Surface nutrient enrichment in the California Current off Southern California: description and possible causes

Concentrations of inorganic plant nutrients in California Current waters are frequently greater near the surface than deeper in the euphotic zone. This surface nutrient enrichment (conversely, shallow subsurface nutrient minima) is poorly described. We used data from the 1983–1995 quarterly California Cooperative Oceanic Fisheries Investigations surveys to characterize surface nutrient enrichment better and to examine causal hypotheses. Averaged over the survey area, frequency of occurrence (% of stations) of enrichment was: phosphate 28%; silicate 51%; nitrate 8%; nitrite 2%. Enrichment frequency, strength and depth, and the thickness of minima layers showed spatial (latitudinal, onshore-offshore) and temporal (seasonal and year-to-year) trends. Co-occurrence, at a given station, of enrichment in two nutrients was nearly random. Apparently, enrichment of different nutrients is driven by independent factors or may be out of phase. Wind speed, wave height, and cloud cover had little effect on enrichment strength, depth, and thickness of minima layers. Potential causes of surface nutrient enrichment include atmospheric deposition, bubble concentration, vertical and horizontal circulation, differences between rates of nutrient uptake and regeneration, photoinhibition, photochemistry, and vertical migrations. Relative importance of these possible contributors is unknown, but some appear minor based on arguments using the CalCOFI data. We believe an imbalance between nutrient consumption and regeneration by euphotic zone plankton is the most important factor in producing surface nutrient enrichment; least important are atmospheric deposition, bubbles, horizontal advection and vertical circulation.