Eutrophication and Algal Proliferation in Florida’s Springs

Forest HydrologySpring 2014

Water Quality and Aquatic Health

• Tenet #1: Contaminants from land end up in the water– Industrial, urban, agricultural chemicals

• Tenet #2: Aquatic systems may respond, often in undesirable ways

• Habitat viability• Aesthetics (color, aroma, clarity)• Function (support C storage, N removal, flow)• Human use potential (e.g., drinking or irrigation water)

Eutrophication

• Def: Excess C fixation– Primary production is

stimulated. Can be a good thing (e.g., more fish)

– Can induce changes in dominant primary producers (e.g., algae vs. rooted plants)

– Can alter dissolved oxygen dynamics (nighttime lows)• Fish and invertebrate impacts• Changes in color, clarity, aroma

More P

Less P

http://www.sjrwmd.com/publications/pdfs/fs_lapopka.pdf

Reduction in Water Clarity = Changes

in Bottom Habitats

Eutrophication may stimulate the growth of algae that produce harmful toxins

Red Tide

Dead Zone in the Gulf of Mexico

http://serc.carleton.edu/microbelife/topics/deadzone/

Scope of the Problem in Florida

Source: USEPA (http://iaspub.epa.gov/waters10/state_rept.control?p_state=FL&p_cycle=2002)

What Causes Eutrophication?

• Leibig’s “Law of the Minimum”– Some element (or light

or water) limits primary production

– Adding that thing will increase yields (GPP)

– What is limiting in forests? Crops? Lakes? Pelagic ocean?

Justus von Liebig

What Limits Aquatic Production?

Typical Symptoms: Alleviation of Nutrient Limitation

(GPP)

• Phosphorus limitation in shallow temperate lakes

• Nitrogen limitation in estuarine systems

V. Smith, L&O 2006V. Smith, L&O 1982

Global Nitrogen Enrichment

• Humans have massively amplified global N cycle– Terrestrial Inputs

• 1890: ~ 150 Tg N yr-1

• 2005: ~ 290+ Tg N yr-1

– River Outputs• 1890: ~ 30 Tg N yr-1

• 2005: ~ 60+ Tg N yr-1

• N frequently limits terrestrial and aquatic primary production– Eutrophication

Gruber and Galloway 2008

Local Nitrogen Enrichment• The Floridan Aquifer (our

primary water source) is:– Vulnerable to nitrate

contamination– Locally enriched as much as

30,000% over background (~ 50-100 ppb as N)

• Springs are sentinels of aquifer pollution– Florida has world’s highest

density of 1st magnitude springs (> 100 cfs)

Arthur et al. 2006

Weeki Wachee20011950’s

Mission Springs Chassowitzka (T. Frazer)

Weeki Wachee

Mill Pond Spring

GROW FASTER LOST MORE SLOWLY

Core Question:

What Causes Algae to Reach Nuisance Levels?

• Hnull: N loading alleviated GPP limitation, algae exploded (conventional wisdom)

• Evidence generally runs counter to this hypothesis– Springs were light limited even at low concentrations (Odum 1957)– Algal cover/AFDM is uncorrelated with [NO3] (Stevenson et al. 2004)

– Flowing water mesocosms show algal growth saturation at ~ 110 ppb (Albertin et al. 2007)

– Nuisance algae exists principally near the spring vents, high nitrate persists downstream (Stevenson et al. 2004)

N Enrichment in Springs

From Stevenson et al. 2004 Ecological condition of algae and nutrients in Florida Springs DEP Contract #WM858

Fall 2002 (closed circles) and Spring 2003 (open triangles)

No correlation between algae and N

N Enrichment and Primary Production[No Significant Association]

• More N does not mean more GPP

(GPP)

Alexander Springs (50 ppb N-NO3)

Visualizing the Problem

Silver Springs (1,400 ppb N-NO3)

Qualitative Insight: Comparing Assimilatory Demand vs. Load

• Primary Production is very high– 8-20 g O2/m2/d (ca. 1,500 g C/m2/yr)

• N demand is proportional– 0.05 – 0.15 g N/m2/day

• N flux (over 5,000 m reach) is large– Now: ca. 30 g N/m2/d (240 x Ua)

– Before: ca. 2.5 g N/m2/d (20 x Ua)

• In rivers, the salient measure of availability may be flux (not concentration)

• Because of light limitation, this is best indexed to demand

• When does flux:demand become critical?

Back to First Principles:Controls on Algal Biomass

bottom up effects

top down effects

Algae Biomass

Grazers Flow RatesDissolved Oxygen

Nutrients Light

mediating factors

Algal Loss Rates - Scouring

• Flow has widely declined, in areas a lot– Silver Springs– White Springs– Kissingen Spring

• Lower discharge means lower scour

• Algal cover varies with flow velocity (King 2014)

Algal Loss Rates - Grazing

• Algal cover is predicted by:– Dissolved oxygen (DO)– Grazer density

• DO is keystone variable for aquatic animal health– Proxy for groundwater age?

Observational Evidence:Grazers and Algae are Correlated

Liebowitz et al. (in review)

Threshold effect?

~ 20 g m-2

Combined model (snails, flow, light) explains over 50% of algae variation

Snail Biomass (g m-2)

Alg

ae B

iom

ass

(g m

-2)

Experimental Evidence: Snails Control Algae

• Enclosed & excluded snails

Liebowitz et al. (in prep)

0 50 100 150 200 250 300 3500

5

10

15

20

25

30

35

40

45

f(x) = 2.46086606987551 exp( − 0.00313473225145049 x )R² = 0.640491892532922

f(x) = 12.8376995134515 exp( − 0.00459539280306784 x )R² = 0.55239167058656

f(x) = 14.9535593057998 exp( − 0.00782615105298781 x )R² = 0.847684879293451

f(x) = 38.1271888766727 exp( − 0.00923800694915261 x )R² = 0.934313949655305

HS

Expo-nential (HS)

GF

Expo-nential (GF)

MP

Expo-nential (MP)

ST

Gastropod wet weight (g m-2)

Alg

ae

AF

DM

(g

m-2

)

Observational Evidence:What Controls Snails?

• Changes in DO– Flow varying?

• Changes in salinity & [Ca++]• Human disturbance• Snail density model r2 > 0.6

– Dissolved oxygen– Salinity– pH– Light– SAV

Liebowitz et al. (in review)

Strong (2004)

Evidence of Alternative States

Experiment 1 – Low Initial Algae: Intermediate density of snails able to control algal accumulation.

Experiment 2 – High Initial Algae: No density of snails capable of controlling accumulation.

Shape of hysteresis is site dependent.

Summary

• Nitrate is a poor predictor of algal abundance– Load >> Demand (N is, and may have always been

sufficient to satisfy all ecosystem demand)• Grazers exert a dominant control on algae

– Evidence of “escape density” thresholds that are really important for management

• Dissolved oxygen (among other things) impacts grazers– Even short term stress has lasting impacts, in part

because algal biomass can escape control

Complex Ecological Causes

Questions?mjc@ufl.edu