Multiple carbon substrate utilization by bacteria at the sediment-water interface: seasonal patterns in a stratified eutrophic reservoir

Sediment-water interfaces (SWIs) in lakes and reservoirs harbor diverse assemblages of heterotrophic bacteria that influence organic matter cycling. Seasonal reservoir mixing and stratification affect the rate and type of dissolved organic carbon (DOC) utilized by SWI bacteria, however these seasonal carbon utilization dynamics remain poorly understood. We conducted an exploratory multi-seasonal study using Biolog EcoPlates to measure SWI bacterial carbon substrate utilization rates (CSURs) and patterns in a monomictic eutrophic reservoir. Principal components analysis (PCA) was used to elucidate seasonal CSUR variation. Carbohydrate utilization was greatest during early (onset of) stratification and also was high during winter mixing. Amino acid utilization was greatest during late (prolonged) stratification. Carboxylic acids had greatest utilization during late stratification, but also had moderate to high utilization during all other seasons. Amines and polymers exhibited moderate utilization during all seasons. We related seasonal variation in these CSURs to SWI bacterial abundance, temperature, dissolved oxygen, and redox potential. Collectively these environmental variables accounted for 49.4-62.3% of the total substrate utilization variance by season. Several individual substrate CSURs were significantly (P < 0.05) positively and/or negatively correlated with individual environmental variables, with L-asparagine and putrescine exhibiting significant positive and negative correlations with every measured environmental variable. Lastly, seasonal changes in CSURs corresponded to various changes in SWI bacterial assemblage composition, with highest similarities among early stratification and autumnal overturn assemblages and lowest similarities among late stratification and winter mixing assemblages.