Pollen transport through distributaries and depositional patterns in coastal waters

In this study we compare seasonal pollen and spore assemblages in river waters from the Mississippi River at Vicksburg, Mississippi to those downstream at Belle Chasse, Louisiana and in a distributary, the Atchafalaya River at Morgan City, Louisiana. Assemblages of pollen in waters at Vicksburg and Belle Chasse are similar to each other, but distinctively different from those in the Atchafalaya River. Local pollen inputs are more important in the anastomosing Atchafalaya River channel, as compared to the meandering main stem Mississippi channel which discharges through the bird-foot delta. Riverine pollen and spore assemblages are also compared to those of surface sediments from the Louisiana continental shelf in the vicinity of the Mississippi River plume. In this area the primary source of marine pollen and spore assemblages appears to be the discharge from the main stem Mississippi, with limited input from the Atchafalaya. Thus, pollen deposited in these coastal waters provides a record of vegetation in the Mississippi drainage basin, rather than local coastal areas. Pollen assemblages on the continental shelf vary with distance from the river source, but not in a systematic manner. The pattern is explained by biological controls on fine-particle deposition in marine waters, i.e., deposition of pollen in copepod fecal pellets. As copepod grazing is dependent upon phytoplankton production, pollen deposition from the river plume will be limited by high turbidities proximal to the river mouth. We hypothesize that in neritic environments, pollen is preferentially deposited in areas corresponding to high phytoplankton production, which are characterized by reduced turbidity and high nutrient availability.