The diel and seasonal effects of submerged aquatic vegetation on nutrient dynamics and organic carbon bioavailability in a southeastern reservoir

Background/Question/Methods

The concentration and bioavailability of dissolved organic carbon (DOC) can be altered by the autochthonous production of macrophytes, and this alteration can influence microbial processes in aquatic ecosystems.  This study evaluated the effects both spatially and temporally of submerged aquatic vegetation (SAV) upon carbon and nitrogen dynamics in Lake Seminole on the Georgia-Florida border.  Concentrations of DOC and NO₃ within SAV beds (Hydrilla verticillata) were measured in situ throughout the water column over a 24-hour period.  Carbohydrate concentrations were measured because they comprise a labile component of DOC.  The effects of labile DOC upon the microbial community and the resulting implications were also investigated by measuring O₂ consumption during whole water incubations.  Finally, nutrient limitation of the microbial communities within SAV beds was also assessed with a series of nutrient amendment experiments.

Results/Conclusions

DOC concentrations exhibited the greatest variability among depths during August (mean diurnal surface concentration = 3.91 mg/L and above the sediment concentration = 3.05 mg/L) and September (mean surface concentration = 2.88 mg/L and benthic concentration = 4.18 mg/L).  Diurnal carbon bioavailability was higher near the sediment during September based on higher O₂ consumption rates observed during the day (1.09 µmol/h) than at night (0.52 µmol/h).  Monosaccharide concentrations also followed the same pattern as DOC and could explain the increased carbon bioavailability.  Therefore, it is likely that an increase in bioavailable carbon led to an increase in microbial metabolism.  NO₃ concentrations were consistently lower during the growing season near the sediment (mean = 189 µg/L) compared to the surface (mean = 705 µg/L) or the intermediate depth (mean = 838 µg/L).  These lower NO₃ concentrations indicate that nitrate was being removed from the system either through active uptake and storage or denitrification.  These findings demonstrate that a portion of the DOC (monosaccharides) produced by SAV is labile which results in the alteration of nutrient cycling within the SAV bed.