BACKGROUND Hurricanes routinely damage mangrove forests; winds break and topple trees, storm surges and waves erode sediments. Normally forests recover through seedling recruitment. However, the passage of two category 4-5 hurricanes across the Cape Sable peninsula in SW Florida (1935, 1960) resulted in localized conversion of mangrove forests to unvegetated mudflats (Wanless et al., 1994). We ask the question "What is the consequence of the conversion of mangrove to mudflat habitat on intertidal assemblages of fish and decapod crustaceans within the creeks of the Big Sable Creek complex?"

Figure 1. (top left) Aerial view of south Florida. [larger image of Figure 1 and 2]   Figure 2. (bottom right) Satellite Image of the Big Sable Creek Complex including locations of nekton sampling sites.

SITE DESCRIPTION

• The Big Sable Creek complex (BSC) consists of six tidal creeks that are a mosaic of mangrove forest (Figure 2, red areas) and mudflats (Figure 2, gray areas).
• Both habitats are intertidal, i.e., inundated at high tide and exposed at low tide.
• Tidal range is approximately 2 meters; salinity is 33-35 psu.

Figure 3. (left) Intertidal rivulet drains a forest in the Big Sable Creek Complex. [larger image]	Figure 4. (right) Intertidal rivulet drains a mudflat in the Big Sable Creek Complex. [larger image]

EXPERIMENTAL DESIGN

Figure 5. Red Mangrove prop roots. [larger image]

• The statistical design is a repeated measures analysis of variance (ANOVA) with creek as the unit of replication.
• The dependent variable is density-fish per 100m³ of water volume, the independent variables are habitat type and season.
• We sampled rivulets at 3 replicate creeks, each with a fixed forested and a fixed mudflat site.
• Samples were taken every 2 months for 18 months.
• We compared species composition between habitat types using an ordination technique, multidimensional scaling (MDS), followed by analysis of similarity (ANOSIM) to ascertain statistical significance of species groupings.
• We compared fish densities with a repeated measures ANOVA.

NULL HYPOTHESES

H₀1: No difference in density of fish and decapod crustaceans between habitats

H₀2: No difference in species composition

METHODS & RATIONALE

• Intertidal rivulets drain the mangrove forests and unvegetated mudflats of the Big Sable Creek complex (Figures 3 & 4).
• Rivulets are depressions in the substrate that are up to 1 meter deeper than the forest floor or mudflats around them, but are smaller than first order tidal creeks.
• Rivulets are "hotspots" for the entry and exit of fish, shrimp, and crabs from intertidal habitats (Rozas et al., 1988).
• Block nets with wings (Figure 6, 7, & 8) were set across intertidal rivulets to compare nekton (fish, decapod crustaceans) leaving replicate forest and mudflat habitat.

Figure 6. (left) BSC forest site #3 at low tide, net ready for retrieval. [larger image]	Figure 7. (right) BSC mudflat site #1 at high tide, net deployment. [larger image]

Figure 8. BSC mudflat site #1 at low tide, net ready for retrieval. [larger image]

Figure 9. Technician takes discharge measurements across an intertidal rivulet. [larger image]

HYDROLOGICAL METHODS

• A Sontek YSI FlowTracker was used to measure discharge on the ebb portion of representative spring tides at all 6 net sites (Figure 9).
• Volume was calculated from rivulet-specific, cumulative stage volume relations (Figure 10, Gordon et al., 1992).
• Calculated volume was used to convert fish catch per net to fish density.

SELECTED RESULTS

• On average nekton were more abundant during the wet season (Figures 11, 12).
• Fish density (fish per cubic meter) was much greater in forested habitats than from unvegetated mudflats (Figure 11).
• Species composition was significantly different between forest and mudflat samples, ANOSIM p value < 0.001 (Figure 12).
• Schooling species characterized mudflats:

  Anchovy densities were greater on mudflats than in forests (Figure 12A).

  Atlantic thread herrings and scaled sardines were captured only from mudflats (Figure 12B-C).

• Mojarra and grass shrimp comprised 85% of organisms captured in the forests; their densities were much greater in forests (Figure 12D-E).
• Densities of frillfin gobies were significantly greater in the forests (Figure 12F).

Figure 10. Results from rivulet discharge showing the relation between water level and cumulative volume discharged. [larger image]

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Figure 11. Density of all fish captured throughout the study, untransformed arithmetic mean by habitat and season, + 1 S.E. [larger image]

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Figure 12. Multidimensional scaling plot (MDS) of standardized forest and mudflat samples (square-root transformed). Samples from the two habitats differed on average by 75%. Differences were statistically significant at < 0.001 (ANOSIM, Global R = 0.532). Characteristic species of each habitat were identified using program SIMPER. Figures A-F are densities of six species ± 1 S.E. [click on separate images for larger version]

CONCLUSIONS AND IMPLICATIONS

• Hurricane-induced conversion of forest to mudflat results in a lower overall fish density in the intertidal zone, as well as changes in the species composition.
• Conversion of forest to mudflat favors water-column schooling fishes over substrate-associated fishes.
• Results are consistent with previous studies regarding the influence of vegetation structure on fish density (Heck et al., 1989).
• In this era of increased hurricane intensity (Emmanuel, 2005), there is an increased potential for mangrove conversion to mudflats.

ACKNOWLEDGEMENTS: Noreen Buster, Gary L. Hill, Adam Brame, Lauren Yeager, Betsy Boynton, Ed Matheson, B.J. Reynolds, Dave Wegener, Everglades National Park. Funded by USGS Global Climate Change and Priority Ecosystems Science Programs.

References Cited

Emmanuel, K. 2005. Increasing destructiveness of tropical cyclones over the past 30 years. Nature 436: 686-688.

Gordon, N. D., T. A. McMahon, and B. L. Finlayson. 1992. Stream Hydrology: An Introduction for Ecologists, John Wiley & Sons, New York, 526 pages.

Heck, K. L. Jr., K. W. Able, M. P. Fahay and C. T. Roman. 1989. Fishes and decapod crustaceans of Cape Cod eelgrass meadows: species composition, seasonal abundance patterns and comparison with unvegetated substrates. Estuaries 12: 59-65.

Rozas, L. P., C. C. McIvor, and W. E. Odum. 1988. Intertidal rivulets and creekbanks: corridors between tidal creeks and marshes. MEPS 47: 303-307.

Wanless, H. R., R. W. Parkinson, and L. P. Tedesco. 1994. Sea level control on stability of Everglades wetlands. IN: Davis, S. M. and Ogden, J. C., (Eds), Everglades: The Ecosystem and Its Restoration, St. Lucie Press, 199-223.