11. Algal Dynamics: Alternate Stable States of Reefs in the Eastern Tropical Pacific

Algae from two kingdoms and four major phyla form productive benthic communities that play key ecological roles on coral reefs of the eastern tropical Pacific (ETP). The diversity of algae that comprise these communities is being actively explored throughout the region, with new species still being discovered. Due to its physical setting, there are several distinct algal habitats in the ETP, including reef crests dominated by crustose coralline algae and algal turfs, reef flats that support a higher abundance of foliose macroalgae, unconsolidated accumulations of dead coral rubble that may support rhodoliths and cryptic macroalgae, and deeper-water refuges from herbivory dominated by more of the palatable forms of macroalgae rarely found on reefs. Past work has identified a number of ecological processes that control algal populations and communities at play in the ETP, but research elucidating their relative roles is still limited in this region. Recent work suggests that these reefs are supported by allochthonous nutrient supplies such as upwelling and thermocline shoaling, though reduced light during these events may limit productivity to shallow reef zones. Though at least partially sheltered from major disturbances that structure other reef systems, such as hurricanes and outbreaks of crown-of-thorns sea stars, reefs of the ETP are strongly affected by sea surface temperature extremes associated with El-Niño-Southern Oscillation (ENSO) leading to major coral mortality with the opening of space for colonization by algae. Biotic processes that control algal communities include herbivory and competition with coral, both of which limit algal proliferation and may enhance recovery after disturbance. However, facilitative interactions within algal communities, such as reduced herbivory and amelioration of nutrient limitation by internal recycling that act as positive feedbacks within areas dominated by algae, may stabilize algal communities after disturbance. There is some empirical and modeling evidence that suggests these processes may drive reefs of the ETP to display alternative stable states. This evidence includes a bifurcated pattern of recovery after ENSO disturbance, the patchy nature of this recovery within certain reefs, and the existence of stabilizing feedbacks that support resilience of both coral and algal states. Further, a simulation model (cellular automaton) of ETP reefs parameterized with experimentally-derived feedback relationships suggests these reefs exist under environmental conditions that produce bistability. However, much research is still needed to fully understand the physical and ecological processes that structure algal communities, and how these controls may shift with anthropogenic impacts.