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 × 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.