Antonella Cattaneo

How Artificial Are Artificial Substrata for Periphyton

Artificial substrata are increasingly used to study periphyton, but their ability to reproduce natural substrata remains controversial. Although many studies have made contemporaneous comparisons of periphyton assemblages on artificial and natural substrata at one or a few sites, no broadly based comparison exists. We therefore surveyed the literature to establish conditions under which artificial substrata satisfactorily mimic both the quantity and the quality of natural periphyton assemblages. In general, epilithon was underestimated by the artificial substrata; epiphyton was overestimated, but less severely. These trends were significantly affected by the time available for colonization of the artificial substrata before sampling, site trophy, ambient temperature, and whether the study was conducted in a lake or in running water. Neither the composition of the substratum nor its orientation appeared important. Natural diatom assemblages were usually well simulated by those on artificial substrata, whereas both epilithic and epiphytic green and bluegreen algae were severely underrepresented on the artificial substrata. Since artificial substrata often misrepresent both the quantity and the quality of natural periphyton, they should be used with more caution, especially in intersite and interseason surveys.

Paleoecological evidence for diatom response to metal pollution in Lake Orta (N. Italy)

Diatom remains were analysed in two short sediment cores from a subalpine Italian lake (Lake Orta), known for its major industrial pollution dating from the late 1920s, which has only recently been stopped. Copper was recognised as the main toxic agent for diatoms during the first 30 years of pollution (peak value: 100 μg l-1 in the late 1950s). A diatom community similar to other deep subalpine lakes existed in the past, and was disrupted by the pollution events. Acute and long-term effects of Cu contamination were tracked by changes in three distinct groups of species around the sharp boundary corresponding to the onset of the pollution. These groups were respectively composed of: (1) Species quickly extirpated by the discharge, mostly belonging to Fragilaria and Cyclotella and never reappearing; (2) Species apparently not affected, or not immediately affected, by the pollution, showing no definite trends with time. Synedra species, with various deformities, were conspicuous among these; (3) Species with accumulation rates increasing with time irrespective of pollution, mostly belonging to Achnanthes. Properties and tolerances of these groups (e.g. Synedra and Achnanthes) are discussed in detail.

Control of Periphyton Biomass in Laurentian Streams (Québec)

Hypotheses concerning the factors controlling periphyton biomass are mostly based on experimental evidence. To examine their application under natural conditions, we sampled periphyton and invertebrate biomass in 12 Laurentian streams (Québec) covering a range of total phosphorus from 5 to 60 μg/L. We sampled at open and shaded sites to explore light limitation by canopy cover. Periphyton biomass measured as chlorophyll a or ash-free dry mass was not related to nutrient concentration or canopy cover. Only current velocity and depth explained a significant but relatively small fraction (10% and 20% respectively) of periphyton variation among sites. Grazer biomass and mean grazer size were positively correlated with phosphorus concentration, which explained 48% and 45% of their variation respectively; canopy cover was not significant. These results indicate that, at least in summer under stable flow, grazer biomass rather than periphyton biomass would increase along a nutrient gradient typical in Laurentian streams. The results suggest top-down control of periphyton biomass.

Periphyton as an indicator of water quality in the St Lawrence River (Québec, Canada)

The performance of various algal indices to document improvements in water quality across a low nutrient concentration gradient was assessed during 2 years in the St Lawrence River (Quebec, Canada). Water-quality variables and periphyton samples were collected on navigational buoys near Montreal during the spring, summer and fall of 1994 and 1995. Exposure to urban wastewater varied widely within the sector surrounding the island of Montreal, with some areas upstream receiving no direct effluents and areas further downstream receiving treated and untreated wastewater. Faecal coliform concentrations provided a good tracer of effluents and were significantly correlated to nutrient concentrations (r = 0.33-0.72, p < 0.001) and water transparency (r = 0.70, p < 0.001). Despite a strong gradient in faecal coliform concentration (< 2 to > 20 000 UFC/100 ml), algal biomass and diversity did not reflect differences between sites with varying levels of urban wastewater. Taxonomic composition of periphyton communities, particularly the presence of the cyanophyte Plectonema notatum Schmidle, was related (r = 0.48, p = 0.004) to exposure to urban effluents. Variables describing seasonal changes (temperature, Julian day, river discharge, conductivity, NO2-NO3) explained a large fraction of total variance (38-52% of total variance) and thus exerted the predominant influence on algal biomass and species composition in the St Lawrence River. Variables describing the presence of effluents explained 1-22% of the variance in compositional data. Subtle changes in periphyton species composition were the only response to different levels of exposure to urban wastewater in the Montreal area, which represented relatively small differences in comparison to natural seasonal variability.

Periphyton distribution and abundance on substrata of different size along a gradient of stream trophy de Montréal

In stream, substrata of different size present different degree ofstability, current, erosion, and deposition to colonizingorganisms. In this study, we tested the importance of substratumsize ranging from sand to small boulders for periphytondistribution and abundance. Because trophy strongly affect streamorganisms, we sampled at nine sites chosen to represent the rangeof nutrients typical of Eastern Ontario and Western Québec. Alarge part of the variability in algal biomass (as chlorophyll)among sites was explained by trophy (as seston or totalphosphorus). However, there was also an effect of substratum size.Cobbles had the highest biomass, and gravel the lowest; sand andboulders were intermediate. Assemblages on different substrata weredifferent in taxonomy and life forms. Cyanobacterial colonies andmotile diatoms dominated the finer substrata while adnate andfilamentous algae were more developed on the larger ones.Consequently periphyton on fine sediments was more loosely attachedthan on rocks. Average algal size was not related to substratumsize but increased significantly with trophy confirmingobservations in benthic and planktonic assemblages inlakes.

Empirical analysis of the removal rate of periphyton by grazers

To establish a general model for the removal rate of periphyton by grazers, we identified 27 publications in which removal rates could be estimated from grazer enclosure or exclosure experiments. When all the measurements obtained under different experimental conditions were extracted, these publications provided 107 data points. Multiple regression of these data showed that periphyton removal rate increased significantly with grazer body mass and food availability, and decreased with grazer crowding. Grazer body mass explained 65% of the variation, while crowding and food availability explained 7 and 6% respectively. Except for the significantly lower removal rate of amphibians, neither taxon of the grazer nor algal composition significantly affected removal rate. Experiments in the laboratory and in outdoor channels tended to give higher removal rates than experiments performed in streams or lakes. A comparison with previous allometric equations predicting the ingestion rate of other invertebrate guilds, and with experiments in which periphyton ingestion rate was measured as incorporation of labelled food, indicated that a large portion of periphyton is removed by the activity of the grazer rather than by direct ingestion. These results could be utilized to predict the impact of grazing on periphyton biomass.

A test of the integrity of metal records in sediment cores based on the documented history of metal contamination in Lac Dufault (Québec, Canada)

There is an important volume of published information on Lac Dufault (Québec) which describes the history of metal inputs over 70 years and the changes that occurred in the lake as a result of this contamination. We used this abundant source of chronological markers to test the hypothesis that lake sediments can provide true historical records of trace metal loading from metal mining. Sediment cores were obtained from the deepest zone of the lake (19 m). The sediments were dated using 210Pb and 137Cs and they were analyzed for total elemental concentrations (Ca, Cd, Cu, Fe, P, Pb, S, Zn). Metal profiles in the sediment core preserved the distinct signatures of different mine exploitations documented in the lake watershed. In particular, the core recorded: the beginning of industrialization in 1926; increasing sedimentation rates associated with perturbations in the lake watershed; the maximum of sediment [Cu] and the contemporary exploitation of ore bodies rich in Cu; the maximum of sediment [Fe] and the contemporary production of pyrite by a mining operation; the low sediment [Cu] and [Zn] in the 1950s and the low contemporary production of these metals by mining operations; the maximum of sediment [Cd] and the contemporary production of Cd by a subsequent mining operation. Anomalies in the distribution of 214Pb activities in sediments reflected the intensity of acid mine drainage (AMD). There is good evidence that the lake resisted acidification from this AMD for the last thirty years. Overall, our results support the thesis that profiles of sedimentary Cd, Cu, Zn, Pb and Fe levels reflect the past history of metal input to Lac Dufault.

Shift from chlorophytes to cyanobacteria in benthic macroalgae along a gradient of nitrate depletion

A survey of the spatial distribution of benthic macroalgae in a fluvial lake of the St. Lawrence River (Lake Saint‐Pierre, Quebec, Canada) revealed a shift in composition from chlorophytes to cyanobacteria along the flow path of nutrient‐rich waters originating from tributaries draining farmlands. The link between this shift and changes in water quality characteristics was investigated by sampling at 10 sites along a 15 km transect. Conductivity, current, light extinction, total phosphorus (TP; >25 μg P · L−1), and ammonium (8–21 μg N · L−1) remained fairly constant along the transect in contrast to nitrate concentrations, which fell sharply. Filamentous and colonial chlorophytes [Cladophora sp. and Hydrodictyon reticulatum (L.) Bory] dominated in the first 5 km where nitrate concentrations were >240 μg N · L−1. A mixed assemblage of chlorophytes and cyanobacteria characterized a 1 km transition zone where nitrate decreased to 40–80 μg N · L−1. In the last section of the transect, nitrate concentrations dropped below 10 μg N · L−1, and cyanobacteria (benthic filamentous mats of Lyngbya wollei Farl. ex Gomont and epiphytic colonies of Gloeotrichia) dominated the benthic community. The predominance of nitrogen‐fixing, potentially toxic cyanobacteria likely resulted from excessive nutrient loads and may affect nutrient and trophic dynamics in the river.

Physical variables driving epiphytic algal biomass in a dense macrophyte bed of the St. Lawrence River (Quebec, Canada)

The variables affecting epiphyton biomass were examined in a sheltered, multispecies macrophyte bed in the St. Lawrence River. Alteration of light penetration, resulting from the presence of dense macrophytes forming a thick subsurface canopy, primarily determined epiphyton biomass. Seasonal decrease of water levels also coincided with major increases in biomass. Plant morphology was the next important variable influencing epiphytic biomass, whereas the contribution of other variables (sampling depth, macrophyte species, relative abundance of macrophytes, and temperature) was low. Groups of lowest epiphyte biomass (0.1–0.6 mg Chla g−1 DW) were defined by the combination of a low percentage of incident light (<13% surface light) and simple macrophyte stem types found below the macrophyte canopy. Highest epiphyte biomass (0.7–1.8 mg Chla g−1 DW) corresponded to samples collected in mid-July and August, under high irradiance (>20% surface light) and supported by ramified stems. Our results suggest that epiphyton sampling should be stratified according to the fraction of surface light intensity, macrophyte architecture, and seasonal water level variations, in decreasing order of influence.

Resource partitioning in a grazer guild feeding on a multilayer diatom mat

Abstract The gut contents of a guild of invertebrate grazers inhabiting the moss Fontinalis and feeding on epiphytic diatoms in a small Québec stream were analyzed to characterize resource partitioning and food selection. A multivariate approach (RLQ analysis coupled with a revised version of 4th-corner analysis) identified distinct diet patterns among co-occurring grazers. These patterns were mainly explained by differential ingestion of diatoms that differed in their spatial positions within the multilayered periphyton mat. When the size range of available diatoms was large, diet differences were partly explained by diatom size. Comparison of diatoms in grazer guts with diatoms available in the environment indicated selective feeding in different levels of the periphyton mat by grazers. Some grazers (scrapers) fed preferentially on tightly attached diatoms, whereas others (surfers) favored overstory diatoms. Spatial segregation of feeding within the periphyton mat by members of the grazer guild was more evident in a period of potential resource limitation (July) than when food was abundant (May). Our results suggest that all layers/growth forms in the diatom mat are used, resulting in spatial partitioning of the resource when considering the entire grazer community. Therefore, foraging theories already established for other ecosystems are confirmed in the unique context of stream benthos.