Demography and dynamics of giant kelp cohorts across four decades: Lessons for conservation and resilience planning

Kelp forests throughout many temperate zones are in decline due to various human stressors, chiefly marine warming. Conservation measures including restoration are presently of great interest and focus on both historical and novel methodologies. Of paramount importance for these efforts is an understanding of the mechanics of kelp decline to identify the factors and triggers leading to stepwise declines and thus support the development and spatial prioritization of strategic intervention to facilitate resilience. Here, we utilized a unique dataset documenting the demographic dynamics of giant kelp, Macrocystis pyrifera, in response to multiple disturbances across >40 years off San Diego (California, USA). The recruitment and life history of >14,000 individuals were used to evaluate cohort structure, size, and longevity forced by algal community structure and disturbance. Cohort dynamics varied spatially by depth and study subregion, thus aiding identification of areas to prioritize intervention to foster resilience. Five algal assemblages were characterized providing context for cohort dynamics in response to physical disturbances and sea urchin grazing. A trend of decreasing cohort size and resilience was observed over time accentuated by the marine heat wave of 2014-2015 (MHW) after which competition with understory canopies increasingly interfered with giant kelp cohort development and plant size structure. Cohort recruitment ranged on a continuum from discrete ("pulsed") to more gradual ("trickled") episodes. Pulsed cohorts mainly produced single cohort-dominated age stands punctuated by major disturbances. Pulsed events were more common than trickled recruitment, especially at deeper sites. Trickled cohorts resulted in relatively mixed age stands, especially when individual cohorts overlapped within sites. Trickled recruitment increased over time as understory dominance increased. Cohort longevity was highly variable among sites and among cohorts within a site, with high first-year mortality mostly due to warming, waves, or their combination. Longevity was inversely related to temperature and sea urchin density, and was greatest at deeper sites, especially after the MHW. The downward trend of single cohort dominance and individual plant size over time and its step downward after the MHW suggest that deeper areas should be prioritized for restoration. Regardless, understory canopies will increasingly dominate Southern California with continued warming.