Another Fact: This variability makes the kelp signature difficult to distinguish from other primary producers such as macroalgae and phytoplankton, throwing a wrench into isotope-based studies of food chains and trophic structure.
Working Hypothesis: The two carbon uptake strategies – mostly CO2 vs. mostly bicarbonate use – of M. pyrifera cause this variation.
WH #2: Light intensity and flow velocity regulate which strategy a given kelp blade uses.
So what do I expect if I break the kelp forest up into dark (deeper blades), light (surface blades), low flow (protected embayments) and high flow (exposed coastal forests) regions?
- If high light and high flow => bicarbonate use, then bicarbonate use => enriched ∂13C signatures(~-15)
- While low light and low flow => CO2 use, and CO2 use => depleted ∂13C signatures (~-25)
This data, collected by myself, Celine, and PISCO divers in April, shows these interesting patterns, albeit with a lot of variation. On the x-axis are collection sites. Hopkins and Maccabee are protected, calm sites whereas Otter Point and Point Piños are exposed sites experiencing a lot of wave action year round. The golden bars are apical meristems and canopy (surface) blades, and are enriched in 13C, as I predicted. The blue green bars are deep and juvenile blades, which experience mostly darkness, and are depleted in 13C. Interestingly, these deep and juvenile blades have a ton of variation in their ∂13C. Preliminary data shows that this is likely due to long distance transport of carbohydrates in their phloem network, which largely brings enriched surface carbon down to the base of the sporophyte. This enriched subsidy muddies the signal of photosynthesis in the deep dark blades. Pretty fascinating. We just repeated this collection blitz this week in September…we’ll see what it looks like soon!