The giant kelp, Macrocystis pyrifera, forms productive and foundational forests bordering the Pacific coasts of four continents. As an ecosystem engineer and the largest source of biomass within coastal kelp forests, M. pyrifera is regarded as a major primary producer of the food chain. Because of this, M. pyrifera tissue is frequently used as the source component of trophic and metapopulation studies. Stable isotope discrimination can differ greatly (5 - 15‰) between different species of autotrophs or slightly (1-3‰) between individuals of the same species who may be experience different levels of stress (i.e. drought). However, M. pyrifera has ∂13C values that differ up to 6‰ within a single individual. In macroalgae, use of the different inorganic carbon species available in seawater (bicarbonate and CO2) are thought to determine species-specific ∂13C values; exclusive bicarbonate users have a ∂13C of +5 to -15‰, whereas species relying only on diffusive CO2 are much more depleted in 13C (a ∂13C more negative than -20‰). M. pyrifera, like many macroalgae, is capable of utilizing both dissolved CO2 and bicarbonate. However, recent work indicates that bicarbonate uptake varies strongly in response to of carbon supply (flow speed) and demand (light intensity). If ∂13C values are a function of bicarbonate uptake in M. pyrifera, the observed variation in tissue ∂13C could be explained by kelp residence in one of several micro-environments representing ecological extremes of current speed and light exposure. To test this hypothesis, we cultivated meristematic tissue in an outdoor mesocosm at Long Marine Lab and in the field for 3 weeks in 4 fully-crossed treatments—photosynthetically saturating irradiance, sub-saturating irradiance, high flow velocity, and low flow velocity. We also collected M. pyrifera tissue from several microhabitats bordering the Monterey peninsula to test our ability to predict ∂13C values in the field.
Results/Conclusions
Preliminary results indicate that final tissue ∂13C incubated under saturating irradiance is enriched relative to tissue incubated at sub-saturating irradiances, whereas flow velocity does not significantly impact ∂13C signatures. Early field-collected samples also support a partitioning of ∂13C signatures into microhabitats differing in light intensity and wave exposure. These data support our hypotheses regarding ecological drivers of bicarbonate uptake in M. pyrifera, potentially accounting for much of the variation in ∂13C observed in the field.