Coastal Upwelling Influences Hypoxia Spatial Patterns and Nearshore Dynamics in Lake Erie

TLDR

Lake Erie hypoxia, defined as dissolved oxygen <2 mg L⁻¹, has been studied since the mid‑1900s, yet spatial and episodic nearshore patterns remain poorly characterized because of limited observations and short‑term dynamics. The study aimed to evaluate a physically based dissolved‑oxygen model against observed hypoxia patterns in Lake Erie. The model incorporated temperature‑dependent, spatially uniform sediment and water‑column oxygen demand rates, was coupled to NOAA’s FVCOM hydrodynamic forecast, and its temperature and DO outputs were compared with ship‑based, real‑time sensor, and 2017 moored observations. The model reproduced observed spatial and temporal hypoxia patterns, showing that southwesterly winds cause downwelling and earlier south‑shore hypoxia, while occasional northeast winds trigger upwelling that transports hypoxic water to nearshore intakes; it also identified episodic hypoxia hotspots near Ohio, north of Pelee Island, and the Bass Islands, though its skill was limited by uncertainties in thermal structure and oxygen demand simulations.

Abstract

Abstract Hypoxia, defined as dissolved oxygen (DO) &lt; 2 mg/L, in the central basin of Lake Erie has been studied since the mid‐1900s. Even so, spatial patterns of hypoxia, and episodic hypoxia in nearshore areas where drinking water plant intakes are located, are not well characterized owing to limited observations and short‐term dynamics. We evaluated a physically based, DO model with respect to patterns of hypoxia observed in Lake Erie. The DO model used assigned rates of sediment and water column oxygen demand that were temperature dependent but otherwise spatially and temporally uniform. The DO model was linked to National Oceanic and Atmospheric Administration's (NOAA) Lake Erie Operational Forecasting System hydrodynamic model, an application of the Finite Volume Community Ocean Model (FVCOM). Model temperature and DO were compared with observations from ship‐based studies, real‐time sensor networks and an array of moored sensors that we deployed in 2017. In years with dominant southwesterly winds, persistent downwelling occurred along the south shore, which resulted in a thinner thermocline and earlier initiation of hypoxia along the south shore than the north. Occasional northeast winds temporarily reversed this pattern, causing upwelling along the south shore that brought hypoxic water to nearshore locations and water intakes. The DO model reproduced observed spatial and temporal patterns of hypoxia and revealed locations subject to episodes of hypoxia, including nearshore Ohio, north of Pelee Island, and near the Bass Islands. Model skill was limited in some respects, highlighting the importance of accurate simulation of the thermal structure and spatial patterns of oxygen demand rates.