Temporal variations of C, N, δ13C, and δ15N in organic matter collected by a sediment trap at Cuenca Alfonso, Bahía de La Paz, SW Gulf of California

To monitor the composition and the vertical flux of particulate matter from the sea surface, a sediment trap was moored in Cuenca Alfonso, Bahía de La Paz, a zone of high productivity in the southwestern Gulf of California. Carbonate-free samples from 2002 to 2005 were analyzed for Corg, N, δ13C, and δ15N. The results show seasonal and interannual variability, with the δ13C and δ15N values larger in spring and summer than in fall and winter. The C:N ratio and δ13C increased by 1.5 units from 2002 to 2003–2005, suggesting a change in the supply of organic matter and-or the use or preferential degradation of Norg. There was no interannual variation in δ15N. The occasional high δ15N values suggest that physical mechanisms, such as the shoaling and advection into the bay of 15N-rich subsurface equatorial water, occur over short time periods. The latter is presumed to be related to the periodic development of a significant cyclonic gyre in the southern Gulf.