Fig 1.
Regional discount rates for comparing historical and contemporary kelp canopy surveys.
Regional mainland examples of (A) historical maps and noted harvestable beds (map images provided from [34], and in the public domain), (B) composite of contemporary (2014–16) CDFW aerial surveys, (C) their reframing at comparable scale (or as harvestable beds), and (D) proportional canopy cover distributions derived from the intersections of (B) and (C) throughout California. The 1911–12 kelp survey represents an effort by the US Department of Agriculture to assess potash resources from California’s summer to fall seaweed canopy. Similarly, during the mid-summer to fall seasonal peak, CDFW periodically conducted annual statewide aerial surveys of kelp canopy from 1989 through 2016. Map base layer provided by ArcGIS Hub (https://hub.arcgis.com/datasets/1612d351695b467eba75fdf82c10884f/explore) with U.S. Census data and licensed as public domain.
Fig 2.
Century-scale, mainland kelp canopy losses throughout northern and southern regions of California slightly surpassed increases along the central coastline.
Mainland kelp canopy resources depicted by (A) total area (ha), and (B) changes within nearshore habitat (≤ 30m depth) during 1911–12 and 2014–2016 (composite) from (C) the Mexico to Oregon state border (0 to 1620 km) [68]. Canopy area gains along central California nearly offset losses within northern and southern coastal regions (see Table 1). To better visualize broad regional trends, we fit a locally weighted regression (LOESS, span 0.075) to these kelp features. Kelp canopy changes between contemporary and historical surveys are indicated by circles, with gains in blue and losses in red. All measurements reflect peak seasonal abundance in kelp from mid-summer through fall. Southern-central and central-northern region dividing landmarks are Point Conception and Pigeon Point, respectively, with San Francisco Bay, Monterey Bay, Santa Barbara Channel, Los Angeles Basin, and San Diego Bay noted as geographic features. Map base layer provided by ArcGIS Hub (https://hub.arcgis.com/datasets/1612d351695b467eba75fdf82c10884f/explore) with U.S. Census data and licensed as public domain.
Table 1.
Statewide and regional changes in California kelp over the last century.
At the state level, the total area (-6.9%), carbon biomass (+5.3%), and social costs (+5.3%) of harvestable kelp beds (see Methods) were not considerably different from 1910–1912 to 2014–2016 surveys. These trends, however, obscure stark regional differences that encompass a dramatic shift of California kelp over this period. In central California, kelp increased 57.6%, growing 19.7 km2 and adding an estimated 145.6 kt CO2. In all other regions kelp declined. Most notably, northern California saw 63% declines in kelp amounting to an estimated 8.1 km2 and 63.2 kt CO2 lost. The overall decline in kelp canopy area with a simultaneously estimated increase in kelp carbon biomass over time highlights regional differences in species composition and associated bed density and carbon content. The estimated social cost of kelp carbon follows the biomass trends, and in both periods exceeds $US 100M.
Fig 3.
Potential coastal sources of influence to statewide kelp canopy area.
(A) Sea surface temperature (SST) heat extremes and (B) kelp climate maximum events (≥ 20° C) occurred most frequently throughout the southern or low latitude portion of (H) California. We estimated occurrence of coastal heat extremes by calculating mean-monthly frequency of events (1983–2016) within the 95th percentile of historical SSTs recorded from 1870 to 1919. (C) Hard seafloor substrate (≤ 30-meter depth) is more abundant throughout northern and central coastal regions, nearly the reverse distribution of (D) human population density. (E) Sea otter population densities are greatest within the central portion of the state’s coastline, where recovery is occurring. (F) Monthly and (G, J) annual net primary productivity (NPP) variability distributions are nearly mirror opposites, corresponding with greater seasonality in northern California and longer cycles of extreme climate conditions in the southern coastline. Raw data are indicated by circles and smoothed using a uniform-span, locally weighted regression (LOESS, α = 0.075). During analysis, we used smoothed data to characterize both non static factors (i.e., sea otter, humans) and environmental data derived from coarser scale models (i.e., SST, NPP). Map base layer provided by ArcGIS Hub (https://hub.arcgis.com/datasets/1612d351695b467eba75fdf82c10884f/explore) with U.S. Census data and licensed as public domain.
Fig 4.
Large-scale SST anomalies and net primary productivity variability corresponded most with overall kelp canopy declines, but sea otter density mitigated statewide losses.
(A) raw, pair-wise comparisons of kelp changes to model factors, and (B) modeled relationships of individual conditional expectations (ICE) from the Random Forest (RF) model outputs for the highest ranked variables (please note the varying y-axis scaling). Predominantly soft seafloor substrate, moderately high temperature heat extreme frequency and NPP variabilities, and densely populated coastlines related most strongly with canopy kelp losses. By contrast, sea otters corresponded with minimal to low declines, or even kelp gains at higher population densities (> 0.03 ha -1). We assigned (C) variable importance rankings from comparative increases in model MSE when each factor was removed. Overall, this six factor RF model explains 71% of variability related to century-scale kelp canopy area changes. (D) Two-way partial dependency plots describe the predicted interactions between impact of selected factors on kelp canopy changes. Here kelp increases with y^, symbolized with cool colors. Among all environmental factors, only sea otters consistently correspond with predicted gains in kelp canopy area.