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Effects of Atmospheric N Deposition on Freshwater Nutrient Enrichment

This report discusses the effects of atmospheric nitrogen deposition on nutrient enrichment in freshwater ecosystems. It examines empirical evidence from experiments, historical records, and monitoring studies, highlighting increased productivity, shifts in algal communities, and loss of rare macrophyte species. The report also suggests further development of critical loads models for nutrient nitrogen in freshwater systems.

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Effects of Atmospheric N Deposition on Freshwater Nutrient Enrichment

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  1. ITEM 4 Acidification and eutrophicationICP WatersECE/EB.AIR/WG.1/2010/6

  2. Effects of atmospheric N deposition on biology in oligotrophic surface watersICP Waters report 101/2010 • Has atmospheric N deposition contributed to nutrient enrichment of lakes and rivers?

  3. Background-1: is algal growth limited by phosphorus, nitrogen, CO2? • Debate on causes of eutrophication in 1970s • Schindler’s ‘whole lake fertilization experiments’ in Experimental Lake Area in Canada • Conclusion: P controls primary productivity

  4. 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

  5. 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

  6. 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

  7. Lake ecosystem Water plants Free-floating algae Benthic algae Lake sediments

  8. 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

  9. 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

  10. 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 15N 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

  11. 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

  12. Higher productivity with higher N deposition • More NO3 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 [NO3] Algal biomass N deposition ---- Bergstrøm and Jansson 2006

  13. 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)

  14. 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

  15. Empirical CLs

  16. 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 NO3 leaching as a function of catchment characteristics • Use FAB model also in modelling of nutrient-N CLs for freshwaters?

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