Hydrobiologia

Two distinct morphotypes ofChydorus sphaericus are recognised in Ireland. A reference collection of scanning electron micrographs of male and femaleC. sphaericus sens. str. has been compiled. This collection is used to compare the varying morphological features of Danish and Irish material to determine their taxonomic significance. However, no taxonomic decisions can be made about the Irish morphotypes until mature males and ephippial females become available. Comparison of population structures and stages of reproduction are accomplished by size-frequency distribution. This study illustrates the ever increasing value of scanning electron microscopy (SEM) in chydorid taxonomy, where characters such as shell-reticulation features may easily be misinterpreted using only light microscopy.

This study addresses protozoan population dynamics of two southeastern blackwater rivers in Georgia: the sixth order Ogeechee River and fourth order Black Creek. The density and composition of the protozoan community was assessed by counting the protozoans into size classes. Particularly abundant flagellate types (bloom forms) and ciliates were enumerated separately. Protozoan density in these blackwater rivers is high. Observations during a two week and a ten month census showed that the rate of community change is rapid and that the range in diversity is great. A backwater site showed the highest percentage of bloom forms, while the main channel sites of Black Creek and the Ogeechee River had fewer bloom forms. Rapid increases in density during a two week microcosm study were attributed to isolation of the water-column from filter-feeders, the benthos, and different source areas. An analogous situation may exist in the low order swamp streams and backwater habitats where many dense blooms occur. The high protozoan population densities observed were partially due to rapid changes in the physical characteristics of the environment, reactions of the protozoa to such changes, growth in the swamp tributaries and backwater areas, subsequent washing into the river, and further growth as the water moves toward the sea.

Hybridization is a common phenomenon in Daphnia species complexes. Hybrids often dominate in Daphnia populations; therefore it is worthwhile to look for principal differences between parental and hybrid populations with respect to their genetic structure and clonal differentiation. We studied natural populations of members of the Daphnia galeata/hyalina/cucullata complex in three lakes. In one of these lakes, one parental species (D. galeata) and one hybrid (D. galeata × cucullata) were investigated more intensively. The frequency of sexual reproduction was higher in parental populations, whereas clonal diversity was higher in hybrid populations. Ecological differentiation among clonal groups was more pronounced in the D. galeata × cucullata hybrid compared to D. galeata, whereas selection intensity was weaker. These results are discussed with respect to stability of clonal groups, multiple hybridizations and selective constraints.

Rates of inorganic nutrient and oxygen fluxes, and gross community primary productivity were investigated using incubated cores in July, August and September 2001, in a seagrass meadow of Lake Illawarra, a barrier estuary in New South Wales, Australia. The results indicated that rates of gross primary productivity were high, varying from C = 0.62 to 1.89 g m−2 d−1; low P/R ratios of 0.28–0.48 define the system as heterotrophic and indicate that more carbon is respired than is produced. In order to determine the effect of macroalgae on O2 and nutrient fluxes, measurements were also conducted on cores from which the macroalgae had been removed. The results showed that the O2 fluxes during light incubations were significantly lower in the cores without macroalgae (P<0.01), indicating that macroalgae could be a significant contributor to the primary production in the lake. In general, nutrient fluxes showed a typical diurnal variation with an efflux from sediments in the dark and a reduced efflux (or uptake) in the light. Dissolved inorganic nitrogen (NO2 −+ NO3 −+NH4 +) net fluxes were directed from the sediments towards the water column and dominated by the NH4 + fluxes (>80%). NO2 −+ NO3 − and o-P fluxes were always very low during the sampling period. The increasing tendency of net nutrient effluxes, especially NH4 + from July to September, is consistent with the increase of the water temperature and seagrass biomasses. However, in September, significantly lower light, dark and net NH4 + effluxes were found in the cores with macroalgae (SA-sediments) compared with the cores without macroalgae (S-sediments). These results support the hypothesis that actively-growing dense macroalgal mats (i.e., algal blooms in September) may act as a filter reducing the flux of nutrients to the water column.

We compare ‘temperate data’ with information contained in 49 surveys of cryptomonads from Brazilian tropical regions. These surveys contain a total of 351 records relative to a total of 38 taxa at the species level or below. All taxa were identified using traditional morphological characters visible with the light microscope. Two taxa appear to be new species, and amongst the known species Cryptomonas erosa Ehrenberg is the most frequently recorded one. To date two species of Pseudocryptomonas Bicudo & Tell (all containing multiple photosynthetic endosymbionts/chloroplasts) and the species Cryptomonas brasiliensis Castro, C. Bicudo & D. Bicudo have not been found outside Brazil. With respect to their geographical distribution in Brazilian tropical regions, cryptomonads are better represented in reservoirs and coastal lagoons located in the south-eastern part of the country. Some species appear to be restricted to brackish coastal ecosystems (oligo- to mesohaline and meso- to eutrophic waters). There are only four records of marine cryptomonads. A comparative study of three different kinds of shallow aquatic environments in Brazil – north-eastern reservoirs, a flood-plain Amazonian lake, and brackish lagoons on the south-eastern coast – shows that cryptomonads are to be found constantly, and their species number density, and biomass are generally low (usually <10%) in relation to the other phytoplankton. The case of Plagioselmis Butcher emend. Novarino, Lucas & Morrall, amongst others, exemplifies the great difficulties encountered while attempting to interpret biogeographical information based on light microscopy within the context of a modern systematic and taxonomic framework based on electron microscopy.

Molecular data were used to evaluate the validity of the genus Neopisosoma Haig, 1960. Comparisons of morphological features within Neopisosoma suggest the existence of two lineages, represented among others, by N. angustifrons (Benedict, 1901) and N. neglectum Werding (1986). Both lineages of Neopisosoma are more similar to two morphologically different species groups of the genus Pachycheles, than to congeners of the other group. Comparative morphology of larvae from N. angustifrons, N. neglectum and species of Pachycheles shows that N. angustifrons closely resembles Pachycheles species, whilst N. neglectum is set apart. Sequences of a 465 bp segment of the mitochondrial gene cytochrome oxidase I (COI) were obtained and used to infer phylogenetic relationships among N. angustifrons, N. neglectum, one species of Pachycheles and seven other species of porcellanids, representing three other genera. Results of the molecular analysis were congruent to results of comparative morphological studies of larvae: N. angustifrons grouped with the Pachycheles species, whereas N. neglectum is placed apart. This led us to the conclusion that the genus Neopisosoma is probably paraphyletic and that the criterion used by Haig (1960) is not reliable to define the genus. A revision on a world-wide basis of the genera included, and additional sequence information will be necessary to satisfactorily resolve relationships within the Porcellanidae.

In the summer of 1994, floodplain habitats of the Atchafalaya River Basin were surveyed for cladocera and copepoda. Collection sites were grouped into three distinct habitat types (black-water, brown-water and green-water) based on a principal components analysis of five hydrographic variables (current velocity, Secchi disk depth, surface percent saturation of dissolved oxygen, dissolved oxygen differential and surface water temperature). An ANOVA of four community indices (total abundance, Shannon–Weiner diversity (H'), richness, and evenness) was performed on both cladoceran and copepod communities among the three floodplain habitats. Common species were compared among habitats (χ2 goodness-of-fit) to determine where they were most abundant. Green-water habitats had the greatest overall abundance of cladocerans and copepods (dominated by Diaphanosoma birgei, Moina micrura and Mesocyclops edax), but ranked lower in diversity and evenness than black-water and brown-water habitats where Ilyocryptus spinifer, Simocephalus serrulatus, Macrocyclops albidus (black-water) and Bosmina longirostris and Acanthocyclops vernalis (brown-water) were most abundant, respectively. These results indicate that the mosaic of floodplain habitats within large temperate river systems support unique zooplankton communities, and that these habitats are largely a function of seasonal hydrographic features.

Numbers of oligotrophic bacteria (on ‘Microcosm-agar’, a low nutrient medium) fluctuated between 4.4 and 88.103 cfu · ml−1 during 1977–1979. The measured environmental variables (water level, suspended solids, temperature, conductivity, pH and phosphate), explained 79% of this variability. The epipsammic, free-living, mesophilic (35 °C), oligotrophic and psychotrophic bacteria (3 °C) were also quantified. The partial correlation analysis showed that the main factor influencing the epipsammic population was one directly linked to rainfall: the suspended solids concentration (r = 0.826, p < 0.001). This suggests that these oligotrophs are predominantly terrestrial in origin and that the river would mainly represent a means of transport. The presence of free-living was more closely related to the nutritional charateristics of the environment, thus allowing to consider them as ‘autochtonous’ oligotrophic bacteria. The significant correlations between oligotrophs and some other ecological-trophic groups (hyper- and mesotrophic) suggest that their presence is controlled by common environmental factors.

Les auteurs comparent un lot de 114 bactéries prélevées dans des eaux eutrophes situées devant le delta du Rhône à deux lots de souches appartenant également au bactérioplancton méditerranéen, le premier prélevé dans des eaux oligotrophes (200 souches), le deuxième constitué par des germes épiphytes-saprophytes d'algues planctoniques (57 souches). Plus de 50% des bactéries épiphytes-saprophytes ont des besoins vitaminiques; peu d'entre elles se développent sur le glucose et les acides organiques; la plupart exigent des milieux complexes. Au contraire, les souches de pleine eau présentent un grand pouvoir de synthèse qui leur permet de croître sur des substrats simples. Dans l'ensemble, le glucose n'est pas un substrat carboné universellement assimilé en présence d'azote minéral, au contraire des acides aminés, qui sont rapidement et efficacement utilisés. Cette utilisation préférentielle des acides aminés serait plus marquée pour les bactéries des eaux oligotrophes que pour celles des eaux eutrophes.