The impact of a plunging inflow on extracellular enzyme activity in a reservoir ecosystem

Reservoirs are interesting limnetic systems in that they are generally dominated by lentic conditions but can be influenced by advective processes driven by inflowing river water or by the release o f water through the dam. This combination o f conditions can produce interesting water column stratification patterns an d zonation along the length o f the reservoir ( GROEGER & KIMMEL 1984). Distinct changes in the biological communities along the length of reservoirs have frequently been observed to correspond to these zones: limited phytoplankton biomass in the up-reservoir riverine zone; peak biomass in the transitional zone; and reduced biomass down-reservoir in the lacustrine zone. Less emphasis has been placed on determining the changes in broader microbial community composition and dynamics. TIETJEN & WETZEL (2003) studied the distribution of bacterial cells, total counts, an d the proportion o f viable cells in an Alabama reservoir and found little variation along the length of the reservoir when water residence times were extremely long. Seasonal dynamics of sediment water interface bacteria were evaluated by CHRISTIAN & LIND (2007), who related bacterial substrate utilization to a suite of environmental parameters but did not consider longitudinal position. GASOL et al. (2002) considered bacterial dynamics at the longitudinal extremes of a reservoir system and found that nutrient supply was the dominant factor controlling bacterial production. Lacking in these past studies is an attempt to assess microbial community metabolism along the entire length of a reservoir encountering the changes in physical structure associated with the riverine, transitional, and lacustrine zones. To accomplish this we assayed extracellular enzyme activity from 7 sites along a 12-km transect, focusing on the changes in lake structure occurring where the inflowing river plunges under the warm surface waters ofthe lacustrine zone. This approach has been used by other researchers seeking to address different aspects of microbial activity. HARBOTT & GRAcE (2005) used extracellular enzyme activity to investigate the bioavailability of dissolved organic carbon along a gradient of urbanization, and suggested that even with stable concentrations the organic matter differences could be detected. This condition may be analogous to many reservoir systems where dissolved organic matter concentrations vary little compared to other constituents. Similarly, FINDLAY et al. (1998) used extracellular enzyme activity to characterize disso1ved organic matter in the Hudson River (New York). W e extrapolated from this approach; rather than considering organic matter composition we sought to characterize the microbial community using similar suites of enzyme activity measurements. During August 2006, Lake Dunlap reservoir in central Texas, U. S. , had long water residence times, -lO days as compared to the long term average of -5.5 days, as a result of a prolonged period ofbelow normal rainfall. This established the conditions necessary for the formation of a plunge point because the cold, inflowing river water was denser than the warmer waters of the epilimnion of the reservoir. This plunging inflow produced a strong physical gradient, which was easily observed in surface chlorophyll a fluorescence and temperature patterns and clearly separated the reservoir into distinct riverine, transitional, and lacustrine zones. We investigated the impact of this physical gradient on functional attributes of the microbial community. The distinctive temperature and chlorophyll fluorescence patterns clearly identified the physical gradient (the barrier imposed by the water density gradient) and one level ofbiological gradient (the phytoplankton peak associated with the transitional zone). This research expands the understanding of kilometer-scale perturbations of the physical environment of reservoirs and how these perturbations influence, segregate, or otherwise affect the activity ofmicroorganisms.