Spatial and temporal response of stream bacteria to sources of dissolved organic carbon in a blackwater stream system

Abstract

1. We hypothesized that changes in bacterial colony growth would be correlated to shifts in riparian vegetation (via leachate quality) along a river continuum of a south‐eastern, blackwater stream (U.S.A.). Spatially, we expected bacterial assemblages from downstream reaches to utilize more sources of leachate and at higher concentrations than bacteria collected from headwater reaches. Temporally, we predicted higher colony growth on leachate from autumn‐shed (senescent) leaves compared with leachate from fresh, green leaves. 2. We examined spatial differences in assemblage growth by culturing bacteria sampled along the stream continuum on gradient plates using leachates from four common riparian species (Taxodium distichum, Carya spp., Acer rubrum and Decumaria barbara). Bacteria from the lowest site were able to use all sources provided and at all concentrations, whereas bacteria from upper reaches could not. Colony density was correlated to relative leachate concentration at all sites along the continuum. 3. Leachates from fresh and senescent A. rubrum leaves were used to determine temporal differences. Winter assemblages of bacteria could not grow on fresh leaf leachate at any concentration but grew well on autumn leaf leachate at higher concentrations. Differential response of bacterial assemblages indicated local adaptation to potential sources of dissolved organic matter. 4. Growth response of stream bacterial colonies appeared to be dependent on the timing and source of leachate as well as on sources of dissolved organic carbon from further upstream. Growth of bacterial assemblages exhibited ‘generalist’ characteristics in headwater reaches and ‘specialist’ characteristics at the mouth of our study stream drainage. Thus, our findings lend support to the argument that variable resource habitats favour a small, generalist assemblage, while environments with stable resource supplies allow for highly diverse assemblages dominated by specialists.

Seasonal Response of Stream Biofilm Communities to Dissolved Organic Matter and Nutrient Enrichments

“…The source, quality, and type of DOM, as well as the quantity, may influence the abundance and distribution of bacteria in stream ecosystems (20,21,24). In addition, other aspects of microbial community function such as respiration, biomass, and extracellular enzyme activity may be limited by dissolved inorganic nutrients and organic matter in streams (49, 51).…”

“…This is because earlier investigations were typically based on assemblage-level responses, such as examining total bacterial numbers (e.g., references 16, 24, 31, and 45), with few examining spatial and temporal changes in specific bacterial populations (4,20,32). In this study, we examined how different bacterial taxa in biofilms responded to DOM and inorganic nutrients by using nutrient diffusing artificial substrates (clay flowerpots), an approach commonly used to demonstrate nutrient limitation in streams (e.g., references 11 and 43).…”

Abundance of Three Bacterial Populations in Selected Streams

“…The nine streams examined in this study were selected because of their use in other studies and differences in their sources and quantities of organic matter [6,7,9,13,29]. They are second-to third-order streams with different vegetation types, located in different regions of the USA.…”

“…They are second-to third-order streams with different vegetation types, located in different regions of the USA. The streams are as follows: Meyers Branch, located on the Savannah River Site near Aiken, South Carolina a blackwater stream with dense vegetation dominated by cypress, tupelo, maple, hickory, willow, and birch [9]; Black Creek, another blackwater stream, on the lower coastal plain of Georgia with cypress-dominated floodplains [15] [7]; and the West Branch of the Mahoning River located within Jenning's Woods in Portage County, Ohio [13], a highly shaded stream with riparian vegetation that includes oak, maple, sycamore, and poplar.…”

“…Streams were selected based on differences in their organic matter dynamics and use in prior studies [3,7,9,13,15,22,26,29]. Streams examined included coastal plain blackwater streams (with high amounts of DOM, in VA, GA, and SC), well-canopied streams (Appalachian Mountain, NC, and northeast OH), more open canopied streams (prairie, KS, and agricultural, IA), a stream with large populations of macrophytes (WI), and a stream in a watershed with forests, agricultural fields, and wetlands (MI).…”