Tidal creek surface film structural and metabolic dynamics

Surface films on marsh creeks form water-air interfaces of high biological activity. The development, movement, deposition, and breakup of the tidal creek surface film in a naturalSpartima alterniflora-dominated salt marsh in Delaware were followed seasonally over tidal cycles. The metabolic activity, i.e., photosynthesis and respiration, of the surface film and underlying water were determined in the field at the time of peak film formation, just prior to high tide, and the particulate material and chlorophylla were quantified over the tidal cycles. The quantities of organic and inorganic components of the particulate material were all significantly higher in the surface film than in the underlying water (on a volume basis). Numbers of algal cells and quantities of chlorophyll were orders of magnitude greater in the surface film than in the water column. Photosynthesis and respiration were significantly higher in the surface film than in the underlying water. The spectrum of fatty acids was more diverse and their total content was significantly greater in the surface film than in the water column, indicating a concentrated food source contributed by the film as well as a biological richness of the film. When water in the creek flooded the marsh plain at high tide, film deposition was greatest on simulated creek bankS. alterniflora stems, compared to stems along rivulets in the marsh and those in the marsh plain. Using surface film dry weight measurements on an areal basis and film velocity in the creek, both measured throughout a tidal cycle during the summer, it was determined that approximately 12 kg (dry weight) of particulate material moved on the creek surface (2 m wide) past a given point on the flood tide, and 14 kg moved in the opposite direction on the ebb tide. The biological and physical data collected in this study illuminate the contribution of the surface film to the biological (food web) and physical (sediment transport and deposition) functions of a salt marsh.