The dynamics of foundation species in ecosystems are key to the fate of many species. Kelp forests are foundation species in temperate ocean ecosystems and contribute to carbon storage, macronutrient dynamics, primary production and biodiversity of a myriad of associated species. Downward trends in their abundance globally have been of concern. We analysed 26 years of aerial censuses (1989–2015) of two canopy kelp species in Washington State (USA) waters. We compared these modern censuses with censuses in 1911 and 1912 to determine the persistence of kelp cover over the past century. Using Autoregressive Integrated Moving Average (ARIMA) models, we compared kelp dynamics with likely environmental drivers, including local environmental variables and ocean indices for this region. Kelp remains at historic levels in many areas, although some eastern populations in proximity to greater human populations are the exception to this pattern. Over the last 26 years, kelp abundance showed high spatial autocorrelation in western areas of Straits of Juan de Fuca, with more variable populations in the annual species and eastward towards Puget Sound. Both species covaried positively in their abundance throughout most of the study area, suggesting that environmental factors rather than competition, drove their dynamics. The population dynamics of these kelp species showed that the abundance 1 year previously was an important predictor, and cyclic dynamics were not indicated using ARIMA models. Kelp abundance correlated inversely with the Pacific Decadal Oscillation and the Oceanic Nino Index, and positively with the North Pacific Gyre Oscillation, indicating that large‐scale processes associated with colder seawater temperatures were associated with greater relative abundance of kelp. Synthesis. Kelp beds in the northern California Current Large Marine Ecosystem have mostly remained persistent over the past century and over many kilometres, but some areas may have decreased in abundance. The sensitivity of these populations to indices of ocean climate, our demonstration that a historic 93‐year sea surface temperature record (Race Rocks, Canada) showed a 0.72°C increase and the classification of some areas as high variability‐low abundance, suggest that the viability of these foundational species remain a concern into the future.

Kelp forests form an important biogenic habitat that responds to natural and human drivers. Global concerns exist about threats to kelp forests, yet long-term information is limited and research suggests that trends are geographically distinct. We examined distribution of the bull kelp Nereocystis luetkeana over 145 years in South Puget Sound (SPS), a semi-protected inner basin in a fjord estuary complex in the northeast Pacific Ocean. We synthesized 48 historical and modern Nereocystis surveys and examined presence/absence within 1-km segments along 452 km of shoreline. Compared to the earliest baseline in 1878, Nereocystis extent in 2017 decreased 63%, with individual sub-basins showing up to 96% loss. Losses have persisted for decades, across a range of climate conditions. In recent decades, Nereocystis predominantly occurred along shorelines with intense currents and mixing, where temperature and nutrient concentrations did not reach thresholds for impacts to Nereocystis performance, and high current speeds likely excluded grazers. Losses predominated in areas with elevated temperature, lower nutrient concentrations, and relatively low current velocities. The pattern of long-term losses in SPS contrasts with stability in floating kelp abundance during the last century in an area of the Salish Sea with greater wave exposure and proximity to oceanic conditions. These findings support the hypothesis that kelp beds along wave-sheltered shorelines exhibit greater sensitivity to environmental stressors. Additionally, shorelines with strong currents and deep-water mixing may provide refugia within sheltered systems.

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21Understanding the historical extent of biogenic habitats can provide insight into the nature 22 of human impacts and inform restoration and conservation actions. Kelp forests form an 23 important biogenic habitat that responds to natural and human drivers. Global concerns exist 24 about threats to kelp forests, yet long term information is limited and research suggests that 25 trends are geographically distinct. We examined distribution of the bull kelp Nereocystis 26 luetkeana over 145 years in South Puget Sound (SPS), a semi-protected inner basin in a fjord 27 estuary complex in the northeast Pacific Ocean. We synthesized 48 historical and modern 28 Nereocystis surveys and examined presence/absence within 1-km shoreline segments along 452 29 km of shoreline. Over the last 145 years, Nereocystis has been documented in 26% of the 30 shoreline segments. Its extent decreased 62% basin-wide between the 1870s and 2017, with 31 extreme losses in the two out of three sub-basins (96% in Central and 83% in West). In recent 32 years, almost all Nereocystis occurred in the East sub-basin. In the majority of segments where 33 Nereocystis disappeared, the most recent observation was 4 decades ago, or earlier. Multiple 34 natural and human factors that are known to impact kelp could have contributed to observed 35 patterns, but limited data exist at the spatial and temporal scale of this study. In some areas, 36 recent environmental conditions approached thresholds associated with decreased kelp 37 performance. Longstanding Nereocystis losses occurred exclusively in areas with relatively low 38 current velocities. Remaining Nereocystis predominantly occurred in areas where circulation is 39 stronger. Exceptions to this pattern demonstrate that additional factors outside the scope of this 40 study contributed to trajectories of Nereocystis persistence or loss. 42 Humans have altered coastal ecosystems for centuries, yet we frequently lack long-term 43 datasets to define a baseline that precedes significant human impacts and to identify changes 44 from that baseline. The need for long-term reference points was initially identified in the context 45 of global fisheries [1], but it is equally important to understand changes in biogenic habitats as 46 losses can trigger changes to broader ecosystem structure and services. Historical maps have 47 been used to estimate changes in the spatial extent of coastal habitats such as salt marshes, oyster 48 reefs, coral reefs and kelp forests over century time scales [2][3][4][5]. Extended temporal baselines 49 can enhance understanding of habitat variability and change. For example, historical nautical 50 charts were used to spatially and temporally extend baseline information for coral reef extent in 51 the Florida Keys, revealing greater loss than was previously quantified, along with newly 52 identified areas of intact offshore reefs [4]. Enhanced understanding of the magnitude and spatial 53 patterns of change in biogenic habitats can guide restoration and conservation...

for their cataloging and scholarship on the Foslie collection and for their mentorship of the next generation of corallinologists.