Dylan S. Ahearn

Summary: 1. Chlorophyll a (Chl a) distribution across a 0.36 km² restored floodplain (Cosumnes River, California) was analysed throughout the winter and spring flood season from January to June 2005. In addition, high temporal‐resolution Chl a measurements were made in situ with field fluorometers in the floodplain and adjacent channel.2. The primary objectives were to characterise suspended algal biomass distribution across the floodplain at various degrees of connection with the channel and to correlate Chl a concentration and distribution with physical and chemical gradients across the floodplain.3. Our analysis indicates that periodic connection and disconnection of the floodplain with the channel is vital to the functioning of the floodplain as a source of concentrated suspended algal biomass for downstream aquatic ecosystems.4. Peak Chl a levels on the floodplain occurred during disconnection, reaching levels as high as 25 μg L⁻¹. Chl a distribution across the floodplain was controlled by residence time and local physical/biological conditions, the latter of which were primarily a function of water depth.5. During connection, the primary pond on the floodplain exhibited low Chl a (mean = 3.4 μg L⁻¹) and the shallow littoral zones had elevated concentrations (mean = 4.6 μg L⁻¹); during disconnection, shallow zone Chl a increased (mean = 12.4 μg L⁻¹), but the pond experienced the greatest algal growth (mean = 14.7 μg L⁻¹).6. Storm‐induced floodwaters entering the floodplain not only displaced antecedent floodplain waters, but also redistributed floodplain resources, creating complex mixing dynamics between parcels of water with distinct chemistries. Incomplete replacement of antecedent floodplain waters led to localised hypoxia in non‐flushed areas.7. The degree of complexity revealed in this analysis makes clear the need for high‐resolution spatial and temporal studies such as this to begin to understand the functioning of dynamic and heterogeneous floodplain ecosystems.