Particle fluxes in San Pedro Basin, California: A four-year record of sedimentation and physical forcing

Abstract

Moored sediment traps were deployed from January 2004 through December 2007 at depths of 550 and 800 m in San Pedro Basin (SPB), CA (33°33.0′N, 118°26.5′W). Additionally, floating sediment traps were deployed at 100 and 200 m for periods of 12–24 h during spring 2005, fall 2007, and spring 2008. Average annual fluxes of mass, particulate organic carbon (POC), δ 13C org, particulate organic nitrogen (PON), δ 15N-PON, biogenic silica (bSiO 2), calcium carbonate (CaCO 3), and detrital material (non-biogenic) were coupled with climate records and used to examine sedimentation patterns, vertical flux variability, and organic matter sources to this coastal region. Annual average flux values were determined by binning data by month and averaging the monthly averages. The average annual fluxes to 550 m were 516±42 mg/m 2 d for mass (sdom of the monthly averages, n=117), 3.18±0.26 mmol C/m 2 d for POC ( n=111), 0.70±0.05 mmol/m 2 d for CaCO 3 ( n=110), 1.31±0.21 mmol/m 2 d for bSiO 2 ( n=115), and 0.35±0.03 mmol/m 2 d for PON ( n=111). Fluxes to 800 and to 550 m were similar, within 10%. Annual average values of δ 13C org at 550 m were −21.8±0.2‰ ( n=108), and δ 15N averages were 8.9±0.2‰ ( n=95). The timing of both high and low flux particle collection was synchronous between the two traps. Given the frequency of trap cup rotation (4–11 days), this argues for particle settling rates ≥83 m/d for both high and low flux periods. The moored traps were deployed over one of the wettest (2004–2005, 74.6 cm rainfall) and driest (2006–2007, 6.6 cm) rain years on record. There was poor correlation (Pearson's correlation coefficient, 95% confidence interval) of detrital mass flux with: C org/N ratio ( r=0.10, p=0.16); δ 15N ( r=−0.19, p=0.02); and rainfall ( r=0.5, p=0.43), suggesting that runoff does not immediately cause increases in particle fluxes 15 km offshore. δ 13C org values suggest that most POC falling to the basin floor is marine derived. Coherence between satellite-derived chlorophyll a records from the trap location (±9 km 2 resolution) and SST data indicates that productivity and export occurs within a few days of upwelling and both of these parameters are reasonable predictors of POC export, with a time lag of a few days to 2 weeks (with no time lag—SeaWiFS chlorophyll a and POC flux, r=0.25, p=0.0014; chlorophyll a and bSiO 2 flux, r=0.28, p=0.0002).