Temporal and spatial scales of kelp demography: The role of oceanographic climate

This paper integrates long-term descriptive and experimental studies of the effects of ocean climate on inter- and intraspecific competition, as expre ssed by recruitment, density, survivorship, growth, and reproduction of the most conspicuous kelp species in the Point Loma kelp forest community off San Diego, California, USA. The species included Macrocystis pyrifera, with a floating canopy; Pterygophora californica and Eisenia arborea, which rel y on stipes to support their canopy; Laminaria farlowii, with a prostrate c anopy; and a speciose red algal turf. To evaluate the roles of large-scale oceanographic processes on biological processes across important depth grad ients, the study was carried out over nine years during a cold-water, nutri ent-rich La Nina event (1988-1989) and a warm-water, nutrient-stressed El N ino period (1992-1994), over a depth range of 8-23 m. This depth range enco mpassed strong physical gradients involving factors that are critical for k elp growth, including bottom temperatures (correlated with nutrients) and l ight levels. To examine interactions among these kelps, we established clearings across the depth gradient and then manipulated Macrocystis recruit densities. The demographic responses offer an understanding of the "fundamental" vs. "real ized" niches of these species. Evaluating these patterns, as they are influ enced by inter- and intraspecific competition, offers insights into the "re alized niches" of the kelps. With the exception of some understory effects on Macrocystis recruitment and some evidence of intraspecific competition d uring the nutrient-rich La Nina conditions, we found little influence of co mpetitive effects on Macrocystis. The response of Pterygophora to manipulat ions and disturbances suggests light-limited recruitment, and competition w ith Macrocystis was exhibited via reduced growth and reproduction, but not survivorship. No nutrient stress was observed in Pterygophora reproduction. Eisenia recruitment is rare, but once established, juveniles had very good survivorship, with growth and reproduction reduced by depth; the Macrocyst is treatment was more important than depth, suggesting the importance of li ght to Eisenia recruitment and growth. In general, Macrocystis had massive effects on Laminaria growth and reproduction, the strength varying with dep th. In particular, there were very strong effects of competition with Macro cystis during the nutrient-rich La Nina period when Macrocystis had a dense surface canopy. In addition to the Macrocystis effects, there were some si gnificant Pterygophora effects on Laminaria growth during El Nino. The strongest biological definition of realized niches occurred during the nutrient-rich La Nina period, especially in shallow depths. One of the most important conclusions of this paper is the appreciation of the importance of scaling in time to include oceanographic climate. There are many seasona l patterns, but the interannual scales that encompass Er Ninos and La Ninas , and ultimately the interdecadal-scale oceanographic regime shifts that af fect the intensity of canopy competition with Macrocystis, are critical for this system because surface-water nutrients have pervasive long-term effec ts an the other kelps. Small-scale patterns are driven by local processes ( competition, disturbance, dispersal, etc.) that potentially are important a t larger scales; however, our most lasting effects result from very large-s cale, low-frequency episodic changes in nutrients, with cascading competiti ve consequences to the other algal populations in the community.