Eutrophication and Algal Proliferation in Florida’s Springs Forest Hydrology Spring 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 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

Scope of the Problem in Florida Source: USEPA (

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 2006 V. Smith, L&O 1982

Global Nitrogen Enrichment Humans have massively amplified global N cycle –Terrestrial Inputs 1890: ~ 150 Tg N yr : ~ 290+ Tg N yr -1 –River Outputs 1890: ~ 30 Tg N yr : ~ 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 (~ ppb as N) Springs are sentinels of aquifer pollution –Florida has world’s highest density of 1 st magnitude springs (> 100 cfs) Arthur et al. 2006

Weeki Wachee ’s Mission SpringsChassowitzka (T. Frazer) Weeki Wachee Mill Pond Spring

GROW FASTERLOST MORE SLOWLY Core Question: What Causes Algae to Reach Nuisance Levels?

H null : 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 [NO 3 ] (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 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 O 2 /m 2 /d (ca. 1,500 g C/m 2 /yr) N demand is proportional –0.05 – 0.15 g N/m 2 /day N flux (over 5,000 m reach) is large –Now: ca. 30 g N/m 2 /d (240 x U a ) –Before: ca. 2.5 g N/m 2 /d (20 x U a ) 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 Rates Dissolved 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 ) Algae Biomass (g m -2 )

Experimental Evidence: Snails Control Algae Enclosed & excluded snails Liebowitz et al. (in prep)

Observational Evidence: What Controls Snails? Changes in DO –Flow varying? Changes in salinity & [Ca ++ ] Human disturbance Snail density model r 2 > 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?