Fifty years of hyporheic zone research have shown the important role played by the hyporheic zone as an interface between groundwater and surface waters. However, it is only in the last two decades that what began as an empirical science has become a mechanistic science devoted to modeling studies of the complex fluid dynamical and biogeochemical mechanisms occurring in the hyporheic zone. These efforts have led to the picture of surface-subsurface water interactions as regulators of the form and function of fluvial ecosystems. Rather than being isolated systems, surface water bodies continuously interact with the subsurface. Exploration of hyporheic zone processes has led to a new appreciation of their wide reaching consequences for water quality and stream ecology. Modern research aims toward a unified approach, in which processes occurring in the hyporheic zone are key elements for the appreciation, management, and restoration of the whole river environment. In this unifying context, this review summarizes results from modeling studies and field observations about flow and transport processes in the hyporheic zone and describes the theories proposed in hydrology and fluid dynamics developed to quantitatively model and predict the hyporheic transport of water, heat, and dissolved and suspended compounds from sediment grain scale up to the watershed scale. The implications of these processes for stream biogeochemistry and ecology are also discussed.

Hyporheic flow and transport processes: Mechanisms, models, and biogeochemical implications

Assessment of the condition of ecosystems is a critical prerequisite for alleviating effects of the multiple anthropogenic stresses imposed on them. For stream ecosystems, a multitude of approaches has been proposed for this purpose. However, they all rest on the assessment of structural attributes, even though it is generally recognized that adequate characterization of ecosystems requires information on both structure (pattern) and function (process). Therefore, we propose a complementary approach to stream assessment based on evaluating ecosystem level processes. Leaf litter breakdown is a prime candidate to consider in this context. This is because of the pivotal role that allochthonous litter plays in streams, the demonstrated effects of anthropogenic perturbations on litter breakdown, and the relative ease of implementation. Leaf breakdown is governed by a variety of internal and external factors that complicate the partitioning of effects due to anthropogenic stress and natural variability (background noise), thus potentially limiting the sensitivity and robustness of litter breakdown assays. However, internal regulation factors can be controlled by standardizing assessment procedures, while variability due to external factors can be accounted for by stream classification and/or a comparative approach (e.g., downstream-upstream comparisons). Composite parameters such as ratios of break- down rates in fine-mesh and coarse-mesh bags may further increase the power of litter breakdown assays. Analyses may also be extended to include both leaf-associated decomposer assemblages (i.e., structural measures) and processes (i.e., additional functional measures). Significant efforts are required for developing standard assessment schemes as refined as extant procedures based on structural stream attributes (e.g., structure of macroinvertebrate assemblages). These efforts are nevertheless worthwhile in view of the new dimension that is added to current assessment procedures when functional elements are incorporated.

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A case for using litter breakdown to assess functional stream integrity

Introduction Assessments of environmental conditions in rivers and streams with diatoms have a long history, which has resulted in the development of the two basic conceptual and analytical approaches used today. First, based on the work of Kolkwitz and Marsson (1908), autecological indices were developed to infer levels of pollution based on the species composition of assemblages and the ecological preferences and tolerances of taxa (e.g., Butcher, 1947; Fjerdingstad, 1950; Zelinka & Marvan, 1961; Lowe, 1974; Lange-Bertalot, 1979). Second, Patrick's early monitoring studies (Patrick, 1949; Patrick et al ., 1954; Patrick & Strawbridge, 1963) relied primarily on diatom diversity as a general indicator of river health (i.e., ecological integrity), because species composition of assemblages varied seasonally and species richness varied less. Thus, the concepts and tools for assessing ecosystem health and diagnosing causes of impairment in aquatic habitats, particularly rivers and streams, were established and developed between 50 and 100 years ago. The many advances in the use of diatoms and other algae for monitoring stream and river quality have been reviewed by Patrick (1973) and, more recently, by Stevenson and Lowe (1986), Round (1991), Whitton et al . (1991), Coste et al . (1991), Whitton and Kelly (1995), Rosen (1995), and Lowe and Pan (1996). There are three major objectives for this chapter. First, we emphasize the importance of designing environmental assessments so that many approaches are used and results are based on rigorous statistical testing of hypotheses. Second, we review the many characteristics of diatom assemblages that could be used in assessments and the methods and indices of assessment.

Assessing environmental conditions in rivers and streams with diatoms

Rapid urbanization threatens the biota of streams and rivers around the globe Efforts to manage urban streams traditionally take an engineering approach focused on stormwater runoff, physical channel condition, and chemical water quality Our objective was to use the biology of streams—measured with the multimetric benthic index of biological integrity (B-IBI) based on benthic macroinvertebrates—to assess stream health From 1997 to 1999, we sampled invertebrates at 45 sites in second- and third-order streams in the Puget Sound lowlands of Washington State Land cover upstream of each site was characterized by analy- sis of a 1998 satellite image We evaluated associations between five land cover categories and biological con- dition across three spatial scales The relationships between B-IBI (and its component metrics) and stream substrate and hydrologic features were also analyzed at a subset of sites Across all study sites, B-IBI declined as the percentage of urban land cover increased ( r � � 071, p � 0001, n � 31) Most metrics were better pre- dicted by sub-basin rather than local-scale urbanization Within individual basins, however, local land-cover urbanization and B-IBI were strongly correlated ( r � � 091, p � 0001, n � 9) The biological condition of a site was also related to measures of hydrologic alteration and stream substrate The aquatic biota is sensitive to a variety of urban effects, expressed at both large and small spatial scales Biological assessment tools such as B-IBI can identify areas of excellent biological condition for conservation and guide the design and evalu- ation of efforts to restore the biota of degraded streams

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Assessing and Restoring the Health of Urban Streams in the Puget Sound Basin

Utility and limitations of species richness metrics for conservation planning

Periphyton assemblage data collected from 233 stream site-visits (49 in 1993, 56 in 1994, and 128 in 1995) throughout the Mid-Appalachian region were used to develop a periphyton index of biotic integrity (PIBI) based on 1) algal genera richness; 2) the relative abundances of diatoms, Cyanobacteria, dominant diatom genus, acidophilic diatoms, eutraphentic diatoms, and motile diatoms; 3) chlorophyll and biomass (ash-free dry mass) standing crops; and 4) alkaline phosphatase activity. Thirty-seven diatom genera and 38 non-diatom genera were collected. The relative richness and relative abundance (RA) of these genera were used to calculate the RA metrics of the PIBI. PIBI scores ranged from 48.0 to 85.1 among the 233 site-visits with an overall regional mean (±1 SE) of 66.1 ± 0.5. The 10 metrics and the PIBI were correlated with 27 chemical, 12 physical habitat, and 3 landscape variables. Overall, PIBI was inversely correlated with stream depth, stream water color, and Fe. Component metrics were sign...

C. Burch Johnson

Papers

Use of periphyton assemblage data as an index of biotic integrity