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 priorities 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 the identification of areas to prioritize for
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 2014-15 (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.