Effects of shading on calcareous benthic periphyton in a short-hydroperiod oligotrophic wetland (Everglades, FL, USA) |
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Authors: | Serge Thomas Evelyn E Gaiser Franco A Tobias |
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Institution: | (1) Southeast Environmental Research Center, Florida International University, Miami, FL 33199, USA;(2) Department of Biology, Southeast Environmental Research Center, Florida International University, Miami, FL 33199, USA |
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Abstract: | The effects of shade on benthic calcareous periphyton were tested in a short-hydroperiod oligotrophic subtropical wetland
(freshwater Everglades). The experiment was a split-plot design set in three sites with similar environmental characteristics.
At each site, eight randomly selected 1-m2 areas were isolated individually in a shade house, which did not spectrally change the incident irradiance but reduced it
quantitatively by 0, 30, 50, 60, 70, 80, 90 and 98%. Periphyton mat was sampled monthly under each shade house for a 5 month
period while the wetland was flooded. Periphyton was analyzed for thickness, DW, AFDW, chlorophyll a (chl a) and incubated in light and dark BOD bottles at five different irradiances to assess its photosynthesis–irradiance (PI) curve
and respiration. The PI curves parameters P
max, I
k and eventually the photoinhibition slope (β) were determined following non-linear regression analyses. Taxonomic composition and total algal biovolume were determined
at the end of the experiment. The periphyton composition did not change with shade but the PI curves were significantly affected
by it. I
k increased linearly with increasing percent irradiance transmittance (%IT = 1−%shade). P
max could be fitted with a PI curve equation as it increased with %IT and leveled off after 10%IT. For each shade level, the
PI curve was used to integrate daily photosynthesis for a day of average irradiance. The daily photosynthesis followed a PI
curve equation with the same characteristics as P
max vs. %IT. Thus, periphyton exhibited a high irradiance plasticity under 0–80% shade but could not keep up the same photosynthetic
level at higher shade, causing a decrease in daily GPP at 98% shade levels. The plasticity was linked to an increase in the
chl a content per cell in the 60–80% shade, while this increase was not observed at lower shade likely because it was too demanding
energetically. Thus, chl a is not a good metric for periphyton biomass assessment across variously shaded habitats. It is also hypothesized that irradiance
plasticity is linked to photosynthetic coupling between differently comprised algal layers arranged vertically within periphyton
mats that have different PI curves. |
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Keywords: | periphyton algae shade photosynthesis– irradiance Everglades wetlands |
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