Oxygen and nitrate availabilities impact the marine nitrogen cycle at a range of spatial and temporal scales. Here, we demonstrate the impact of denitrifying foraminifera on the nitrogen cycle at two oxygen and nitrate contrasting stations in a fjord environment (Gullmar Fjord, Sweden). Denitrification by benthic foraminifera was determined through the combination of specific density counting per microhabitat and specific nitrate respiration rates obtained through incubation experiments using N₂O microsensors. Benthic nitrate removal was calculated from submillimeter chemical gradients extracted from 2D porewater images of the porewater nitrate concentration. These were acquired by combining the DET technique (diffusive equilibrium in thin film) with chemical colorimetry and hyperspectral imagery. Sediments with high nitrate concentrations in the porewater and oxygenated overlying water were dominated by the non-indigenous species (NIS) Nonionella sp. T1. Denitrification by this species could account for 50 %–100 % of the nitrate loss estimated from the nitrate gradients. In contrast sediments below hypoxic bottom waters had low inventories of porewater nitrate, and denitrifying foraminifera were rare. Their contribution to benthic nitrate removal was negligible (< 5 %). Our study showed that benthic foraminifera can be a major contributor to nitrogen mitigation in oxic coastal ecosystems and should be included in ecological and diagenetic models aiming to understand biogeochemical cycles coupled to nitrogen.

A growing body of observations has revealed rapid changes in both the total inventory and the distribution of marine oxygen over the latter half of the 20th century, leading to increased interest in extending oxygenation records into the past. The use of paleo-oxygen proxies has the potential to extend the spatial and temporal range of current records, constrain pre-anthropogenic baselines, provide datasets necessary to test climate models under different boundary conditions, and ultimately understand how ocean oxygenation responds beyond decadal-scale changes. This review seeks to summarize the current state of knowledge about proxies for reconstructing Cenozoic marine oxygen: sedimentary features, sedimentary redox-sensitive trace elements and isotopes, biomarkers, nitrogen isotopes, foraminiferal trace elements, foraminiferal assemblages, foraminiferal morphometrics, and benthic foraminiferal carbon isotope gradients. Taking stock of each proxy reveals some common limitations as the majority of proxies functions best at low-oxygen concentrations, and many reflect multiple environmental drivers. We also highlight recent breakthroughs in geochemistry and proxy approaches to constraining pelagic (in addition to benthic) oxygenation that are rapidly advancing the field. In light of both the emergence of new proxies and the persistent multiple driver problem, the need for multi-proxy approaches and data storage and sharing that adhere to the principles of findability, accessibility, interoperability, and reusability (FAIR) is emphasized. Continued refinements of proxy approaches and both proxy–proxy and proxy–model comparisons are likely to support the growing needs of both oceanographers and paleoceanographers interested in paleo-oxygenation records.

Coastal environments are exposed to numerous pressures that potentially affect marine soft bottom faunas. Among these pressures, organic matter enrichment is very frequent. Several indices based on living (Rose Bengal-stained) foraminiferal communities have recently been developed to assess the induced impact and determine the environmental quality of these environments. Here, we use an open waters Mediterranean Sea data set to test the benefits and drawbacks of four foraminiferal indices: (1) the effective number of species (exp(H’_bc)), (2) the Tolerant Species Index for the Mediterranean (TSI-Med), (3) the Foraminifera AMBI (Foram-AMBI), and (4) the Foram Stress Index (FSI). These indices all intend to measure the response of the foraminiferal communities to organic matter enrichment, and therefore, their results should be very similar. We found that the diversity-based index, exp(H’_bc), was not suitable to evaluate the environmental quality of our Mediterranean coastal settings, as it had a non-monotonic relationship with the organic matter enrichment gradient. The three indices based on groups of indicator species (TSI-Med, Foram-AMBI, and FSI) yielded fairly similar results. For Foram-AMBI, using only assigned species to calculate proportions of indicator species partly avoided to misinterpret the results. In TSI-Med, a correction based on sediment grain size is applied to distinguish between natural and anthropogenic organic enrichment. Such a correction could also be added to Foram-AMBI and FSI. The limits between the Ecological Quality Status (EQS) classes used for macrofauna were not adapted for Foram-AMBI. Here, we propose new EQS class limits for the Foram-AMBI as follows: very good: 0 ≤ x < 1.4; good: 1.4 ≤ x < 2.4; moderate: 2.4 ≤ x < 3.4; poor: 3.4 ≤ x < 4.4; bad: 4.4 ≤ x ≤ 6. These limits could evolve in the future as new data become available, especially with a poor and bad environmental quality.

Marine environments worldwide are increasingly threatened by warming, deoxygenation, and ocean acidification. These stressors can be recorded as alterations in foraminiferal geochemistry and morphology. We integrated morphological features with assemblage and geochemical records of benthic foraminifera from the Danish Straits at the entrance to the Baltic Sea, covering the Last Interglacial period (LIG, MIS5e), to assess potential indications of environmental changes. Using synchrotron radiation X‐ray microcomputed tomography (SRµCT), we measured Elphidium clavatum in terms of diameter, height, surface area, volume, thickness, and number of pores. Pore patterns and wall thickness were evaluated simultaneously to assess both metabolic and mechanical constraints under environmental changes. The proloculus (initial chamber) size may indicate the reproduction mode of foraminifera in response to environmental stress, including salinity and oxygen variations. We show that during the early‐mid LIG, the physical resistance of E. clavatum against mechanical constraints remained relatively strong, characterized by high wall thickness and low porosity, coinciding with higher bottom water salinity and oxygen content. In the mid‐late LIG, pore density and wall thickness decreased, while porosity and proloculus size increased, fueling metabolism and increasing survival rates. These traits reflect adaptation to an increasingly stressful environment with lower salinity and oxygen levels, as indicated by declining faunal diversity and increasing E. clavatum abundance. Our study demonstrates that benthic foraminiferal morphological features, including proloculus size, wall thickness, and pore patterns, serve as indicators for assessing stress levels and reconstructing bottom water conditions in brackish and potentially hypoxic environments during the LIG.

The trophic ecology of benthic foraminifera in intertidal mudflats is closely linked to diatoms, a dominant component of the microphytobenthos (MPB). Although experimental studies and metabarcoding have clarified foraminiferal diets, in situ assessments of the temporal dynamics of diatoms and foraminifera remain limited. In this study, we examined the seasonal dynamics of adult (> 150 µm) foraminiferal species over a notable 3.5-year monthly monitoring period at the La Coupelasse mudflat (Bay of Bourgneuf, French Atlantic coast). We related these dynamics to 25 environmental variables and to diatom assemblages, focusing on their traits (size, shape, and life-form). La Coupelasse exhibited a clear seasonal pattern driven by bay hydrodynamics, which regulated the availability of redox-sensitive metals, nutrients, and MPB biomass, thereby shaping the environmental context for benthic communities. Diatom traits, whether considered individually or in combination (“size + shape + life-form”), revealed distinct seasonal strategies that complemented species-level analyses. While species-level data provided a detailed understanding of foraminiferal temporal dynamics, combining diatom traits offered a more effective way to identify seasonal dietary shifts. The four dominant foraminiferal species occupied different seasonal niches, with Ammonia confertitesta and Haynesina germanica showing synchronized biannual peaks in spring and autumn, but differed in dietary responses, as H. germanica responded only to diatom shape. Elphidium oceanense displayed a single annual peak in early autumn, corresponding to a broader trophic flexibility across diatom traits, while Elphidium selseyense showed a late spring peak and remained enigmatic regarding its diatom food preferences. Overall, using combined diatom traits outperformed both species identity and MPB biomass in predicting foraminiferal patterns, highlighting their potential to simplify diatom–foraminifera trophic ecology by overcoming diatom taxonomic constraints. These findings shed light on our understanding of benthic ecology and suggest that trait-based approaches, when integrated with spatial and microbiome data, can enhance predictions of ecosystem responses to environmental change.