Diatoms from two macro-tidal mudflats in Chignecto Bay,Upper Bay of Fundy,New Brunswick,Canada

Mudflat research is dispersed among several fields (ecology, sedimentology), each with its own focus and methodology. Consequently, although the volume of mudflat literature is considerable, our understanding of mudflat ecology remains fragmented. For example, little is known about the structure of microbial communities outside Western Europe. Here we present the first North American specific composition and densities of live mudflat diatoms and relate them to properties of their environment on two closely located flats. The two flats (Daniel’s and Buck’s) were similar until the mid–1980s. After this time the biological and sedimentary environment on Buck’s Flats began to change and resulted in a precipitous decline of the keystone invertebrate Corophium volutator (Pallas). The specific diatom composition on each of the two flats examined was still very similar. Tychoplanktonic diatoms were numerically dominant on both flats. The flats differed significantly in the relative contribution of epipelic diatoms, which were about an order of magnitude greater on Buck’s Flats. CCA analysis suggests that very few of these species exist within their optimal habitat. Some of the differences appeared small, but were statistically and biologically significant. Daniel’s Flats sediments had a 30% larger mean grain size, less water and organic carbon compared to Buck’s Flats sediments. Buck’s Flats had more variable depths of the oxygenated layer, often with anoxic inclusions throughout. Daniel’s Flats supported more C. volutator, while Buck’s Flats contained greater densities of diatoms. The importance of preserving environmental conditions (sedimentary and biotic) prevailing on flats such as Danielȁ9s Flat in order to foster populations of Corophium at a level necessary to support foraging migratory shorebirds is also discussed.     

PERIPHYTIC DIATOM ASSEMBLAGES FROM ULTRA‐OLIGOTROPHIC AND UV TRANSPARENT LAKES AND PONDS ON VICTORIA ISLAND AND COMPARISONS WITH OTHER DIATOM SURVEYS IN THE CANADIAN ARCTIC1

Periphytic diatoms are potentially powerful indicators of environmental change in climatically‐sensitive high latitude regions. However, only a few studies have examined their taxonomic and ecological characteristics. We identified and enumerated diatom assemblages from sediment, rock, and moss habitats in 34 ultra‐oligotrophic and highly transparent lakes and ponds on Victoria Island, Arctic Canada. The similar limnological characteristics of the sites allowed us to examine the influence of habitat, independent of water chemistry, on the diatom assemblages. As is typical in shallow arctic water bodies, benthic taxa, including species of Achnanthes, Caloneis, Cymbella, Navicula, and Nitzschia, were most widely represented. Minor gradients in our measured environmental variables did not significantly explain any variance in diatom species, but there were marked differences in diatom assemblages among sites. Pond ephemerality seems to explain some diatom variation, because aerophilic taxa such as Achnanthes kryophila Petersen and A. marginulata Grunow were dominant in shallow sites that had undergone appreciable reductions in volume. We identified several taxa that exhibited strong habitat preferences to sediment, moss, or rock substrates and also found significant differences (P < 0.01) in diatom composition among the three habitats. In comparisons with three similar diatom surveys extending over 1200 km of latitude, we determined that surface sediment assemblages differed significantly (P < 0.001) among all regions examined. Diatom species diversity was inversely related to latitude, a result likely explained by differences in the lengths of growing seasons. These data contribute important ecological information on diatom assemblages in arctic regions and will aid in the interpretation of environmental changes in biomonitoring and paleolimnological studies.     

Structure and diversity of intertidal benthic diatom assemblages in contrasting shores: a case study from the Tagus estuary1

The structure of intertidal benthic diatoms assemblages in the Tagus estuary was investigated during a 2‐year survey, carried out in six stations with different sediment texture. Nonparametric multivariate analyses were used to characterize spatial and temporal patterns of the assemblages and to link them to the measured environmental variables. In addition, diversity and other features related to community physiognomy, such as size‐class or life‐form distributions, were used to describe the diatom assemblages. A total of 183 diatom taxa were identified during cell counts and their biovolume was determined. Differences between stations (analysis of similarity (ANOSIM), R = 0.932) were more evident than temporal patterns (R = 0.308) and mud content alone was the environmental variable most correlated to the biotic data (BEST, ρ = 0.863). Mudflat stations were typically colonized by low diversity diatom assemblages (H′ ~ 1.9), mainly composed of medium‐sized motile epipelic species (250–1,000 μm³), that showed species‐specific seasonal blooms (e.g., Navicula gregaria Donkin). Sandy stations had more complex and diverse diatom assemblages (H′ ~ 3.2). They were mostly composed by a large set of minute epipsammic species (<250 μm³) that, generally, did not show temporal patterns. The structure of intertidal diatom assemblages was largely defined by the interplay between epipelon and epipsammon, and its diversity was explained within the framework of the Intermediate Disturbance Hypothesis. However, the spatial distribution of epipelic and epipsammic life‐forms showed that the definition of both functional groups should not be over‐simplified.     

SPATIAL AND TEMPORAL VARIATION OF EPIPSAMMIC DIATOMS IN A SPRING-FED BROOK1

Epipsammic diatom communities were sampled and quantified bimonthly from June 1983 to April 1985 at four sites in a small brook in northwest Ohio. The primary objective was to determine the extent of temporal and spatial variability in the epipsammon. The four sites, approximately 250 m apart, differed in current velocity, illumination and the amount of sand-associated detritus. Diatoms were significantly more abundant in the portion of the brook flowing through an unshaded marsh than at three heavily shaded sites located upstream. Fragilaria leptostauron (Ehr.) Hust. and Achnanthes lanceolata var. dubia Grun. dominated the epipsammic assemblage at all sites throughout the entire study period. Only Meridion circulare (Grev.) Ehr. displayed a marked seasonal distribution. Although the same species were generally found at all sites, there were sufficient differences in relative abundance that communities could be discriminated according to site. Using two canonical variate axes, all 48 samples were correctly assigned to their proper sampling site based on community composition. Data from this epipsammic assemblage support the idea developed from the river continuum concept that species comprising riverine benthic assemblages continually persist and rarely become completely absent.     

VARIATION IN DIATOM COMMUNITY STRUCTURE AMONG HABITATS IN SANDY STREAMS1

Community structure of benthic diatoms in two sandy streams was studied for two summers to assess differences in abundances and species composition among habitats and between years and streams. The greatest differences in abundances (cells·cm^?2) were lower abundances on rocks than on sand, whether the sand was clean or covered with flocculent organic material. Relatively little variation in abundances occurred between years and streams. Species composition of diatom communities varied more among habitats and between streams than from year to year. Species composition was most unique in floc-covered sand when communities in clean and floc-covered sand, rock, plant, and plankton were compared. Diatom species composition in these sandy streams was most similar on sand and rocks.     

The Role of Substrate Type on Benthic Diatom Assemblages in the Daly and Roper Rivers of the Australian Wet/Dry Tropics

The selection of one or more river substrata for the collection of benthic diatoms is fundamental to any monitoring or research programme because it can potentially bias the diatom data set. In the wet/dry tropics of Australia, where the use of benthic diatoms for river health assessment is in its infancy, the comparability of diatom assemblages on river substrata has been assessed. Benthic diatoms were sampled from seven river sites, with a range of ionic chemistries (conductivities 27–6500 μS cm⁻¹) but low nutrient concentrations. At each site, triplicate samples were collected from 3 to 6 substrata. The diatom assemblages sampled were: epilithon (assemblages on rock), epiphytes on macroalgae and macrophytes, epidendron (assemblages on wood), epipsammon (assemblages on sand), epipelon (assemblages on mud) and bacterial slime. The variability between substrate assemblages, at each site, were assessed according to the following: (1) a multivariate analysis of diatom assemblages, (2) ANOVA tests of species richness, (3) ANOVA tests of the relative abundance of common species (defined by an abundance of at least 10% in any one sample), and (4) a comparison of the number of species unique to a substrate. A total of 198 taxa were identified, with some taxa common to temperate Australia. Common species were found on all substrata, with sometimes statistically significantly different relative abundances. Taxa unique to a substrate had low relative abundances (0.1–2%), were most often found on only one replicate, and are unlikely to be substrate specific because many are known to occur on other substrata. The assemblages on hard substrata, epilithon and epidendron, were found to be most similar. Diatom assemblages on macroalgal and macrophyte substrata, compared to other substrata, were highly variable. This is attributed to the loss of diatoms from grazing and sloughing, followed by recolonisation of newly exposed substrate. Other assemblages, notably epipsammon, were similar to epilithon and epidendron but sometimes differed in their relative abundance of common species. The principal finding of the study was the similarity of the epilithon and epidendron, which are considered to be indistinguishable. Rock and wood hard substrata can be substituted for one another during field surveys, thereby increasing the number of potential sample sites available for monitoring activities that standardise to a hard substrate.     

Sexual reproduction in British Ectocarpus Siliculosus (Phaeophyta)

One male and one female gametophyte of Ectocarpus siliculosus have been detected in a sample of 19 plants taken from the Isle of Man. Fertilization takes place between gametes formed in plurilocular gametangia. Clear cross-fertility exists with Mediterranean strains. This result confirms the notion that the concept of the gametic character of meiospores in Ectocarpus should be abandoned.     

Minutes of the Ninth Annual General Meeting

In unialgal culture, Gymnodinium pseudopalustre Schiller (G.p.) and Woloszynskia apiculata sp. nov. (W.a.) multiply respectively by binary fission in the motile state and by motionless zoosporangia, releasing 2, 4 or 8 zoospores. Both species are isogamous, but G.p. is homothallic, W.a. heterothallic. Fusion of the planogametes leads to long-lived planozygotes, which retain two posterior flagella and, while enlarging, assume specific morphologies. The motile stage of the zygotes is terminated by formation of hypnozygotes (resting spores), globular and spiny in G.p., grossly fusiform (‘horned’) and tubercled in W.a. The composition of the hypnozygote walls is described. After their dormancy has been broken by a cold treatment of several weeks in the dark, hypnozygotes of both species germinate when brought back to higher temperature and light. In so doing, those of G.p. excyst one posteriorly biflagellate swarmer as a meiocyte, which, after a stage of nuclear cyclosis or, in karyological terms, zygotene through postzygotene, undergoes two steps of binary fission in the motile stage, separated by several days. In W.a., cyclosis as well as the first meiotic cell division occur inside the closed wall of the hypnozygote (now a meiocyte); thereafter either two swarmers escape and undergo the second meiotic division in a separate zoosporangium or, alternatively, second meiotic nuclear and cell divisions also take place inside the spore wall and four swarmers are finally excysted. Some aspects of dinoflagellate life cycles and taxonomic questions are discussed.