Seasonal dynamics of mudflat foraminifera linked to diatom species and traits

Abstract. 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.