Sophia I. Passy

SPATIAL PARADIGMS OF LOTIC DIATOM DISTRIBUTION: A LANDSCAPE ECOLOGY PERSPECTIVE

Spatial distributional patterns of benthic diatoms and their relation to current velocity were investigated in an unshaded cobble‐bottom reach of White Creek (Washington County, NY). On 27 August 1999, diatoms were sampled and current velocity and depth were measured on a regular square sampling grid with a grain size of 0.01 m2, interval of 0.5 m, and extent of 16 m2. The relative abundance of the 18 common diatom species enumerated in the 81 samples was subjected to detrended correspondence analysis (DCA). The first axis (DCA1) explained 51% of the variance in diatom data and separated the samples according to current regimes. The spatial autocorrelation of DCA1 sample scores in deposition and erosion regions of White Creek was determined by Morans I statistic to indicate patch size. In White Creek the patch length of all diatom communities was more than 3.1 m, whereas the patch width was 1 m in the deposition region and 0.5 m in the erosion region. There were 5 dominant diatom taxa, Achnanthes minutissima Kütz. et vars, Fragilaria capucina Dezmazières et vars, F. crotonensis Kitt., Diatoma vulgaris Bory, and Synedra ulna (Nitz.) Ehr. et vars. The patch length of the dominant species varied from 1 to more than 4.1 m, whereas the patch width, if defined, was 0.5 m. Achnanthes minutissima and F. capucina, the two diatom species with the highest relative abundance, displayed spatially structured patches of low abundance and comparatively random patches of high abundance, suggesting broad scale abiotic control of species performance in low abundance regions and finer scale biotic control of high abundance areas. Another objective of this study was to test the hypothesis that higher current velocities, which generally impede immigration, would increase randomness and complexity (i.e. homogeneity of diatom distributional patterns). The spatial complexity in low versus high velocity transects was determined by calculating the respective fractal dimension (D) of DCA1 scores. D of DCA1 was higher in the higher current velocity transects, suggesting that spatial complexity and homogeneity of diatom communities increased in faster currents. Partial canonical correspondence analysis was conducted on diatom, environmental, and spatial data to assess how much of the variance in species distribution could be attributed to environmental (current velocity and depth) versus spatial factors. The variance of species data, explained by the environment (exclusively current velocity), was 38%; whereas space alone contributed only 10%, indicating that 1) current velocity was the major factor that controlled diatom distribution in streams and 2) there were other spatially dependent variables, most likely biotic, but their role in shaping diatom communities was minor.

Continental diatom biodiversity in stream benthos declines as more nutrients become limiting.

Biodiversity of both terrestrial ecosystems and lacustrine phytoplankton increases with niche dimensionality, which can be determined by the number of limiting resources (NLR) in the environment. In the present continental study, I tested whether niche dimensionality and, with this species, richness scale positively with NLR in running waters. Diatom richness in 2,426 benthic and 383 planktonic communities from 760 and 127 distinct localities, respectively, was examined as a function of NLR, including basic cations, silica, iron, ammonia, nitrate, and dissolved phosphorus. The patterns found in the two communities were opposite: as more resources became limiting, diatom richness declined in the benthos but increased in the phytoplankton. The divergence of benthic from both planktonic and terrestrial communities is attributed to the complex spatial organization of the benthos, generating strong internal resource gradients. Differential stress tolerance among benthic diatoms allows substantial overgrowth, which greatly reduces nutrient transport to the biofilm base and can be supported only by high ambient resource levels. Therefore, niche dimensionality in the benthos increases with the number of resources at high supply. These findings provide a mechanistic explanation of the well documented phenomenon of increased species richness after fertilization in freshwater as opposed to terrestrial ecosystems. Clearly, however, new theoretical approaches, retaining resource availability as an environmental constraint but incorporating a trade-off between tolerance and spatial positioning, are necessary to address coexistence in one of the major producer communities in streams, the algae.

Diatom model affinity (DMA), a new index for water quality assessment

A new index, diatom model affinity (DMA) was erected as a reference-based community metric, derived from generic and species composition. DMA provides an assessment of water quality by the calculation of percent similarity to a model community, which can be viewed as a reference standard. High similarity to the model indicates communities that are minimally disturbed, while lower similarity suggests water quality problems. The index is similar to that developed for macroinvertebrate communities by Novak & Bode (1992, Journal of the North American Benthological Society 11: 80-85). The index correlated well with environmental factors operating on hierarchical scales. Compared to other commonly used diatom indices, DMA-based community assessments demonstrated high correspondence to patterns derived from multivariate statistics.

A hierarchical theory of macroecology.

The relationships of local population density (N) with body size (M) and distribution (D) have been extensively studied because they reveal how ecological and historical factors structure species communities; however, a unifying model explaining their joint behaviour, has not been developed. Here, I propose a theory that explores these relationships hierarchically and predicts that: (1) at a metacommunity level, niche breadth, population density and regional distribution are all related and size-dependent and (2) at a community level, the exponents b and d of the relationships N ~ M (b) and N ~ D (d) are functions (f) of the environment and, consequently, species richness (S), allowing the following reformulation of the power laws: N ~ M (f(S)) and N ~ D (f(S)) . Using this framework and continental data on stream environment, diatoms, invertebrates and fish, I address the following fundamental, but unresolved ecological questions: how do species partition their resources across environments, is energetic equivalence among them possible, are generalists more common than specialists, why are locally abundant species also regionally prevalent, and, do microbes have different biogeography than macroorganisms? The discovery that community scaling behaviour is environmentally constrained calls for better integration of macroecology and environmental science.

DIATOM COMMUNITY DYNAMICS IN STREAMS OF CHRONIC AND EPISODIC ACIDIFICATION: THE ROLES OF ENVIRONMENT AND TIME1

Community dynamics of epiphytic diatoms were studied for 3 years in a chronically and an episodically acidified tributary of Buck Creek, Adirondacks. Both streams experienced pulses of acidity during hydrologic events but these pulses were more pronounced in the episodically acidified stream, where pH decreased over two units (between 4.53 and 6.62) and the acid‐neutralizing capacity (ANC) became negative. In the chronically acidified stream, pH was below 4.9 and the ANC was negative 94% of the time. In this stream, high inorganic acidity following SO42− enrichment from snowmelt or rainstorms alternated with high organic acidity derived from a headwaters wetland during base flow. The fluctuating water chemistry generated shifts in diatom community composition: from exclusive dominance of Eunotia bilunaris (Ehrenberg) Mills during periods of high inorganic acidity to proliferation of several subdominant species during periods of high organic acidity. In the episodically acidified stream, the pulses of acidity were associated with high NO3− concentrations and the corresponding high ratios of inorganic monomeric Al (Alim) to organic monomeric Al (Alom). Diatom communities there were dominated exclusively by E. exigua (Brébisson) Rabenhorst year round; however, this species peaked during periods of low acidity. Periods of high acidity and Alim:Alom ratios were marked by a decline in E. exigua and a concomitant increase in the subdominant species. Variance partitioning into terms of environmental and temporal variance, and their covariance, suggested that diatom communities in the chronically acidic stream were governed primarily by environmental factors while in the episodically acidic stream environmental and temporal factors had equal contributions.

A distinct latitudinal gradient of diatom diversity is linked to resource supply

For over 200 years, scientists have recognized the nearly ubiquitous poleward decline of species richness, but none of the theories explaining its occurrence has been widely accepted. In this continental study of U.S. running waters, I report an exception to this general pattern, i.e., a U-shaped latitudinal distribution of diatom richness (DR), equally high in subtropical and temperate regions. This gradient is linked unequivocally to corresponding trends in basin and stream properties with impact on resource supply. Specifically, DR distribution was related to wetland area, soil composition, and forest cover in the watershed, which affected iron, manganese, and macronutrient fluxes into streams. These results imply that the large-scale biodiversity patterns of freshwater protists, which are seasonal, highly dispersive, and sheltered by their environment from extreme temperature fluctuations, are resource driven in contrast to more advanced, perennial, and terrestrial organisms with biogeography strongly influenced by climate. The finding that wetlands, through iron export, control DR in streams has important environmental implications. It suggests that wetlands loss, already exceeding 52 million hectares in the conterminous United States alone, poses a threat not only to local biota, but also to biodiversity of major stream producers with potentially harmful consequences for the entire ecosystem.

SPECTRAL FINGERPRINTING OF ALGAL COMMUNITIES: A NOVEL APPROACH TO BIOFILM ANALYSIS AND BIOMONITORING†

A new technique for spectral fingerprinting of major algal groups in the freshwater periphyton (i.e. cyanobacteria, green algae, and diatoms) was developed using confocal laser scanning microscopy. This technique used the differential spectral emission signatures of photosynthetic algae and allowed their spatially explicit quantification and community three‐dimensional reconstruction. Algal biovolume measurements, carried out with this technique, are superior to existing protocols involving chl and ash‐free dry mass assessments because they are nondestructive, localized, and specific at a group level. This technique can be used to generate depth profiles of the periphytic mat with various applications in aquatic ecology and biofilm analysis.

The relationship between species richness and evenness: a meta-analysis of studies across aquatic ecosystems

Biological diversity comprises both species richness, i.e., the number of species in a community, and evenness, measuring how similar species are in their abundances. The relationship between species richness and evenness (RRE) across communities remains, however, a controversial issue in ecology because no consistent pattern has been reported. We conducted a systematic meta-review of RRE in aquatic ecosystems along regional to continental gradients and across trophic groups, differing in body size by 13 orders of magnitude. Hypotheses that RRE responded to latitudinal and scale variability across trophic groups were tested by regression analyses. Significant correlations of species richness and evenness only existed in 71 out of 229 datasets. Among the RRE, 89 were negative and 140 were positive. RRE did not vary with latitude but showed a positive response to scale. In a meta-analysis with ecosystem type as a single explaining variable, RRE did not vary among ecosystem types, i.e. between marine and freshwater. Finally, autotrophs had more positive RRE than heterotrophs. The weak RRE in many aquatic datasets suggests that richness and evenness often reflect independent components of biodiversity, highlighting that richness alone may be an incomplete surrogate for biodiversity. Our results further elucidate that RRE is driven by organismal and environmental properties, both of which must be considered to gain a deeper understanding of large-scale patterns of biodiversity.

Taxonomic and functional composition of the algal benthos exhibits similar successional trends in response to nutrient supply and current velocity

In an effort to identify the causes and patterns of temporal change in periphytic communities, we examined biomass accumulation, taxonomic and functional composition, rate of species turnover, and pairwise species correlations in response to variability in current velocity and nutrient supply in artificial stream flumes. Divergent patterns in community growth and succession were observed between nutrient treatments and, to a lesser extent, between flow treatments best described by shifts in taxonomic and functional composition. Specifically, understory low profile species, tolerant to low resource supply, became dominant under low nutrients, while overstory high profile and motile species with higher nutrient demands dominated the high nutrient treatments. Increased resource supply or current velocity did not influence the species turnover rate, measured by a time-lag analysis. Interspecific interactions, especially competition, did not appear to be driving community dynamics, as the number of positive and negative pairwise species correlations ranged between low and extremely low, respectively. The overwhelming majority of correlations were not significant, indicating that species within the biofilm matrix were not perceptibly influencing one another. Thus, temporal trends in taxonomic and functional composition were largely environmentally driven, signifying that coexistence in biofilms is defined by the same mechanism along the hierarchy from species to functional groups.

Are algal communities driven toward maximum biomass

In this continental-scale study, we show that in major benthic and planktonic stream habitats, algal biovolume—a proxy measure of biomass—is a unimodal function of species richness (SR). The biovolume peak is observed at intermediate to high SR in the benthos but at low richness in the phytoplankton. The unimodal nature of the biomass–diversity relationship implies that a decline in algal biomass with potential harmful effects on all higher trophic levels, from invertebrates to fish, can result from either excessive species gain or species loss, both being common consequences of human-induced habitat alterations. SR frequency distributions indicate that the most frequent richness is habitat-specific and significantly higher in the benthos than in the plankton. In all studied stream environments, the most frequent SR is lower than the SR that yields the highest biovolume, probably as a result of anthropogenic influences, but always within one standard deviation from it, i.e. they are statistically indistinguishable. This suggests that algal communities may be driven toward maximum biomass.