2.3.1. Pulsing, shading, and calcium seed addition
A bell-siphon was used in conjunction with a 0.3175 cm (0.125 in.) air release valve and Idec GT3W timer to create pulsed inflow conditions for certain experiments (see Table 1). The bell-siphons were constructed from 7.62 cm (3 in.) and 3.175 cm (1.25 in.) PVC pipes. The 3.175 cm PVC pipe was placed inside the larger 7.62 cm pipe using nylon screws. A 7.62 cm (3 in.) cap was placed on top of the 7.62 cm pipe with a hole drilled in the top. A 0.635 cm (0.25 in.) nylon hose barb was sealed into the hole using silicone. This barb fitting was then connected to a length of 0.635 cm (0.25 in.) PVC vinyl tubing. As water filled the inflow tank, air was trapped inside the bell siphon. The Idec GT3W timer released the trapped air at calibrated intervals, creating a pulse of water of known volume. Pulse frequency and volume are given in Table 1.
Shading was accomplished by covering the raceways with 20 × 20 mesh using clamps made of sections of 3.175 cm (1.25 in.) PVC pipe. A LI-COR LI-1400 PAR meter measured a 60% reduction in photosynthetically active radiation under the mesh. For experiments involving Ca seed addition, 12 g of calcium carbonate (Mississippi Lime Vitacal® PCC) was sprinkled across the last 9.3 m of the raceway using a powdered sugar shaker. Multiple passes were made to ensure an even distribution of the CaCO3 seed material.