Contrasting effects of sand burial and exposure on invertebrate colonization of leaves.
Abstract: Leaf detritus in streams fills dual resource roles as habitat and as food. Unless retained by some structural component, detritus gets transported to downstream reaches out of the local system. Low gradient sandy-bottomed streams retain leaf' detritus via burial in sand, but this mechanism of retention limits the availability of detritus as a resource for the benthic community. We hypothesized that burial of leaf litter in sand would impact invertebrate colonization by reducing density and richness on leaf litter. We conducted a short-term experiment (i.e., 2 wk) in a sandy-bottom stream in which leaves were subject to either burial in sandy substrate, exposure to the water column, or a sequential combination of both. Results showed that 2 wk burial of leaf litter significantly impacted the colonization of benthic invertebrates. Burial or exposure status of leaves at the time of collection represented the major factor influencing invertebrate abundances on leaf litter. Leaves exposed to the water column had the highest abundance of invertebrates, dominated by collector- gatherers, that suggests the primary role of leaf litter as refugia in this system. Burial of leaf litter in sand had a significantly negative effect on invertebrate colonization of leaf litter. Furthermore, no difference existed in invertebrate colonization on leaf litter that had never been buried versus leaf litter that had been buried for 1 wk and then exposed and collected after a week in the water column. This suggests short-term burial of leaf litter does not influence colonization by invertebrates once leaf litter is exposed to the water column. The results of this study suggest that the benthic colonization on newly exposed leaf litter is rapid, potentially due to a lack of habitat structure availability in the sandy-bottomed stream.
Article Type: Report
Subject: Leaves (Environmental aspects)
Invertebrates (Environmental aspects)
Authors: Scott, Susanna E.
Zhang, Yixin
Pub Date: 01/01/2012
Publication: Name: The American Midland Naturalist Publisher: University of Notre Dame, Department of Biological Sciences Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Earth sciences Copyright: COPYRIGHT 2012 University of Notre Dame, Department of Biological Sciences ISSN: 0003-0031
Issue: Date: Jan, 2012 Source Volume: 167 Source Issue: 1
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 278400280
Full Text: INTRODUCTION

In-stream structural properties, including substrate type and woody debris availability, affect organic matter retention, which in turn, influences benthic community dynamics (Bunn and Davies, 2000). Leaf litter retained by instream structures increase habitat patches and food availability (Anderson et al., 1978), which can enhance invertebrate diversity (Richardson and Neill, 1991; Lemly and Hildebrand, 2000; Zhang et al., 2003). Amalgamations of retained leaf litter also provide benthic invertebrates refugia from predators (Johnson et al., 2003). Much research has been conducted concerning the retention and processing of leaf litter in high-gradient, cobble- and gravel- dominated streams (Peterson and Cummings, 1974; Webster et al., 1994; Kobayashi and Kagaya, 2005), but low gradient sandy-bottomed streams have garnered considerably less attention (Metzler and Smock, 1990; Yamamuro and Lamberti, 2007).

Low gradient, sandy-bottomed streams commonly occur across the Gulf Coastal Plain in the Southeastern United States. Unlike cobble- and gravel-dominated streams, in which the substrate provides physical structure for organic matter retention and influence breakdown rate (Hoover et al., 2006), sandy-bottomed streams have highly mobile substrate and little stable structure for retaining organic matter (Webster et al., 1994; Jones, 1997). In these systems, litter retaining woody debris can be ephemeral and infrequent (Roeding and Smock, 1989; Jones, 1997). Due to this lack of instream structure for primary producers (e.g., algae), benthic invertebrate consumers may be reliant on detrital inputs as the primary basal resource (Roeding and Smock, 1989) and as habitat (Johnson et al., 2003). Disturbances, such as storms and anthropogenic activity, can lead to sediment transport that may entrain and bury organic matter (Smock, 1990; Schofield et al., 2004), possibly limiting its availability to invertebrate colonizers. The dynamic shifting of sand buries organic matter under the stream bed and espouses organic matter from sand, providing a unique mechanism for organic matter storage and retention in sandy streams. However, storage of detritus by burial reduces its availability to the benthic biota (Smock, 1990).

Leaf litter retention via sand often occurs dynamically, in which leaf litter gets buried and re-exposed within a short time frame. When bedload shifting storms occur in short temporal succession, the mechanical and biotic processing of the leaf litter as it is alternately buried and exposed likely influences invertebrate utilization of leaf litter. For instance, it is likely leaves conditioned in the water column may be a preferred resource to leaf litter conditioned while buried beneath the sand, even after re-exposure to the water column (Herbst, 1980). Furthermore, the mechanical abrasion of leaf litter subject to repeated changes in burial or exposure state may fragment leaf litter, increasing its decomposition rate and availability to collector-gatherer invertebrates (Boling et al., 1975; Webster and Benfield, 1986; Heard et al., 1999).

Several studies have investigated differences in invertebrate colonization and leaf litter breakdown of buried versus unburied leaf litter (Mayack et al., 1989, Smith and Lake, 1993; Tilman et al., 2003). Nevertheless, to the best of our knowledge, no studies in sandy-bottomed streams have explored invertebrate dynamics on leaf litter subject to shifts between burial and exposed states. The dynamic process of leaf burial and exposure may influence invertebrate colonization dynamics. For brevity, we use the term "burial/ exposure shifts" to include either burial and subsequent exposure to the water column or exposure to the water column followed by burial in sand.

We conducted a field experiment to investigate effects of the short-term periodic covering and uncovering of leaf litter by sand on the colonizing invertebrate community structure. We tested the influence of short term changes in burial/exposure state due to sand movement on invertebrate colonization on leaf litter in a sandy-bottomed stream. Our treatments included experimental leaf bags buried within the sandy substrate (hereafter treatment B), leaf bags exposed in the water column for the first half of the experiment (treatment U) or either buried or exposed for the duration of the experiment (treatments BB and UU). To determine the influence of burial/exposure shifts on leaf pack colonization we also included treatment manipulations in which leaf bags underwent buried/exposed or exposed/buried shifts (treatments BU and UB, respectively) in sand. Of particular interest were differences in colonization. Differences between BU treatments and UU treatments would indicate that short term burial influences colonization dynamics of leaf litter. We hypothesized that short term burial of leaf litter would decrease invertebrate abundances on leaf bags. These manipulations allowed us to examine how such a burial/ exposure shift and temporal variation between the treatments influenced invertebrate colonization on leaf detritus during a short period under a hypothetical storm scenario, simulating conditions during which leaf litter might be subject to changes in burial state.

METHODS

This study was conducted in Clear Creek (30[degrees]05'44.33"N, 96 [degrees]03'28.77"W), a second-order stream and a tributary of the Brazos River (Waller County, Texas, USA). This stream was approximately 3-5 in wide with variable flow. Sandy substrates were highly mobile. The 34 m study reach was classified from one downstream pool tail to the next upstream pool tail. The study was conducted in the "run" section of this reach. Water quality parameters were measured on each sampling date and intermittently between sampling dates (Table 1). American elm (Ulmus americana), sycamore (Plantus occidentalis), sweetgum (Liquidamber styraciflua), loblolly pine (Pinus taeda), and herbaceous vegetation dominated the riparian vegetation.

Dominant invertebrate taxa in Clear Creek included Diptera (Chironomidae and Simulidae), Ephemeroptera (Baetidae and Leptohyphidae), Trichoptera (Hydropsychidae) and small bivalves (i.e., Corbicula). Collector-gatherers represented the most abundant functional feeding group in Clear Creek, with mean densities of 5608.8 ([+ or - ]1 SE: [+ or -]1076.0) ind. [m.sup.-2]. Scraper and filter feeder mean densities were 284.6 ([+ or - ]158.4) ind. [m.sup.-2] and 260.0 ([+ or -]160.5) ind. [m.sup.-2]. Shredders were the least abundant functional feeding group in Clear Creek, with only 4.7 ([+ or -]4.7) ind. [m.sup.-2].

This short-term study took place during 2 wk in January 2009. We collected naturally fallen, American elm (Ulmus americana) leaves on site from the dry sandy bank of Clear Creek. Leaves used to stuff leaf bags were transported back to the laboratory and dried at 60 C for 48 h to a constant weight. To obtain initial dry mass, leaf bags (20 cm x 20 cm) were constructed with large aperture plastic mesh (3 cm mesh) and contained 2-3 elm leaves (mean [+ or -] 1 SE: 1.21 g [+ or -] 0.02). Although this is smaller than leaf bags generally used in colonization studies (e.g., Leroy and Marks, 2006; Entrekin et al., 2008; Tiegs et al., 2008), our pilot studies collected as many as 1800 invertebrates on leaf bags of this size, so they were deemed of adequate size for colonization.

We secured leaf bags to planks of wood measuring 1.5 m in length. Planks were staked to the substrate, with flow moving horizontally over each leaf pack. Each randomly assigned treatment was tied to a different plank of wood and had five replicates. Leaf bags were collected on days 7 and 14. This experiment consisted of six treatments: (1) leaf bags completely buried approximately 15 cm in depth within the stream substratum for 1 wk (treatment B); (2) unburied leaf bags, which were exposed to the water column for 1 wk (U); (3) leaf bags completely buried for the entire 2 wk duration of the experiment (BB); (4) leaf bags left exposed in the water column for the 2 wk duration of the experiment (UU); (5) leaf bags left exposed for the first half of the experiment, and buried for the second half of the experiment (UB); and (6) leaf bags buried for the first half of the experiment, and uncovered and exposed for the second half of the experiment (BU). The burial depth was within the burial-depth range through sandy substrate movement after a flood or storm event in this area (pers. obs.). Burial state changes for treatments UB and BU took place after 1 wk.

On each sampling date, water current velocity was measured at each leaf pack as well as water temperature and temperature within the substrate. Mean water velocities on exposed leaf bags were 26.5 ([+ or -]1.2) cm [s.sup.-1]. Mean water temperature at sampling times was 13.3 ([+ or -] 3.0[degrees]) C. Mean sand temperature in the substrate at the time of sampling was 11.0 ([+ or -] 1.5[degrees]) C. Leaf bags were collected individually, with a 250 [micro]m sieve positioned immediately downstream of the leaf bag, to capture any invertebrates knocked loose during the removal of the leaf pack. To collect bags buried in sand, the top layer of sand was first gently brushed away from the leaf pack before using the methods mentioned above. To capture any invertebrates inadvertently removed from leaf bags during the sand removal process, the several centimeters of sand immediately covering buried leaf packs was brushed into the 250 [micro]m sieve. This method might increase the probability of including invertebrates associated with sand, and not our leaf bags, into samples. All leaf bags were transferred on ice in separate containers back to the laboratory for processing. Leaf bags were gently washed with tap water over a 250 [micro]m sieve, and all invertebrates were removed and preserved in 95% EtOH for identification. All invertebrates were identified to the lowest practical taxonomic level for the determination of functional feeding group (Merritt et al., 2007). Leaves collected from each treatment were dried at 60 C for at least 48 h, and reweighed to determine percent mass reduction (percentage difference of initial total dry mass and final total dry mass of the leaf pack) and exponential decay rate (-k).

To fully investigate the influence of burial/exposure shifts on invertebrate community composition in leaf bags, we examined taxa composition, which included total abundances of invertebrates in all leaf bags, abundances of dominant taxa, and total taxa richness as well as functional feeding group abundances and richness in all leaf bags. In an attempt to meet all assumptions of normality and homoscedasticity, data were log transformed and analyzed with a one-way analysis of variance (ANOVA) (Zar, 1999). Data with significantly different treatment effects (P < 0.05) were then analyzed post hoc with a Tukey's HSD (Honestly Significant Difference) test to determine similarity among treatment means. Percent dry mass remaining of leaf bags was analyzed with a one-way analysis of covariance (ANCOVA) using sampling date as a covariate, to examine the influence of time in our analysis. Leaf litter exponential decay (-k) was modeled after Peterson and Cummings (1974), using the equation [M.sub.t] = [M.sub.0] x [e.sup.-kt]. This was calculated from Mt, the dry mass at time t and Mo, the original dry mass. All statistics were performed using SPSS 15.1 for Windows (SPSS Inc., Chicago, IL).

RESULTS

Invertebrate abundance and taxa richness differed depending on the treatments of leaf bags. We found significant effects of sand burial on invertebrate composition in leaf bags significant in the total abundance and total richness of all invertebrates, functional feeding groups' abundance and richness (except shredder abundance and richness), and major taxa abundance (Table 2). Total abundance of invertebrate individuals on leaf bags ranged from 15.4 [+ or -] 1.5 ind. (mean [+ or -] 1 SE) in UB treatment to 2057.8 [+ or -] 387.2 ind. in UU treatments (Fig. la). Presence of Chironomidae larvae, the most abundant taxa in all treatments, primarily drove difference in overall abundance and functional feeding group abundances (Fig. 2). Collector-gatherers were the most abundant functional feeding group in all leaf bags, for all six treatments, due to the high abundances of collector-gatherer Chironomidae (Chironominae and Orthocladiinae). In both burial treatments (B and BB), the abundance of Chironomidae was higher than all other taxa groups. Filter feeder abundances were high, due to high abundances of Simulidae in treatments exposed to the water column. Predator abundances were low in buried leaf bags compared to predators in exposed leaf bags (U). This was primarily due to the presence of predator Chironomidae (Tanypodinae). Scraper abundances across all treatments were low. Shredder densities were low as well, with only three individual shredders, all Amphipoda, found among all leaf bags.

Bags that were buried had significantly fewer invertebrates ([F.sub.1,5] = 64.728, P < 0.001) and invertebrate taxa (Table 2). Tukey's HSD tests revealed that invertebrate colonization on leaf bags depended on burial state of the leaf pack at the time of sample collection. Leaf bags which were buried at the time of collection (B, BB, and UB) generally did not significantly differ. Likewise, leaf bags which were exposed at the time of collection (U, UU, and BU) also usually did not significantly differ in invertebrate abundance (Figs. 1, 2). There were two exceptions to these groupings. Collector-gatherer abundances and predator richness each divided into three subsets. However, they followed the same general patterns of all other functional feeding groups, with treatments buried at the time of collection having significantly lower abundance (Collector-gather abundance; Fig. 2a) or richness (Predator richness; Fig. 2g) than treatments that were exposed at the time of collection.

There was a significant effect of time ([F.sub.1,24] = 6.110, P = 0.021) on percent leaf litter remaining for all treatments. However, there was no detectable treatment effect on leaf litter breakdown (ANCOVA, [F.sub.3,24] = 1.223, P = 0.323) during the 2 wk experimental period. Mean mass reduction for all leaf bags in all treatments was 9.13% dry mass after 7 d and 14.64% dry mass after 14 d. Mean decomposition rate (-k) for all treatments was calculated as 0.011 [+ or -] 0.0010.

DISCUSSION

This experiment revealed a significantly negative effect on invertebrate colonization of sand burial of leaf detritus (Rulik et al., 2001; Crenshaw and Valett, 2002; Tillman et al., 2003). Buried leaf bags were colonized by few invertebrates and unburied leaf bags were colonized by more invertebrates. Densities of primary consumers shifted dramatically in response to burial status of leaf bags upon time of collection. The buried or exposed status of the leaves at the time of collection was the primary factor influencing invertebrate community composition. This supports the prediction that burial in sandy substrate decreases invertebrate taxon richness and densities on leaf detritus. However, this also leads to the preliminary conclusion that burial in sand does not influence invertebrate utilization of leaf litter, once it is exposed to the water column.

[FIGURE 1 OMITTED]

Shifts in burial state did not show any effect on leaf breakdown rate during the experiment, which we attribute to the short experimental time and winter temperatures. American elm leaves are considered a rapidly decomposing species of leaf litter (Findlay and Arsuffi, 1989). Our decomposition rates occupy the low end of the range of American elm decomposition rates in the literature (0.01 [less than or equal to] k [less than or equal to] 0.06; Gazzera et al., 1993; Eichem et al., 1993; Tate and Gurtz, 1986; Sangiorgio et al., 2010). Winter stream temperatures may have contributed to the low (for elm) decomposition rates of the leaf litter in this experiment (Mayack et al., 1989; Tank et al., 1993) as well as the lack of burial effect on decomposition rates. Mayack et al. (1989) found that during the winter portion of a leaf burial experiment, the breakdown rate of buried leaf litter of sweetgum (Liquidamber styraciflua) did not differ significantly from the leaf litter suspended in the water column. Alternativly, in the spring, decomposition of buried leaf litter occured significantly slower than exposed leaf litter decomposition (Mayack et al., 1989). Other studies have shown decreased decomposition rates of buried leaf litter in comparison to exposed litter (e.g., Metzler and Smock, 1990; Smith and Lake, 1993; Tillman et al., 2003). However, during a naturally occurring bedload moving event, physical fragmentation of leaf litter due to shear stress and sand abrasion would likely increase leaf litter decomposition rates. While we took care not to increase leaf fragmentation through handling when shifting leaf burial state, our experiment could not address decomposition due to the mechanical factors that would be present during a natural storm scenario.

[FIGURE 2 OMITTED]

Additionally, colonization dynamics of leaf litter did not influence leaf breakdown rate. Though invertebrate abundance and richness significantly differed between treatments, this was not reflected in leaf litter decomposition rates. This indicates that the leaf litter is primarily utilized as habitat, rather than a food resource. If leaf litter were the primary food source for colonizing invertebrates, we would have expected higher leaf litter decomposition rates in treatments with higher invertebrate densities, particularly higher shredder densities. However, shredder densities were low in all treatments. In addition, scraper densities were influenced by leaf burial. Significantly higher scraper density and richness occurred in unburied treatments, indicating utilization of microbes on leaf litter as a food source. It is likely that 2 wk is too short of a time for leaf litter to be conditioned adequately for shredder consumption, but 2 wk appears as a short enough timeframe for sufficient microbial growth for scraper consumption (Hieber and Gessner, 2002).

Differential conditioning of leaf litter has been shown to influence detrital palatability and influence invertebrate colonization dynamics (Peterson and Cummings, 1974; Chauvet et al., 1993). For this reason, we were particularly interested in the comparison between BU treatments and UU treatments. However, we did not observe colonization differences between BU and UU treatments. It is possible that had our treatments been conducted over a longer period of time, e.g., had BU treatments been conditioned under the sandy substrata for several weeks before being exposed to the water column; treatments might have yielded different results. Conditioning of leaves under the sandy substrata may alter their quality as a food source. Herbst (1980) found that invertebrate shredders consumed greater amounts of surface incubated leaf litter than leaf litter conditioned while buried in sand, which was not reflective of the nutrient quality of the leaf litter. Leaves buried in sand generally had higher caloric and organic content than leaf litter incubated in the water column. However, this may vary with leaf species and season. Carbon to nitrogen ratios (C:N) of sweetgum leaf litter did not differ between litter buried in sand and litter in the water column during winter months but did during the spring when burial slowed the reduction of C:N ratios (Mayack et al., 1989). In that particular study, the difference in C:N ratios during the spring could be due to enhanced microbial conditioning of leaf litter in the water column but not on buried litter (Mayack et al., 1989).

Invertebrate colonization depended on whether leaves were buried or exposed at the time of collection, but colonization was not influenced by treatment (buried or exposed) of leaves during the first week of the experiment. There was no significant difference in taxa and functional feeding group abundance and richness in leaves that had been buried and subsequently exposed and leaves that remained exposed. This suggests that colonization rates of unburied leaf litter were relatively rapid. We speculate that the subsurface community in sandy streams had lower densities and diversity than the surface (e.g., benthic) community due to low dissolved oxygen (Whitman and Clark, 1984; Strommer and Smock, 1989). Few invertebrates are adapted to these conditions, but chironomids, nematodes, and crustaceans usually represent the dominant taxa in sandy subsurface zones (Poole and Stewart, 1976; Jeffery et al., 1986; Tilman et al., 2003; Yamamuro and Lamberti, 2007). Buried leaf litter in our study was primarily colonized by chironomids as well as a few simuliids and ephemeropterans (Family Baetidae). Though subsurface dissolved oxygen (DO) of our experiment was not measured, we infer that potential low DO in the sandy subsurface, due to small pore space could be one of factors to cause the low diversity of taxa present on buried leaf litter.

Allochthonous inputs provide both habitat and food to stream inhabitants (Richardson, 1992, Wallace et al., 1997). Little evidence existed that leaf detritus in our experiment functioned as a food source for shredder consumers. Our results showed high abundances of collector-gatherers and filter feeders compared to shredder abundances. New leaf litter most likely offers structure and refugia for drifting invertebrates (Richardson, 1992). We did not examine bag effects in this experiment. It is possible that some invertebrate colonization was associated with the mesh bags. However, most of the colonizing invertebrates were found within the leaf detritus. The results of this study suggest that the benthic colonization on newly exposed leaf bags is rapid, potentially due to a lack of habitat structure availability in the sandy-bottomed stream. While retention of leaf litter in debris dams may create an infrequent patchy distribution of organic matter within the stream, leaf litter stored beneath the sandy benthos is less spatially and temporally variable (Smock, 1990). The shifting of the sand influences the availability of this resource to invertebrates.

Leaf litter inputs often occur in large pulse events, with high temporal variability, which supply a dynamic heterogeneous resource and habitat for benthic invertebrates (Moore et al., 2004). While sandy-bottomed streams generally have little surface structure to retain organic inputs, resulting in a lower surface retention of organic matter in comparison to streams with cobble substrates (Jones and Smock, 1991; Jones, 1997), the burial of organic matter in sandy-bottomed streams may be substantial. Metzler and Smock (1990) estimated that over 20% of the annual litter fall entering sandy-bottomed streams were covered and retained by sand. This creates the dynamic of a system able to store organic matter, but the organic matter is largely unavailable to the benthic community. We found that invertebrates rapidly colonized leaf litter once it was exposed in the water column, regardless of the burial of leaves during the first week of the experiment. However, the consequences and ecological impacts of short-term storage of leaf litter in the sandy substrate remain poorly understood. While this study addressed the immediate, small scale impacts of short term storage, the resulting detrital dynamics of short term burial may have larger ecosystem level effects.

Acknowledgments.--We would like to thank Francis Lash and Diego Araujo for their assistance in the field. We would also like to thank Drs. Weston Nowlin and Timothy Bonnet for their helpful notes on the manuscript. We also would like to thank the anonymous reviewers and the Associate Editor, Romi Burks, whose comments and suggestions greatly helped improve the clarity and quality of the manuscript.

SUBMITTED 4 NOVEMBER 2010

ACCEPTED 29 JULY 2011

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SUSANNA E. SCOTT (1) AND YIXIN ZHANG (2)

Department of Biology, Texas State University, San Marcos 78666

(1) Present address: Department of Zoology, Miami University, Oxford, Ohio 45056

(2) Corresponding author: e-mail: yz11@txstate.edn; Telephone: (512) 245-3552
TABLE 1.--Water quality parameters in Clear Creek
during experiment

Parameter                Mean [+ or -] SE

Depth (cm)               13.1 [+ or -] 3.0
Water velocity (cm       26.6 [+ or -] 1.0
 [s.sup.-1])
DO (mg [L.sup.-1])       8.83 [+ or -] 1.3
pH                       7.63 [+ or -] 0.05
Specific conductivity     406 [+ or -] 3.0
 ([mu]s [cm.sup.-1])

TABLE 2.--Results from one-way ANOVA for invertebrate community
composition in leaf bags. All abundance data were analyzed as
number of individuals in each leaf pack. Taxon richness was
analyzed at the family level

                                df      F        P

Total abundance                 5     64.728   <0.001
Total taxon richness            5     66.822   <0.001
Collector-gatherer abundance    5     61.387   <0.001
Filter-feeder abundance         5     42.021   <0.001
Predator abundance              5     60.411   <0.001
Scraper abundance               5     46.897   <0.001
Shredder abundance              5      0.630    0.678
Collector-gatherer richness     5     29.274   <0.001
Filter-feeder richness          5     25.504   <0.001
Predator richness               5     47.199   <0.001
Scraper richness                5     24.092   <0.001
Shredder richness               5      0.600    0.700
Diptera abundance               5     63.437   <0.001
Ephemeroptera abundance         5     81.732   <0.001
Trichoptera abundance           5    150.043   <0.001
Plecoptera abundance            5     24.990   <0.001

[alpha] = 0.05
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