Heidi Hirsh successfully defended her thesis "Spatiotemporal variability in kelp forest and seagrass ecosystems: Can local biogeochemistry combat acidification stress?". A recording of her pH.D. defense can be found at bit.ly/hh_defense. Below the abstract of her presentation. Also check out her YouTube talk on Drone Surveys of Giant Kelp.
Abstact:
Anthropogenic carbon dioxide (CO2) emissions have driven widespread ocean acidification (OA). However, both kelp forests and seagrass beds may have the potential to locally ameliorate OA by removing CO2 during photosynthesis. To understand the contributions of kelp forest and seagrass metabolism (photosynthesis/respiration) to their respective local biogeochemistry, we must determine the natural variability in these systems and disentangle physical and biological drivers.
In Chapter 1, I deployed an extensive instrument array in Monterey Bay, CA, inside and outside of a kelp forest to assess the degree to which kelp locally ameliorates present-day acidic conditions. Interactions between upwelling exposure, internal bores, and biological production shaped local biogeochemistry inside and outside of the kelp forest. Significantly elevated pH, attributed to kelp canopy productivity, was observed at the surface inside the kelp forest. This modification was largely limited to a narrow band of surface water, implying that while kelp forests have the potential to locally ameliorate ocean acidification stress, this benefit may largely be limited to organisms living in the upper part of the canopy.
In Chapter 2, I quantified net community production over a mixed seagrass-coral community on Ngeseksau Reef, Republic of Palau. We observed a net heterotrophic daily signal, but dissolved oxygen (O2) fluxes during the day were largely positive, illustrating daytime autotrophy. The relationship between tidal regime and time of day drove the magnitude of the signals observed.
Remote sensing can help us capture and describe the biomass responsible for the changes observed in biogeochemical records. In Chapter 3, I established a low altitude unmanned aerial vehicle (UAV) record of giant kelp canopy extent over 18 months in Monterey Bay, CA. I compared my UAV kelp classification to canopy cover determined from Landsat images obtained over the same period. A similar temporal trend was evident in both records with maximum canopy area in the mid-late summer, but the higher resolution UAV imagery allowed better classification of kelp canopy. There was a linear relationship between the drone kelp ratio and Landsat kelp canopy fraction for spatially-matched pixels; a Landsat kelp fraction of 0.64 was equivalent to 100% kelp cover in the drone data.
