Near-Bottom Currents at DeepInsight Hill (Mohns Ridge) from In Situ Measurements, Global Ocean Models, and Biological Flow Indicators

Characterizing near-bottom ocean currents at deep-sea topographic features requires observations across multiple scales, from basin-wide circulation models to local in situ measurements. This study addresses this challenge at DeepInsight Hill, on Mohns Ridge, by combining eight months of in situ current measurements, global ocean model predictions, and novel ROV-based biological flow indicators. The flow regime shows a striking decoupling between tidal pressure and near-bottom currents. The semi-diurnal tide (M2) accounts for ~50% of tidal pressure amplitude yet contributes less than 0.5% of current kinetic energy, while the diurnal tide (K1) drives the largest tidal currents. Tidal energy accounts for only 8.8–9.4% of total current kinetic energy, consistent with other published current records along Mohns Ridge. A monthly comparison with the Copernicus global model shows that the model reproduces the seasonal modulation of the flow at the base of the hill but not the flow at the summit. During winter, sustained directional transport occurs simultaneously in opposite directions at the summit and base of DeepInsight Hill (S-SE and N-NW respectively), accounting for most of the effective water displacement over the record. Foliaceous sponges at the base of the hill show consistent W-SW orientations that do not match the long-term mean flow direction. Instead, these orientations align with episodic peak currents recorded during summer, suggesting that sponge morphology records specific high-energy flow events rather than time-averaged conditions, possibly amplified by peak seasonal nutrient supply. Bedforms observed at the same location eight months apart show no significant change, indicating that sediment-mobilizing currents are episodic and may not recur annually. These results highlight the value of combining physical and biological proxies for characterizing near-bottom flow regimes in topographically complex deep-sea settings.