Point Loma Kelp Forest
The Point Loma kelp forest, one of the largest kelp forests in California, is located offshore of the City of San Diego . This urban setting is between the entrances to two large bays, Mission Bay, a recreational park, and the much larger San Diego Bay, a major naval and commercial port. The kelp forest is crossed by the outfall from the Point Loma sewage treatment plant; discharge takes place 4.5 miles offshore through multiple diffusers in 320 foot depths. Within the forest there is intense sport and commercial fishing for sea urchins, spiny lobsters, and fin fishes, and the kelp itself is harvested for the production of alginates. This multi-use resource is also important to San Diego 's large diving community. Thus, the health of this ecosystem is of concern to all aspects of society.
Like all kelp forests, the Point Loma forest is highly dynamic (Dayton et al. 1992). Dredging the bays in the early 20 th century transported sand onto the kelp habitat and restricted both the north and south sides of the kelp forest. In the 1950s the kelp forest was stressed by poorly treated sewage released within the San Diego Bay and finally the giant kelp itself virtually collapsed in the face of a massive El Nino in the late 1950s. In the early 1950s Scripps Institution of Oceanography began some of the first coordinated scientific diving research in the world with various projects by Connie Limbaugh, Wheeler North, and Jim Stewart amongst many. The Scripps research has continued in the kelp forest through the present. Since 1970 the long-term study has focused on permanent transects and study sites that cover all the habitats within the forest, but many of these sites were chosen to continue as closely as possible to those sites studied by the earlier workers. The study of these permanent sites is now well into the fourth decade, and because the sites were chosen to be as close as possible to earlier sites there is even longer continuity. Except for the CalCOFI program of the California Current, now in its sixth decade, the Point Loma kelp program may be the longest continued marine time-series in the world.
The present program was started in 1971 (Dayton et al., 1984). In 1983 it was expanded to include population data on kelps and benthic macroinvertebrates at five permanent sites. This program was expanded again in the early 1990s to include many more sites throughout the kelp forest. Natural disturbances, notably storms, El Niños, and grazing, caused major fluctuations in the distribution and abundance of kelps, especially the giant kelp, Macrocystis pyrifera . Plants in this large forest are affected by gradients in depth, light, temperature, water motion, nutrient availability, and planktonic propagule supply. Storm mortality is strongly depth dependent; the inner edge of the M. pyrifera forest appears to be defined by the height of breaking waves (Dayton et al. 1992, Seymour et al. 1989). Kelp recruitment density also decreases with depth. In addition to cross shore gradients, there is significant longshore variability as well. Giant kelp plants on the two longshore ends of the forest suffered much higher mortality than plants in the center of the forest at the same depth during two major storm episodes. Conversely, the end of the forest sites had dramatically better kelp survivorship than the central site at the same depth during the 1983 El Niño summer; these sites face into longshore currents where they may be exposed to water not depleted of nutrients by the rest of the forest (Tegner & Dayton 1987).
The Point Loma kelp forest continues to face potential threats from natural and anthropogenic impacts. There has been a long-term increase in ocean temperatures since 1977. The productivity of the forest is strongly affected by the low nutrients associated with higher temperatures. Average giant kelp plant size and productivity have declined significantly since the early 1970s, and will continue to decline if the warming continues. The strong El Niño of 1997/1998 devastated the Point Loma kelp forest, but was quickly followed by a La Niña event, which initiated recovery. Intense fish trapping of important sea urchin predators has the potential to lead to more destructive grazing events. Non-point source pollution from terrestrial runoff and the bays that bracket Point Loma remain a concern. It is important to understand all sources of variability affecting the kelp community at Point Loma to separate potential outfall impacts from other disturbances.