Degrading seagrass (<Emphasis Type="Italic">Posidonia oceanica</Emphasis>) ecosystems: a source of dissolved matter in the Mediterranean |
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Authors: | Eugenia T Apostolaki Marianne Holmer Núria Marbà Ioannis Karakassis |
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Institution: | 1.Marine Ecology Laboratory, Biology Department,University of Crete,Heraklion,Greece;2.Institute of Oceanography,Hellenic Centre for Marine Research,Heraklion,Greece;3.Institute of Biology,University of Southern Denmark,Odense M,Denmark;4.Department of Global Change Research,Institut Mediterrani d’Estudis Avan?ats (CSIC-UIB),Esporles (Illes Balears),Spain |
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Abstract: | Diurnal variation of dissolved oxygen (DO), organic and inorganic carbon (DOC, DIC), nitrogen (DON, DIN), and phosphorus (DOP,
DIP) flux across the sediment–water interface was assessed in fish farm impacted and pristine seagrass (Posidonia oceanica) meadows in the Aegean Sea (Greece). DIC consumption decreased by 52% and DO production decreased by 60% in the light, suggesting
reduced photosynthetic performance of the plant community under the fish cages probably due to organic matter loading. In
light there was 4 and 15 times higher release of dissolved inorganic and organic matter, respectively, compared to dark incubations
under the cages, indicating that fish farming impact is more intense during daytime. DO was taken up, while DIC was released
in the dark in both stations, representing a direct measure of mineralization. Dissolved inorganic matter flux (as the sum
of DIN and DIP fluxes) was positively related to DIC flux, rendering mineralization as the main driver of nutrient flux under
the cages. On average, the impacted meadow released DIN and DIP both in light and dark, while efflux of dissolved organic
matter (as the sum of DOC, DON, and DOP fluxes) increased by 132% in the light and by 21% in the dark, implying that the degrading
seagrass meadow is a source of dissolved matter to the surrounding water. Shoot density and leaf production were negatively
correlated with both diel DIN and DIP fluxes, showing that meadow regression is accompanied by DIN and DIP release from the
sediment. Hence, nutrient efflux can adequately illustrate meadow deterioration and, therefore, can be used as indicator of
P. oceanica community health. |
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