Sampling and experiments were performed at the Askö Laboratory in Trosa archipelago, photo: Stefano Bonaglia

Title: Control factors of the marine nitrogen cycle. The role of meiofauna, macrofauna, oxygen and aggregates

Opponent: Nils Risgaard-Petersen, Center for Geomicrobiology, Aarhus University (Denmark)

Abstract:The ocean is the most extended biome present on our planet. Recent decades have seen a dramatic increase in the number and gravity of threats impacting the ocean, including discharge of pollutants, cultural eutrophication and spread of alien species. It is essential therefore to understand how different impacts may affect the marine realm, its life forms and biogeochemical cycles. The marine nitrogen cycle is of particular importance because nitrogen is the limiting factor in the ocean and a better understanding of its reaction mechanisms and regulation is indispensable. Furthermore, new nitrogen pathways have continuously been described. The scope of this project was to better constrain cause-effect mechanisms of microbially mediated nitrogen pathways, and how these can be affected by biotic and abiotic factors.

This thesis demonstrates that meiofauna, the most abundant animal group inhabiting the world’s seafloors, considerably alters nitrogen cycling by enhancing nitrogen loss from the system. In contrast, larger fauna such as the polychaete Marenzelleria  spp.  enhance  nitrogen  retention,  when  they  invade  eutrophic  Baltic  Sea  sediments.  Sediment  anoxia, caused  by  nutrient  excess,  has  negative  consequences  for  ecosystem  processes  such  as  nitrogen  removal  because  it stops nitrification, which in turn limits both denitrification and anammox. This was the case of Himmerfjärden and Byfjord, two estuarine systems affected by anthropogenic activities, such as treated sewage discharges. When Byfjord was artificially oxygenated, nitrate reduction mechanisms started just one month after pumping. However, the balance between denitrification and nitrate ammonification did not favor either nitrogen removal or its retention.

Anoxia is also present in aggregates of the filamentous cyanobacteria Nodularia spumigena. This thesis shows that even in fully oxic waters, millimetric aggregates can host anaerobic nitrogen processes, with clear implications for the pelagic compartment. While the thesis contributed to our knowledge on marine nitrogen cycling, more data need to be collected and experiments performed in order to understand key processes and regulation mechanisms of element cycles in the ocean. In this way, stakeholders may follow and take decisions in order to limit the continuous flow of human metabolites and impacts on the marine environment.

Keywords: Nitrogen cycle, denitrification, DNRA, anammox, anoxia, hypoxia, eutrophication, meiofauna, macrofauna,
aggregates, cyanobacteria, Baltic Sea.

About thesis in DiVA

About thesis on IGVs homepage

About BEAM, Baltic Ecosystem Adaptive Management