Phosphate balance and spatial variability on the continental shelf off the western US-Mexico border region

Abstract

 A cruise was conducted along the northwestern coast of Baja California, off the US-Mexico border region, to determine the physical, biological and anthropogenic factors affecting the concentration and distribution of phosphates (PO₄^3–). The vertical distribution of temperature and PO₄^3– showed isograms (13ºC and 0.75 µM, respectively) rising towards the shore. Although this phenomenon was observed in all the study area, it was more abrupt in the southern part than in the northern, resulting in lower phosphate concentrations in the north (0.50 µM) associated with high chlorophyll a concentrations (10.0 mg m^–3). A principal components analysis indicated that the PO₄^3– concentrations were related to upwelling (51%) and phytoplankton biomass (23%). A PO₄^3– mass balance indicated that sewage discharge had a very local effect, contributing 3.0 ± 0.12 t d^–1 (5%), while horizontal advection contributed 20.0 ± 2.2 t d^–1 (32%) and upwelling supply was 41.1 ± 2.3 t d^–1 (63%). If the PO₄^3– sewage contribution trend does not change, we predict that in 2035 the anthropogenic contribution will be as high as the amount delivered to the area by the Californian Current. Conversely, if Mexico´ s sewage treatment systems improve to a level similar to that in the USA, we estimate that by 2012 the anthropogenic sources will be insignificant.