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Biogeochemical consequences of sewage discharge on mangrove environments

Monday, 20 June, 2011 - 16:00
Campus: Brussels Humanities, Sciences & Engineering campus
Faculty: Science and Bio-engineering Sciences
Perrine Mangion
phd defence

The rapid population growth and associated activities has dramatically increased the
nutrient delivery to coastal waters thereby threatening coastal ecosystems with
eutrophication. Fortunately, part of these nutrients is retained by coastal wetlands, such
as mangrove systems, that help in maintaining water quality and prevent coastal
eutrophication. This ecosystem function could be particularly useful in the surroundings
of coastal African cities that lack adapted wastewater treatment systems and typically
discharge their sewage in creeks lined by mangrove forests. However, many questions
remain to get the full picture of how the phytoremediation function works in mangrove
environments and of the ecological consequences of sewage discharge on mangrove biota.
Moreover, methods to trace anthropogenic N inputs and to assess their impact on
mangrove ecosystems are required. This PhD research thus investigated the carbon
oxidation mechanisms and nitrogen cycle in peri-urban and pristine mangrove settings
and also used carbon and nitrogen stable isotopes of mangrove biota as an indicator of
environmental conditions and anthropogenic influence. The main achievements of the
present work were:
(i) a confirmation that natural peri-urban mangrove systems can act as a sink for dissolved
inorganic nitrogen.
(ii) a better understanding of the potential filtering function of mangrove ecosystems and
the processes involved. Denitrification was stimulated by nutrient enrichment but
accounted for limited N-removal (12 – 19 %) of DIN inputs in the sediment. Thus most of
the dissolved inorganic nitrogen was removed through other processes such as sediment
retention or trees and/or phytoplankton uptake.
(iii) a contribution to the description of the different carbon oxidation pathways in periurban
and pristine mangrove which didn't appeared affected by these amounts of nutrient
(iv) the demonstration that ä15N of mangrove leaves, sediment and epifauna could be used
as an ecological indicators of sewage discharge in mangrove environments.