Benthic community metabolism in three habitats in an Ozark stream

Hydrobiologia 437: 165–170, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands.

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Benthic community metabolism in three habitats in an Ozark stream Gregory W. Whitledge1 & Charles F. Rabeni2,∗ 1 The

School of Natural Resources, Department of Fisheries and Wildlife Sciences, University of Missouri, Columbia, MO 65211-7240, U.S.A. 2 U.S. Geological Survey, Missouri Cooperative Fish and Wildlife Research Unit, Department of Fisheries and Wildlife Sciences, University of Missouri, Columbia, MO 65211-7240, U.S.A. (∗ Author for correspondence) Received 15 November 1999; in revised form 13 July 2000; accepted 7 August 2000

Key words: primary production, respiration, periphyton, in situ, Missouri

Abstract Benthic community metabolism was measured in three habitats (riffles, runs and pools) during spring (May), summer (July) and fall (October) in the Jacks Fork River, Missouri, using an in situ chamber technique. Net community productivity (NCP) and gross community productivity (GCP) were highest in riffles, lowest in pools and intermediate in runs. Rates of NCP and GCP during spring and fall were similar for both riffles and runs, but NCP and GCP increased significantly during summer in both habitats. Pool substrates were always heterotrophic and exhibited no significant seasonal changes in NCP or GCP. Community respiration (CR) was highest in riffles, intermediate in runs and lowest in pools, but interhabitat differences in CR were generally smaller than for NCP. Rates of CR during spring and fall were similar, but CR increased significantly during summer. Results indicate that the physical conditions associated with each habitat strongly affect benthic community metabolism in this stream and that the relative proportions of these habitats will influence the ratio of living algal:detrital organic matter potentially available for consumers. Introduction Streams are physically heterogeneous environments, consisting of a diverse assemblage of habitats at multiple spatial scales (Pringle et al., 1988). One of the most apparent forms of spatial heterogeneity in streams is the juxtaposition of hydraulic habitats such as riffles, runs and pools. The arrangement and relative proportion of these habitats is governed by geomorphic and hydrologic processes operating in a nested hierarchy of spatial scales (Frissell et al., 1986). Hydraulic habitats are characterized by specific combinations of depth, current velocity and substrate, as well as other more complex hydraulic factors (Rabeni & Jacobson, 1993). Riffles are usually associated with relatively large substrate particle sizes, shallower depths and higher current velocities than pools during baseflow conditions; runs are typically intermediate with respect to these characteristics. These hydraulic habitats often possess distinct assemblages

of primary producers (Stockner & Shortreed, 1976; Keithan & Lowe, 1985), benthic invertebrates (Barmuta, 1989) and fishes (Peterson, 1996). Thus, the relative abundance of habitats can strongly influence biological structure and function in the stream as a whole. Understanding the influence of habitat on biological patterns and processes is, therefore, necessary for predicting how changes to streams and their watersheds might affect structure and function of stream ecosystems. Despite the importance of habitat in regulating benthic community structure and bioenergetic characteristics of streams (Benke et al., 1988), very few studies have explicitly compared benthic community metabolism in different habitats in the same stream, and none have examined riffles, runs and pools simultaneously. However, various studies suggest that primary production is generally higher in riffles than in pools (Keithan & Lowe, 1985; Rosenfeld & Hudson, 1997) and is higher on rock (Bott et al., 1985; Brown

166 & King, 1987) than on sand and sediment (Antoine & Benson-Evans, 1985; Rier & King, 1996). Understanding the influence of habitat on benthic community metabolism is important for predicting how changes in relative abundance of habitats resulting from land-use or climate changes will affect overall stream metabolism. The purpose of this study was to assess the influence of habitat (riffles, runs and pools) on benthic community metabolism in an Ozark stream.

Methods Study area This study was conducted on a segment of the Jacks Fork River in Shannon County, Missouri, U.S.A., approximately 2 km upstream from Alley Spring (37◦ 90 N, 91◦ 270 W). The Jacks Fork River has a mean annual discharge of 12.5 m3 ·s−1 and is heavily springfed with a gradient of about 1 m·km−1 . Stream width is typically 5–25 m, and pools >2 m deep are common. The substrate consists predominantly of cherty dolomitic limestone, with gravel, pebble and cobble particle sizes dominating (Rabeni, 1992). The study segment consisted of three reaches, each containing a riffle, run and bluff pool. Bluff pools are a common habitat in Ozark streams and are distinguished by the presence of steep limestone bluffs which border one side of the stream (Rabeni & Jacobson, 1993). The three habitats studied are characterized by non-overlapping 95% confidence intervals for mean depth (0.16–0.22 m for riffles, 0.33–0.43 m for runs and 1.41–1.68 m for bluff pools) and current velocity (0.62–0.81 m/s for riffles, 0.26–0.41 m/s for runs and 0.13–0.22 m/s for bluff pools; Peterson, 1996). Measurement of benthic community metabolism Benthic community metabolism measurements were made on three dates each during summer (7–9 July 1998), fall (12–14 October 1998) and spring (25– 27 May 1999). Collection sites within each habitat were chosen based on the presence of a characteristic dominant substrate type (large cobble in riffles, small cobble and large pebble in runs, and small gravel and coarse sand in pools). Within a particular habitat, substrate was collected from areas where these substrate types were visually determined to be dominant. Areas within runs and riffles which were too swift and deep to permit effective collection of smaller intersti-

tial particles along with the dominant substrates were not sampled. Benthic community metabolism was measured by enclosing substrate in production and respiration chambers and measuring changes in dissolved oxygen concentrations within chambers over a set time interval (Bott et al., 1978). Substrate was removed from the stream using a 27 × 20 cm shovel and carefully placed in 23-cm diameter aluminum pans so that the orientation of the substrate remained as close as possible to its orientation in the stream. Substrate-filled pans were placed into incubation chambers submerged in the stream. Chambers were constructed from 250 mm inside diameter plastic desiccators with either clear lids (for measuring net community productivity) or with lids painted black (for measuring community respiration). A Mabuchi submarine motor (Edmund Scientific) powered by a AA battery was placed in each chamber to circulate water. Chamber lids fitted with watertight cork gaskets were clamped to the chamber bottoms with large binder clips to enclose the substrate. Dissolved oxygen concentrations within chambers were measured using a polaragraphic oxygen sensor (YSI model 5739) inserted through an access port in the top of each chamber lid. Chambers were moved to a depth in the stream such that their access ports protruded above the water surface, and rubber stoppers were placed in each access port to prevent stream water from entering chambers during incubations. Substrate-filled pans were not left in the stream for several weeks prior to incubations, which is sometimes done in benthic community metabolism studies to allow periphyton to recover from sampling disturbance before metabolism measurements are performed (Bott et al., 1978; Rier & King, 1996). Substrate pans were not left in the stream due to the high risk of burial or theft of pans in the Jacks Fork River and because the design of our chambers required pans to be moved to an area of appropriate depth and current velocity immediately prior to incubations. On each sampling date, six net production and three respiration measurements were made for each habitat from one of the three reaches. The three reaches were sampled on each of three consecutive days during each season for a total of 18 net production and nine respiration measurements per habitat per season. Measurements were made on habitats from a different reach each day during each season to ensure true replication for statistical analysis (Hurlbert, 1984). Chambers containing substrate from each habitat from a given reach were incubated side-by-side in

167 an unshaded pool so that light and temperature conditions would be essentially identical for all chambers during each day’s incubations. Incubations lasted approximately 2 h and were conducted from around 1100 to 1300 each day. Initial dissolved oxygen concentration was measured in the stream near the chambers using a portable dissolved oxygen meter (YSI model 51B), since oxygen concentrations in the stream were not substantially different from those in the chambers at the beginning of the incubations. Dissolved oxygen concentrations at the end of the incubations were measured in each chamber and the change in dissolved oxygen concentration was calculated. Net community productivity (NCP) was calculated for each clear chamber and community respiration (CR) calculated for each dark chamber as: NCP or CR (mg O2 ·m−2 ·h−1 ) = 1 O2 · V · S−1 · T−1 where 1 O2 is the change in oxygen concentration (mg·L−1) during the incubation, V is chamber volume (L) corrected for displacement by substrate, S is surface area of the substrate pan (0.028 m2 ), and T is the length of the incubation (h). To correct chamber volume during incubations for displacement by substrate and to characterize substrate composition within each habitat, substrate from each chamber was separated into size groups of 64 mm (cobble) diameter using sieves. The volume of each size class of substrate within each chamber was determined by measuring water displacement. To characterize substrate composition from the areas we sampled, the mean proportion of the total substrate volume from each habitat contained within each substrate size class was calculated for each season. Measurements of NCP and CR from each habitat on a given sampling date (n=6 for NCP per habitat; n=3 for CR per habitat) were averaged to generate mean values of NCP and CR for each habitat on each date. Gross community productivity (GCP) was calculated as the sum of NCP and CR for each habitat on each date. This yielded three replicate values of NCP, GCP and CR per habitat for each season. These data were analyzed for differences in NCP, GCP and CR between habitats and seasons using ANOVA. Tukey’s multiple comparison test was used when ANOVA indicated significance. A p-value of ≤0.05 was considered significant.

Figure 1. Net community productivity in riffles, runs and pools during May (a), July (b), and October (c) in the Jacks Fork River. Values presented are means of three replicate habitats (one habitat from each reach) and associated 95% confidence intervals.

Results Significant differences in NCP were found among all three habitats (p < 0.001; Figure 1). Overall, NCP was highest in riffles, lowest in pools and intermediate in run habitats. NCP was greater in riffles than in pools on every date sampled, but runs were more variable from day to day, exhibiting values from near 0 mg O2 ·m−2 ·h−1 to values characteristic of riffles during a given season. Significant differences in NCP were also detected among seasons (p < 0.001). Overall, NCP declined significantly between July and October. During May, NCP was also significantly lower than in July, but was not significantly different from values measured during October. This trend was evident for both riffles and runs, which had rates of NCP that averaged 2–13 times greater in July than in May or October. However, pools exhibited net heterotrophy on all dates sampled, and showed less change in NCP among seasons than riffles or runs. Significant differences were also observed in CR among habitats, although the differences were less

168 Table 1. Gross community productivity (GCP) (mg O2 /m2 /h) in riffles, runs and pools during May, July and October 1998–1999 in the Jacks Fork River. Values presented are means of three replicate habitats (one habitat from each reach) during each season and associated standard errors Habitat

Riffle Run Pool

May

Month July

October

339 (7) 136 (9) 57 (7)

749 (73) 670 (89) 111 (41)

315 (14) 137 (46) 30 (4)

Figure 2. Community respiration in riffles, runs and pools during May (a), July (b), and October (c) in the Jacks Fork River. Values presented are means of three replicate habitats (one habitat from each reach) and associated 95% confidence intervals.

pronounced than for NCP (p < 0.05; Figure 2). Overall, CR was significantly greater in riffles compared to pools. Runs had intermediate rates of CR, but mean CR in runs was not significantly less than mean CR in riffles nor significantly greater than mean CR of pools. Trends in CR among seasons were similar to those observed for NCP. Rates of CR were significantly higher in July than during the other two months (p < 0.05) but were not significantly different for October and May. This seasonal trend was evident for all three habitat types. Mean GCP ranged from 30 mg O2 ·m2 ·h−1 for pools in October to 749 mg O2 ·m2 ·h−1 for riffles in July (Table 1). Trends in GCP were similar to those observed for NCP. Overall, GCP was highest in riffles, lowest in pools and intermediate in runs. Rates of GCP were highest in July, but were not significantly different for October and May. Substrate composition was similar within each habitat across all three seasons (Figure 3). Pool substrates consisted mainly of small gravel and sand, while large pebble usually comprised