TEMPORAL AND SPATIAL SCALES OF KELP DEMOGRAPHY: THE ROLE OF OCEANOGRAPHIC CLIMATE

Abstract

This paper integrates long-term descriptive and experimental studies of the effects of ocean climate on inter- and intraspecific competition, as expressed 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 rely on stipes to support their canopy; Laminaria farlowii, with a prostrate canopy; and a speciose red algal turf. To evaluate the roles of large-scale oceanographic processes on biological processes across important depth gradients, the study was carried out over nine years during a cold-water, nutrient-rich La Niña event (1988–1989) and a warm-water, nutrient-stressed El Niño period (1992–1994), over a depth range of 8–23 m. This depth range encompassed strong physical gradients involving factors that are critical for kelp growth, including bottom temperatures (correlated with nutrients) and light 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. “realized” niches of these species. Evaluating these patterns, as they are influenced by inter- and intraspecific competition, offers insights into the “realized niches” of the kelps. With the exception of some understory effects on Macrocystis recruitment and some evidence of intraspecific competition during the nutrient-rich La Niña conditions, we found little influence of competitive effects on Macrocystis. The response of Pterygophora to manipulations and disturbances suggests light-limited recruitment, and competition with 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 Macrocystis treatment was more important than depth, suggesting the importance of light to Eisenia recruitment and growth. In general, Macrocystis had massive effects on Laminaria growth and reproduction, the strength varying with depth. In particular, there were very strong effects of competition with Macrocystis during the nutrient-rich La Niña period when Macrocystis had a dense surface canopy. In addition to the Macrocystis effects, there were some significant Pterygophora effects on Laminaria growth during El Niño. The strongest biological definition of realized niches occurred during the nutrient-rich La Niña 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 seasonal patterns, but the interannual scales that encompass El Niños and La Niñas, and ultimately the interdecadal-scale oceanographic regime shifts that affect the intensity of canopy competition with Macrocystis, are critical for this system because surface-water nutrients have pervasive long-term effects on the other kelps. Small-scale patterns are driven by local processes (competition, disturbance, dispersal, etc.) that potentially are important at larger scales; however, our most lasting effects result from very large-scale, low-frequency episodic changes in nutrients, with cascading competitive consequences to the other algal populations in the community.