Tides Boost Primary Production in the Indian Ocean

Tides are a critical source of energy and influence transport in the ocean, with potential implications for biogeochemical cycling and biological production. Here, we quantify the influence of tides on nutrient transport and primary production in the northern Indian Ocean, where topographic features generate hotspots of tidal processes and primary production supports important regional food supply via aquaculture and fisheries. Using a high-resolution regional ocean model, we investigate the effects of diabatic tidal mixing (i.e., irreversible mixing that stirs nutrients) and adiabatic motions associated with internal tides (i.e., reversible motions that shift water masses without mixing) on primary production. We find that tides increase regional primary production by 5% on average, with seasonal increases reaching up to 10%–15% in open ocean regions and 30% in coastal regions. Tidal mixing sets the magnitude of tide-driven production by supplying nutrients to the euphotic zone, and controls the contrast between the stronger coastal response and milder open ocean response. Background stratification and nutricline depth control the seasonality in tide-driven production at a given location: tides accelerate the onset and delay termination of blooms in productive regions (e.g., Arabian Sea) and reinforce bloom peak in less productive regions (e.g., Bay of Bengal). Adiabatic motions have only a small effect, indicating that tidal influences can be effectively parameterized in global models without costly high spatio-temporal resolution and explicit tidal forcing. We discuss the influence of tidal mixing on biogeochemistry beyond primary production and how it may change in a warmer and more stratified ocean.