Effects of El Niño on local hydrography and growth of the giant kelp, Macrocystis pyrifera, at Santa Catalina Island, California1

Abstract

Deepened isotherms associated with El Niño resulted in severe nutrient limitation and very low kelp productivity during the last half of 1983. Frond growth rates were so low that terminal blades formed before reaching the surface, eliminating the canopy. Frond initiation rates were also extremely low, resulting in significant reductions in mean plant size. Plants growing above 10 m were more severely affected than plants at 20 m. These results suggest that nutrient pulses associated with internal waves are critical for survival of Macrocystis pyrifera in nutritionally marginal habitats in southern California.

Post-glacial redistribution and shifts in productivity of giant kelp forests

“…Analysis of southern California kelp canopy area and biomass records from 1968 to 2002 reveals predictably higher biomass density during periods of high productivity potential (La Niñ a) versus low productivity potential (El Niñ o; figure 3a), presumably owing to nutrient deficiency during El Niñ o events (Zimmerman & Robertson 1985;Tegner et al 1997). Furthermore, recently published palaeooceanographic records (Hendy et al 2002(Hendy et al , 2004Friddell et al 2003) for the last 20 000 years reveal an abrupt shift in late Quaternary oceanographic productivity potential, from cold unproductive conditions at the LGM to warmer, more productive conditions from 15 600 yr BP to present (figure 3b).…”

“…In addition to biomass, the area-specific productivity of giant kelp strongly depends on the availability of nutrients, particularly nitrate, in shallow surface waters. In presentday southern California waters, nitrate concentration exhibits a strong, consistent relationship with sea temperature (Zimmerman & Robertson 1985;Dayton et al 1999), and is correlated with the upwelling of cold sub-thermocline water driven by coastal winds. However, over millennial time scales, the supply of nutrients to surface waters also depends on the source of upwelled water.…”

“…In southern California, high irradiance requirements for sexual reproduction generally restrict giant kelp populations to depths of less than 25 m, where giant kelp is the competitive dominant throughout its range (Foster & Schiel 1985). Peak giant kelp productivity occurs during seasonal upwelling (Zimmerman & Robertson 1985), when local wind forcing brings deep, nutrient-rich water to the surface. Grazing by sea urchins (Strongylocentrotus purpuratus, S. franciscanus) can also locally limit giant kelp distribution in California (Behrens & Lafferty 2004), although not in the company of the primary urchin predator, sea otters (Enhydra lutris), which were present in the region prior to extirpation by Europeans in the 1700s (Kenyon 1969;VanBlaricom 1988); sea otter and sea urchin remains have been continuously observed in southern California archaeological records from 9800 to 220 years before present .…”

“…In addition to the areal extent of kelp populations, kelp forest productivity depends primarily on the biomass density of kelp canopies (Reed et al 2008), nutrient flux to surface waters (Zimmerman & Robertson 1985) and incident solar irradiance. High-frequency oceanographic warming and cooling events (i.e.…”

Deep-water kelp refugia as potential hotspots of tropical marine diversity and productivity

“…Because kelp is more vulnerable to nutrient limitation than temperature stress (27,28), we used the depth of the seasonal thermocline to estimate the vertical extent of the mixed layer, and thus the zone within which deep-water nutrient replenishment occurs (Fig. 1b) (29). Vertical temperature profiles from the World Ocean Database (WOD2001) (30) were optimally interpolated onto a 0.5°geographic grid at standard depth levels (30) and used to estimate the monthly climatological mixed-layer depth, defined as the shallowest local maximum of second derivative of the temperature-depth function (i.e., the shallowest maximum that was at least 80% of the global maximum gradient).…”