Changes in invertebrate assemblage composition in benthic and hyporheic zones during a severe supraseasonal drought

Droughts are unpredictable disturbances characterized in streams by declining flow, reduced habitat availability, and deteriorating abiotic conditions. Such events typically reduce benthic invertebrate taxon richness and modify assemblage composition, but little is known about how hyporheic invertebrate assemblages respond to drought or how these responses relate to changes in benthic assemblages. We hypothesized that taxon richness (diversity) and variability (as within-site diversity) in benthic assemblage composition would decline as drought proceeded, whereas concurrent changes in hyporheic assemblages would be lower in this more stable environment. We predicted that benthic assemblage composition between sites would converge as epigean taxa were selectively eliminated, whereas between-site hyporheic diversity would change little. We sampled benthic and hyporheic invertebrates concurrently from 4 sites along a groundwater-fed stream during the final stages of a severe supraseasonal drought punctuated by a record heat wave. Abiotic conditions in benthic habitats deteriorated as flow declined, but changes were less pronounced in the hyporheic zone. Benthic diversity declined during drought, whereas hyporheic diversity changed little. However, benthic within-site diversity increased as the drought progressed because of localized variation in the abundance of common taxa. Temporal trends in hyporheic diversity were less consistent. Benthic assemblages at individual sites became more similar, especially during the heat wave, reflecting low diversity and abundance. Hyporheic assemblages changed markedly because of temporary increases in abundances of epigean and hypogean amphipods. These contrasting responses of benthic and hyporheic assemblages to drought should be recognized when developing management strategies for drought-impacted streams

Efficient indexing of spatiotemporal objects

Abstract — Spatiotemporal objects i.e., objects which change their position and/or extent over time, appear in many applications. This paper addresses the problem of indexing large volumes of such data. We consider general object movements and extent changes. We further concentrate on “snapshot ” as well as small “interval ” historical queries on the gathered data. The obvious approach that approximates spatiotemporal objects with MBRs and uses a traditional multidimensional access method to index them is inefficient. Objects that “live ” for long time intervals have large MBRs which introduce a lot of empty space. Clustering long intervals has been dealt in temporal databases by the use of partially persistent indices. What differentiates this problem from traditional temporal indexing is that objects are allowed to move/change during their lifetime. Better methods are thus needed to approximate general spatiotemporal objects. One obvious solution is to introduce artificial splits: the lifetime of a long-lived object is split into smaller consecutive pieces. This decreases the empty space but increases the number of indexed MBRs. We first introduce two algorithms for splitting a given spatiotemporal object. Then, given an upper bound on the total number of possible splits, we present three algorithms that decide how the splits should be distributed among the objects so that the total empty space is minimized. I

Red-Black Prefetching: An Approximation Algorithm for Parallel Disk Scheduling

. We address the problem of I/O scheduling of read-once reference strings in a multiple-disk parallel I/O system. We present a novel online algorithm, Red-Black Prefetching (RBP), for parallel I/O scheduling. In order to perform accurate prefetching RBP uses L-block lookahead. The performance of RBP is analyzed in the standard parallel disk model with D independent disks and a shared I/O buffer of size M . We show that the number of parallel I/Os performed by RBP is within a factot \Theta(maxf p MD=L;D 1=3 g) of the number of I/Os done by the optimal offline algorithm. This ratio is within a canstant factor of the best possible when L is L = O(MD 1=3 ). 1 Introduction Continuing advances in processor architecture and technology have resulted in the I/O subsystem becoming the bottleneck in many applications. The problem is exacerbated by the advent of multiprocessing systems that can harness the power of hundreds of processors in speeding up computation. Improvements in I/O tech..