Stéphane Campeau

Diatoms as quantitative paleodepth indicators in coastal areas of the southeastern Beaufort Sea, Arctic Ocean

Abstract Surface sediment diatom assemblages in depth profiles along the southeastern Beaufort Sea coasts (Canada) were analysed to describe the relationship between species distribution and water depth of deposition. The 74 coastal stations sampled were distributed in several sedimentary environments, from salt marshes to the inner shelf. The relationship between diatom species distribution and water depth was examined using canonical correspondence analysis (CCA) and partial CCA. The water depth accounted for 9.7% of the variance in the data set. The diatom/water-depth relationship is believed to be controlled by the shoreface circulation which results in a shift in the relative abundance of the epipsammon, epipelon, and plankton along the water depth gradient. Upper shoreface assemblages are dominated mainly by epipelic species and a few number of epipsammic taxa, whereas deeper environments are dominated by planktonic species. A transfer function was derived using weighted averaging regression and calibration to reconstruct the water depth of deposition over the coastal interval from mean sea level to a maximum of 14 m depth on the coastal shelf of the southeastern Beaufort Sea. The transfer function allows relative sea level to be predicted from fossil diatom assemblages with a root mean square error of 1.43 m. This model is likely to produce reliable water depth inferences along the southeastern Beaufort Sea coasts and in other shallow sandy microtidal environments dominated by low energy waves and influenced by freshwater discharge. The transfer function was used to provide quantitative paleodepth inferences for late Holocene sediments from the Atkinson Point area based on fossil diatom assemblages.

Diatoms and biomonitoring: should cell size be accounted for?

Despite the fact that biovolume calculation is a common procedure in most phytoplankton and periphyton studies, diatom community analyses are usually based on relative abundance data. In a biomonitoring context, a community metric that accounts for cell size could be of interest due to the potential differences that might exist in nutrient uptake between large and small-sized species. This paper addresses the question of whether diatom community analysis should be based on relative abundance, biovolume or cell surface. The results show that although community structure expressed as relative proportion of taxa varied according to the metric used, the ordinations conducted with each metric were similar. The explained percentage of species variance was slightly higher with the relative abundance metric compared to the metrics based on relative biovolume or cell surface area. Partial CCAs showed that each water chemistry variable generally explained a higher portion of species variance when the relative abundance was used. The analyses conducted with two size groups (small and large taxa) expressed as relative abundance and relative biovolume showed similar results. Moreover, our data showed that there is no significant relationship between diatom size and total phosphorus. According to these results, it seems that relative abundance would be the most appropriate metric to use for biomonitoring purposes. The biovolume and cell surface area calculations added substantially to the total analysis time due to the numerous measurements required, but did not improve the variance explained in community structure, and site ordinations were not significantly different.

Diatom typology of low-impacted conditions at a multi-regional scale: combined results of multivariate analyses and SOM

Benthic diatoms have been used for decades as indicators of stream water quality and environmental stress. While classification systems and monitoring methods have been developed mostly in Europe, the search for main factors determining assemblages at various scales has been mainly conducted on the American continent. We analysed a selection of 467 diatom records from stream with minimal human impact, from several countries and regions of Western Europe, using different multivariate techniques and artificial neural networks (ANN). The data matrix contained 123 diatom taxa X 23 environmental variables, and covered 35 major catchments. Data processing involved the use of PCA (Principal Component Analysis), DCA (Detrended Correspondence Analysis), CCA (Canonical Correspondence Analysis) and SOM (Self Organizing Maps). Multivariate analyses were useful for identifying the main environmental gradients, and combination of these analyses and SOM enabled to define 10 ecological groups, composed of key indicator taxa. Some of these groups could be identified as corresponding to near-natural conditions, allowing the definition of a biotypology of benthic diatom along a gradient of alkalinity, conductivity, pH – mainly determined by geological features – and a temperature/elevation gradient. Sensitivity analysis and box-plots of environmental variables helped identify the main factors determining stream conditions for these assemblages, and slightly altered conditions or particular situations were easily detected. Several possible bias were identified, either from imbalance among river types in the database, or from taxonomic and identification problems, or from collection of records from various sources. Taxa distribution maps, obtained from the SOM, have been used as a useful mean for representing auto-ecological properties of benthic diatoms and for identifying dual distributions resulting either from errors, from incorrect taxonomic status or from actual ecological differences within a same taxon. On the basis of available information, factors determining diatom assemblages are similar in different regions and even continents, which raises the question of the relevance of the eco-regional approach for this stream community.

A large-scale stream benthic diatom database

A relational database linking benthic diatom records, taxonomic nomenclature including synonyms, and corresponding environmental data has been built in MS Access. It allowed flexible and long-term use of a relatively important amount of data (∼3000 records) gathered in the framework of the EC-funded PAEQANN project, gathering precise and documented information both about benthic diatoms and quantitative or semi-quantitative environmental data. Such a database has been shown to be a useful tool for the definition of benthic diatom typology at a multi-regional scale, the prediction of the impact of environmental characteristics on the structure of diatom communities, and additionally for a new insight on the auto-ecology of some taxa. This database could serve as a template for further work on diatoms and, after some implementation, on other freshwater communities. It could also be the basis for wider typology of stream diatoms, extended to other regions.

Three-dimensional hydrostratigraphical modelling of the regional aquifer system of the St. Maurice Delta Complex (St. Lawrence Lowlands, Canada)

In the central part of the St. Lawrence Lowlands (Mauricie, Québec), Late Quaternary deglacial events led to the formation of a series of complex granular aquifers, such as those in the (1) Saint-Narcisse morainic complex, (2) paleodelta formed by the Saint-Maurice River, and (3) sandy littoral terraces left during marine regression. The aquifers are an important supply of potable water for most municipalities in the region, including the City of Trois-Rivières, which is a mid-size city where groundwater accounts for 46% of the water supply. The main objectives of this study were to build a three-dimensional (3D) model of the Quaternary deposits to define the main hydrogeological contexts of the Mauricie region and to characterize the regional aquifers. The compilation of existing hydrogeological data led to the selection of 5386 well logs that contained stratigraphic information of variable quality, ranging from only surficial sediment thickness to descriptions of fully cored boreholes. To supplement the existing data, fieldwork was undertaken in areas where few data were available, including 63 km of high-resolution seismic reflection surveys and 34 new boreholes. The final 3D model consists of six layers, from the bedrock surface to the upper littoral and deltaic sands. The total thickness of the deposits ranges from zero, on bedrock outcrops, to 150 m beneath central Trois-Rivières. Taking into account the thickness of the saturated layer and the porosity of the sand and gravel, the upper unconfined aquifer contains an estimated 364 million m3 of water. The 3D model helped refine the understanding of regional aquifers and was used to identify unexploited aquifers, notably around the Saint-Narcisse morainic complex and along the St. Cuthbert Fault. The model clarified the regional stratigraphic architecture, especially topography of the bedrock surface, the lateral extent of Late Quaternary sands and development of Holocene post-glacial sediment sequences.

Gomphonema caperatum sp. nov., G. obstipum sp. nov. and similar taxa from rivers of North America

Two new species of Gomphonema with linear-lanceolate valves, narrow cells (width ≤6 µm), wide axial areas, short uniseriate striae, and one stigmoid per valve are described. Gomphonema caperatum sp. nov. has narrow, almost linear valves with short marginal striae, apically elongated slit-like areolae, and transapically oriented linear markings on the external valve surface, which are only observable under scanning electron microscopy. This is a common and often abundant species in rivers across Eastern North America, especially in the Appalachian region, but is also found on the West Coast. Gomphonema obstipum sp. nov. has lanceolate valves with bent apices, variously shaped apically elongated areolae, and shallow circular depressions on the external valve surface. This species has only been found in two Virginian rivers. We compare the new taxa with several species of Gomphonema from North American rivers that are morphologically similar, such as Gomphonema amerhombicum Reichardt, Gomphonema stoermeri Kociolek & Kingston, and other species with linear-lanceolate valves and wide axial areas. In addition, we report here for the first time the occurrence of Gomphonema incognitum in North America. This study underscores the insufficiency of our knowledge of diatom diversity in North America and the need for more detailed taxonomic investigations.