Data to support "delta13C and delta15N values from a mesocosm experiment showing sea urchins mediate the availability of kelp detritus to benthic consumers"

The purple sea urchin Strongylocentrotus purpuratus is an important herbivore and detritivore in southern California giant kelp (Macrocystis pyrifera) forests that grow on shallow rocky reefs (~3-20m depth) off southern California, including Santa Barbara where this study was done. To investigate the shredder activity of purple urchins, we assembled communities of common detritivores and suspension feeders from local reefs in mesocosms. Three species of brittle stars (Ophiopteris papillosa, Ophioplocus esmarki, and Ophiothrix spiculata), one vermetid gastropod (Thylacodes squamigerus), two barnacles (Chthamalus sp. and Megabalanus californicus), one polychaete worm (Chaetopterus sp.), and three sea cucumbers (Cucumaria piperata, Pachythyone rubra, and Cucumaria salma) were collected from the seafloor of local kelp forests at 5-15m depth in the Santa Barbara Channel. Ten of each species were placed in each of six 50 L (51x38x27 cm) flow-through unfiltered seawater tanks, each containing two concrete bricks for hard substrate on top of 3 cm of sand. Because sea urchins are highly mobile, capable of bulldozing other occupants, we isolated them above the experimental communities on plastic mesh (1cm). Mesh dividers were secured horizontally across each mesocosm 16 cm above the bottom. Ten individuals of each consumer species, along with 2-3 blades, totaling 47.3 g (±1.2 SE), of isotopically labeled kelp (see below) were placed on the floor of each tank, below the divider. On top of the mesh divider, we placed an additional 229.8 g (±6.1 SE) of enriched kelp blades, and in half of the tanks, 10 adult urchins (total mass 343.3 g ±1.3 SE) per tank. The kelp below the mesh ensured that the detritivores had direct access to degrading kelp detritus regardless of the urchin treatment, similar to the situation in the kelp forest, allowing us to more clearly ascertain the degree to which the detritivores were dependent on urchins for kelp detritus assimilation. The experiment was run for 28 days (Jul 26 - Aug 24, 2016); above-mesh kelp was removed and replenished once after 13 days. The duplicated data table has been published in Dryad: Yorke, Christie E.; Page, Henry M.; Miller, Robert J. (2019), Data from: Sea urchins mediate the availability of kelp detritus to benthic consumers, Dryad, Dataset, https://doi.org/10.5061/dryad.38m3dc6