Pore-water nutrient concentrations variability under different oxygen regimes: A case study in Elefsis Bay, Greece

Eleni Rousselaki; Panagiotis Michalopoulos; Alexandra Pavlidou; Helen Kaberi; Eleni Prifti; Manos Dassenakis

doi:10.1016/j.scitotenv.2023.169830

Pore-water nutrient concentrations variability under different oxygen regimes: A case study in Elefsis Bay, Greece

Sci Total Environ. 2024 Mar 10:915:169830. doi: 10.1016/j.scitotenv.2023.169830. Epub 2024 Jan 6.

Authors

Eleni Rousselaki¹, Panagiotis Michalopoulos², Alexandra Pavlidou², Helen Kaberi², Eleni Prifti², Manos Dassenakis³

Affiliations

¹ Institute of Oceanography, Hellenic Centre for Marine Research, Anavyssos, Greece. Electronic address: erousel@hcmr.gr.
² Institute of Oceanography, Hellenic Centre for Marine Research, Anavyssos, Greece.
³ Laboratory of Environmental Chemistry, Department of Chemistry, National and Kapodistrian University of Athens, Panepistimioupolis, Athens, Greece.

PMID: 38190920
DOI: 10.1016/j.scitotenv.2023.169830

Abstract

Anthropogenic pressures considerably affect coastal areas, increasing nitrogen and phosphorous loads that lead to eutrophication. Eutrophication sometimes results in hypoxic and/or anoxic conditions near the bottom water. Dissolved oxygen (DO) concentrations influence redox-sensitive nutrients, which can alter the benthic flux of nutrients. We retrieved sediment cores from two sites in the eastern and western parts of Elefsis Bay, a semi-enclosed area of the Eastern Mediterranean, Greece, during winter and summer. In the western part, seasonally hypoxic or anoxic conditions occurred. We analysed pore-water samples under normoxic, hypoxic and anoxic bottom water conditions to study the pore-water nutrient concentrations variability under different oxygen regimes. Ex situ incubation experiments were conducted at the site experiencing oxygen deficiency by manipulating the DO concentrations. The pore-water nutrient concentrations showed higher variability at the site experiencing oxygen deficiency. Notably, elevated ammonium concentrations were observed in the pore water during anoxic conditions, in the 2-20-cm sediment layer. However, the benthic fluxes of ammonium and phosphate at the 0-2-cm sediment layer were comparable under hypoxic and anoxic conditions. The results of the incubation experiments demonstrate a direct decrease in nitrate concentrations as the DO concentrations diminished in the overlying water. The incubations after re-oxygenating the overlying water show that phosphate was more efficiently scavenged when anoxic conditions prevailed in the bottom water. The incubation experiments indicate the rapid response of the seafloor to oxygen availability, particularly concerning processes that influence nitrate and phosphate concentrations. These observations highlight the dynamic nature of nutrient cycling in shallow, seasonally anoxic environments, such as Elefsis Bay, and emphasise the sensitivity of the seafloor ecosystem to changes in bottom water oxygen availability.

Keywords: Anoxia; Early diagenesis; Hypoxia; Nutrient dynamics; Pore water; Shallow ecosystems.