Nutrient enrichment shapes depth-resolved bacterial community assembly in mangrove sediments

Abstract Microbial communities in mangrove sediments are critical for carbon and nutrient cycling; yet despite their importance, responses to eutrophication — a pervasive global threat to coastal ecosystems — remain poorly understood. Here, we tested whether nutrient enrichment restructures mangrove sediment bacterial communities more strongly in the subsurface than at the sediment surface. We assessed bacterial community responses to an in-situ slow-release NPK (Osmocote®) enrichment experiment in a temperate mangrove (Avicennia marina subsp. australasica) forest in Aotearoa New Zealand. Using 16S rRNA gene sequencing, we compared surface (0–2 cm) and subsurface (5–7 cm) community diversity and functional potential, 4.5 months after enrichment. Differences in porewater nutrient concentrations between ambient and enriched plots were subtle and spatially heterogeneous; however, bacterial communities exhibited significant shifts relative to controls. Community shifts in subsurface samples were stronger compared to the surface, with enrichment reducing diversity and altering community composition to Proteobacteria and Bacteroidota favoured communities. Conversely, taxa from Desulfobacterota, Chloroflexi, and Nitrospirota were more abundant in controls. Similarly, shifts in inferred functional potential in the surface remained limited, while the subsurface showed strong, selective changes: a proportional decline of sulphur-cycling and hydrogen-oxidising taxa and a relative increase of heterotrophic guilds. Community assembly was more deterministic in subsurface compared to surface sediments, and under nutrient addition subsurface assembly shifted, mainly through reduced dispersal limitation, to more closely resemble the surface communities. These results underscore the need to better understand and integrate microbial responses at relevant spatial scales into predictions of ecosystem resilience under increasing anthropogenic pressures.