Response of Diatom Assemblages to the 20 th Century Changes in Water Quality

Transcription

1 Response of Diatom Assemblages to the 20 th Century Changes in Water Quality Conditions in Biscayne Bay, FL, USA Anna Wachnicka, Evelyn Gaiser and Lynn Wingard Florida International University, FL, USA U.S. Geological Survey, VA, USA

2 The 20 th Century Modifications of South Florida Hydroscape 1920s 1960 sig. reduction of natural surface and groundwater flow 1990s saltwater encroachment into coastal wetlands Florida City Canal highly pulsed point source freshwater discharges from canals unnatural fluctuations in salinity and nutrient levels increased pollutant concentration changes in water circulation

3 Ecosystem State Changes Resilience Threshold Resista ance State A State B Valleys of Stability Hollings 1973

4 Questions What can fossil diatoms tell us about consequences of watershed modifications on the mainland on environmental conditions in Biscayne Bay? Were the changes in environmental conditions large enough to cause ecosystem regime shifts? If yes, when that happened and which factors should be blamed for the shifts?

5 Why Diatoms? ubiquitous distribution relevant to ecosystem form base of food web easy to identify unique valve ornamentation diverse and species rich provide large amount of ecological information and statistical power in analyses short reproduction rates (hours to days) respond quickly to environmental changes strong relationships between diatoms and water quality parameters good valve preservation

6 Sampling Sites 81 W 25 N Florida Snapper Creek Cutler Drainage Canal Central Bay No Name Bank Black Point Black Creek Canal Biscayne Bay Featherbed N Bank Princeton Canal Military Canal Mowry Canal North Canal Florida City Canal Turkey Point Power Plant South Bay Atlantic Ocean X-rayed N Radiometric Dating 210 Pb, 14 C Card Sound 80 09'W Wingard et al. (2003)

7 Compositional Similarity among Sites based on Non-metric Multidimensional Scaling (NMDS) with Sørensen Similarity Index dry season wet season Axis North of Mowry Canal STP South of Mowry Canal WTOC STN Off-shore sites O2 S D Ax xis Nearshore N of Mowry Canal WTP Nearshore Off-shore S of sites Mowry Canal WTOC STPWTN STN STC Turb T D ph S stress= Axis Axis 1 stress=11.9 Mowry Canal off-shore site nearshore sites North of Mowry Canal nearshore sites South of Mowry Canal Mowry Canal

8 Trajectory of Change of Diatom Assemblage Structure over the last ~ 130 years No Name Bank 1999 Stress: Card Sound Bank Stress: % Similarity % Similarity

9 Principle of Quantitative Paleoenvironmental Reconstruction Calibration biological data = diatoms 1, m taxa Modern Data (training Y set) n samples X=Ȗ (Y) environmental variables (e.g., salinity, nutrients) 1, p variables n samples X Ȗ = ecological response function % ab bundance Taxon abund dance (y) Optimum (u) Tol. (t) Environmental variable (x) (e.g. salinity) Reconstruction y ik abund. of taxon k in sample i x i value of env. variable in site i m xˆ 0 = y 0k uˆ k / y 0k k=1 m k=1 1, m taxa 1, Fossil Data t samples Y 0 X 0 = ˆ Ȗ (Y 0 ) ˆ t samples X 0 Unknown env. variable to be reconstructed red variable (x) Diatom-inferr ˆRMSEP=(xi-xi) ˆ r 2 Measured variable (x) Birks (1995)

10 Evaluation of Weighted Averaging Partial Least Squares Inference Models alinity (ppt) tom-inferred S Dia R 2 = 0.96 RMSEP=2.82ppt 1: Observed Salinity (ppt) Dia atom-inferred TN (ppm) R 2 = 0.85 RMSEP=0.09ppm 1: Observed WTN (ppm) Diatom-Inferre ed TP (ppm) R 2 = 0.81 RMSEP=0.003ppm 1: Observed WTP (ppm) TOC (ppm) Diatom-Inferred R 2 = 0.80 RMSEP=1.93ppm 1: Observed WTOC (ppm)

11 Diatom-based Estimation of Past Salinity and Nutrient Concentration Levels at No Name Bank Age Depth (c cm) Salinity (ppt) WTN (ppm) WTP (ppm) WTOC (ppm) % similarity

12 Diatom-based Estimation of Past Salinity and Nutrient Concentration Levels at Card Sound Bank Salinity (ppt) WTN (ppm) WTP (ppm) WTOC (ppm) % Similarity Ag ge Depth (cm)

13 Conclusions Watershed modifications on the mainland destabilized the Biscayne Bay ecosystem and decreased its ability to absorb the disturbances The amount of disturbance needed to change an ecosystem state decreased (ecological resilience declined) The rate of diatom assemblage restructuring at both locations sig. increased after the late 1950 s (NNB) and late 1920 s (CSB) compared to earlier periods (up to 65% vs. ~ 30%), implying that anthropogenic modifications of watershed on the mainland had a sig. effect on Biscayne Bay env. cond. The 1960 s largest restructuring t of diatom assemblages occurred also in Eastern and Central Florida Bay, implying large-scale shift in ecosystem state in South Florida estuaries Not one, but combination of multiple factors (anthropogenic and natural in nature) should be blamed for the changes

14 Acknowledgments Dissertation Committee Members (E. Gaiser, L. Collins, J. Boyer, W. Anderson, B. Clement) Peter Harlem, Jack Meeder & Mike Ross for their valuable comments Colleagues from Periphyton Lab for fieldwork assistance SERC for logistic support Biscayne National Park, USGS for financial support