Meiofaunal diversity across a sharp gradient of fjord oxygenation: Insights from metabarcoding and morphology approaches in foraminifera

Fjords are important global carbon sinks, yet their ecosystems are increasingly impacted by anthropogenic eutrophication and oxygen depletion. Understanding how benthic meiofaunal communities respond to environmental parameters is essential for environmental impact assessment and biomonitoring. Here, we examine how benthic foraminiferal communities respond to contrasting bottom-water oxygen conditions in three Swedish west coast fjords (long-term anoxic By Fjord, seasonally hypoxic Gullmar Fjord, and permanently oxic Hake Fjord) using parallel morphological identification and environmental DNA (eDNA) metabarcoding methods. Multidimensional scaling revealed site-specific structuring in both datasets. By Fjord showed the lowest diversity in both datasets, dominated by monothalamids and opportunistic, stress-tolerant taxa adapted to anoxic conditions. Gullmar Fjord supported more diverse assemblages, indicated by abundant Globothalamea, notably Bulimina marginata and Stainforthia fusiformis, reflecting moderate oxygenation conditions. Shallow Hake Fjord supported a slightly lower diversity benthic community and less molecular signal preservation. Both Hake- and Gullmar fjords exhibited occurrence of Nonionella sp. T1, a recently established putatively invasive species, indicating ecological competitiveness across variable oxygenation conditions. Complementary use of molecular and morphological analyses highlighted methodological biases: eDNA captured broader taxonomic diversity, particularly elusive monothalamids, but was influenced by variable DNA preservation under different redox conditions and was limited by sequence database resolution. The morphological approach targeted mineralised foraminifera of a certain size (> 63 μm), providing clearer ecological associations related to immediate oxygen conditions and salinity, while metabarcoding of bulk sediment more effectively traced potential bidirectional responses of a wider range of cryptic and soft-bodied foraminifera. We conclude that used together, these two approaches provide critical insights into species ecology necessary for interpreting palaeoecological archives and assessing contemporary environmental status in oxygen-stressed coastal ecosystems.