André Cardinal

Community development following removal of urchins, Strongylocentrotus droebachiensis , from the rocky subtidal zone of the St. Lawrence Estuary, Eastern Canada

SummaryThe role of sea urchins, Strongylocentrotus droebachiensis, in structuring the rocky subtidal community was examined at Anse aux Basques on the north shore of the St. Lawrence Estuary, Québec. In an experimental area, measuring 20x20 m and extending from 0 to 10 m in depth, we greatly reduced the intensity of urchin grazing by eliminating all urchins larger than 10 mm in test diameter. This area was observed for two years and compared to an adjacent control area. In the upper portion of the experimental area during the first month after urchin removal, mid-July to mid-August 1978, a dense diatom cover developed, and during the second month the diatoms were overgrown by Ulvaria obscura. After four months (November) an Alaria esculenta overstory was present from near low water level to 3 m deep. Community development was much slower at greater depths and it took a year for the Alaria zone to extend to 4–5 m deep, and two years to extend to 6 m deep. The low light penetration at this estuarine location was probably the main factor for the slow algal development at 6–10 m deep. At the end of the experiment Agarum cribrosum was second in importance after Alaria and was most common at 3 to 6 m in depth. Laminaria spp. was found in low numbers in the first year and did not show an increase during the second year. There was a dramatic increase in the number of species and abundance of algae in the experimental area. Also, there was a marked increase in many animal species, particularly Acmaea testudinalis, Mytilus edulis and Margarites helicinus, and a decrease in Metridium senile. By contrast, in the control area, the number of algal and invertebrate species remained low. In the experimental area a sharp increase in the growth rate of a cohort of very small urchins, which was not eliminated by our removal effort, demonstrated that there is strong intraspecific competition amongst urchins when the food supply is limited. In the St. Lawrence Estuary, there are few predators of urchins and the urchin dominated community appears to be a stable situation.

Réponse d'une communauté de diatomées de glace à un gradient de salinité (baie d'Hudson)

Sea ice offers a physical support to the growth of microalgae (epontic community). Almost all the studies on ice microflora in the Arctic and the Antarctic have been performed in waters of high salinities, and they generally reported a very high standing crop. From February through May 1978, 15 stations were sampled in the southeastern part of Hudson Bay (Manitounuk Sound), Quebec, Canada. The peculiar physical condition of the water bodies establishes a salinity gradient in the underlying waters from the mouth of the Great Whale River to the upper reaches of Manitounuk Sound, leading to a parallel gradient in the ice dynamics. The main goal of this study was to examine the effect of these gradients on the biomass and the taxonomic composition of the epontic community. Thus, there are two distinct gradients in the ice, one ruled by the surface-water salinity gradient controlling the ice thickness and the settlement efficiency, which triggers the biomass of the epontic community (higher standing crop in the upper reaches of Manitounuk Sound). In addition, the ice salinity gradient influences the taxonomic composition (lower number of microalgae taxa toward the mouth of the Great Whale River). Salinity of the underlying surface waters, following discriminant analyses of species-based clusters of observations, emerges as a major environmental variable controlling the distribution and abundance of species. In addition to adequate light intensity and nutrient supply, salinity of the underlying waters is therefore another limiting factor to the settlement and growth of an ice microflora. This factor is of great potential significance in coastal and esturine glacial waters.

FRAGILARIA AND SYNEDRA (BACILLARIOPHYCEAE) A MORPHOLOGICAL AND ULTRASTRUCTURAL APPROACH

In this paper we discuss and illustrate the morphological and ultrastructural features of several Fragilaria and Synedra representatives. These features are often very similar in the two genera, raising once again the question of generic circumscription. New observations are presented for: Fragilaria virescens var. subsalina f. oviformis, Synedra arctica, S. camtschatica and its var. finnmarchica, S. pulchella var. macrocephala, S.fulgens, and S. undulata. Ardissonia and Toxarium should be reinstated as genera because of distinct valve characters.

AN ALTERNATE GROWTH PATTERN FOR LAMINARIA LONGICRURIS1,2

A population of Laminaria longicruris de la Pylaie was followed for a year at Bic Island, Quebec, Canada where nutrient levels in the seawater were elevated throughout the year. Tagged kelp were measured each month for growth and analyzed for alginic acid, laminaran, mannitol, carbon, nitrogen, and nitrate. Maximum growth (3.5 cm · d−1) was observed in June, and minimal growth (0.18 cm · d−1) from December to February, when ice cover limited light levels. No reserves of carbon or nitrate were formed. Laminaran levels remained below 2.7% dry weight while tissue nitrate did not exceed 0.75 μmol · g−1 dry weight. Total carbon produced per plant was 40 g C · yr−1. Nutrient availability enables the kelp to take advantage of summer light and temperature conditions to grow rapidly.

Brooding behaviour of the northern sea star Leptasterias polaris

The northern sea star Leptasterias polaris (Müller and Troschel) has a unique mode of brooding its young. The mother curves her arms along the plane of the disc, the whole individual assuming a flattened plate-like shape. In this form she covers the larvae, which are attached to the substratum beneath her. The central disc is not raised to form a brood chamber, as in related species, and there is no tendency to hide while brooding. In the St. Lawrence Estuary, brooding L. polaris are common from February through May and decline in numbers during June and early July. The genus Leptasterias originates in the Arctic and probably all species brood their young.

Intertidal microalgal production and the auxiliary energy of tides

We monitored the photosynthetic response of an estuarine epilithic microflora exposed to natural variations in water level and light intensity. The experimental community developed on an artificial substrate in the intertidal zone of the St. Lawrence estuary. Fragilaria striatula dominated the assemblage. Samples for the determination of the Photosynthesis-Irradiance curve were collected at intervals of 2 h over a period of 11 d. The initial slope of the curve (αB) and the maximum photosynthetic rate (PmB) per unit Chl a were estimated. During spring tide, wave-induced turbulence reaching the experimental substrate at low tide eroded the arborescent stratum of the cell mat. The physiological condition of the remaining prostrate stratum was poor (low Chl a/ phaeopigment ratio). The photosynthetic response of the community was weak and showed little variability. During neap tide, the arborescent stratum of the permanently inundated community persisted. The community showed a stronger and more variable photosythetic response. During this period fluctuations in the magnitude of αB and PmBwere dominated by a 24-h periodicity, but also presented a secondary semidiurnal rhythm. The ciradian periodicity in the photosynthetic response was best explained by postulating an endogenous control. Circatidal variations in PmBwere perhaps related to tidal fluctuations in nutrient availability. The fortnightly renewal of space by the auxiliary energies of wind and tides apparently controlled the dynamics of the community.

Influence of physical gradients on the structure of a northern rocky subtidal community

AbstractWe have examined the effect of the estuarine and depth gradients on community structure (diversity, richness, density, biomass) and biomass of trophic guilds in the rocky bottom subtidal community of the St. Lawrence Estuary, eastern Canada. The study was based on 199 samples (50 cm × 50 cm quadrats) distributed along transects placed perpendicular to the shore at 6 locations. Some 315 species of macrobenthos (> 1 mm) were identified. Community structure varied markedly with the depth gradient but less so with the estuarine gradient (20-28‰). Generally, the density of organisms decreased moving up the Estuary ( ≈ 300 individuals 0.25 m-2 near the mouth, 175 ind. 0.25 m−2 on intermediate transects, and 65 ind. 0.25 m−2 at the most brackish sites). Invertebrate biomass did not show a corresponding decrease along the estuarine gradient. No consistent effect of the estuarine gradient on invertebrate diversity was observed. Macroalgal diversity and biomass varied in an irregular pattern along the estua...

Variations de la teneur en acide alginique des Laminariales de l’estuaire maritime du Saint-Laurent (Québec).

Variations in the Content of alginic acid in Laminariales of the St. Lawrence Estuary (Quebec) The present study deals with a three-year analysis of variations in alginic acid content in Laminariales (Laminaria longicruris, L. digitata, L. saccharina and Alaria esculenta) in two maritime regions of the St. Lawrence Estuary (Quebec), in terms of seasons, exposure mode, size and anatomical location in the algae. A brief analysis of seasonal variations in the proportion of mannitol is also given for L. digitata and L. longicruris, in an effort to e stabil sh a correlation with seasonal variations in alginic acid content.