Contrasting Effects of Sand Burial and Exposure on Invertebrate Colonization of Leaves

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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.

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 Utter 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-grathent, cobble- and gravel- dominated streams (Peterson and Cummings, 1974; Webster et al, 1994; Kobayashi and Kagaya, 2005), but low grathent 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 andiropogenic 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. …