Christiane Hudon

THE ECOLOGICAL IMPLICATIONS OF GROWTH FORMS IN EPIBENTHIC DIATOMS

This paper evaluates the utilisation of space by epibenthic diatom cells, as a response to environmental variations. The aggregation pattern of five species of epibenthic diatoms was quantified and compared to provide evidence for the significance of cell motility as an adaptive mechanism for space occupation and monopoly. The epibenthic diatoms included (1) non‐mobile colonial species forming either fan‐shaped (Synedra tabulata (Ag)Kz.) or arborescent (Gomphonema kamtschaticum var. californicum Grun.) colonies; (2) slow‐moving (Cocconeis costata Greg, and Amphora pusio Cl.), and (3) fast‐moving (Navicula direct a (W. Sm.) Ra.) non‐colonial species. The aggregation pattern of S. tabulata did not vary significantly among six different light intensities manipulated in nature. The major patterns of aggregation were identified using analysis of covariance and dummy‐variable regression. Highly mobile N. directa are significantly less aggregated than the four other diatom species. Non‐mobile and slow‐moving species show a similar, highly aggregated pattern. The occurrence of two patterns of spatial dispersion indicates that growth forms bear far‐ranging ecological implications with respect to colonization strategies, immigration, and possibly impact by grazers. An integrated model of growth form characteristics, biological properties, and ecological implications is presented for epibenthic diatoms.

Macroinvertebrate size–mass relationships: how specific should they be?

Abstract.  We assessed the relative magnitude of various factors (year, preservation method, continent, investigator, and taxonomic level) affecting prediction of invertebrate dry mass (DM) in light of the variability of assessments of invertebrate density. We developed 34 length (L)–DM relationships for Oligochaeta and 17 freshwater invertebrate families belonging to Mollusca, Crustacea, and Insecta. Comparison of our predicted DM for reference-size individuals with values from 120 other published equations revealed that 31% of predicted values were within our 95% CI and 73% were within a 2× DM range (i.e., between 0.5× and 2×). Interannual differences in exponent (slope) or scale factor (intercept) of L–DM relationships were detected for 6 of the 7 taxa investigated, but represented only 3% of total variance in predicted DM. Similarly, preservation methods and measured body dimension each accounted for a small (0–3%) fraction of total variance. Variation among investigators (12–50%) and continents (1–17%) were more important and might have reflected methodological or regional and latitudinal differences. Increasingly precise taxonomical levels explained progressively lower proportions of the total variance, a result indicating that family or a more precise taxonomic level provided a robust estimate of most invertebrate DM even if the equations were derived from other sites. However, overall variability induced by L–DM relationships was smaller than variability in total invertebrate density among replicate samples (coefficient of variation [CV]  =  19–97%), a result indicating that more effort should be devoted to improving the accuracy of invertebrate density estimates than to developing site-specific L–DM relationships to assess benthic biomass in freshwater.

Increasing occurrence of the benthic filamentous cyanobacterium Lyngbya wollei: a symptom of freshwater ecosystem degradation

Abstract: The filamentous cyanobacterium Lyngbya wollei (Farlow ex Gomont) comb. nov. forms dark green to black mats on the bottom of rivers and lakes. Benthic mats often remain inconspicuous until they float to the surface because of trapped gas bubbles or until high winds and wave action dislodge and wash mats ashore. Mats induce dark, anoxic conditions conducive to nutrient mineralization, atmospheric N2 fixation, and heterotrophic metabolism. Lyngbya wollei has been found historically in southeastern USA, but genetically similar subgroups have been proliferating more recently in the Laurentian Great Lakes and the St Lawrence River. This taxon is found under contrasting environmental conditions, including very clear, thermally and chemically stable, and heavily mineralized Florida Springs and turbid, high dissolved organic C, and seasonally variable conditions, influenced by agricultural tributaries in the St Lawrence River. Lyngbya wollei produces a number of unique saxitoxins and volatile organic compounds that are responsible for a musty-earthy taste and odor in water, which affect aesthetics and recreational water uses. Mats of L. wollei are less palatable than other vegetation but provide shelter for invertebrates, which hide in dark mats of filaments. In the St Lawrence River, wetlands dominated by L. wollei tend to be characterized by a lower biomass of invertebrates and large fish, lower fish species richness, and slower-growing juvenile fish than macrophyte-dominated wetlands. Replacement of macrophytes by L. wollei mats induces a shift in trophic structure and coincides with a decrease in carrying capacity for fish, and significantly alters the dynamics of freshwater ecosystems.

Benthic cyanobacteria and filamentous chlorophytes affect macroinvertebrate assemblages in a large fluvial lake

Abstract Proliferations of filamentous chlorophytes and mats of cyanobacteria (hereafter termed metaphyton) are increasingly observed in rivers, lakes, wetlands, and estuaries undergoing eutrophication, but their contribution to invertebrate production and overall ecological significance remains poorly understood. In Lake Saint-Pierre, a shallow widening of the St. Lawrence River (Québec), vascular macrophytes (mainly Vallisneria americana) grow in combination with filamentous chlorophytes (Hydrodictyon, Oedogonium) in the upstream reach, which is fed by nutrient-rich waters from the tributaries, and in association with filamentous cyanobacteria (Lyngbya wollei) in the chronically NO3−-depleted downstream reach. We hypothesized that different vegetation types (macrophytes, filamentous chlorophytes, and cyanobacteria) would support macroinvertebrate communities with different biomasses and taxonomic compositions. We expected a higher invertebrate biomass in the upstream reach and, within the reach, a higher biomass on metaphyton than on macrophytes. Total macroinvertebrate biomass was significantly higher at the enriched stations in the upstream reach (75–100 mg/g vegetation, dry mass) than farther downstream (8–38 mg/g). In addition, macrophytes and metaphyton in the upstream reach sustained taxonomically different invertebrate assemblages. Gastropods dominated the fauna associated with macrophytes throughout the lake (43–73%) and probably benefitted from a structurally simple and solid substratum on which to crawl and feed. Small mobile taxa, such as cladocerans, copepods, chironomids, and ostracods, were more abundant on filamentous metaphyton, both up- and downstream. Amphipods were dominant (59%) in metaphytic mats of L. wollei. At the scale of the river reach, macrophytes supported most of the invertebrate biomass. Chlorophytes in the upstream reach contributed <5% of the total biomass, representing an alternative, albeit temporary, habitat. In contrast, in the downstream reach where macrophytes were scarce, cyanobacterial mats hosted a significant fraction of macroinvertebrates (36%). Shifts in vegetation between the 2 reaches affected the quantity and availability of prey items for fish predators.

Wave exposure and current regulate biomass accumulation of the benthic cyanobacterium Lyngbya wollei in a large fluvial lake

Harmful proliferations of the mat-forming cyanobacterium Lyngbya wollei are increasingly reported in North American lakes, springs, and reservoirs. We examined the hypothesis that currents generated by waves and river flow control spatial and temporal variations of L. wollei biomass in a large river system. We measured L. wollei biomass together with meteorological, physical, and chemical variables during 2009–2011 at 10 sites along a gradient of exposure to current and wind in Lake Saint-Louis, a large (148 km2) fluvial lake of St Lawrence River. Where water chemistry was conducive to L. wollei growth, wave exposure and current velocity controlled spatial and temporal biomass variations. Biomass increased from May to November and persisted during winter. Interannual variations were primarily controlled by river flow (water level), and high spring discharge dislodged mats from the previous growing season. Under climate-change scenarios, anticipated declines in water level and rising storm intensity may lead to an increase in the areas colonized by L. wollei, more frequent than present-day episodes of mat disruption, water-use impairment, and beach fouling.