Spatio-Seasonal Hypoxia/Anoxia Dynamics and Sill Circulation Patterns Linked to Natural Ventilation Drivers, in a Mediterranean Landlocked Embayment: Amvrakikos Gulf, Greece

Nikos Georgiou,Konstantinos-Marios Vaziourakis,Elias Fakiris,Dimitra Antoniou,Xenophon Dimas,Zacharias G Kapellonis,George Ferentinos,Constantin Koutsikopoulos,Dimitris Christodoulou,Alexandra Noti,George Papatheodorou,Maria Geraga

doi:10.3390/geosciences11060241

Nikos Georgiou, Konstantinos-Marios Vaziourakis + Show 10 more

Open Access

https://doi.org/10.3390/geosciences11060241

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Abstract

Amvrakikos Gulf is a Mediterranean landlocked, fjord-like embayment and marine protected area suffering from natural, human-induced hypoxia/anoxia and massive fish mortality events. Seasonal marine geophysical and oceanographic surveys were conducted focusing on the water-circulation patterns at the sill and the spatial-seasonal distribution of dissolved oxygen (DO) in the gulf. Detailed surveys at the sill, the only communication route between the gulf and the open sea, revealed a two-layer water circulation pattern (top brackish outflow–bottom seawater inflow) and the role of the tide in the daily water exchange. Statistical analysis of the known natural drivers of DO distribution (density difference between the Ionian Sea and Amvrakikos, river inflow, wind) revealed that horizontal density gradients strongly affect anoxia reduction and seafloor oxygenation, while river inflow and wind mainly oxygenate volume/areas located above or within the pycnocline range, with DO concentrations > 2 mg/L. Complex geomorphology with well-formed internal basins contributes to the development and preservation of low DO conditions below the pycnocline. Finally, 43% of the seafloor and 36% of the gulf’s total water volume are permanently hypoxic, and reach a maximum of 70% and 62%, respectively, in September and July. This work is tailored to future ecosystem management plans, decisions, and future research on coastal ecosystems.

Highlights

Coastal ecosystems, despite covering 2% of the ocean surface, offer high productivity and numerous ecosystem functions [1,2]
The results are described in two main topics: (a) The water circulation patterns at the sill during the high- and low-density difference period between the open sea and the gulf (Section 3.1), and (b) the hypoxia/anoxia dynamics linked to natural ventilation drivers and geomorphology in the whole gulf (Sections 3.3–3.5)
A geophysical survey was performed for the first time at the sill, showing that a two-layer water circulation pattern was formed at the area of the sill due to the seasonal density difference between the Ionian Sea and Amvrakikos Gulf (Figure 10A), which acts as the main natural driver that generates horizontal pressure gradients and forces the denser oxygenated seawater to intrude [14,27]

Summary

Introduction

Despite covering 2% of the ocean surface, offer high productivity and numerous ecosystem functions [1,2]. Coastal water bodies with restricted flow and exchange of waters are susceptible to the generation of low dissolved oxygen (DO) conditions Despite that, they are widely protected by the European network of protection nature areas “Natura 2000” (https://natura2000.eea.europa.eu/, accessed on 10 February 2021), overfishing, urbanization, agriculture, sewage, riverine inputs, and consequent pollution, resulting in eutrophication, habitat loss, and species invasion. Over 500 dead zones have been related to human-induced activities [8], including a broad degradation of health status and the viability of the goods and services of the ecosystems [9] Their future seems ominous due to climate change impacts, and meteorological condition intensification (temperature rise, ocean current acceleration, wind, precipitation) is believed to disturb the DO dynamics of coastal areas [8,10,11,12]

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