García-Reyes M et al. (2022), Winter oceanographic conditions predict summer ...

ECB-ART-50348

PLoS One 2022 Jan 01;175:e0267737. doi: 10.1371/journal.pone.0267737.

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Winter oceanographic conditions predict summer bull kelp canopy cover in northern California.

García-Reyes M , Thompson SA , Rogers-Bennett L , Sydeman WJ .

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Bull kelp, Nereocystis luetkeana, is an iconic kelp forest species of the Northeast Pacific that provides a wide range of ecosystem services to coastal marine species and society. In northern California, U.S.A., Nereocystis abundance declined sharply in 2014 and has yet to recover. While abiotic and biotic stressors were present prior to 2014, the population collapse highlights the need for a better understanding of how environmental conditions impact Nereocystis. In this study, we used a newly-developed, satellite-based dataset of bull kelp abundance, proxied by canopy cover over 20 years, to test the hypothesis that winter oceanographic conditions determine summer Nereocystis canopy cover. For the years before the collapse (1991 through 2013), wintertime ocean conditions, synthesized in a Multivariate Ocean Climate Indicator (MOCI), were indeed a good predictor of summer Nereocystis canopy cover (R2 = 0.40 to 0.87). We attribute this relationship to the effects of upwelling and/or temperature on nutrient availability. South of Point Arena, California, winter ocean conditions had slightly lower explanatory power than north of Point Arena, also reflective of spring upwelling-driven nutrient entrainment. Results suggest that the Nereocystis gametophytes and/or early sporophytes are sensitive to winter oceanographic conditions. Furthermore, environmental conditions in winter 2014 could have been used to predict the Nereocystis collapse in summer 2014, and for kelp north of Point Arena, a further decline in 2015. Importantly, environmental models do not predict changes in kelp after 2015, suggesting biotic factors suppressed kelp recovery, most likely extreme sea urchin herbivory. Conditions during winter, a season that is often overlooked in studies of biophysical interactions, are useful for predicting summer Nereocystis kelp forest canopy cover, and will be useful in supporting kelp restoration actions in California and perhaps elsewhere in the world.

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	Fig 1. Bull Kelp geographical location and canopy extent.(A) Study region from 38–40°N on the northern California Coast, U.S.A. Green dots indicate kelp canopy data locations. Extensive sandy beaches are located north of Point Arena and Fort Bragg; no measurable kelp canopy occurs there. (B) Summer kelp canopy extent data (km2), 1991–2020, aggregated regionally for north (orange) and south (gray) of 39°N (Point Arena). Data derived from [34], freely available under CC BY 4.0 License at https://doi.org/10.6073/pasta/89b63c4b49b80fb839613e9d389d9902.
	Fig 2. Linear regression between canopy coverage north of Point Arena and MOCI.(A) Linear regression between central California MOCI (CenCal MOCI) and kelp canopy extent north of Point Arena, 1991–2013, red-dotted lines indicate the confident intervals; R2 indicates the explained variance and the color indicates the year. (B) Time series of summer kelp canopy extent north of Point Arena: measured data are in red, modeled pre-collapse data are black, and predicted data for collapsed years are shown in grey asterisks (*). Modeled and predicted kelp uses the linear regression in (A). (C) Same as (A) but for winter southern California MOCI (SoCal MOCI). (D) Same as (B) but model and predicted data used the linear regression with southern California MOCI shown in (C). Regressions details are in Table 2.
	Fig 3. Linear regression between canopy coverage south of Point Arena and MOCI.Same as Fig 2 for regressions between kelp canopy south of Point Arena and northern California MOCI (NorCal MOCI; (A) and (B)) and central California MOCI, (CenCal MOCI; (C) and (D)).
	Fig 4. Linear regressions between canopy coverage and nutrient-related variables.Linear regression for the pre-collapse period 1991–2013, red-dotted lines indicate the confident intervals; R2 indicates the explained variance and the color indicates the year. (A) Summer kelp canopy extent north of Point Arena and winter SSTN14; summer kelp canopy extent south of Point Arena and (B) winter SSTN14, (C) spring BEUTI37N. Regressions details are in Table 2. SSTN14: sea surface temperature at buoy N14, BEUTI37N: BEUTI index at 37°N.

References [+] :

Beas-Luna, Geographic variation in responses of kelp forest communities of the California Current to recent climatic changes. 2020, Pubmed