Nutrients and Ecosystems

Fertilizer Application Rates

• Lawns: 80-240 kg N/ha/yr• Athletic Fields: 200-280 kg N/ha/yr• Pastures (Dairy): 240-360 kg N/ha/yr• Vegetables: 180-200 kg N/ha/yr• Corn: 150-210 kg N/ha/yr• Citrus: 140-200 kg N/ha/yr• Forests: 160-220 kg N/ha/20+

yrs

Nutrient Loads to the LandLanduse Nitrogen Load Phosphorus LoadsCrops 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 ArmTN= 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 AliceTN = 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.

2,000,000 m3 x 20 mg/m3 = 40,000,000 mg P = 40kg

40,000,000 mg P / (5 ha x 10,000m2/ha) = 800 mg / m2

(watershed export= 40 kg / 50 ha = 0.80 kg/ha/yr)

Export

Load

1,000,000 mg = 1 kg

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 mg10 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

TP (mg/m3)Load (mg/m2/yr lake surface)

+Mean lake depth (m) Flushing rate (y) Sedimentation ( )

Loads to Lake Concentrations – Using the Das Vollenweider Model

=

TPL

(ρ+σ)Z=

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?

TP

L

Z (ρ+σ)

=

600

2 (2.5 +0.5)TP=

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

Water Body Load

Reduce Watershed Exports

Water Body Concentration

Improvements in Use/ Appearance / Biology

(convert total export to mg and divide by lake surface area)

(Models: concentration function of load, flushing out puts, sediment outputs & dilution factor)

(Regressions, models, wishful thinking)

TMDL Implementation

Calculate Load that will yield desired concentration

Set Watershed Export

Measure Water Body Concentration

Observe the desired condition

(elsewhere?)

TP<50mg/m3

What TMDL level?

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

EPA or State (WMDs) set TMDL

TMDL reset

Load reduction allocation

Law Suits

Law Suits

Law Suits

Criteria Based on Geography

3 Level III Ecoregions in Florida

47 Lake 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.