Environmental Drivers of Giant Kelp Biomass and Physiological Condition through Space and Time
The giant kelp (Macrocystis pyrifera) is a globally distributed foundation species, which supports an incredibly productive ecosystem. While this species has been well studied over the past several decades, there exists much debate on the relative roles of external and intrinsic drivers of canopy dynamics. Much of this debate may stem from the geographic differences in the environments of this well-adapted species. In the first part of this dissertation I used multispectral satellite imagery to help build a dataset of giant kelp canopy biomass dynamics along the coast of California across nearly three decades. We were then able to decompose this spatiotemporal matrix into orthogonal modes that allowed for the ranking of the most important environmental drivers of the kelp canopy: wave disturbance, seasonal nutrient supply, and the North Pacific Gyre Oscillation. We then used generalized additive models to determine the nonlinear effect shapes of each potential biomass driver. In the next chapter, we explored the physiology of the giant kelp canopy and found that photosynthetic pigment state and the chlorophyll a to carbon ratio (Chl:C) of the kelp canopy more closely resembles changes in available nitrate rather than changes in available light at locations in southern California, while the reverse is true along the more nutrient-replete central California coastline. Temporally lagged Chl:C was positively related to changes in kelp biomass and net primary production along the periodically nutrient-limited southern California coast. These results open the possibility of estimating net primary production of giant kelp over large spatial and temporal scales using present and planned remote sensing technologies and the modeling of Chl:C based on well measured environmental variables. In the final chapter, we used hyperspectral imagery to examine the physiological condition of the giant kelp canopy over a variety of scales to identify and elucidate ecological processes related to external environmental drivers and demographics. We found that regional patterns of Chl:C were associated with large-scale fluctuations in sea surface temperature, and by extension ambient nutrient concentration. Local scale variability in Chl:C across a single kelp forest equaled the regional variability, implying that local scale processes also play a role in the physiological condition of this species. Local scale examples showed that canopy Chl:C was related to the date when kelp canopy first emerged, suggesting that demographic patterns in kelp frond age influence the local physiological condition and persistence of giant kelp canopy.