WGE 29th session, September Brit Lisa Skjelkvåle ITEM 4 Acidification and eutrophication ICP Waters ECE/EB.AIR/WG.1/2010/6

WGE 29th session, September Brit Lisa Skjelkvåle Effects of atmospheric N deposition on biology in oligotrophic surface waters ICP Waters report 101/2010 Has atmospheric N deposition contributed to nutrient enrichment of lakes and rivers?

WGE 29th session, September Brit Lisa Skjelkvåle Background-1: is algal growth limited by phosphorus, nitrogen, CO 2 ? Debate on causes of eutrophication in 1970s Schindler’s ‘whole lake fertilization experiments’ in Experimental Lake Area in Canada Conclusion: P controls primary productivity

WGE 29th session, September Brit Lisa Skjelkvåle Background-2 P-limitation of algal growth in freshwaters is textbook knowledge Effects of N deposition studied in relation to acidification Recent studies indicate that N supply affects algal growth ICP Waters report: – to give overview of literature on effects of reactive N in freshwaters – to improve/extend critical loads for nutrient nitrogen for freshwaters

WGE 29th session, September Brit Lisa Skjelkvåle Effects of atmospheric N deposition on N leaching N deposition leads to increased leaching of nitrate to surface waters in natural ecosystems Leaching of N highly dependent on catchment type Kaste ea 2008

WGE 29th session, September Brit Lisa Skjelkvåle Effects of N in freshwaters Experimental evidence – Whole-lake experiments – Smaller scale in-situ experiments – Laboratory studies Historical records: – Lake sediment studies (paleolimnology) Monitoring studies – Region-wide lake surveys

WGE 29th session, September Brit Lisa Skjelkvåle Lake ecosystem Free-floating algae Benthic algae Lake sediments Water plants

WGE 29th session, September Brit Lisa Skjelkvåle Experimental evidence ’Whole-lake’ experiments – Expensive, but high ecological realism – Additions with P and N show changes in algal growth and algal species – N can be a limiting factor Mesocosm experiments – Less expensive and less realistic – Pelagic (”free-floating”) algae can respond to N addition only – Benthic (”rock-dwelling”) algae can also respond but less clearly

WGE 29th session, September Brit Lisa Skjelkvåle Experimental evidence Nitrogen is (co)limiting factor for growth of phytoplankton and benthic algae No direct link between response to N and N deposition Critical loads need a relation between N deposition and a certain biological response

WGE 29th session, September Brit Lisa Skjelkvåle Historical records: paleolimnological evidence Most studies from arctic and alpine lakes N deposition below 5 kg N/ha/yr Types of response: – Sediments enrichment with 15 N which indicates an atmospheric source – Changes in algal community structure (species diversity) – Statistical correlations between species diversity and N enrichment – Climate change is also partly responsible for observed changes

WGE 29th session, September Brit Lisa Skjelkvåle Monitoring: lake surveys Strength: – data from many regions – Possible to relate N deposition to a response Weakness: – other factors than N deposition may control the response Bergstrøm and Jansson 2006

WGE 29th session, September Brit Lisa Skjelkvåle Higher productivity with higher N deposition More NO 3 at higher N deposition Higher algal production where N deposition was high (corrected for P) Clearest effect at N dep < 5 kg N/ha/yr N deposition ----  [NO3] Algal biomass Bergstrøm and Jansson 2006

WGE 29th session, September Brit Lisa Skjelkvåle Nitrogen enrichment effects on water plants (macrophytes) Worldwide loss of typical acid-sensitive communities adapted to low N availability (isoetides) Increased growth of more nitrophilous species Especially well-documented in the Netherlands Related to N-availability and –form Also related to alkalinity and nutrients in sediments Critical load 5-10 kg N/ha/yr (Bobbink ea 1995 WASP)

WGE 29th session, September Brit Lisa Skjelkvåle Conclusions Atmospheric N deposition leads to N-enrichment of oligotrophic lakes in arctic, alpine and boreal regions Evidence from paleolimnology, whole-lake and smaller scale experiments, regional surveys – increased productivity – shifts in algal communities – loss of rare macrophyte species, increase of other (nuisance) species – data scarcity on effects on benthic algae and effects at higher trophic levels

WGE 29th session, September Brit Lisa Skjelkvåle Empirical CLs

WGE 29th session, September Brit Lisa Skjelkvåle Suggestions for further development CLs for nutrient-N for freshwaters Catchment characteristics determine catchment N retention capacity FAB model is presently used to model critical loads for acidity by simulation of NO 3 leaching as a function of catchment characteristics Use FAB model also in modelling of nutrient-N CLs for freshwaters?