Does eutrophication control need to include nitrogen? If so, under which conditions?
So far, where controlling eutrophication and cyanobacterial blooms has been successful, this has largely been achieved through reducing P while examples of an explicit focus on measures that reduce N are rare – they almost seem to be limited to Müggelsee. Despite impressive success stories with P-control, frustration with internal loading preventing sufficient responses of TP-concentrations to external load reduction is also impressive (particularly for shallow lakes). It is even argued that successful upstream reduction of biomass through inducing stringent P limitation can lead to N being “left over”, increasing downstream N-concentrations and thus fertilising previously N-limited estuarine and coastal systems. Also, denitrification can permanently remove N from a lake, while P can only be diluted or buried in the sediment. This situation has led to renewed debate on a need to also reduce N loading. Surprisingly often, this need is concluded from observing co-limitation to be common in field samples enriched in the laboratory. The debate is further fuelled by experimental results showing N-concentrations in culture media to influence the cellular concentrations of N-rich peptide toxins in the producing cyanobacterial genotypes.
Happy to now be part of IGB as guest scientist, I will present an overview of this current debate on the background of a brief introduction to my own work on lake restoration in Berlin, on collating case studies of P-control, and on our freshly published debate published in Science about the impact of a “P-only approach” on cellular cyanotoxin contents. In particular, I will introduce a new initiative, resulting from a session at SIL in Berlin in August 2022, of about 30 limnologists across the globe who will evaluate the evidence around the N&P debate to provide a better consolidated scientific basis for deriving management guidance.