History of kelp beds (Macrocystis) in Orange and San Diego Counties, California

The response of giant kelp (Macrocystis pyrifera) in southern California to low‐frequency climate forcing

“…Many studies have at least partly attributed localized kelp-forest disturbances to human disturbances such as pollution and sedimentation (North et al 1993;Foster and Schiel 2010) and resource utilization (Tegner and Dayton 2000). However, pollution and sedimentation are not likely a major cause of the kelp response we describe here because their effects on kelps are localized in time and space (Dayton et al 1992) and cannot have affected kelp forests over the length scale of our study region.…”

“…Previous work investigating the importance of nutrient control of giant kelp used temperature, which accounted for no . 46% to 24% of the variance (North et al 1993; Fig. 13.…”

“…The greater variation of nitrate as a function of temperature is evident in Fig. 13, especially at nitrate concentrations most frequently observed on the inner shelf (, 2 mmol L 21 ); (2) North et al (1993) analyzed kelp data lumped across the regime shift; (3) North, by necessity, used canopy data, which can be biased by the episodic submergence of canopy by currents and the seasonality of canopy disappearance without associated kelp mortality. The combination of these factors has deemphasized the importance of nutrient control of M. pyrifera after the regime shift.…”

“…Kelp-forest canopy cover in southern California, from the Mexican border to the southern border of Los Angeles County, has been estimated annually from aerial photographs since 1967 (North et al 1993). Aerial surveys have been conducted two to four times each year over that period, and the annual maximum coverage observed for each kelp forest was used as the annual estimate of canopy cover.…”

“…''resource'' or ''bottom-up''), cool high-nutrient periods such as La Niñ as are typically associated with increased levels of giant kelp productivity, growth, survival, and reproduction (Dayton et al 1999). Yet despite the well-established relationship between temperature and nitrate (Zimmerman and Kremer 1984), temperature accounts for less than half of the variability in proxies of giant kelp abundance such as canopy area, density, or harvest within the CCS (North et al 1993;Tegner et al 1996). Despite this, macroalgal stands at kelp-forest scales and greater are generally considered climate (cf.…”