What is Eutrophication? What are its effects?
process by which a body of water becomes enriched in nutrients that stimulates excessive aquatic plant growth leading to depletion of DO
effects: reduced fisheries, loss of aesthetic and recreational value, loss of economic value, decreased biodiversity
Why are we interested in Phosphorus?
- P in excess causes eutrophication
- often the limiting nutrient in freshwater systems
- fertilisers: P is a key strategic resource for fertilisers and fertilisers are a primary cause of water quality problems
P is a major algal cellular component but occurs at low levels in freshwater systems
What are safe planetary boundaries?
- values for control variables that are a safe distance from thresholds
- identifying and quantifying planetary boundaries that must not be transgressed if we want to prevent unacceptable environmental global change
- boundary has been exceeded for N, almost exceeded for P
What are major sources of Phosphorus?
phosphate mining, fertilizer, weathering (leaching due to rain), sewage treatment plants, direct deposition from atmosphere, mobilization of P in lakes under low O2, run off from land (e.g. leaves in gutters)
What causes release of P in sediments?
Change in internal conditions of lake causes massive release of P in sediments
- Mixing of sediment
- Physical, biological
- Changes in pH
- Whole lake anoxia or just anoxia below sediment/water interface
Lake observations of P
- reservoirs/lakes that receive a higher P loading also retain more P per m2
- water bodies with longer HRT retain more of the external P
- large lakes retain more P with greater depth
- stratified lakes retain less external P if their depths are greater
- BOD5 and PO4 match trends of increasing waste treatment over time (concs decreased)
Lake Eerie and Phosphorus
Lake Eerie is a large shallow and eutrophic lake
inputs of P: agriculture, WWTP
has substantial P release from bottom sediment when DO in overlying water is around 0 mg/L
Explain Nutrient Limitation
The least available nutrient in any system is the limiting nutrient; one nutrient is totally consumed before the others.
In complex ecosystems even if one nutrient is depleted other organisms that need little to none of this nutrient can be favoured and still flourish
Common ratio of P:N:C is 1:16:106
P is often limiting in lakes, N is often limiting in marine environments
Effect of Household detergents on P concentrations
WWTP discharges are major contributor to P loading and eutrophication of lakes and streams. Another major source was household detergents
- Bans in US decreased phosphorus loading
- Bans in 2013 in Belgium decreased P in lakes
What is the study on US lakes and streams relating to phosphorus?
- There has been a continental scale increase in lake and stream total phosphorus (TP) concentrations in hundreds of lakes and streams (2000-2014)
- Most notable, lakes and streams all initially at low TP and occurred in relatively undisturbed watersheds (few sources of P)
- Streams and lakes with < 10 microgram/l decreased in TP
- Increasing TP therefore cannot be attributed to common non-point or point sources of TP
Plausible mechanisms include:
- Increase in extreme precipitation events
- Increase in extreme hydrological events
- Increased atmospheric deposition of P-rich dust particles from increased frequency of droughts and biomass-burning
- Increased temperature of water bodies (e.g. increased internal P remobilisation; increased OM production and less oxygen in lower waters)
What are the operational measures of Phosphorus?
- Total Phosphorus: measures all the forms of phosphorus in the sample (orthophosphate, condensed phosphate and organic phosphate).
Determined by: first ‘digesting’ (heating and acidifying) the sample to convert all the other forms to orthophosphate. Then the orthophosphate is measured - Dissolved/Soluble Reactive P: fraction of the total phosphorus which is in the solution in the water (as opposed to being attached to the suspended particles)
Determined by: first filtering the sample then analysing the filtered sample for phosphorous
