Effects of Urbanization on Stream Ecosystems in the Lower Basin of the St. Johns River

Transcription

1 Effects of Urbanization on Stream Ecosystems in the Lower Basin of the St. Johns River Dean R. Dobberfuhl, Ph.D. Division of Environmental Sciences, St. Johns River Water Management District

2 Collaborators Dr. Art Benke Dr. Alex Huryn Dr. Jonathan Benstead Dr. Michael Chadwick John Hendrickson LSJRB Field Crew University of Alabama SJRWMD

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6 Initial Hypotheses H : No relationship between impervious surface and stream ecosystem function H 1 : A relatively smooth decline in stream ecosystem function with increasing impervious surface H 2 : An a apparent threshold of impervious surface beyond which stream ecosystem function declines markedly Function Function Function Impervious Surface Impervious Surface Impervious Surface What is ecosystem function?

7 Objectives (Phase I) Quantify detrital inputs and utilization across a rural-urban gradient Litter fall, retention Leaf litter breakdown (mass, ergosterol) Elemental contents Determine trophic changes Invertebrate biomass, production Downstream effects Stable isotope analyses Water quality

8 Targeted 1 st and 2 nd order headwater streams 2 sites selected to represent a range of % urban cover using GIS landuse Impervious surface (PTIA) estimated using NDVI model Four focal streams

9 Normalized Difference Vegetation Index (NDVI) NDVI = λ λ NIR NIR λ + λ red red

10 Benthic Organic Matter Inventory 3 25 BOM Imp Surf 6 5 Benthic Organic Matter (g AFDM m -2 ) % Impervious Surface DET26 DET27 DET13 DET2 DET12 DET17 DET11 DET1 DET8 DET2 Site Reduction in BOM to 3% impervious Variable BOM in urban streams

11 Benthic Organic Matter Inventory Benthic Organic Matter (g AFDM m -2 ) seeds/cones other leaf other wood sweetgum oak maple needles hydrilla DET26 DET27 DET13 DET2 DET12 DET17 DET11 DET1 DET8 DET2 Site Loss of Sweetgum, Oak, other leaves Reduction in detrital diversity

12 Trophic Changes in Focal Streams More Individuals in Hydrilla & Urban Attenuated seasonal response

13 Trophic Changes in Focal Streams # taxa higher in Hydrilla and lower in Urban Appearance of scraper guild Diversity decreases

14 Trophic Changes in Focal Streams C-G increase, shredders decrease, scrapers appear, F-F decrease Seasonality of guilds is different Implications for invertebrate predators?

15 Urban Stream (Det 17) :2:4 22:3:4 3:4:4 8:5:4 14::4 19:1:4 :2:4 5:3:4 1:4:4 15:5:4 21::4 2:1:4 7:2:4 time Stream Function Diurnal O 2 patterns for two focus streams Hydrilla Stream (Det13) DO (mg/l) 6:55:4 12::39 17:5:39 22:1:4 3:15:4 8:2:4 13:25:4 18:3:4 23:35:4 4:4:4 9:45:4 14:5:4 19:55:39 1::4 6:5:4 time DO (mg/l) Urban stream has mostly intact riparian corridor and floodplain Hydrilla stream has open canopy and maintained corridor

16 Leaf analogs: Stream Function Movement and recovery highly correlated with discharge and PTIA. Percent leaf analog retention % urban land-use Transport distance (m) % urban land-use Average Q (L/sec) Average Q (L/sec) litter retention important for conditioning Increased impervious results in increased flow and decreased litter retention

17 Stream Function Hydrilla Stream (Det13) System diurnal P:R ratio Primary Productivity. 15:45 18:15 2:45 23:15 1:45 4:15 6:45 9:15 11:45 14:15 16:45 19:15 21:45 :15 2:45 5:15 7:45 1:15 12:45 15:15 17:45 2:15 22:45 1:15 3:45 6: Respiration time Net heterotrophic stream-microbial loop Reduced carbon export PP or R (mg DO/m2/15 min)

18 Stream Function Litter Processing Rates.4.3 red maple sweetgum k dd PTIA Processing rates per degree days (> C) for red maple and sweet gum leaves versus percent impervious land-cover. Error bars are ± 1 SE. (Chadwick et al. 26) Processing increases to ~ 3% PTIA then declines Classic subsidy-stress response

19 average biomass (mg bag -1 ) Stream Function Litter Bag Colonization Average macroinvertebrate biomass (mg bag -1 ) and total taxa richness versus PTIA. Error bars are ± 1 SE. (Chadwick et al. 26) taxa richness (bag -1 ) PTIA

20 Downstream Effects Water Quality Parameters Significantly Correlated with % Impervious Surface Positively Related Alkalinity Ca Chlorophyll a Conductivity Mg ph Sr Water temperature Negatively Related Al Color Cr DOC Fe TN TOC

21 Al (µg liter -1 ) Downstream Effects Water quality thresholds? % 1% 2% 3% 4% 5% 6% 7% % Impervious Surface Cr (µg liter -1 ) Dissolved O2 (mg liter -1 ) % 1% 2% 3% 4% 5% 6% 7% % Impervious Surface % 1% 2% 3% 4% 5% 6% 7% % Impervious Surface

22 Downstream Effects Organic Carbon Constituents NMDS of Organic Carbon Forms Stress =.5 LTOC Dim 2 Impervious Surface > 17% Impervious Surface < 11% Dim 1 TOC, DOC, Color, RTOC, R:L Impervious Surface

23 Summary BOM changes with PTIA with respect to both amount and composition. Invertebrate guild representation and seasonality change with PTIA. Hydrilla is detrimental. Litter processing and colonization show a subsidy-stress pattern (~3% threshold?)

24 Summary (cont.) Nutrients and metals do not show a consistent pattern with PTIA. Some constituents show a ~ 17% threshold. Organic carbon shows a strong response to PTIA at a ~ 17% threshold.

25 Phase II Interface between tributaries and mainstem Net organic export Rate, 15 N Quantity Form System metabolism Utilization by biota Fish Invertebrates Nutrient uptake and cycling, 15 N