EFFECTS OF A LARGE SEWAGE SPILL ON A KELP FOREST COMMUNITY - CATASTROPHE OR DISTURBANCE

San Diego's sewage outfall broke during winter 1992, spilling 7.1 x 10 (8) litres/d of treated effluent in kelp forest depths for a two month period during an El Nino event. The ecological implications for the P oint Loma kelp forest community were studied by comparing long term da ta with conditions during and after the spill. Surface ammonium concen trations within I km of the break were at potentially toxic levels, an d light levels were reduced enough to have inhibited kelp germination and growth. However, because of El Nino conditions, it is unlikely tha t kelp would have germinated in the absence of a spill. Beyond 1 km, h igh ammonium concentrations benefitted the nutrient-depleted surface c anopy of giant kelp (Macrocystis pyrifera). Measured sedimentation rat es were significantly higher near the outfall during the spill and wer e strongly I elated to wave height; water motion, however, prevented s ediment accumulation. Bioassays were conducted on a grid of stations s urrounding the outfall. There were significant reductions in the densi ty and growth of microscopic sporophytes of Macrocystis outplanted nea r the outfall during the spill, but this pattern disappeared in sample s collected 11 d after the repair was completed and was not observed a gain. Sediments collected near the outfall during the spill significan tly reduced Macrocystis germ tube elongation; a post repair assay show ed no differences with respect to the outfall. No significant effects were observed in outplants of juvenile Macrocystis sporophytes, cup co rals, and juvenile abalones. Video transects during the spill and subs equent diving observations provided no evidence of sediment accumulati on or negative impacts on established animal populations. Kelp populat ion dynamics at the permanent sites were predictable from existing pop ulation structure and El Nino conditions. Damage to kelps, apparently resulting from a combination of low light and nutrient conditions with mechanical damage from storms, construction activity, and barge ancho r cables, was observed along the outfall immediately adjacent to the b reak point. Shortly after the outfall was repaired, ztpwelling improve d conditions for kelp germination and growth, and the zone of maximum impact developed into a dense kelp forest. Suspension feeders, detriti vores 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 o f population studies at Point Loma, the spill was a modest disturbance similar to the natural vagaries of kelp recruitment. We emphasize tha t this spill was an intense but not chronic impact during an EI Nino e vent that also stressed control areas. However, it is representative o f 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.