Effects of phosphorus availability on aquatic food webs and community structure in the Everglades

Transcription

1 Effects of phosphorus availability on aquatic food webs and community structure in the Everglades Shawn Liston, Joel Trexler, Scot Hagerthey, Ryan King & Brian Garrett

2 Outline Food webs Our Everglades food web Observed effects of P on the food web Impact of methodology on interpreting food web data Conclusions & future directions

3 Food Webs Predation Combined effects of predation (top( top-down) ) and resource availability (bottom-up)) control: Distribution of biomass among trophic levels Resource availability

4 Food Webs Combined effects of predation (top-down) and resource availability (bottom-up) control: Distribution of biomass among trophic levels Chase & Ryberg (2004) Community structure species richness species diversity

5 Food Webs Additional factors: Food quality Edibility Grazers Omnivory Habitat complexity Spatial pattern & scale of habitat Connectivity, patches, refuges Interactions with other variables (e.g. hydrology) Indirect effect + C B A Morin (1999)

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7 Everglades Food Web Unique distribution of biomass High algal biomass + Low consumer biomass Fish Everglades (Turner et al. 1999) Kalf (2002)

8 Everglades Food Web Everglades Food Web Unique distribution of biomass 100 High algal biomass + Low consumer biomass Extremely low biomass of grazers relative to other systems karstic sub-tropical wetlands ditch river 6 th order 4 th order 2 nd order 3 rd order 1 st order pond stream temperate wetland lake gravel pit 7 th order 5 th order Belize wetland Mexican wetland WCA enriched WCA pristine Everglades National Park 0 Cliff Ruehl, personal communication Snail Biomass (g/m 2 )

9 Everglades Food Web 3 Consumer Macrophytes (habitat structure) Nutrient excretion colonies) 2 Consumer Periphyton, epiphyton (refuge) (habitat) 1 Consumer Floc, Detritus 1 Producer

10 Everglades Food Web ROLE OF CALCIUM CARBONATE Physiological Limitations (Direct) maintain osmotic balance, construct shells, skeletons, etc. mat-forming periphyton Cascading Trophic Effects (Indirect) CaCO 3 in periphyton mats affects consumers algae edibility, palatability critical for detritus production important consumer habitat CaCO 3 is responsible for this low nutrient, P-limited P system

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12 Effects of P on food webs Effects on Community Structure: FISH Data suggest more herbivores MACROINVERTEBRATES Shift in community structure with P enrichment Apparent at low levels of enrichment More oligochaetes,, more predators Indicator species (esp. Chironomidae) (McCormick et al. 2004)

13 Effects of P on food webs FISH Generally, INCREASE in fish (large & small) density and biomass with enrichment (Rehage & Trexler 2006) (Turner et al. 1999) (Liston 2006)

14 Effects of P on food webs FISH Generally, INCREASE in fish (large & small) density and biomass with enrichment May decrease at high P levels (subsidy-stress) stress) Log 10 (mg/m 2 ) Fish Biomass King & Richardson 2007 r 2 =0.49, F 2,11 =5.35, p= Sediment TP (mg/kg)

15 Effects of P on food webs MACROINVERTEBRATES Kendall et al. (2000) Macroinvertebrate density & biomass responses to P enrichment are complex

16 Effects of P on food webs MACROINVERTEBRATES Macroinvertebrate density & biomass responses to P enrichment are complex Driving factors Higher trophic levels (cascade) (Turner et al. 1999)

17 Effects of P on food webs MACROINVERTEBRATES Macroinvertebrate density & biomass responses to P enrichment are complex Driving factors Higher trophic levels (cascade) Habitat structure (Turner et al. 1999)

18 Effects of P on food webs MACROINVERTEBRATES Macroinvertebrate density & biomass responses to P enrichment are complex Driving factors Higher trophic levels (cascade) Habitat structure Hydroperiod ( Liston 2006)

19 Effects of P on food webs MACROINVERTEBRATES Macroinvertebrate density & biomass responses to P enrichment are complex Driving factors Higher trophic levels (cascade) Habitat structure Hydroperiod Sample method vs. microhabitat variation McCormick et al. 2004

20 Effects of P on food webs MACROINVERTEBRATES 6 Floating Mat Macroinvertebrate density & biomass responses to P enrichment are complex Driving factors Higher trophic levels (cascade) Habitat structure Hydroperiod Sample method vs. microhabitat variation Ln (total invertebrates/m 2 + 1) Benthic Floc OC C LO INT HI P Loading Level (Liston, Newman & Trexler 2008)

21 Effects of P on food webs MACROINVERTEBRATES Macroinvertebrate density & biomass responses to P enrichment are complex Driving factors Higher trophic levels (cascade) Habitat structure Hydroperiod Sample method vs. microhabitat variation P range Log 10 (mg/m 2 ) Macroinvertebrate Biomass C6 C4 r 2 =0.50, F 2,11 =5.57, p= Sediment TP (mg/kg) C1

22 Wading Bird Response to P Greater overall abundance of wading birds at moderately enriched sites (WHIB, GREG, WOST, GBH) Preference for foraging in more openly-vegetated slough-like habitats (most typical of unenriched locations) Annual hydrologic conditions may greatly affect foraging locations: - Wet years may increase foraging by wading birds in enriched areas - Dry years may increase foraging by wading birds in unenriched areas

23 Wading Bird Response to P Eutrophic Conditions Oligotrophic Conditions Increased prey abundance Increased habitat structure reduces prey availability Limited prey abundance Less habitat structure increases prey availability

24 SCALE

25 Assessing these effects is tricky SPATIAL SCALE Patchiness Landscape-level vs. mesocosms Open vs. closed systems (oasis of enrichment) TEMPORAL SCALE Short-term term vs. long-term dynamics Are we studying a system that is at equilibrium?

26 High Dose Low Dose Time Difference from Natural State

27 Conclusions Everglades aquatic consumers are P-limited P and this limitation relaxes with P enrichment The Everglades food web is typical of karstic tropical ecosystems; P enrichment alters the unique distribution of biomass characteristic of these systems Increased P influences Everglades aquatic consumer communities through a combination of direct and indirect factors Disappearance of periphyton mats results in habitat loss for many macroinvertebrates; ; prior to mat collapse, change in relative abundance of algae in mats may make edible algae more available to grazers Increased macrophyte density with enrichment provides increased habitat for aquatic consumers (increased density of fish), but makes m prey less available to wading birds

28 Conclusions Scale (spatial & temporal) is a critical consideration when designing experiments and interpreting data on food web effects of P enrichment Future research is needed to understand the interaction of hydroperiod and P enrichment on Everglades food webs

29 Special thanks to Numerous funding sources for supporting our research Ramesh Reddy & Ronnie Best for inviting us to participate in this symposium