Metadata

Description
The EPA periphyton samples were collected annually as part of the Monitoring and Assessment Program of the Comprehensive Everglades Restoration Plan (CERP 2020). Generalized random-tessellation stratification (Stevens and Olsen 2004) was used to determine the location of three unique, sampleable Primary Sampling Units (PSUs) each year within 800 × 800 m grid cells (Philippi 2005). To be considered sampleable habitat, the PSUs had to contain vegetation that was not too dense for the sample device to enclose one-m3 of the water column and less than one m deep, which were primarily wet prairies and sloughs. Samples were collected from each PSU during a four-month window (September–December) representing the peak wet season in the EPA. At each PSU, a one-m3 enclosure with mesh sides that was open on the top and bottom was used to collect samples. All periphyton within the enclosure was collected and measured for biovolume using a graduated cylinder. If no calcareous benthic, epiphytic, or metaphytic periphyton was present, flocculent detritus or filamentous green algae were collected. A 120 mL homogenized subsample of the periphyton was returned to the lab on ice, frozen, and then thawed before processing.
The BICY periphyton samples were collected annually by the South Florida/Caribbean Inventory and Monitoring Network (SFCN) of the National Park Service in wetlands in the northwest corner of the Preserve because surface water monitoring in that area indicated high concentrations of P (Urgelles et al. 2019). The monitoring area was delineated into seven hydrologically distinct basins separated by natural and artificial barriers. Some of these basins had anthropogenically elevated surface water P concentrations, while other basins had ambient surface water P concentrations. A restricted stratified sampling design was used to establish six to seven permanent, random sampling sites per basin, targeting freshwater broadleaf marsh habitat. To be considered sampleable habitat, site selection was constrained by the following criteria: the site must be located within 250 m of freshwater broadleaf marsh habitat, at least 1000 m from other sites within the same basin, located greater than 100 m from road or trail; the site must be accessible by road or helicopter and without obvious human disturbance or vegetation that is too dense to sample. Samples were collected from each site during October–March. For samples collected between hydrologic years 2013 and 2019, the preferred substrate collection order for calcareous periphyton was (1) floating mat, (2) epiphytic, (3) benthic, and (4) epidendric. At each site, a minimum of five grab samples of periphyton were collected within a 5 m radius and composited into two 120-mL homogenized samples. For samples collected in the hydrologic year 2020, the periphyton samples did not have a preferred substrate collection order, but instead were collected in relative proportion of the substrates represented at the site. Each sample collected was divided into two 125-mL Nalgene (Thermo Fisher Scientific Inc., Waltham, Massachusetts) bottles on ice and returned to the lab. For each sample, a subsample (one 125-mL Nalgene bottle) was preserved in a 3% formalin solution for diatom assemblage analysis and the other subsample (one 125-mL Nalgene bottle) was frozen and thawed before further processing.
Water depth was measured at each site using a one-m measuring stick (cm), and surface water conductivity (µS cm-1), temperature (°C), and pH were measured using a multimeter probe. Days since the last dry down (number of days since flooding of the marsh surface after the latest drying even when water levels were <5 cm) and hydroperiod (days flooded) of the sample sites were estimated by calibration measured water depths to nearby continuous water level gauges using digital elevation models provided by the Everglades Depth Estimation Network (“EDEN”; https://sofia.usgs.gov/eden/stationlist.php).
Sample processing
In the lab, animals, plant matter, and other debris were removed from the periphyton, and subsamples were taken for the measurement of dry mass, periphyton mat total P (TP) concentrations, and diatom taxonomic composition analysis. To obtain periphyton dry mass (g), the biomass subsample was dried at 80°C for >48 h and then weighed. To obtain mat TP, the TP subsample was dried at 80°C, pulverized with a mortar and pestle, and then processed using colorimetric analysis to estimate TP concentration expressed as µg/g mat dry mass (Solórzano and Sharp 1980). We used mat TP as a proxy for TP in the environment because mat TP has been shown to have a stronger correlation with P loading than traditional water column P measurements (Gaiser et al. 2004). Excess P delivered to the ultraoligotrophic Everglades is rapidly assimilated by periphyton and vegetation, making TP almost undetectable in the water column even when the system has been exposed to enriched inputs for years (Gaiser et al. 2004; Gaiser 2009).
Diatom samples were cleaned of mineral debris and calcite organic matter using sulfuric acid oxidation methods following Hasle and Fryxell (1970) and a known volume was then permanently affixed to a glass slide using Naphrax (PhycoTech Inc., St. Joseph, Michigan) mounting medium. A minimum of 500 valves was counted and identified per slide (Weber 1980) using a compound light microscope at 1000× magnification under oil immersion. Identifications were made to the lowest taxonomic level possible using Diatoms of North America (diatoms.org), a database of South Florida diatom taxa (https://fce-lter.fiu.edu/data/database/diatom/), and other regional references (Slate and Stevenson 2007; Lee et al. 2014). Raw diatom counts were converted to relative abundance through standardizing by the number of valves counted for each taxon by the total number of valves counted.
Taxonomic harmonization
While a consistent photo-documented voucher flora was generated for both the BICY and EPA datasets to guide taxonomic decisions, different taxonomic sources and conventions were used for the two wetlands, requiring a taxonomic harmonization step prior to analysis. Frequent discussions between taxonomists responsible for the BICY and EPA floras enabled the harmonization of distinct morphological taxonomic units (MOTUs) for the most common taxa, and a taxonomic assignment for these MOTUs was agreed upon. When agreement on a taxonomic assignment for a MOTU could not be achieved, taxa were lumped into a new composite MOTU (Table S1). Instead of dropping difficult taxa from an analysis, inclusion through lumping can improve transfer functions involving multiple analysts, provided that the lumping decision is consistent throughout the combined datasets (Lee et al. 2019). Harmonization resulted in consistent assignments of 97.2% and 98.8% of diatom MOTUs in the BICY and EPA datasets, respectively, and the remaining non-harmonized taxa and those not identified to the species level or below were removed from the analysis. Since the presence of rare species in data sets can create noise and reduce the clarity of underlying patterns, and because we were interested in comparing the mat TP optima of taxa that were present in both wetlands, we removed taxa occurring in < 1% of the samples in each of the regional datasets (BICY and EPA) and taxa with a mean relative abundance of < 0.5% in each of the regional datasets. When harmonization and screening were complete, we merged the two taxonomic and environmental datasets into a common combined dataset.
References:
CERP (Comprehensive Everglades Restoration Plan) (2020) 2020 CERP Report to Congress. Department of the Army, Washington DC
Gaiser EE, Scinto LJ, Richards JH, Jayachandran K, Childers DL, Trexler JC, Jones RD (2004) Phosphorus in periphyton mats provides the best metric for detecting low-level P enrichment in an oligotrophic wetland. Wat Res 38:507–516
Gaiser EE (2009) Periphyton as an indicator of restoration in the Florida Everglades. Ecol Indic 9:S37–S45
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Philippi T (2005) Adaptive Cluster Sampling for Estimation of Abundances Within Local Populations of Low-Abundance Plants. Ecol 86:1091–1100
Lee SS, Gaiser EE, Van De Vijver B, Edlund MB, Spaulding SA (2014) Morphology and typification of Mastogloia smithii and M. lacustris, with descriptions of two new species from the Florida Everglades and the Caribbean region. Diatom Res 29:325–350
Lee SS, Bishop IW, Spaulding SA, Mitchell RM, Yuan LL (2019) Taxonomic harmonization may reveal a stronger association between diatom assemblages and total phosphorus in large datasets. Ecol Indic 102:166–174
Slate JE, Stevenson RJ (2007) The diatom flora of phosphorus-enriched and unenriched sites in an Everglades marsh. Diatom Res 22:355–386
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