Background
Ladah et al. (1999) described how the El Niņo of 1997-98, which produced positive sea surface temperature anomalies that exceeded 2.5°C from September 1997 to February 1998, totally decimated giant kelp "forests" located near Bahia Tortugas, Baja California, Mexico, and how they suddenly reappeared in July 1998.  At the time, they attributed the return of the giant kelp to "a microscopic stage that was not visible during dive surveys [that] survived the stressful conditions of ENSO and caused the recruitment event."  Now, they reveal a second and more likely predominant means by which the return was made possible.

What was done
Dive surveys that recommenced after the El Niņo had run its course revealed the other source of the returning kelp.

What was learned
Although all giant kelp growing at 15 m depth or shallower died during the peak warmth of the El Niņo, Ladah and Zertuche-Gonzalez discovered there were numerous large fertile adults located between 25 and 40 m depth that were unaffected by the high surface water temperatures.  Although the upper 15 meters of these plants died and sloughed off, regenerated fronds had reached the surface of the sea by the fall of 1998.

What it means
The two scientists say that "survival in deep water during this extreme El Niņo may have been due to local hydrography, such as internal waves bringing cool nitrate-rich water into the deeper regions of the shelf from below the thermocline, providing a refugium against the warm temperatures, low nutrients, and heavy wave action associated with warming events."  They also note that "the increased light that often occurs after canopy removal apparently resulted in ... recruitment events ... from newly produced spores from nearby fertile individuals surviving in deeper waters."  Hence, they conclude that "deep-water populations may regularly survive El Niņo warming in this region due to internal wave activity, and go undetected due to the depth at which they occur and the sloughing of the shallow (<15 m) biomass."

Reference
Ladah, L.B., Zertuche-Gonzalez, J.A. and Hernandez-Carmona, G.  1999.  Giant kelp (Macrocystis pyrifera, Phaeophyceae) recruitment near its southern limit in Baja California after mass disappearance during ENSO 1997-1998.  Journal of Phycology 35: 1106-1112.