As human impacts on the ocean have largely increased in the last century, a question of primary interest now facing scientists is whether changes in environmental variables can be attributed to human input or are simply natural. Answering this question is crucial in assessing human impact on the environment, yet is complicated by the fact that the natural variability of oceanic properties is very high and still largely undefined.

Currently about 4500 million liters of treated sewage effluent are discharged daily in the Southern California Bight, mostly through three major submarine outfalls, a freshwater input greater than the natural input from rivers, runoff and storms in the region. It is however unclear whether these artificial rivers are affecting the water column physical properties and biology. This research investigates the relation between variations in "anthropogenic" properties, i.e. properties input by humans in the ocean (sewage flow and suspended solid discharge), and water column properties (Secchi depth transparency, percent transmissivity, temperature and dissolved oxygen) measured near three sewage discharge sites in the Southern California Bight: the Santa Monica Bay outfall of the city of Los Angeles; the Palos Verdes Peninsula outfall of Los Angeles County; and Point Loma outfall of the city of San Diego. Data on the same variables had been collected for 15 years at control and test locations with comparable methods in all three areas.

Time-series analyses were used to investigate the variability of both anthropological and water column properties at the three locations (50-200 km apart). Comparisons among the areas and correlations with the sewage discharge within each area were used to distinguish the natural from anthropogenic sources of variability. In fact, previous studies have shown that many physical and biological water column variables (including temperature, zooplankton biomass and phytoplankton abundance) are coherent at interannual and annual frequency bands over the entire Southern California Bight. In other words, they tend to rise and fall simultaneously from year to year. The question was: Does the same happen also in areas with a large anthropogenic input?

My analysis of the sewage inputs showed that the three discharges were not coherent in any frequency band. Thus, coherence in the water column across the Bight would indicate a Bight-wide (natural) driving force, while correlations with the local discharges would indicate anthropogenic driving force. The analysises of the water column variables showed no correlations between any water column properties and the sewage properties. Morevover, the interannual components of transparency, transmissivity and (particularly) temperature were correlated, within each area, between test and control stations, and were also correlated across areas. Thus, the dominant mode of the fluctuations of these three properties was natural (Bight driven) rather than driven by the local anthropogenic discharges. On the other hand, oxygen did not show common temporal patterns even between the stations within the same area, thus cross-Bight comparisons of this property could not be made.