Hydrobiologia 386: 45–53, 1998. 45 © 1998 Kluwer Academic Publishers. Printed in the Netherlands. Relationships between bacterial productivity and organic carbon at a soil–stream interface William V. Sobczak1;2;∗, Lars O. Hedin1 & Michael J. Klug3;4 1Section of Ecology and Systematics, Cornell University, Ithaca, NY 14853, U.S.A. 2Institute of Ecosystem Studies, Box AB, Millbrook, NY 12545, U.S.A.; Tel: 914 677 5343; Fax: 914 677 5976; E-mail: [email protected] (∗author for correspondence) 3W. K. Kellogg Biological Station and Department of Zoology, Michigan State University,Hickory Corners, MI 49060, U.S.A. 4Center for Microbial Ecology, Michigan State University, East Lansing, MI 48824, U.S.A. Received 24 April 1998; in revised form 15 September 1998; accepted 30 September 1998 Key words: stream sediments, riparian zone, dissolved organic carbon (DOC), particulate organic carbon (POC), FAME yield, bacterial productivity Abstract Microbial communities at soil–stream interfaces may be particularly important in regulating amounts and forms of nutrients that leave upland soils and enter stream ecosystems. While microbial communities are thought to be responsible for key nutrient transformations within near-stream sediments, there is relatively little mechanistic in- formation on factors that control microbial activities in these areas. In this study, we examine the roles of dissolved organic carbon (DOC) vs. particulate organic carbon (POC) as potential controls on rates of bacterial productivity (measured as incorporation of [3H]thymidine into bacterial DNA) and amounts of bacterial biomass (measured as fatty acid yield) in sediments along a transect perpendicular to a soil–stream interface. We hypothesized that spatial patterns in bacterial productivity would vary in response to strong and persistent patterns in pore-water concentrations of DOC that were observed along a soil–stream transect throughout a 2-year period. Our results did not support the existence of such a link between pore-water DOC and bacterial productivity. In contrast, we found bacterial productivity and biomass were related to small-scale spatial variations in sediment POC on 3 of 4 sample dates. While our results indicate that total bacterial productivity in near-stream sediments is not consistently linked to spatial variations in pore-water DOC, it is likely that DOC and POC are not mutually exclusive and the relative contribution of DOC and POC to sedimentary microbes varies temporally and spatially in different riparian habitats. Introduction crobial communities on nutrient flux is thought to be particularly important in regions where nutrients enter Microbial processes that occur at the interface between streams primarily via subsurface flows of soil-pore terrestrial and lotic ecosystems can influence both water and groundwater (McDowell & Likens, 1988; amounts and forms of nutrients that move from upland McClain et al., 1994; Dosskey & Bertsch, 1994). soils to surface waters (Likens, 1984; Wetzel, 1990; Despite the potential biogeochemical importance Hedin et al., 1998). For example, denitrifying bacteria of microbial processes within soil–stream interfaces, at the interface between upland soils and streams can we know relatively little about factors that control have an important impact on amounts of dissolved in- the activities of microbial communities within these organic nitrogen that are exported from terrestrial to habitats. Other aquatic habitats have in comparison aquatic ecosystems (Groffman et al., 1992; Hanson received more attention. In many freshwater and mar- et al., 1994; Hedin et al., 1998). Such control by mi- ine sediment habitats bacterial productivity appears to 46 be primarily controlled by the availability of decom- posable organic matter supplied as sediment-bound, particulate organic carbon (POC) (see reviews by Cole et al., 1988; Sander & Kalff, 1993); however addi- tional factors (e.g., temperature, inorganic nutrients, and grazing) can influence production rates (White et al., 1991). In stream and riverine sediments dis- solved organic carbon (DOC) (Findlay et al., 1993; Vervier et al., 1993; Jones et al., 1995) and POC (Hedin, 1990; Pusch & Schwoerbel, 1994; Marxsen, 1996) have both been shown to influence the activ- ity of bacteria. DOC has received increased attention in stream and riverine sediments, relative to lentic aquatic habitats, because it frequently dominates the flux of organic matter through lotic ecosystems (Thur- man, 1985). However, only a fraction of the DOC transported in streams appears to be readily available for bacterial uptake (Thurman, 1985; Kaplan & New- bold, 1993), and transformations between DOC and POC are known to occur (Metzler & Smock, 1990; Findlay & Sobczak, 1996). Hence, the relative role of Figure 1. Persistent trend in DOC in pore waters across the DOC vs. POC as sources of carbon for stream bacteria soil–stream interface at our study site. Values are means based on 13 is difficult to resolve. sample events. For each sample event we sampled between one to four parallel transects of sample wells. Error bars indicate standard While several studies have addressed the regula- deviations based on n = 13 sample events. See Hedin et al. (1998) tion of microbial activity in stream sediments (Findlay for additional pore water chemistry at this site. et al., 1993; Hendricks, 1996; Marxsen, 1996), no study has examined the importance of organic matter in controlling patterns and rates of bacterial productiv- which physical and chemical conditions vary widely ity in near-stream (i.e., riparian) sediments: along or are poorly characterized. In addition, we employed hydrologic flow paths from upland soils to stream sed- a novel sampling protocol that allowed us to estim- iments. In this study we address whether patterns of ate bacterial productivity in anoxic sediments in situ. bacterial productivity and biomass in riparian sedi- Because this is, to our knowledge, the first study ments vary as a function of changes in concentrations of bacterial productivity in anoxic sediments along a of DOC in pore waters or amounts of sediment-bound riparian transect we compare our estimates of bac- POC. We took advantage of a strong cline in pore- terial productivity to those reported from other aquatic water DOC concentrations (Figure 1) which consist- habitats. ently occurred across a riparian soil–stream interface for over two years (Hedin et al., 1998). Based on previous suggestions of the importance of DOC in Study site controlling bacterial activity in streams (see Kaplan & Newbold, 1993), we hypothesized that bacterial pro- We conducted this study along a forested section ductivity would decrease in concert with DOC across of Smith Creek, a first-order stream within the Au- the riparian interface. In addition, we examined the hy- gusta Creek drainage basin in southwestern Michigan pothesis that bacterial productivity was more closely (Hedin et al., 1998). Near-stream soils consist of three coupled to variations in standing stocks of POC within distinct layers: a highly organic upper horizon (ca. 0– riparian sediments. We employed an experimental 5 cm), a mixed inorganic-organic horizon dominated design that aimed to provide variation in DOC and by sand (ca. 5–60 cm), and a tightly packed inor- POC along a discreet riparian transect in which phys- ganic sand horizon (below ca. 60 cm). The Smith ical and chemical conditions were well characterized Creek drainage basin and water chemistry have been (Hedin et al., 1998), as opposed to relying on variation described elsewhere (Wetzel & Manny, 1977; Hedin in DOC and POC from an array of sites or streams in & Brown, 1994; Hedin et al., 1998). 47 We focused on a riparian area that was part of a more extensive study on biogeochemical processes that occur within soil–stream interfaces (Hedin et al., 1998). We sampled sediments that were located imme- diately downstream of a field of 24 wells that were po- sitioned in four rows along an ca. 3 m transect across the soil–stream interface. These wells were used to monitor pore-water chemistry and hydraulic heads. In- dividual wells were constructed from 19 mm (inner diameter) polyvinyl pipes with a 10 cm deep sample port located 40 cm below the soil surface (Hedin et al., 1998). Sediments were continually inundated with wa- ter. Flow paths of pore waters through the soil–stream interface were characterized by measuring hydraulic Figure 2. Discharge during 1 June 1993 to 1 September 1993 at the potentials by using the sampling wells as piezomet- USGS gauging station located on Augusta Creek, MI, USA. Smith Creek is a tributary of Augusta Creek. Arrows indicate sample dates. ers, and by following the movement of additions of a conservative tracer (Br− as NaBr) (Hedin et al., 1998). Additions of inert tracers showed that pore wa- (i.e., summer base-flow) (Figure 2). Each of the 2 tran- ter moved from upland wells towards the stream at sects consisted of 5 sampling sites (i.e., coring sites) − velocities ca. 0.4–0.9 cm hr 1 and that rates of vertical within a 3 m distance from the stream (Figure 1). We movement of pore waters (upwelling) increased with shifted coring transects within a 1 m area between proximity to the stream (Hedin et al., 1998). DOC sample dates in order to ensure that a given sample in shallow pore waters consistently decreased along site was previously undisturbed and to minimize po- the flow path (Figure 1). Concentrations of electron tential autocorrelation within the data set. Hence, we − 2− acceptors (e.g., O2,NO3 ,SO4 ) varied in a thermo- examined 40 independent sediment samples. We used dynamically predictable pattern as pore water moves a one inch soil corer with plastic sleeve to sample sed- from anoxic upland soils to oxic stream-surface sed- iments. To prevent O2 intrusion into anoxic sediments iments (Hedin et al., 1998). Sampling and analysis we immediately capped the plastic sleeves with rubber of pore water chemistry is described in Hedin et al. stoppers and positioned them in deoxygenated water (1998).