-equilibrium. It also echoes a recurring theme in mesocosm research: the earliest days of a manipulation often capture the most dramatic effects, while communities buffer and reorganise over time. For ecosystem managers and modellers, this suggests that the consequences of pulsed terrigenous organic matter inputs, such as those following storm-driven runoff events, may be most pronounced in the days immediately following the pulse, even if the standing community recovers subsequently.
Perhaps the most consequential phase of the experiment came with the crash of both phytoplankton and microzooplankton populations under intense mesozooplankton grazing. This top-down control illustrates a fundamental feature of plankton ecosystems: the fate of bottom-up stimulation, whether from nutrient enrichment or organic matter subsidisation, is ultimately determined by the grazing community. Copepods and other mesozooplankton graze not only the phytoplankton directly but also, through their consumption of microzooplankton, exert cascading control over the microbial loop. The authors’ hypothesis that browning and altered iron availability could favour the vertical migration of mesozooplankton toward the surface in a darkening ocean adds a behavioural dimension to these biogeochemical and optical effects. Zooplankton vertical migration is among the largest organised animal movements on Earth, and it plays a central role in the biological carbon pump through the active transport of carbon to depth. If darker surface waters alter the trade-off between feeding opportunity in the surface and predation risk in the illuminated layer, the depth, timing, and intensity of migration could shift, with cascading consequences for carbon export, food web coupling, and the efficiency of energy transfer to fish larvae that depend on grazing zooplankton.
The broader climatic context amplifies the significance of these coastal findings. Coastal seas disproportionately matter for global biogeochemical cycling relative to their area; they host a large share of global fisheries landings, bury substantial amounts of organic carbon in sediments, and mediate the transfer of terrigenous material to the open ocean. Changes in the structure of coastal plankton communities therefore ripple outward, affecting carbon sequestration, the productivity of commercially important stocks, and the transport of organic matter and metals across the shelf. Furthermore, browning interacts with other simultaneous stressors, including ocean warming, acidification, deoxygenation, and eutrophication. Darker waters warm differently because absorbed radiation is trapped nearer the surface, stratification patterns change, and the balance between heterotrophic and autotrophic processing of carbon may tip toward respiration and CO2 outgassing. The demonstration that CDOM and iron jointly reshape phytoplankton succession and zooplankton behaviour provides an empirical foundation for incorporating these linkages into Earth system models, which historically represent coastal processes in highly simplified ways.
Finally, the study highlights the value of long-running European mesocosm infrastructure and international collaboration in addressing questions that no single nation or discipline could resolve alone. The combination of expertise in algal physiology, trace metal chemistry, optical ecology, and zooplankton behaviour was essential to disentangle the mechanisms at play. As climate change continues to intensify the hydrological cycle and mobilise terrigenous carbon from soils, experiments of this kind, conducted over ecologically meaningful timescales with realistic manipulations, will be indispensable for anticipating how the microscopic communities at the base of the marine food web will reorganise. The picture that emerges is one of an ocean whose colour is not merely an aesthetic signature of change but an active driver of ecological and biogeochemical transformation, mediated as much by metal-ligand chemistry and animal behaviour as by the amount of light reaching the cells that anchor nearly all marine food webs.
Subject of Research: …
Article Title: Coloured Dissolved Organic Matter and Iron Rewire the Marine Plankton Food Web in a Changing Climate
Article References: Segovia, M., Ramírez, L., Vázquez, V., Arnone, V., González-Santana, D., León, P., Santana-González, C., Santana-Casiano, M., González-Dávila, M., Tsagaraki, T. M., Smerdou, C., López-Parages, M., Macías, M., Nejstgaard, J. C., Cañete, S., Berger, S. A., Egge, J. K., & Larsen, A. (2026). Coloured Dissolved Organic Matter and Iron Rewire the Marine Plankton Food Web in a Changing Climate. Microbial Ecology. https://doi.org/10.1007/s00248-026-02856-6
Image Credits: AI Generated
DOI: 10.1007/s00248-026-02856-6
Keywords: Coloured, Dissolved, Organic, Matter, Iron, Rewire, Marine, Plankton, Food, Changing, Climate, scientific research
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Violet Maxwell. (September 25, 2026). Darker, Iron-Rich Waters Could Rewire Ocean Food Weefs. Scienmag. https://scienmag.com/darker-iron-rich-waters-could-rewire-ocean-food-weefs/
Violet Maxwell. “Darker, Iron-Rich Waters Could Rewire Ocean Food Weefs.” Scienmag, 25 September 2026, https://scienmag.com/darker-iron-rich-waters-could-rewire-ocean-food-weefs/. Accessed 25 September 2026.
Violet Maxwell. “Darker, Iron-Rich Waters Could Rewire Ocean Food Weefs.” Scienmag. September 25, 2026. https://scienmag.com/darker-iron-rich-waters-could-rewire-ocean-food-weefs/
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Tags: ChangingclimateCoastal water browning and marine ecosystem impactsColouredconsequences of permafrost thawing on marine environmentsDissolvedeffects of freshwater runoffeffects of land runoff on marine food websfoodimpact of increased terrestrial organic carbon flux on coastal watersimplications of ocean water browning for marine food websinfluence of coloured dissolved organic matter (CDOM) on ocean chemistryinfluence of organic matter decomposition on marine plankton communitiesironmarineMatterOrganicplanktonRewirerole of trace metals in coastal ocean biogeochemistryScientific Researchterrigenous dissolved organic matter in oceans
