Abstract
Globally, there has been a decrease in dissolved oxygen in the oceans, that is more pronounced in coastal waters, resulting in more frequent hypoxia exposure for many marine animals. Managing hypoxia requires an understanding of the dynamics of dissolved oxygen (DO) where it occurs. The French coast facing the Bay of Biscay (N-E Atlantic Ocean) hosts at least a dozen tidal and turbid estuaries, but only the large estuaries of the Gironde and the Loire, are subject to a continuous monitoring. Here, we compared the DO dynamics in these two systems, in order to evaluate to what extent it is possible to predict the sensitivity to hypoxia of a tidal and turbid estuary based on its most common and easily accessible characteristics (the estuary surface area and length, liquid and solid discharge, suspended particulate load, water renewal time, presence of cities). Whereas the hyperturbid Gironde estuary is the longest (170 km), implying longer water and particle residence times, and is bordered by a large metropole (> 750 000 inhabitants), only episodic summer hypoxia events (DO < 2 mg L-1) were recorded in its urban Garonne subestuary. In contrast, the turbid Loire estuary, smaller in terms of surface area and length, experiences permanent summer hypoxia. This demonstrates the inability to evaluate the susceptibility of a tidal and turbid estuary to hypoxia only by considering its general characteristics. We urge that there is a need for a pertinent DO survey strategy based on an initial assessment of DO in a specific estuary based on longitudinal investigations, in particular during the warm season (as higher water temperatures decrease DO solubility), for detecting the potential zones of minimum DO.
Highlights
The French Atlantic coast hosts numerous macrotidal estuaries that flow into the Bay of Biscay that are natural corridors for migratory fishes (Figure 1A)
This study aimed to propose a strategy to test the hypoxic nature of an estuary as an indispensable prerequisite to further implementation of a dissolved oxygen (DO) survey
The estuaries of the Loire and the Gironde are characterized by the presence of a metropolis in their upstream reaches: 1http://www.loire-estuaire.org/dif/do/network 2http://rogir.epoc.u-bordeaux1.fr/Ayant_Droits/donnees/
Summary
The French Atlantic coast hosts numerous macrotidal estuaries that flow into the Bay of Biscay that are natural corridors for migratory fishes (Figure 1A). The asymmetry of the tidal wave, when propagating upstream, creates a welldeveloped turbidity maximum zone (TMZ), with concentrations of suspended particulate matter (SPM) of several hundred milligrams of dry sediments per liter (Toublanc et al, 2016). This TMZ is defined as an estuarine region where SPM concentrations exceed 0.5 g L−1 in the Loire or 1 g L−1 in the Gironde (Jalón-Rojas et al, 2015, 2017). The degradation of the organic content of the SPM consumes DO This specific feature is likely to promote a chronic undersaturation of dissolved oxygen in turbid estuarine waters, with respect to the expected value in equilibrium with the atmosphere at a given water temperature and salinity (Benson and Krause, 1984). Critical thresholds are even higher: here, we use a threshold of below 3 mg L−1 for hypoxia and between 3 and 5 mg L−1 for critical DO inducing stress for most organisms (Curran and Henderson, 1988). Delage et al (2019) showed that oxygen depletion induces sublethal effects at 70% O2 saturation and lethal effects at 50% O2 saturation during the early life phases of the European sturgeon
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