Traditional fisheries management in southern California has failed, in part because it is based on an assumption of an unvarying environment and is focused on size limits rather than insuring the persistence of aggregations of large fecund individuals. The combined effect of low frequency climatic variability and anthropogenic perturbations can have dramatic consequences for abalone in southern California. Abalone species are tightly linked to kelp forest ecosystems that, besides furnishing habitat, also provide the main food source for abalone. In southern California, kelp canopies are very sensitive to oceanographic climate because the kelp depend upon high nutrients in the water column. Oceanic warming, in turn, results in decreased nutrients in the surface water, and this is correlated with marked reductions in giant kelp biomass.Here we address the additive effects of ocean warming on two species of California abalone (the red abalone, Haliotis rufescens; and the green abalone, H. fulgens) by subjecting them to varied environmental conditions similar to cool, normal, and warm phases of the California current in the southern California Bight. Our experimental design simultaneously tested the synergistic effects of temperature and food quantity and quality on survivorship, growth, and reproduction. For red abalone, warm temperatures increased the onset of withering syndrome, a fatal abalone disease, and halted growth and reproduction. In contrast, green abalone survivorship, growth, and reproduction were relatively robust irrespective of temperature, while their growth and reproduction were most strongly influenced by food quantity. We found clear evidence suggesting that, combined with overfishing, California abalone populations are adversely affected by ecosystem responses to ocean warming: Cool‐water red abalone suffer stronger consequences in warm water than do green abalone. Conservation, restoration, and recovery plans of remnant California abalone populations must consider these relationships when taking any action.

Declines in landings of Southern California abalone fisheries and the eventual collapse of many stocks over the last two decades coincided with a period of greatly increased environmental variability. This included massive storms, an increase in the frequency of warm-water El Nino events after 1976, and an interdecadal-scale increase in sea surface temperatures. Kelp populations may be decimated by severe storms or warm water. Because of the strong inverse relationship between nitrate availability and water temperature, temperature is a good indicator of nitrate availability or stress SINCE kelp growth ceases in warm nutrient-depleted water, tissue decays, and standing stocks may be greatly reduced. Abalones are affected by the availability of the drift kelp on which they feed. Anomalously warm temperatures may affect reproduction, and altered current patterns may affect larval dispersal. Because water temperature varies with location in Southern California and each of the five exploited abalone species has its own thermal preferences, we chose to evaluate the role of environmental variability on population, of red abalone (Haliotis rufescens) on three northern Channel Islands spanning a temperature gradient. We compared water temperature regimes and anomalies, monthly aerial surveys of canopies of giant kelp (Macrocystis pyrifera), and field evidence of poor abalone growth and reproduction during El Nino events. The severity of El Nino disturbances and long-term changes in kelp standing stocks both correlated with the temperature gradient. Declines of red abalone total landings and area-specific landings on the warmer Santa Cruz and Santa Rosa Island began a decade after the large 1957-1959 El Nino. The subsequent collapse of many populations appears related to warm anomalies after the 1976-1977 regime shift, kelp declines, and poor reproduction coupled with fishing-induced declines in adult abalone density. Red abalone populations have persisted on cooler San Miguel Island where thermal anomalies had less effect and kelp canopy biomass has been more stable. Southern California abalones evolved in this disturbance regime, but the combination of extended periods of increased environmental variability with intense fishing pressure may have led to the loss of local populations, especially in warmer areas.

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 Nina event (1988-1989) and a warm-water, nutrient-stressed El Nino 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 Nina 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 Nina 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 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 seasonal patterns, but the interannual scales that encompass Er Ninos and La Ninas, 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 an 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.

The detection of trends in ecosystems depends upon (1) a good description of the foundation or benchmark against which changes are measured and (2) a distinction between natural and anthropogenic changes. Patterns and mechanisms observed over 25 years in a large kelp forest suggest that definition of a meaningful benchmark is impossible, because many of the large animals have been gone for years to decades, and kelps are sensitive to large-scale, low-frequency El Nino-Southern Oscillation events and longer term regime shifts. A shift in the oceanographic climate has significantly reduced the average size and carrying capacity of the dominant plant. The animals that have been functionally removed from the community include sea otters, black sea bass, yellowtail, white sea bass, and abalones. Other species are still present, but fisheries have had huge effects on the abundances, size-frequencies, and/or spatial distributions of sheephead, kelp bass, rays, flatfish, rock fish, spiny lobsters, and red sea urchins. Now even sea cucumbers, crabs, and small snails are subject to unregulated fishing. The plants continue to exist without a hint of the effects of the loss of so much animal biomass. Furthermore, most of the megafauna have been removed with very little documentation or historical understanding of what the natural community was like. Thus, our ability to separate anthropogenic impacts from the "natural" dynamics of the system is severely compromised. We discuss the importance of both an ecosystem focus on productivity and careful monitoring of as many populations as possible. In addition, we show that this community is not tightly integrated with mutual dependencies; hence, many species can be removed without much affecting the rest of the ecosystem.

Two catastrophic-scale storm disturbances of a giant kelp forest community were followed by very different oceanographic conditions, the warm, nutrient-stressed period of the 1982-1984 El Nino and the cold, nutrient-rich La Nina of 1988-1989. Here we compare the fates of the 2 post-disturbance algal communities, or '2 cohorts', under conditions determined by large-scale, low-frequency oceanographic events. Succession and population dynamics of the competitive dominant kelp, Macrocystis pyrifera, and understory kelps, Pterygophora californica and Laminaria farlowii, were followed at 5 permanent sites in the Point Loma kelp forest near San Diego, California, USA, where kelps have been mapped quarterly since 1983. There was intense kelp recruitment after both disturbances. The different oceanographic conditions, however, strongly affected the population dynamics of M. pyrifera and its competitive interactions with the lower standing species. Poor M. pyrifera growth, canopy formation, and survival during the El Nino apparently allowed the persistence of understory populations. Extraordinary conditions for M. pyrifera growth during the La Nina were associated with the near extinction of understory populations. The number of stipes per plant and stipe density are indices of M. pyrifera growth and carrying capacity, respectively, which appear to be very sensitive to environmental conditions. The 2 cohorts exhibited very different stipe patterns. In both cases, the anomalous oceanographic conditions lasted for about 2 yr after the disturbances, but the effects on kelp community structure persisted for the Lives of the M. pyrifera cohorts, despite average or relatively poor conditions later. In summary, these data suggest that large-scale, low-frequency oceanographic phenomena are important to kelp forest successional processes, population dynamics, and competitive interactions among kelp guilds.

Evidence for long-term natural change in coastal ecosystems has to be separated from the effects of intense anthropogenic impacts, especially in heavily populated areas. The kelp forests of southern California, highly productive ecosystems organized around the giant kelp, Macrocystis pyrifera, support a variety of fisheries, and the kelp is harvested for extraction of alginates. Because of the importance of Macrocystis itself to the diverse assemblage within the kelp forest community, research in this ecosystem has focused on changes in giant kelp populations. Canopy maps of the Point Loma kelp forest near San Diego illustrate major changes over the last century; these changes have been ascribed to a variety of different causes, including temperature. To understand the role of physical forcing on interannual variability in the Macrocystis canopy at Point Loma, we compared two 31-year kelp data sets with available physical records. Annual average surface temperature was significantly correlated with kelp harvest, but explained only 24% of the variance. Because the canopy of giant kelp is susceptible to disturbances that may not affect plant survival, we evaluated two subsurface measures-stipe number as an index of individual plant growth, and stipe density as a measure of carrying capacity. Both stipe measures were sensitive to interannual variability in surface temperature for the period 1983-95 and were more sensitive than plant survival. Plant size and carrying capacity were very low following 1992-93 El Nino conditions and the anomalously warm 1994. Comparison with historical stipe data from 1957, 1973, and 1974 indicates up to two-thirds reductions in standing biomass since 1957. There is a strong inverse trend between median plant size and the sums of anomalies in Scripps Institution of Oceanography Pier surface temperature, calculated quarterly for three years. It appears, however, that these large, interdecadal changes in biomass can be explained by the location of the data sets within multiyear warm and cold periods. We conclude that stipe numbers and stipe density--measures of individual plant size and carrying capacity--are useful tools for evaluating long-term change in Macrocystis populations within specific locations. Although limited by the paucity of historical observations, the sensitivity of stipe counts to surface temperature argues strongly for their incorporation into ongoing and future kelp forest research.

Sea urchins, Strongylocentrotus franciscanus (A. Agassiz) and especially S. purpuratus (Stimpson) sheltering in holdfasts of giant kelp, Macrocystis pyrifera, feed on haptera, eventually creating cavitation damage that leads to structural failure of the holdfast when the plants are stressed by large waves. Periodically giant kelp plants on permanent transects in a large Southern California forest were categorized for their degree of urchin infestation and cavitation damage, and subsequent survival followed for 5 yr. Plants with a high degree of urchin damage had significantly higher rates of mortality than plants with little damage during several assessment periods. There was a decreasing gradient in the degree of urchin damage and importance of cavitation from the deep (18 m), outer edge of the Point Loma forest, through the center (15 m), to the inner (12 m) edge of the forest which paralleled urchin abundance and recruitment rates. This gradient acts to reduce the impact of the gradient of giant kelp mortality caused by storms, which is much greater in shallow water and decreases seaward.

San Diego's sewage outfall broke during winter 1992, spilling 7.1 × 108 litres/d of treated effluent in kelp forest depths for a two month period during an El Niño event. The ecological implications for the Point Loma kelp forest community were studied by comparing long term data with conditions during and after the spill. Surface ammonium concentrations within 1 km of the break were at potentially toxic levels, and light levels were reduced enough to have inhibited kelp germination and growth. However, because of El Niño conditions, it is unlikely that kelp would have germinated in the absence of a spill. Beyond 1 km, high ammonium concentrations benefitted the nutrient-depleted surface canopy of giant kelp (Macrocystis pyrifera). Measured sedimentation rates were significantly higher near the outfall during the spill and were strongly related to wave height; water motion, however, prevented sediment accumulation. Bioassays were conducted on a grid of stations surrounding the outfall. There were significant reductions in the density and growth of microscopic sporophytes of Macrocystis outplanted near the outfall during the spill, but this pattern disappeared in samples collected 11 d after the repair was completed and was not observed again. Sediments collected near the outfall during the spill significantly reduced Macrocystis germ tube elongation; a post repair assay showed no differences with respect to the outfall. No significant effects were observed in outplants of juvenile Macrocystis sporophytes, cup corals, and juvenile abalones. Video transects during the spill and subsequent diving observations provided no evidence of sediment accumulation or negative impacts on established animal populations. Kelp population dynamics at the permanent sites were predictable from existing population structure and El Niño conditions. Damage to kelps, apparently resulting from a combination of low light and nutrient conditions with mechanical damage from storms, construction activity, and barge anchor cables, was observed along the outfall immediately adjacent to the break point. Shortly after the outfall was repaired, upwelling improved conditions for kelp germination and growth, and the zone of maximum impact developed into a dense kelp forest. Suspension feeders, detritivores and sea urchins, whose natural history indicates they could have been affected by the spill, showed no unusual population changes. In the context of the continuum of disturbances observed in two decades of population studies at Point Loma, the spill was a modest disturbance similar to the natural vagaries of kelp recruitment. We emphasize that this spill was an intense but not chronic impact during an El Niño event that also stressed control areas. However, it is representative of massive spills in coastal regions, and the fact that a sewage spill of this magnitude had no lasting effects on a kelp forest community is of general interest.