*4.1. Spectral Characteristics of Kelp as It Is Submerged*

A broad R0+ peak across the NIR region was observed for surface measurements of *Nereocystis* as a result of the interaction of light with the cellular structure of the kelp [51]. Once submerged, our experiment showed two key changes in the NIR region of the kelp spectra (Figure 3), both due to characteristic absorption features of water: (1) the splitting of the single broad NIR peak into two narrower RE and NIR peaks due to prominent water absorption feature at 760 nm [52] and (2) higher R0+ at the RE peak versus the NIR peak, resulting from the continually increasing absorption of light by water above 600 nm [22]. Since *Macrocystis* and *Nereocystis* are spectrally similar to one another in the NIR region [15,18,27,50], and the changes seen in the spectra of submerged kelp are due to properties of water absorption, we expect that the spectral results of this experiment are generally applicable to both *Macrocystis* and *Nereocystis* canopies, making these findings relevant for surface-canopy forming kelp species globally.

The results of these experiments were generally in line with our expectations according to similar studies of submerged aquatic vegetation [24,25], although there were some interesting phenomena seen in the spectra that are worth noting. In the visible region of the spectra, R0+ in the red wavelength range decreased with depth, as expected. However, the R0+ at the absorption feature between 400 and 550 nm increased slightly with submersion. We hypothesize that this increase in R0+ is due to the scattering of light by the conditions of the water optical constituents, thus increasing the R0+ with depth. As such, we suspect that this increase in R0+ may be specific to the water conditions during the experiment and may not have occurred if the water had contained more optical constituents that absorb blue light, such as colored dissolved organic matter. Another interesting phenomenon noted in the floating kelp spectra was what appeared to be a sunlight-induced chlorophyll fluorescence (SICF) peak at 761 nm (Figure 4). Within the NIR region, photosynthetic organisms generally have a broad SICF peak centered at 740 nm [53]. However, due to the high magnitude of the NIR reflectance, the SICF is usually only visible as a small, narrow peak centered at 761 nm. Typically, the R0+ within the NIR wavelength range overwhelms the signal from SICF, however, atmospheric gasses highly absorb incoming irradiance at 761 nm, which can create a fill-in effect by the SICF in this region [53]. While this phenomenon has been correlated with photosynthetic output and general health of terrestrial vegetation and phytoplankton [53,54], we are not aware of any publications that report an SICF peak in kelp spectra, and this may present an opportunity for future hyperspectral research. Once kelp was submerged the SICF feature was dampened, and therefore future research should take note of the amount of kelp at the surface if attempting to derive information from an SICF peak.

#### *4.2. NIR Differences between Nereocystis Bulbs and Blades*

The magnitude of reflectance across the NIR region in vegetation is generally due to the cellular structure of the respective tissues [55]. Both *Nereocystis* bulb and blade tissues are composed of the same three cellular layers: the meristoderm, the medulla, and the cortex. The meristoderm is a thin chloroplast-packed epidermal layer that surrounds the entire

individual [56], and the medulla is a complex web of filaments that acts as a transportation system within the kelp, composing the innermost layer of kelp tissue [57,58]. Between these two layers is the cortex, which connects the meristoderm to the medulla, and generally provides structural support for the kelp [56,59]. Given this structural arrangement, we speculate that the NIR signal from bulbs is consistently higher compared to the blades' signal because (1) the bulb cortex is many times thicker than the blade cortex [57,59]; and (2) the gas cavity of the bulb is lined by the medulla [57], creating a high surface area with many large refractive differences—similar to the mesophyll layer of a terrestrial leaf [55,60]. In comparison, the blade medulla is housed in a gelatinous extracellular matrix between cells [57], and with no gas cavities, the refractive differences are much smaller, allowing for increased transmittance of NIR light through the blades [55]. For our experiment, spectral measurements for blades were taken using a single blade wrapped around the polyethylene frame with only slight overlap between the edges of the blade. However, *Nereocystis* individuals may have between 30–60 blades each. Overall, a thicker mass of blade tissues due to high overlap may result in higher R0+ in the RE and NIR wavelength ranges than seen in this experiment.
