Treatments:
Submerged pellet: Sensu Douglas et al. (2016), slow-release P nutrient pellets are deployed 7 cm below the sediment surface using a hand corer. Tiles are placed directly above the deposited pellets on the sediment surface. Pellet amounts are designed to elicit 1) baseline concentration, 2) boosted ion concentration to account for limestone sorptive capacity, and 3) very high ion concentrations to overcome all possible uptakes from limestone tile, marl soil, and soil biota, plus possible higher-than-expected ion release rate to ambient water from the source:
• Baseline:
o P target concentration of 10 ppb, or 0.01 mg (10 ug) P /L/day
1.5 mg of slow-release P nutrient pellets deployed
o Salt target concentration of 1 ppt, or 1 g NaCl /L/day
30 grams of rock salt pellets deployed
• Boosted:
o P target concentration of 20 ppb, or 0.02 mg (20 ug) P /L/day
3 mg of slow-release P nutrient pellets deployed
o Salt target concentration of 5 ppt, or 5 g NaCl /L/day
150 grams of rock salt pellets deployed
• Overkill:
o P target concentration of 40 ppb, or 0.04 mg (40 ug) P /L/day
6 mg of slow-release P nutrient pellets deployed
o Salt target concentration of 10 ppt, or 10 g NaCl /L/day
300 grams of rock salt pellets deployed
Pellet bed: Following the same target ion concentrations as above, slow-release P nutrient pellets and/or rock salt pellets are put in a fine plastic mesh bag and placed on the sediment interface. Tiles are placed directly on top of the pellet bed.
Soaked tile: Limestone tiles are soaked in a solution of each treatment type at ten times the desired release concentration (P = 0.1 mg/L, NaCl = 10 g/L), since it is expected that ion release via passive diffusion will release 90% of the ions in the first 3 days of deployment in non-solution water (Pringle and Bowers 1984, Rugenski et al. 2008). P solution is made from a dilution of 85% phosphoric acid (H3PO4), sensu (Servais et al. 2018). The salt solution is made using pure granulated sea salt, dissolved and diluted sensu (Mazzei et al. 2020). At high salt concentrations, solution should be heated on a hot plate to ensure complete crystal dissolution into solution. Tiles are soaked in each solution in plexiglass tubs for at least 1 week's time. Tiles are transported to the field in individual ziploc bags to prevent cross-contamination.
Special attention to P release will be paid to this method, due to concern that P may adsorb too tightly to the limestone and marl and release poorly, or, in S+P trial, will not adsorb to the tile enough to begin with due to the high concentration of salt ions (Flower et al. 2017, Flower et al. 2016).
• Baseline:
o P soaking concentration of 0.1 mg P /L/
o Salt soaking concentration of 1 ppt, or 10 g NaCl /L
• Boosted:
o 0.2 mg P / L soaking concentration
o 50 g NaCl / L soaking concentration (5 ppt)
• Overkill:
o 0.4 mg P / L soaking concentration
o 100 g NaCl / L soaking concentration (10 ppt)
