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LA. Photo of blocks 3 and 4 in Port Fourchon, LA. Extensive boardwalks were built to access dieback areas, low and high Spartina density treatments, and adjacent healthy areas. GA. Methods.

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Methods

LA

Photo of blocks 3 and 4 in Port Fourchon, LA. Extensive boardwalks were built to access dieback areas, low and high Spartina density treatments, and adjacent healthy areas.

GA

  • Methods.

  • Four plots(7 x 8 m) were setup in 6 blocks each in dieback areas in GA and LA with a range of S. alterniflora densities (reference (vegetated controls), low and high density plantings, and bare areas).

  • Three replicate quadrats were sampled during the fall of 2006-2008 in each treatment per block (3 x 4 x 6 = 72 per state).

  • Meiofauna (>63 µm) and macroinfauna (>500 µm) were collected with a plastic pvc corer (diam. 5.2 cm x 5 cm depth), sieved, preserved, and stained with Rose Bengal dye.

  • Snails and other molluscs were collected from within a 2500 or 5000 cm2 area in each quadrat, counted, and measured with calipers to the nearest 0.1 mm.

  • Fiddler crab holes (>5 mm) were counted in a 625 cm2 area in each quadrat as a proxy for the number of crabs in each plot.

  • Blue crabs abundance was assessed by visual counts in the field and by deploying crab pots in vegetated and bare areas overnight in fall 2008.

GA - size frequency distribution

2006

2008

2007

LD

HD

VC

  • For meiofaunal samples with >1000 individuals, 2 subsamples/rep were taken from a known slurry volume with a goal of attaining ~150-200 animals from the dominant group (copepod or nematode) and then adjusted to core volume. Density centrifugation (Ludox HS40) was used to aid in separating meiofauna from the sediment.

  • Due to the high density of meiofauna, we analyzed 2 reps each from bare and reference (vegetated) samples for 2006 and 1 rep from each of the 4 treatments in 2007.

  • All reps and treatments collected were analyzed for macroinfauna.

LA - snail frequency distribution

2006

2008

2007

LD

HD

Photo shows a core being taken in a bare plot for meiofauna and macroinfauna.

VC

GA

LA

  • Macroinfaunal Presence, Abundance, and Taxon Richness.

  • Fiddler Crab Abundance.

    Blue Crab Abundance.

GA

LA

GA

LA

LA

GA

  • In GA, greater macroinfauna in with increasing vegetation (graph above)

  • In LA, no clear trend with treatment

  • Decrease in abundance over time in both states

    • (GA – drought in 2008?: LA – hurricane Ike 2008?)

GA

LA

  • No evident trend in GA

  • 10x fewer crab holes in LA

  • Oligochaetes and polychaetes were the dominant macroinfauna

  • Overall, LA had greater macroinfaunal densities

  • Species richness generally low in both states, and decreased over time (as did trends in abundance)

  • In GA, taxon richness increased with increasing vegetation

  • Overall, more blue crabs in LA marsh

  • Also, visual counts ~10.5 crabs/block in LA vs. 0 crabs /block in GA

  • More blue crabs in bare areas than in vegetated areas at all blocks in LA (blocks not shown)

GA

LA

4.0

4.0

2006

3.5

2007

3.5

)

-2

2008

3.0

3.0

2.5

Taxa (# 300 m-2)

2.5

Species (# 300 m

2.0

2.0

1.5

1.5

Taxa

1.0

1.0

0.5

0.5

0.0

0.0

Bare

Low

High

Reference

Bare

Low

High

Reference

Density

Density

Density

Density

Filled: Vegetated

Open: Bare

oligochaetes*

oligochaetes*

*no sulfur isotopes measured

Variation in marsh benthic invertebrate presence and abundance related to altered Spartina alterniflora density

Caroline R. McFarlin1, T. Dale Bishop1, Merryl Alber1, Mark Hester2

1Dept. of Marine Sciences, University of Georgia, Athens, GA 30602,

2Dept. of Biology, University of Louisiana at Lafayette, Lafayette, LA 70504

  • Abstract.Sudden loss of a foundation species like the salt marsh grass Spartina alterniflora can drastically affect ecosystem functioning, including altering habitat support. Dieback areas following droughts in GA and LA were opportunistically used to set up manipulation plots encompassing a range of Spartina densities (vegetated/control, low/high density transplants, affected/bare). During fall of 2006-2008, we sampled infauna (meiofauna>63 µm, macroinfauna>500 µm) and epifauna (crabs, snails, bivalves). In GA, higher densities of most organisms (meiofauna, macroinfauna, Littoraria, bivalves) were found in vegetated plots (no trend in infaunal crabs or blue crabs). In LA, there were higher densities of Littoraria and blue crabs in vegetated plots, but meiofauna were more abundant in bare areas (no trend in macroinfauna or infaunal crabs). There were also differences between states: periwinkle and fiddler crab abundances were higher in GA (3x and 10x, respectively), whereas blue crab abundances were higher in LA. Overall meiofaunal density was similar between states with nematodes and copepods comprising >90% of individuals. Macroinfaunal density was 5x greater in LA and dominant taxa differed: oligocheates were most abundant in LA, whereas polychaetes were most abundant in GA. Stable isotopes show that food webs are behaving similarly in each state and in bare and vegetated plots.

  • Hypotheses Tested.

  • Invertebrate abundance and diversity will be greatest with increased Spartina density.

  • 2. Changes in the relative importance of primary producers (Spartina vs. benthic microalgae) will alter the benthic food web.

  • However, both hypotheses will depend on hydrogeomorphic between the states.

Snail Abundance.

Snail Size.

LA

GA

  • Overall, more snails in GA

  • Significantly greater snail densities with increasing vegetation each year in both states

LA

GA

  • No difference in size of GA snails over time

  • Increase in size of LA snails over time

  • Size frequency distribution (blue bar graphs to right) shows distinct cohorts over time in both states

  • Meiofaunal Presence and Abundance.

2006 Nematodes

2006 Copepods

GA

LA

GA

LA

Isotopic Analysis.

  • Discussion and Conclusions.

  • Ecosystem services provided by tidal wetlands include habitat, refugia, and food for invertebrates.

    • In GA, the density of all invertebrates (except for fiddler crabs) was greater in vegetated areas, reflecting the importance of habitat and refugia.

    • In LA, vegetated areas also provide habitat and refugia (snails). However meiofauna were more abundant in bare areas, while macroinfauna showed no real trend.

  • Some of the differences observed between the states are likely due to hydrogeomorphic differences, access to food resources, predators, and habitat suitability.

    • LA sites are at a lower elevation and remain wetter than those in GA and thus, both bare and vegetated areas to remain sufficiently wet to accommodate meiofuanal and macroinfaunal movement, respiratory function, and osmotic regulation. In GA, bare areas tend to be much drier (cracked surfaces were often observed), and both meiofauna and macroinfauna abundances were lower than in vegetated areas (where plants provide shading).

    • Bare areas in LA were covered by 10 x (300 mg m-2) more benthic algae than vegetated areas and either treatment in GA. This provides a potential food resource for meiofauna and may account for the high abundances observed in bare areas in LA.

    • Blue crabs and other migratory predators (birds, observed but not quantified) were more abundant in the LA marsh sites. This may account for the greater densities of snails and fiddler crabs in GA than in LA.

  • Isotopic results suggest that the food webs are behaving similarly in both states and in bare and reference areas:

    • Benthic microalgae is an important food source for infauna

    • Spartina is a source of food for epifauna (snails)

    • - Mobile organisms (fiddler crabs, blue crabs) rely on a combination of BMA and Spartina regardless of plant density

  • In GA and LA, Spartina and benthic microalgae (BMA) in reference areas show distinct δ13C signal signature

  • There is a shift in δ34S value in the BMA in bare areas, although the shift is opposite in GA and LA

  • The oligochaete δ13C signals were closest to benthic microalgae

  • Littoraria signal closer to Spartina

  • Fiddler crabs in both bare and vegetated areas in GA and LA likely rely on combination of BMA and Spartina (and possibly mangroves in LA, as well)

2007 Nematodes

GA

LA

2007 Copepods

GA

LA

Other Meiofauna Present

GA

LA

Annelids

Oligocheata

+

+

Polychaeta (Sabellidae)

-

+

  • In GA, greater number of nematodes and copepods in vegetated areas.

  • In LA, greater number of nematodes and copedpods in bare and low density areas.

  • Nematodes and Copepods account for >95% of meiofauna in both states.

Crustacea

Ostracoda

-

+

  • Blue crabs in both treatment areas of LA also show evidence for both BMA and Spartina as the source of their food chain

Un-ID Arthropod nauplii

+

+

Crab zoea

+

-

Molluscs

Geukensia

+

-

Hydrobiidae

+

-

Insects

Acknowledgements.This work is part of an EPA-funded project, “Climate-Linked Alteration of Ecosystem Services in Tidal Salt Marshes of Georgia and Louisiana”. We would like to thank Kristen Anstead, Jenny Fenton, Christine Hladik, Galen Kaufman, Trey Kenemer, Ashley Rich-Robertson, Erin Romer, Daniel Saucedo, Jennie Seay, Sylvia Schaefer, Jacob Shalack, Caitlin Yeager for help in the field and lab.

Ceratopogonidae

+

+

Collembola

-

+

Acari

+

+

Other

Kinorhyncha

+

-

Foraminifera

-

+


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