Kelp forests are commonly classified within remote sensing imagery by contrasting the high reflectance in the near-infrared spectral region of kelp canopy floating at the surface with the low reflectance in the same spectral region of water. However, kelp canopy is often submerged below the surface of the water, making it important to understand the effects of kelp submersion on the above-water reflectance of kelp, and the depth to which kelp can be detected, in order to reduce uncertainties around the kelp canopy area when mapping kelp. Here, we characterized changes to the above-water spectra of Nereocystis luetkeana (Bull kelp) as different canopy structures (bulb and blades) were submerged in water from the surface to 100 cm in 10 cm increments, while collecting above-water hyperspectral measurements with a spectroradiometer (325–1075 nm). The hyperspectral data were simulated into the multispectral bandwidths of the WorldView-3 satellite and the Micasense RedEdge-MX unoccupied aerial vehicle sensors and vegetation indices were calculated to compare detection limits of kelp with a focus on differences between red edge and near infrared indices. For kelp on the surface, near-infrared reflectance was higher than red-edge reflectance. Once submerged, the kelp spectra showed two narrow reflectance peaks in the red-edge and near-infrared wavelength ranges, and the red-edge peak was consistently higher than the near-infrared peak. As a result, kelp was detected deeper with vegetation indices calculated with a red-edge band versus those calculated with a near infrared band. Our results show that using red-edge bands increased detection of submerged kelp canopy, which may be beneficial for estimating kelp surface-canopy area and biomass.

Surface-canopy forming kelps (Macrocystis pyriferaandNereocystis luetkeana) can be monitored along the Northeast Pacific coast using remote sensing. These kelp canopies can be submerged by tides and currents, making it difficult to accurately determine their extent with remote sensing techniques. Further, both species have morphologically distinct canopies, each made up of structures with differing buoyancies, and it is not well understood whether the differing buoyancies between these species’ canopies affects their detectability with remote sensing technologies. Here, we collectedin situabove-water spectral signatures for the surface-canopies ofNereocystisandMacrocystis, providing the first direct hyperspectral comparison between the structures that make up the canopies of these species. Additionally, we compare the strength of their red-edge and near-infrared band signals, as well as the normalized difference red-edge (NDRE) and normalized difference vegetation index (NDVI) values. At the bed level, we compare detection of kelp canopy extent using both NDRE and NDVI classified unoccupied aerial vehicle imagery. We also characterized how changing tides and currents submerge the canopies of both species, providing insights that will allow remote sensors to more accurately determine the extent of kelp canopy in remote sensing imagery. Observations of canopy structures paired within situhyperspectral data and simulated multispectral data showed that more buoyant kelp structures had higher reflectance in the near-infrared wavelengths, but even slightly submerged canopy structures had a higher reflectance in the red-edge rather than the near-infrared. The higher red-edge signal was also evident at the bed level in the UAV imagery, resulting in 18.0% more canopy classified with NDRE than with NDVI. The area of detected canopy extent decreased by an average of 22.5% per meter of tidal increase at low current speeds (<10 cm/s), regardless of the species present. However, at higher current speeds (up to 19 cm/s),Nereocystiscanopy decreased at nearly twice the average rate of kelp beds in low-current conditions. Apart from the strong differences in high-current regions, a robust linear relationship exists between kelp canopy extent and tidal height, which can aid in understanding the errors associated with remote sensing imagery collected at different tidal heights.

Climate change is restructuring ecological communities globally, yet the impacts are often underestimated or poorly resolved due to the lack of historical baselines. In temperate oceans, biologically diverse and socioeconomically important kelp forests are the marine ecosystem most threatened by climate change. However, long‐term historical baselines for kelp forests are lacking and the processes driving community‐level changes remain poorly resolved. Here, using recently discovered aerial imagery and subtidal quadrat data from 1972, we recreated historical baselines for kelps and associated benthic macroalgae in a global hotspot within the northern Salish Sea (British Columbia, Canada). We resurveyed the same sites in 2023 to quantify community shifts, showing that a half‐century ago, bull kelp ( Nereocystis luetkeana ) formed expansive kelp forests in the region (>550 ha), none of which remain today. Satellite time series of bull kelp show that the majority was lost between 1972 and 1984. These data increase baselines of bull kelp canopy extent in this area by more than 10‐fold. Changes to the benthic kelp forest assemblage were mainly driven by loss of the dominant kelp, Saccharina latissima (−78%), across all depths. Historically abundant species of red algae also decreased substantially (e.g., Mazzaella splendens [−98.5%] and Plocamium pacificum , [−62.1%]), largely above three meters depth. Applying the community temperature index (CTI) to this half‐century comparison, we show that CTI of the kelp forest community (+1.4°C; 95% CI: 0.43–2.37°C) had tracked increases of summer SST (+1.66°C; 95% CI: 1.20–2.13°C) more closely than winter SST (+0.65°C; 95% CI: 0.46–0.84°C), indicating that temperatures during the hottest summer months are likely driving community shifts. The abundance of cold‐affinity species decreased more than warm‐affinity species abundance had increased, indicating that the subtidal kelp forest community was predominantly restructured by deborealization, rather than tropicalization. Community deborealization may be prevalent in temperate hotspots that are disjunct from areas with similar climatology, creating colonization barriers for warm‐affinity species. Our study underscores the importance of historical data for understanding the true magnitude of climate change impacts and suggests that deborealization of temperate kelp forest communities may be more common than has previously been recognized.