Nutrients and Ecosystems
Fertilizer Application Rates
•
•
•
•
•
•
•
Lawns:
Athletic Fields:
Pastures (Dairy):
Vegetables:
Corn:
Citrus:
Forests:
80-240 kg N/ha/yr
200-280 kg N/ha/yr
240-360 kg N/ha/yr
180-200 kg N/ha/yr
150-210 kg N/ha/yr
140-200 kg N/ha/yr
160-220 kg N/ha/20+ yrs
Nutrient Loads to the Land
Landuse
Crops
Nitrogen Load Phosphorus Loads
350 kg/ha/yr 40 kg/ha/yr
Pasture
125 kg/ha/yr
25 kg/ha/yr
Lawn
280 kg/ha/yr
20 kg/ha/yr
Forest
25 kg/ha/yr
2+ kg/ha/yr
USGS, 2000
Nutrient Loading Calculations
Estimating Biological Response from
Concentration
• Recall that TP is
strongly correlated
with chlorophyll a
production in shallow
lakes
• Use measured or
predicted TP to
estimate lake
response
5. Gator Lake TP concentration with a totally forested watershed is 6
mg/m3. What is the predicted Secchi depth?
What is the predicted Secchi for the lake if 10 ha of the watershed are
converted to urban land use?
TP= 100 mg/m3
Biven’s Arm
TN= 2,700 mg/m3
TP= 230 mg/m3
chlA= 130
Secchi ~0.5m
Hypereutrophic
Lake Sheeler
TN= 70 mg/m3
TP= 4 mg/m3
chlA=1 mg/m3
Secchi = 8m
Oligotrophic
Silver Springs
TN= 1400 mg/m3
TP= 50 mg/m3
chlA=1 mg/m3
Secchi = 50 m
Trophic State?
TN= 730 mg/m3
TP= 50 mg/m3
chlA=1 mg/m3
Secchi = 50m
Trophic State?
Lake Santa Fe
TN = 450 mg/m3
TP= 15 mg/m3
chlA = 8
Secchi = 1.5m
Mesotrophic
TN = 450 mg/m3
TP= 15 mg/m3
chlA = 8
Secchi = 1.5m
Lake Alice
TN = 500
TP = 550
chlA = 10
Secchi = 1.7
TN = 500
TP= 550
chlA= 10
Secchi 1.7m
Watershed export is the amount of a
substance exported from a watershed
usually expressed as:
kg / hawatershed / yr
Water body load is the amount of a
substance delivered to a water body
usually expressed as:
mg / m2water body surface area / yr
Watershed Export (Flux)
• Export is a MASS (kg)
– Index time (per year) and area (per hectare)
– Typical units: kg/ha/yr
• To get mass: flow volume x concentration
– Flow (m3/yr) x Conc. (mg/m3) = mass (mg/yr)
• To get mass per hectare, divide by area
Lake Loading
• Mass export from the watershed impacts a
lake (or estuary) in proportion to the area of
that aquatic system
• Lake loading is mass (kg/yr) per lake area (m2)
– Units: kg/m2/yr
• We work on what happens to that load later
Example Export - Load Problem:
•50 ha watershed
•2,000,000 m3/yr runoff
•[TP] in runoff is 20 mg/m3
•Water body is a 5 ha lake.
Export
1,000,000 mg = 1 kg
2,000,000 m3 x 20 mg/m3 = 40,000,000 mg P = 40kg
(watershed export= 40 kg / 50 ha = 0.80 kg/ha/yr)
Load
40,000,000 mg P / (5 ha x 10,000m2/ha) = 800 mg / m2
Example Problem
1. What is the TP export (kg/ha/yr) from the
watershed with total forest cover?
500,000 m3 x 10 mg/m3 = 5,000,000 mg = 5 kg
5 kg / 100 ha = 0.05 kg/ha/yr
2. What is the TP load to the Gator Lake with total
forest cover?
500,000m3 x 10mg/m3 = 5,000,000 mg
5,000,000mg / (10ha x 10,000m2) =
50mg/m2/yr
3. What is the load to Gator Lake when 10ha of the
watershed is converted to urban land use?
90 x 0.05 kg/ha x 1,000,000 mg/kg= 4,500,000 mg
10 x 5.55 kg/ha x 1,000,000 mg/kg= 55,500,000 mg
60,000,000 mg
60,000,000 mg / 100,000 m2 = 600 mg/m2
Lake Surface Area
Loads to Lake Concentrations – Using the
Das Vollenweider Model
Load (mg/m2/yr lake surface)
TP (mg/m3) =
Mean lake depth (m)
( Flushing rate (y)
+ Sedimentation
L
TP
=
Z (ρ+σ)
Models used by WMDs are similar. Estimating sedimentation (σ) in
Florida Lakes can be problematic because many are shallow and
winds can easily re-suspend bottom sediments (internal load).
)
4. What is the predicted TP concentration of
Gator Lake if 10 ha of the watershed are
converted to urban land use?
L
TP
=
Z
TP=
(ρ+σ)
600
2 (2.5 +0.5)
TP= 100 mg/m3
Regulations and Restoration
So…what’s an OK Pollution level?
• Depends on what you want it for
– Designated Use
• Potable (Drinking Water)
• Fishable/Swimmable
• Agricultural/Industrial
• When water fails to meet its designated use,
government intervenes
– Sets Total Maximum Daily Loads (TMDL) to try to
clean up the watershed via equitable
apportionment of responsibility
Water Quality and Designated Use
• The Clean Water Act (1972) mandates protection of
the nations water for “designated uses”
– Water Supply (Class I), Shellfish Harvest (Class II), Fishable
(Class III), Industrial (Class V)
– Current FDEP move to add “splashable” and “boatable”
• Water quality standards are set based on designated
use
– Numeric criteria (pollutant thresholds based on conc.)
– Narrative criteria (ecosystem measures based on effects)
Oversight of Designated Uses
• Regulation of point source permits (NPDES)
• Enforcement of “presumed compliant” activities
– BMPs
• If water body consistently fails to meet designated use,
process of attainment starts
– Total Maximum Daily Load (TMDL)
– Pollutant/Water Body Specific (establish pollutant load
reduction goals based on models and/or historical data)
– Equitable apportionment of responsibility
– Headed by FDEP in Florida; FDACS/DoF have compliance
tasks
TMDL Implementation
Reduce Watershed Exports
(convert total export to mg and
divide by lake surface area)
Water Body Load
(Models: concentration function of load,
flushing out puts, sediment outputs &
dilution factor)
Water Body Concentration
(Regressions, models, wishful thinking)
Improvements in Use/ Appearance / Biology
What TMDL level?
Set Watershed Export
Calculate Load that will yield desired concentration
Measure Water Body Concentration
Observe the desired condition
(elsewhere?)
TP<50mg/m3
Section 305 (b) of the CWA (fishable, swimmable)
Sample water and/or biology to
determine if water meets use for that
class (5 classes in Florida)
305 (b) report
Meets use
Partially meets or fails to meet use
303 (d) list of impaired waters
Law Suits
EPA or State (WMDs) set TMDL
Law Suits
TMDL reset
Law Suits
Load reduction allocation
Criteria Based on Geography
47 Lake Ecoregions in Florida
3 Level III Ecoregions in Florida
Recent Developments – Numeric
Nutrient Criteria
• Nov 14th 2010 – EPA signed into law new rules
about nutrient pollution in Florida
– Becomes effective March 6th 2012
• Result of lawsuit against EPA by Earthjustice
arguing that existing rules were under-protective
– Why?
– 8% of assessed river miles, 26% of assessed lake
acres
– Increased 3% in extent between 2008 and 2010
Narrative vs. Numeric
• Narrative – “shall not cause an imbalance in
the native flora and fauna”
– Intrinsically reactive
• Numeric – a single number for all systems
– Intrinsically over-general
Stressor – Response for Streams
• No association in the data between
indices of ecological condition and
nutrient levels
• Elected to use a reference standard
whereby the 90th percentile of
unimpacted streams is adopted as
the criteria
What’s Happened?
• In a word…lawyers
– Procedural challenges to EPA rules
• Arbitrary and capricious standard of review
• Excess of authority (state’s rights)
– Reference conditions in spite of no cause and
effect relationship in streams
– Insufficient recognition of natural variation
– Florida’s Attorney General, Counties,
Municipalities, Florida Farm Bureau, etc.