Small headwater streams have a suite of physical eccentricities that distinguishes them from the rest of the river system. These differences are now recognized in the study of sediment transport and channel morphology, but their implications for other limnological disciplines are less apparent. We suggest a zonation scheme for the upper stream system that acknowledges obvious geomorphic boundaries while highlighting differences in fine-scale habitat features that are likely to be biologically relevant. The upstream heterogeneous zone (UHZ) is distinguished by a high ratio of structural component size to stream width. Structural components include large rocks exposed from the regolith or from colluvial sources, tree roots, and woody debris, all of which are stochastically distributed and can constrain the morphology, hydraulics, and habitat distribution of small headwater streams. The high ratio of structural component size to stream width is a geomorphic phenomenon linked to the stream's lack of competence to move the material that forms its bed and banks. It follows that the incompetence of streams within the UHZ is ultimately responsible for their greater internal physical heterogeneity than downstream reaches and is a fundamental driver of their physical structure, dynamics, and possibly ecology. Benign hydraulic conditions in small headwater streams have the potential to uncouple the link between physical and biological heterogeneity, resulting in a faunal community composed of highly mobile generalists.

The spiraling concept provides an explicit approach to modeling the longitudinal linkages within a river continuum. I developed a spiraling-based model for particulate organic C dynamics in the Little Tennessee River to synthesize existing data and to illustrate our current understanding of ecosystem processes in river ecosystems. The Little Tennessee River is a medium-sized river flowing ∼100 km through the southern Appalachian Mountains of northern Georgia and western North Carolina (USA). Across this distance, allochthonous inputs decrease and autochthonous production increases, resulting in a U-shaped curve of energy input. The model was set up as an advecting seston compartment interacting with 3 benthic compartments: coarse benthic organic matter, fine benthic organic matter, and autotrophs. Model-estimated ecosystem respiration was consistently lower than measured values, suggesting a need to evaluate our measurements of whole-stream metabolism. Also, model-predicted seston concentrations were generally lower than measured values, reflecting a need to consider additional sources of organic C in the model. For the whole river system, leaves accounted for 19% of inputs, primarily near the headwaters, and the remaining input was from instream primary production in the lower reaches of the river. Almost ½ of the input was respired, 28% by autotrophic respiration and 21% by heterotrophic respiration, and the remaining 51% was transported downstream. Ecosystem efficiency was ∼50% along the length of the river, and turnover length increased from several hundred meters at the headwaters to >100 km downstream. Based on various measures, the transition from heterotrophy to autotrophy ranged from 25 to >100 km downstream from the headwaters. As this model illustrates, a consequence of downstream transport is that much of the particulate C in streams is metabolized a considerable distance downstream from where it enters the stream. This longitudinal linkage is essential to our understanding of stream ecosystems.

The location of a stream reach relative to other landforms in a watershed is an important attribute. We hypothesized that lakes disrupt the frequency of finer, more mobile sediments and thereby change sediment transport processes such that benthic substrates are more stable (i.e., less mobile) below lakes than above lakes. In turn, we hypothesized that this reduced mobility would lead to greater periphyton biomass below lakes. We tested these hypotheses in study reaches above and below lakes in 3 mountain watersheds. To expand this comparison, we analyzed the relationship between sediment attributes and periphyton biomass in one watershed with and one watershed without a lake. We hypothesized that no clear pattern or change in sediment size or chlorophyll a (chl a) would be observed over a 3-km-long study reach without a lake. In contrast, we expected a clear discontinuity in both sediment size and chl a in a 7-km-long study reach interrupted by a lake. Average median sediment size (D₅₀) was significantly larger (p < 0.01) in lake-outlet than lake-inlet reaches (41 mm vs 10 mm). Bed sediments in lake-outlet reaches were immobile during bankfull flows, whereas sediments at lake-inlet reaches were mobile during bankfull flows. Chlorophyll a was ≥10× greater in lake-outlet reaches than in lake-inlet reaches, although this difference was not statistically significant (p = 0.17). The longitudinal analysis clearly showed geomorphic transitions in sediment size and mobility downstream of mountain lakes, and these geomorphic transitions might be associated with changes in periphyton biomass. Geomorphic transitions can alter sediment transport and should be considered in concert with other factors that are considered more commonly in benthic ecology, such as light, nutrients, and temperature.

The effects of drought on stream invertebrates have been reviewed, but the effects of artificially reduced flows have not. We addressed this knowledge gap by reviewing the literature on the effects of natural low flows and artificially reduced flows (without complete cessation of flow). We considered the effects of low water volume on habitat conditions and on invertebrate community structure, behavior, and biotic interactions. Decreases in discharge usually cause decreased water velocity, water depth, and wetted channel width; increased sedimentation; and changes in thermal regime and water chemistry. Invertebrate abundance increases or decreases in response to decreased flow, whereas invertebrate richness commonly decreases because habitat diversity decreases. Invertebrates differ in their environmental tolerances and requirements, and any loss of habitat area or alteration of food resources from decreased flow can influence organism behavior and biotic interactions. Invertebrate drift often increases immediately after flow reduction, although some taxa are more responsive to changes in flow than others. Natural low flows and artificially reduced flows have similar effects on invertebrates, but the severity (duration and magnitude) of the flow decrease can influence invertebrate responses. Certain invertebrate taxa are especially sensitive to flow decreases and might be useful indicators for reduced flows or flow restoration. The effect of low flow on streams is an important issue, but few empirical studies of the impacts of decreased flow on stream ecosystems have been done, and more manipulative experiments are needed to understand the ecological consequences of decreased flow.

Dissolved organic matter (DOM) is an important component of aquatic ecosystems, and it influences a range of physical, chemical, and biological properties. Reactions induced by solar radiation may oxidize DOM to inorganic C or break large molecules into smaller ones. Therefore, photodegradable DOM is removed with exposure to light, and the remaining DOM pool might become less photodegradable as photorecalcitrant DOM accumulates. This possibility has led to speculation that previous light exposure might influence the susceptibility of DOM to photodegradation and that forested low-light streams might have highly photodegradable DOM. To assess this possibility, we measured: 1) the susceptibility of stream DOM to photoreactions and compared our results to studies in other aquatic ecosystems, 2) the relative importance of the ultraviolet (UV) portion of the solar spectrum to DOM photoreactions, and 3) the photoreactivity of DOM collected from streams with and without upstream lakes. We measured DOM properties of stream water exposed for ∼56 h to 1 of 3 treatments: full sunlight, sunlight with the UV portion of the spectrum (<400 nm) removed, and a dark control. Exposure to light reduced the UV light-absorbing ability of DOM and, to a lesser extent, its concentration. Most alterations of DOM properties could be attributed to the UV portion of the solar spectrum. We found no evidence that previous light exposure significantly influenced photodegradability of stream DOM. Our results suggest that other DOM-processing agents, such as heterotrophic uptake, can obscure the effect of upstream photoexposure on downstream DOM photodegradability.

Leaf-litter inputs provide substrate and energy to stream systems. These contributions vary based on species-specific differences in litter quality, but little is known about how differences in litter quality within a species can affect ecosystem processes. Genetic variation within tree species, such as oaks and cottonwoods, affects ecosystem processes including decomposition and nutrient cycling in forest ecosystems and has the potential to do the same in streams. We collected litter from 5 genotypes of each of 4 different cottonwood cross types (Populus fremontii, Populus angustifolia, and natural F₁ and backcross hybrids), grown in a common garden, and measured their decomposition rates using litter bags in the Weber River, Utah. The proportion of 35 species-specific P. fremontii restriction-fragment length polymorphism markers in the genotype explained 46% and genetically controlled phytochemical mechanisms (e.g., % soluble condensed tannin in litter) explained >72% of the variation in leaf-litter decomposition rate, respectively. Understanding how natural genetic variation in plants can affect ecosystem processes will provide baseline information with which to address the loss of genetic variation (through habitat fragmentation and global change) and altered genetic variation through hybridization with cultivars and transgenic manipulations in the wild.

We tested the hypothesis that algae influence the activities of extracellular enzymes involved in mineralization processes within microbial assemblages in streams. We tested the prediction that the factors that influence algal biomass and photosynthesis (i.e., diel fluctuations in photosynthetically active radiation [PAR], long-term variations in light regime, and community development stage) would have a corresponding effect on extracellular enzyme activities. We also tested the prediction that algae would influence enzyme activities on inorganic substrata and in detrital communities where they ultimately would influence plant litter decomposition rates. We allowed microbial communities to develop on inert substrata (glass-fiber filters) or on leaf litter in artificial streamside channels. For each community type, we examined the effects of long-term light manipulations, community development stage, and diel periodicity on the activities of β-glucosidase, alkaline phosphatase, leucine-aminopeptidase, and phenol oxidase. In addition, we measured the decomposition rates of the leaf litter substrata in the low- and high-light treatments. Our results support the prediction that factors that influence algal photosynthesis and biomass in the short (diel fluctuations in PAR) and long (shading, community development stage) term ultimately influence enzyme activities in microbial communities associated with both inorganic substrata and detritus. Furthermore, decomposition rates of organic detritus probably are enhanced by algal colonization and activity. Algal photosynthesis might enhance redox and pH conditions within microbial communities, and in turn, might increase the activities of oxidative and hydrolytic enzymes. As a consequence, photoautotrophic activities might stimulate heterotrophic pathways in stream ecosystems by creating conditions favorable for decomposition of both dissolved and particulate organic detritus.

Widespread declines in biodiversity at both global and local scales have motivated considerable research directed toward understanding how changes in biological diversity may affect the stability and function of the ecosystems on which we rely. However, the research effort devoted to addressing this question in benthic systems has been minimal. In laboratory microcosms, we manipulated the number and composition of 3 functionally distinct benthic invertebrate freshwater species that are bioturbators of sediment over 3 biomass levels. Our objective was to test the effects of bioturbator diversity on rates and reliability of total dissolved P (TDP) flux between benthic and pelagic habitats. Both composition and species richness affected TDP flux. TDP flux was highest in the most species-rich community because of functional complementarity rather than selection effects. Furthermore, species richness enhanced TDP flux reliability by increasing the predictability of the biomass–TDP flux relationship by 30%, on average, for each species added. We attributed these nonadditive effects of invertebrate diversity to a combination of functionally mediated biogeochemical interactions and density-mediated interaction strength. Thus, our results suggest that bioturbator diversity can be important to nutrient cycling in aquatic ecosystems by strengthening benthic–pelagic coupling.

Floodplains of unregulated rivers alternate between aquatic and terrestrial phases, resulting in temporary aquatic habitats. If aquatic invertebrates are to take advantage of such habitats, they must be capable of rapid colonization and growth. Aerial immigration is one means of colonization and may include active dispersal by reproductive and nonreproductive adult insects and passive dispersal of other invertebrates. The aquatic invertebrate assemblage that could develop from aerial colonization during the time of a flood was investigated with floating colonization trays containing natural detritus and water on the water surface of a southeastern USA floodplain during 5 flooded periods in a year. Density and biomass of aquatic invertebrates that resulted from aerial colonization were measured after 17 d. Minimum growth rates and potential secondary production of several chironomid taxa were measured based on increases in larval size during this time period. Egg-mass collection trays, similar to colonization trays, were placed in the floodplain to estimate an egg-laying flux of chironomids (egg masses m⁻² d⁻¹) during the same 5 periods. Aquatic insects, including dipterans, ephemeropterans, and odonates, oviposited in trays during April, June, August, and November, but not January. Adult coleopterans and hemipterans colonized by nonreproductive immigration during June, August, and April. Noninsect invertebrates, such as microcrustaceans, nematodes, and water mites, colonized the trays during all months, apparently by hitchhiking or wind dispersal. The highest density of insects occurred in August (78,600 ind./m²) and the lowest in November (159 ind./m²). Chironomids had the highest relative abundance of insects in June and August (>75%), but did not colonize in November or January. Chironomids had minimum daily growth rates between 0.271 and 0.515/d, and chironomid secondary production ranged between 0 (November and January) and 230.9 mg m⁻² d⁻¹ (August). Egg-laying fluxes were similar in June (12 egg masses m⁻² d⁻¹) and August (6 egg masses m⁻² d⁻¹) but no chironomid egg masses were collected in November, January, or April. Aquatic invertebrates may arise from several sources (including recovery from dormancy or drift) in inundated floodplains, but aerial colonization and rapid growth rates alone can reestablish a diverse assemblage quickly, at least during warmer parts of the year. However, floodplains of regulated rivers managed for biological productivity should be inundated for a time period that enables colonization and completion of life cycles, a minimum of 2 wk for the fastest growing taxa.

Macroinvertebrate secondary production was estimated for 2 reaches in each of 3 adjacent forested headwater streams. We had 3 objectives: 1) to compare macroinvertebrate secondary production and community structure both within and among streams to examine the spatial extent of variability, 2) to explore important habitat variables related to secondary production, and 3) to compare our secondary production values to values from other headwater streams in deciduous forests. Principal components analysis separated study streams on the basis of small differences in substrate composition, organic-matter standing crops, and instream wood, but geology, riparian tree species composition, and fine benthic organic-matter standing crops were similar among streams. Secondary production varied among streams (range ∼1.2 to 3.3 g ash-free dry mass m⁻² y⁻¹) and was low compared to estimates from other streams draining deciduous forest. Macroinvertebrate communities had relatively higher production of scrapers, predators, and collector-filterers and lower production of shredders and collector-gatherers as compared to other perennial eastern deciduous headwater streams. We expected differences in secondary production among streams to be related to leaf-litter standing crops; however, differences among streams were positively related to % cover of gravel and cobble and chlorophyll a concentrations on gravel (R² = 0.87, p = 0.01). Total secondary production was negatively related to the number of large pieces of wood, leaf-litter standing crop, and % cover of sand. A similar, positive relationship between % cover of gravel and cobble and chlorophyll a concentrations was found for primary consumers (R² = 0.89, p = 0.03), collector-filterers (R² = 0.76, p = 0.02), and scrapers (R² = 0.67, p = 0.04). Low amounts and patchy distributions of coarse benthic organic matter, leaf litter, and large wood probably resulted in more variable secondary production of predators and shredders within these streams than among all streams. Inorganic substrate composition, primary production, and water temperature probably were key factors regulating secondary production of the other functional feeding groups and total secondary production among streams.

We used a subsidy–stress model as a basis for predicting macroinvertebrate community response to a steep gradient of P enrichment in the Florida Everglades, a P-limited wetland ecosystem. We tested the hypothesis that consumers were resource limited and their biomass would show a subsidy response (increase) to low-to-moderate levels of P enrichment, but a stress response (decrease) at high levels of P enrichment because dense emergent macrophytes, particularly Typha, might significantly reduce periphyton food resources. We used a spatially extensive sampling design (14 clusters of 9 sites, 126 total) that incorporated vegetation pattern to evaluate consumer responses along the P gradient. We then conducted a 1-y temporal study at 3 of the 14 clusters to evaluate how seasonal hydrological variation interacted with nutrients to influence consumer biomass. Macroinvertebrate community biomass showed a significant unimodal response to increasing P enrichment consistent with a subsidy–stress relationship. Eight of 12 major taxonomic groups (Amphipoda, Decapoda, Diptera, Empheroptera, Gastropoda, Hirudinea, Odonata, Oligochaeta) had this unimodal response, whereas 3 (Coleoptera, Hemiptera, Isopoda) increased monotonically and 1 (Trichoptera) decreased monotonically in response to P. Periphyton C:N and C:P ratios declined with increasing P, but periphyton cover was minimal at high levels of P enrichment where tall invasive macrophytes limited its growth. The temporal study revealed a subsidy–stress response except after marsh reflooding following the dry season when the most P-enriched clusters of sites had the highest consumer biomass, presumably because drought-induced senescence reduced macrophyte cover, which enabled heavy growth of periphyton. Our results suggest that an interaction between increased quality and decreased quantity of periphyton caused the subsidy–stress patterns observed. We suggest our findings could be generalized to other wetland ecosystems where nutrient enrichment leads to invasion of weedy emergent macrophytes, such as Typha, and elimination of open-canopy habitats rich in periphyton.

 Our study is a first attempt to characterize seasonal fatty acid (FA) profiles of foodweb components in a small forested stream. We measured FA content of autochthonous food sources (aquatic primary producers = periphyton, green algae, red algae, bryophytes), allochthonous food resources (terrestrial matter = benthic and transported organic matter [BOM and TOM, respectively]), and macroinvertebrate consumers (Hydropsyche spp., Ephemerella spp., isopods, oligochaetes). We examined whether FAs could be used as trophic markers and tried to identify which food sources best provided macroinvertebrates with essential FAs (EFA, ω3 and ω6 groups). Primary producers consistently had greater content of several EFAs (18:2ω6 and 18:3ω3 in green algae, 20:5ω3 in diatoms, 20:4ω6 in bryophytes) than did terrestrial matter. The ratio of Σω3/Σω6 FAs, a putative marker of the relative amount of autochthonous vs allochthonous matter, was greatest in macroinvertebrates, followed by algae, and was significantly correlated with chlorophyll a content of food sources (periphyton, ultrafine BOM, and TOM). The seasonal dynamics of EFA content of BOM and TOM varied with particle size. Other FAs were identified as specific markers for diatoms (20:5ω3 [eicosapentaenoic acid], 16:1ω7, 16:ω4s, 16C-polyunsaturated FAa [PUFAa]), green algae (18:3ω3 [α-linolenic acid], 18:2ω6 [linoleic acid], 16C-PUFAb), and bryophytes (20:4ω6, 20:3ω3). Terrestrial matter had higher levels of bacterial and fungal FAs than did primary producers. Nonmetric multidimensional scaling analyses based on FA spectra of foodweb components in early spring (open canopy) and mid-summer (shaded canopy) confirmed that Ephemerella and Hydropsyche consumed mainly autochthonous food sources, even during the shaded summer period. Isopods and oligochaetes consumed a mixed diet of terrestrial matter and algae. Autochthonous food sources may be a more important part of the diets of benthic macroinvertebrates in forested streams than previously recognized.

We examined the relationship between land use in the watershed and nutrient release by mayflies in streams in southcentral Ontario (Canada). Mayfly excretion rates and molar ratios of dissolved organic C (DOC), NH₄ , and soluble reactive P (SRP) were measured in 6 streams flowing through watersheds with contrasting agricultural land use. Mass-specific release rates of NH₄ and SRP by mayflies and % agricultural land use (% agriculture) in the watershed were strongly positively related (r² = 0.48, p < 0.01 and r² = 0.58, p < 0.05, respectively). Changes in excretion rates and ratios probably were caused by lower periphyton C:N and C:P ratios associated with elevated total dissolved N and total P concentrations (r²= 0.30, p < 0.05 and r²= 0.38, p < 0.001, respectively) in streams in watersheds with high % agriculture. Mayfly mass-specific excretion rates and periphyton elemental composition were related to % agriculture, but the relatively constant body C, N, and P content in mayflies across all streams suggests strong homeostasis in these consumers. Watershed land use alters nutrient loading, which influences periphyton elemental composition and, ultimately, stoichiometry of nutrient fluxes through invertebrate consumers.

Agriculture has influenced southern Appalachian streams for centuries, but recent socioeconomic trends in the region have led to extensive reforestation of agricultural land. Stream ecosystem metabolism might recover from agricultural influence as watersheds undergo reforestation, particularly when shade from terrestrial vegetation is restored. We determined historical (1950) and current (1993) forest cover in 2^nd- and 3^rd-order watersheds in 4 counties of the southern Appalachians using a geographic information system. Streams were placed into landuse categories based on % forested land cover in watersheds and riparian zones. Categories included forested (FOR; >98% forested) and 3 levels of agriculture (AG; ranging from 95% forest to <60% forest) with no change in % forest over the past 50 y, and 2 levels of recovery from agriculture (REC) indicated by reforestation after land abandonment. We selected 3 streams from each category and measured gross primary production (GPP) and 24-h respiration (R₂₄) using a 2-station diurnal O₂ change technique and gas releases to determine reaeration rates. We calculated net ecosystem production (NEP) and the ratio of GPP to R₂₄ (P/R) to compare ecosystem energetics among landuse categories. We measured nutrient concentrations, photosynthetically active radiation (PAR), temperature (degree-days), suspended particle concentrations, and benthic algae (chlorophyll a and ash-free dry mass) to determine if these factors were affected by current or historical agriculture and were correlated with metabolism. Concentrations of inorganic nutrients, PAR, degree-days, suspended solids, and benthic algae were significantly higher in AG streams than in FOR streams. Nutrient and suspended solid concentrations also were higher in REC than in FOR streams, but PAR, degree-days, and benthic algae were similar in REC and FOR streams. GPP varied from <0.1 g O₂ m⁻² d⁻¹ in FOR streams to 1.0 g O₂ m⁻² d⁻¹ in AG streams. GPP was similar in REC and FOR streams, suggesting that shading caused by reforestation might reduce GPP to pre-agricultural levels. R₂₄ was 4 to 20× greater than GPP in all stream types, resulting in highly negative NEP. NEP was less negative in AG streams than in FOR and REC streams. Negative NEP and P/R consistently <1 could have been caused by allochthonous organic matter from remnant forested land (up to 75% forested) in agricultural watersheds. GPP and P/R were strongly correlated with PAR, degree-days, and algal biomass, suggesting that reduced light limited primary production in the streams studied. R₂₄ was positively correlated with nutrient concentrations. Shading caused by reforestation appears to be an important mechanism by which stream metabolism recovers from historical agriculture. Our results provide support for stream restoration efforts focused on developing and maintaining streamside forests.

Two difficult decisions in the design of any bioassessment program based on stream macroinvertebrates are the number and types of habitats that should be sampled and the taxonomic level to which specimens should be identified. We used a large data set from biomonitoring of streams in the greater Sydney region, New South Wales, Australia, to compare bioassessment results obtained with an average-score-per-taxon type of biotic index among several habitats and between the taxonomic levels of family and genus. We evaluated the sensitivity of family- and genus-level indices calculated for 5 habitats (edges of pools, rocks in pools, riffles, aquatic macrophytes, and submerged wood) by considering relationships to chemical and microbial indicators of anthropogenic stress and capacity to distinguish sites affected by human activities from reference sites. Samples from rocks in pools did best on both criteria, and samples from edges of pools also did well. Genus-level index scores were slightly more strongly correlated with environmental variables than were family-level index scores, but taxonomic resolution had virtually no average effect on the degree to which samples from test sites differed from reference status, even though tolerance values for genera differed widely within some families. We attributed the weak effect of greater taxonomic resolution to the small number of identified genera in most families and the fact that many specimens could be identified to family but not to genus. The cost of discrete sampling from multiple habitats might sometimes be justified by its potential to detect habitat-specific impacts at particular sites. However, we conclude that it might be more cost-effective in broad-scale surveys to restrict sampling to the edges of pools, a habitat that occurs widely, or to consider assessment using composites of samples across multiple habitats. The small difference in sensitivity between the family- and genus-level indices suggests that, given its greater cost, bioassessment with fine-level taxonomy may be justified only in special circumstances, such as detection of subtle impacts. A tiered approach, in which only those families with wide intrafamilial variation in tolerance are identified to finer levels, is likely to be more cost-effective than identifying all taxa with fine-level resolution.

Diatom-based indicators can contribute significantly to comprehensive assessments of stream biological conditions. We used modeling to develop, evaluate, and compare 2 types of diatom-based indicators for Idaho streams: an observed/expected (O/E) ratio of taxon loss derived from a model similar to the River InVertebrate Prediction And Classification System (RIVPACS) and a multimetric index (MMI). Modeling the effects of natural environmental gradients on assemblage composition is a key component of RIVPACS, but modeling has seldom been used for MMI development. Diatom assemblage structure varied substantially among reference-site samples, but neither ecoregion nor bioregion accounted for a significant portion of that variation. Therefore, we used Classification and Regression Trees (CART) to model the variation of individual metrics with natural gradients. For both CART and RIVPACS modeling, we restricted predictors to natural variables unaffected by or resistant to human disturbances. On average, 46% of the total variance in 32 metrics could be explained by CART models, but the predictor variables differed among the metrics and often showed evidence of interacting with one another. The use of CART residuals (i.e., metric values adjusted for the effect of natural environmental gradients) affected whether or how strongly many metrics discriminated between reference and test sites. We used cluster analysis to examine redundancies among candidate metrics and then selected the metric with the highest discrimination efficiency from each cluster. This step was applied to both unadjusted and adjusted metrics and led to inclusion of 7 metrics in MMIs. Adjusted MMIs were more precise than unadjusted ones (coefficient of variation ∼50% lower). Adjusted and unadjusted MMIs rated similar proportions of the test sites as being in nonreference condition but disagreed on the assessment of many individual test sites. Use of unadjusted MMIs probably resulted in higher rates of both Type I and Type II errors than use of adjusted metrics, a logical consequence of the inability of unadjusted metrics to distinguish the confounding effects of natural environmental factors from those associated with human-caused stress. The RIVPACS-type model for diatom assemblages performed similarly to models developed for invertebrate assemblages. The O/E ratio was as precise as the adjusted MMI, but rated a lower proportion of test sites as being in nonreference condition, implying that taxon loss was less severe than changes in overall diatom assemblage structure. As previously demonstrated for O/E measures, modeling appears to be an effective means of developing more accurate and precise MMIs. Furthermore, modeling enabled us to develop a single MMI for use throughout an environmentally heterogeneous region.