R. Jan Stevenson

Using diatoms as indicators of ecological conditions in lotic systems: a regional assessment

Benthic diatoms and water chemistry were sampled from 49 stream sites in the Mid-Atlantic Highlands region of the United States to evaluate the use of diatoms as indicators of environmental conditions in streams across varying geographic and ecoregional areas. Diatom samples were collected from depositional and erosional habitats in a randomly selected reach in each stream site. Patterns of diatom species distributions in relation to environmental variables were determined using canonical correspondence analysis. Diatom species in both habitats were highly correlated with a pH gradient. A second gradient was correlated with variables that were commonly associated with agricultural runoff such as turbidity and total phosphorus. The relationship between diatoms and major environmental variables was quantified with regression and calibration models. The correlation between diatom-inferred and observed pH was high (r2 = 0.90). Cross-validation with jackknifing showed that pH models were reasonably robust (r2 = 0.69 for depositional habitats, r2 = 0.67 for erosional habitats). The regression and calibration models for the depositional habitats had only slightly higher predictive powers than those of erosional habitats. The relationship between diatoms and important environmental variables was robust and quantifiable, and the sensitivity of diatom assemblages to environmental conditions did not differ between erosional and depositional habitats. Therefore we concluded that diatoms can be used as quantitative indicators of environmental conditions in lotic systems.

Resistance and Resilience of Lotic Algal Communities: Importance of Disturbance Timing and Current

We examined effects of disturbance timing on resistance and resilience of epilithic algal communities growing in fast- (29 cm/s) and slow- (12 cm/s) current outdoor experimental stream channels in Kentucky, USA that were either left undisturbed (control) over 33 d following a simulated spate, or were subjected to an additional spate after either 9, 18, 27, or 33 d. On day 33, all channels were subjected to a final spate to assess effects of short-term disturbance history on resistance, independently of seasonal influences. Suc- cession proceeded from a sparsely populated community dominated by a small, monora- phid diatom (Achnanthes minutissima) immediately after the initial spate, to dominance by dense floating mats of filamentous green algae (Zygnematales: Mougeotia and Spirogyra) and Synedra spp. by day 21-24. Resistance was generally lower in slow-current commu- nities, both in terms of cell-density reduction and displacement of taxonomic structure, than in fast-current communities. Resistance in slow-current communities varied tem- porally, with communities least resistant on day 18, when community composition and physiognomy was changing rapidly, and on day 33, when green algal mats began to senesce. On day 33, slow-current communities that had not been recently disturbed (control, D9) exhibited greatest spate-induced loss of algal biomass. Additionally, slow-current com- munities with high pre-disturbance phaeophytin content (an indicator of algal senescence) also changed most in diatom assemblage structure across the final spate. No such rela- tionship was noted in fast current, suggesting that autogenic factors influenced communities in slow current more than those in fast. Resilience was higher in slow-current communities than in fast current, with disturbed communities reaching biomass and taxonomic structure similar to controls after 3-9 d. High resilience in slow current resulted from enhanced reproduction in some populations following spate-induced biomass reduction and pre- sumed release from nutrient and light limitation, and low shear stress relative to fast- current channels where biomass accrual was limited by current. Interactions between dis- turbance timing, successional state, and habitat affect the susceptibility of epilithic algal communities to disturbance and likely influence temporal and spatial heterogeneity in stream ecosystems.

Effects of Current and Conditions Simulating Autogenically Changing Microhabitats on Benthic Diatom Immigration

Diatom immigration rates were affected by changes in current patterns similar to those found around substrates in streams and were increased by changes in microhabitat characteristics similar to those autogenically generated during diatom community development. Diatom abundances on tiles exposed for 24 h, i.e., diatom immigration rates, ranged from 30 to 1500 cells.cm-2 d-1. Immigration rates in areas sheltered from the 27 cm/s currents of two Michigan streams were greater than in areas exposed to the current. Diatom immigration rates increased by a factor of 6 when microhabitat conditions were altered by interrupting currents near the substrate surface, and by a factor of 2 when substrates were coated with agar. These two microhabitat conditions, respectively, simulated changes in current patterns near the substrate, and in adsorptive characteristics of the substrate that could occur autogenically as diatoms accumulate on substrates. Species-specific effects of current velocity and microhabitat conditions were related to size and growth habits of cells.

Scale-dependent determinants and consequences of benthic algal heterogeneity

The objectives of this paper were to introduce frameworks for predicting the determinants and consequences of the great heterogeneity in benthic algal assemblages. A hierarchical framework of environmental factors affecting benthic algal spatial heterogeneity is presented with the ultimate determinants (climate, geology, land use, and biogeography) that are important constraints on benthic algal assemblage structure at large spatial scales. These ultimate determinants constrain expression of intermediate and proximate determinants of benthic algal function and structure. A similar framework for temporal heterogeneity distinguishes how assemblages respond to short-term and long-term environmental changes, referred to as disturbances and stresses, respectively. Assemblages recover from punctuated (short-term) environmental change (disturbance) by immigration and reproduction of both persistent and recolonizing species. Assemblages adapt to permanent (longterm) environmental change (stress) by changing species membership, i.e. loss of some species that cannot survive in the new conditions and invasion of new species that are adapted to the new conditions. Stresses operating on large temporal scales probably constrain effects of disturbance, which operate on small temporal scales. The consequences of interactions among patches are related to benthic algal assemblages and are referred to as the emergent properties of heterogeneity. Enhanced biomass, diversity, and stability of benthic algae are hypothesized to be emergent properties of habitat heterogeneity. A mathematical model is presented that indicates that algal accrual in a reach is enhanced by interactions between algae in riffles and pools. Conceptual models are introduced that hypothesize that dispersal in spatially and temporally heterogeneous habitats enhances diversity and stability of benthic algal assemblages.

PERIPHYTON AS A TOOL FOR ECOLOGICAL ASSESSMENT AND MANAGEMENT IN THE FLORIDA EVERGLADES

The Florida Everglades is an extensive subtropical wetland that formed during the past 5000 years through the process of peat and marl accretion within a limestone depression (Gleason and Stone 1994). This wetland once covered approximately 10,000 km2 of the south Florida peninsula and extended uninterrupted from Lake Okeechobee to Florida Bay (Fig. 1). The freshwater Everglades is composed of extensive stands of sawgrass (Cladium jamaicense Crantz) and other emergent macrophytes interspersed with open-water sloughs containing abundant periphyton. This vegetative mosaic supports a productive food web including some of the largest concentrations of wading birds in the southeastern United States (Robertson and Frederick 1994). The ecological features of the Everglades developed under a hydrologic regime of seasonal inundation and drying coupled with low rates of nutrient loading, which allowed for the maintenance of oligotrophic conditions (Parker 1974, Davis 1994). These conditions have been altered dramatically by the rapid human development in south Florida during the 20th century. Approximately half of the wetland has been drained for agricultural and urban use by a network of canals and levees that transformed the once free-flowing ‘‘river of grass’’ into a series of diked marshes (Fig. 1) and altered the quantity and timing of water flows throughout much of the system. Canals also have served as conduits for agricultural and urban runoff, which contain elevated nutrient concentrations compared to the marsh (Waller and Earle 1975, McCormick et al. 1996). Human-induced changes in marsh hydrology and

DENSITY-DEPENDENT GROWTH, ECOLOGICAL STRATEGIES, AND EFFECTS OF NUTRIENTS AND SHADING ON BENTHIC DIATOM SUCCESSION IN STREAMS1

The importance of immigration, growth, and competition for nutrients and light in benthic diatom succession was studied in experimental channels in a low‐nutrient stream. Diatom accumulation was greater in channels enriched with nitrate and phosphate (NP) than in control channels, reaching about 5 × 106 and 2 × 106 cells‐cm−2, respectively, after 30 d. Shading during late stages of community development reduced algal standing crop.

Spatial patterns and ecological determinants of benthic algal assemblages in mid-atlantic streams, usa

We attempted to identify spatial patterns and determinants for benthic algal assemblages in Mid‐Atlantic streams. Periphyton, water chemistry, stream physical habitat, riparian conditions, and land cover/use in watersheds were characterized at 89 randomly selected stream sites in the Mid‐Atlantic region. Cluster analysis (TWINSPAN) partitioned all sites into six groups on the basis of diatom species composition. Stepwise discriminant function analysis indicated that these diatom groups can be best separated by watershed land cover/use (percentage forest cover), water temperature, and riparian conditions (riparian agricultural activities). However, the diatom‐based stream classification did not correspond to Omernik’s ecoregional classification. Algal biomass measured as chl a can be related to nutrients in habitats where other factors do not constrain accumulation. A regression tree model indicated that chl a concentrations in the Mid‐Atlantic streams can be best predicted by conductivity, stream slope, total phosphorus, total nitrogen, and riparian canopy coverage. Our data suggest that broad spatial patterns of benthic diatom assemblages can be predicted both by coarse‐scale factors, such as land cover/use in watersheds, and by site‐specific factors, such as riparian conditions. However, algal biomass measured as chl a was less predictable using a simple regression approach. The regression tree model was effective for showing that ecological determinants of chl a were hierarchical in the Mid‐Atlantic streams.

Effects of Snail Grazing on Benthic Algal Community Structure in Different Nutrient Environments

The effects of interaction between nutrient availability and grazing intensity on the structure of benthic algai assemblages were investigated in an ephemeral stream. Small (250 ml) enclosures containing substrata releasing either: 1) no nutrient, 2) 0.5 M KH2 PO4 or 3) 0.5 M NaNO3 were exposed for 19 d to one of five densities of the snail Goniobasis ranging from 0 to 160 snails/m2. Algal standing crop (total cell density and community biovolume) on coverslips inside each enclosure decreased with increased grazing pressure in control and nitrate-enriched environments, but increased at low grazing pressure in the phosphate-enriched environment. Responses among constituent populations differed for three reasons: 1) stalked and loosely attached diatoms were more susceptible to snail grazing than firmly attached, prostrate algae; 2) some species were stimulated by phosphorus enrichment and/or grazing while others were not; 3) algal species differed in their response to interactions between grazing and nutrient enrichment. Two alternative hypotheses may explain this interaction: 1) grazers stimulate the growth of understory species by removing overlying cells; 2) excretion of nitrogenous compounds by grazers enhances the effects of phosphorus enrichment alone. Although diversity peaked at intermediate grazer densities in unenriched and nitrate-enriched environments, no significant change was found in phosphate-enriched environments, where the community was dominated either by grazer-sensitive populations that increased faster than they were grazed, or by grazer-resistant algae.

MECHANISMS OF BENTHIC ALGAL SUCCESSION IN LOTIC ENVIRONMENTS

Although seasonal patterns of algal succession have received considerable attention, few studies have attempted to elucidate the processes that contribute to short- term algal succession following disturbance. We framed the present study around four general mechanisms of succession in order to investigate the contribution of species-specific autecologies and interspecific interactions to observed short-term patterns of algal succes- sion in a productive third-order stream in Kentucky, USA. Observations in different current environments in the stream during two consecutive winters were used to classify dominant species as early or late successional based on changes in relative abundance through time on newly exposed substrates. Assemblages of the same age but differing in the relative abundance of early- and late-successional species were developed in streamside channels recreating both current environments to measure growth parameters (e.g., reproduction) of dominant species and to test for interspecific interactions. Of the five dominant species of algae in the winter assemblage, three species were consistently classified as early successional (i.e., decreased in relative abundance with in- creasing assemblage age) and two species as late successional. Early-successional species differed in their growth form and strategy for initially dominating the substrate, having high densities in the water column, high probabilities of attaching, or fast early reproductive rates. Late-successional species exhibited a more extended growth form and had the highest per capita reproductive rates during later growth. Late-successional species reduced the reproductive rate of early species as succession proceeded in both current environments but were not themselves inhibited by biovolume increases. While differences in growth strategies between early- and late-successional species indicated the importance of passive tolerance mechanisms of succession, density-dependent interactions during community development were consistent with active tolerance mechanisms. As in terrestrial plant communities, successional patterns in benthic algal assemblages appear to result from several processes that defy explanation by a single mechanistic model.

Benthic Algal Community Dynamics in a Stream during and after a Spate

Changes in a benthic algal community were observed during and after a major storm. Algal abundance, mostly diatoms, increased slightly during a 3-d interval that included the storm on day 1. Positive effects of the storm on benthic algae were indicated by decreases in the proportions of diatoms that were dead, but not in the dead cell numbers, and by decreases in the proportions of sexually reproducing cells. Increases in current or nutrient supply to periphyton during the storm may have stimulated growth of live cells and retarded sexual reproduction. During two 3-d growth periods after the storm, benthic diatoms grew rapidly from 1.1× 107 cells/cm2 and seemed to reach a carrying capacity near 5.2× 107 cells/cm2 in only 8 days. Density-dependent decreases in growth rates were accompanied by increases in the proportion of diatoms that were dead or in stages of sexual reproduction, indicating nutrient limitation. Immigration was not an important mechanism of accumulation immediately after the storm. However, immigration/colonization rates increased during community development and indicated substantial turnover by cell transport during late succession. Benthic diatom communities are well adapted to storms in temperate climates in streams with gravelbed channel morphology. I hypothesize that benthic diatoms are actually affected positively by most storms and are negatively affected only by the most severe storms that overturn substrates in the stream.