Examining SeaWiFS chlorophyll variability along the Louisiana coast using wavelet analysis

Multiyear time-series satellite derived chlorophyll a concentrations (Chl) were used to examine the dominant scales of Chl variability with distance offshore from the coast in waters influenced by the discharge from the Mississippi and Atchafalaya Rivers in the northern Gulf of Mexico. Wavelet analysis was applied to almost ten years (1998-2007) of 15-day averaged, 2 km resolution Sea-viewing Wide Field-of-view Sensor (SeaWiFS) along three zones influenced by the two large rivers. River discharge and monthly-mean fields of QuikSCAT satellite derived wind stress superimposed on SeaWiFS Chl distributions provided additional insights on the Chl variability in the region. In a zone immediately west of the Mississippi delta at the 20-m isobath, seasonal maxima in peak Chl variance were associated with seasonal peaks in river discharge with maxima in 1999 and 2004. Offshore at the 500 m isobath, similar peaks occurred with less regularity with interannual maxima in Chl variance occurring in 1998, 2003 and 2007 that appeared to be influenced mainly by the offshore wind stress that transported river plume waters with elevated Chl offshore. Although similar, Chl variability was generally lower in the zone off the Atchafalaya River than off the Mississippi River. Periods of Chl variance were generally of longer duration offshore than nearshore. In a zone west of the Atchafalaya River where Hurricane Rita made landfall, a significant peak in Chl variance appeared to be associated with the effects of the Hurricane.