Metadata

Description
Dataset:
PeriphytonCores_Chemistry

Ten mL periphyton mat subsamples were collected bimonthly, starting in April 2015, from each mesocosm using a plastic syringe. In the lab, the mat samples were placed in a 500-mL beaker and homogenized with deionized water (DI) water to a periphyton slurry with a minimum volume of 200-mL. The beaker was placed on a stir plate to facilitate continuous mixing while the slurry was subsampled for biomass, chl-a total carbon (TC), total nitrogen (TN), and total phosphorus (TP) analyses. The biomass and chl-a subsamples were analyzed using the same methods described for the settlement plate samples. The TP subsample was dried at 80 degrees C and pulverized with a mortar and pestle. We processed a known amount of each subsample and used colorimetric analysis to estimate TP concentration, expressed as micrograms gram AFDM, following the methods of Solorzano and Sharp (1980). TC and TN subsamples were ground down to a fine powder by mortar and pestle, dried at 60 degrees C, and analyzed for total C and N content using a Flash 1112 elemental analyzer (CE Elantech, Lakewood, New Jersey, USA) following standard procedures.
PeriphytonPlates_AccumulationRates
Clear, acrylic settlement plates (approximately 104 square centimeters) were deployed in each mesocosm to measure periphyton biomass and chlorophyll-a accumulation rates. The settlement plates were incubated on the soil surface of the mesocosm for two months to allow periphyton mats in the mesocosms to colonize the plates. At the end of the two-month incubation period, settlement plates were retrieved and transported to the laboratory, where they were scrapped clean using a razor blade and deionized water. Settlement plates were sterilized using a bleach solution and redeployed the following month. This was done quarterly from March 2015 to April 2016 for a total of 4 settlement plate samples per mesocosm over the duration of the experiment. The material scraped from the settlement plates was collected in a beaker and homogenized with deionized water (DI) water to a minimum volume of 200-mL. The beaker was placed on a stir plate to facilitate continuous mixing while the slurry was subsampled for biomass, chlorophyll-a, and diatom analysis. The biomass subsample was placed in a drying oven at 80 degrees C for at least 48 h to obtain a dry mass (g) measurement and then in a furnace where it was combusted at 500 degrees C for 1 h to obtain the ash (mineral) mass (g). We calculated ash-free dry mass (AFDM) by the loss-on-ignition method as the difference between the ash mass and the total dry mass. The chl-a subsample was filtered onto Whatman 25-mm diameter glass fiber filter (GFF) paper, frozen, and later subjected to 48 h 90 percent cold acetone extraction at 20 degrees C. Chl-a concentrations were determined using a Gildford FLUORO IV fluorometer (Gilford Instrument, Oberlin, Ohio, USA; excitation 435 nm; emission 667 nm). AFDM and chl-a accumulation rates (mgPerMeterSquaredPerDay) were calculated by standardizing AFDM and chl-a values by the area of the settlement plates and the number of days incubated.
PeriphytonPlates_DiatomRelAbund
A subsample of the periphyton material scrapped from the settlement plates was used to analyze diatom species composition. Diatom subsamples were cleaned of mineral debris and organic matter using the sulfuric acid oxidation methods prescribed by Hasle and Fryxell (1970). We pipetted a known volume of cleaned diatom sample onto a glass coverslip, permanently mounted it on a microscope slide with Naphrax (PhycoTech Inc., St. Joseph, Michigan, USA) mounting medium, and viewed the diatoms under a compound light microscope (Axioskop 2; Zeiss, Thornwood, New York) equipped with differential interference contrast and a digital camera (Leica DFC425, Wetzlar, Germany). We counted at least 500 diatom valves along random transects at 600× magnification under oil immersion and identified them to the species level. Raw diatom counts were converted to relative abundance by standardizing the number of valves counted for each species by the total number of valves counted. We chose to analyze periphyton diatom species composition from the settlement plates rather than samples taken from the mesocosm soil surface because, as diatoms are counted and identified from cleaned (oxidized) samples, testing treatment effects on the diatom assemblage using established mats would have been confounded by the presence of dead diatom frustules; using settlement plates allowed us to better pick up diatom community response to treatments over time.