Nitrogen-fixation in the littoral benthos of an oligotrophic lake

Blue-green algae are common in the benthos of Mirror Lake, New Hampshire (U. S. A.) — on macrophytes and on the lake bottom-and are probably responsible for the variable, sometimes high rates of N-fixation that detected by a series of acetylene-reduction assays during September and October.     

The Seasonal Succession of Epiphytic Communities within an Equisetum fluviatile L. Stand in Lake Pääjärvi,Southern Finland

The seasonal succession of epiphytic communities on Equisetum fluviatile was controlled both directly and indirectly by the macrophyte. Decaying macrophytic material supported rich algal growth and biomass accumulation in spring and early summer. Emergence of the macrophytes severely reduced underwater light availability, and the epiphytic algal biomass declined rapidly as a result of both lower photosynthetic activity of the epiphytic algae, and more intensive grazing by invertebrate herbivores. Epiphytic N: P ratios were lower than those in the water around suggesting that either the water was not the only source of phosphorus for the epiphytic algae or the algae took phosphorus up selectively from the water. Low epiphytic C: N ratios suggested a high potential nutritional value for herbivores.     

RESPONSE OF MICROALGAL EPIPHYTE COMMUNITIES TO NITRATE ENRICHMENT IN AN EELGRASS (ZOSTERA MARINA) MEADOW1

The community structure and productivity of epiphytic microalgae on field populations of eelgrass (Zostera marina L.) from a high flow regime were characterized under water-column nitrate enrichment over a 30–d period during the autumn growing season for the macrophyte. Epiphyte communities in replicate low-nitrogen sites (LOW-N, median water-column nitrate concentrations below detection) were compared to communities in replicate N-enriched sites wherein nitrate was leached from clay pots filled with enriched agar (N-ENRICH, median concentration ca. 6 μM NO₃^?_-N; pots replaced at 8– to 12–d intervals). In experimental chambers, total epiphyte community productivity as ¹⁴C-bicarbonate uptake was determined from short-term (3–h) laboratory assays. Track light microscope-autoradiography enabled estimates of species-specific productivity for abundant algal taxa. After 6 d in the LOW-N and N-ENRICH communities, the crustose adnate red alga Sahlingia subintegra (Rosenvinge) Kornmann was dominant in terms of cell number and codominant in biovolume. Photosynthetic dinoflagellates, not previously reported as abundant in eelgrass epiphyte communities, were dominant in biovolume contribution after both 6 and 30 d in LOW-N communities. Nitrate enrichment stimulated the adnate monoraphid diatom Cocconeis placentula Ehr. but apparently inhibited dinoflagellates and the diatom Melosira sp. Total productivity of the epiphyte communities remained comparable in both the LOW-N and N-ENRICH sites. Shifts in community structure and species-specific productivity, however, indicated a controlling influence of nitrate supply on microalgal epiphytes in the field eelgrass community.     

Selenicereus wittii (Cactaceae): An epiphyte adapted to Amazonian Igapó inundation forests

The biology, ecology, and distribution ofSelenicereus (Strophocactus)wittii, one of the least known taxa ofCactaceae, are described. This epiphyte climbs appressed to tree trunks with leaf-like, flattened stems and is found exclusively along the high waterline of black water rivers (Rio Negro, Vaupés, Apaporis) in the Igapó inundation forests of Amazonia. Ecophysiologically,S. wittii is a crassulacean acid metabolism (CAM) plant. It bears white, nocturnal flowers 25 cm in length which emit a fragrance consisting mainly of benzylalcohol, benzyl benzoate, and benzyl salicylate. They exhibit an extreme sphingophilous syndrome as an adaptation to pollination by probably only two species of hawkmoth from the generaAmphimoena andCocytius. The seeds, aberrant for the family, contain air-filled chambers and are water-dispersed. Thus,S. wittii represents the paradoxical life form of an hydrochorous epiphytic cactus which withstands periodical inundation.Dedicated to emer. Univ.-Prof. DrFriedrich Ehrendorfer on the occasion of his 70th birthday     

Elements of disturbance that affect epiphyte vitality in a temperate rainforest: an experimental approach

Aims Epiphytes are an abundant and diverse component of many wet temperate forests and have significant roles in ecosystem processes. Little is known about the processes and rates of their death and decomposition when they fall from the canopy, which limits our understanding of their role in forest carbon sequestration and nutrient cycling. In the temperate rainforest of the Quinault River Valley, Washington State, our aim was to test hypotheses regarding four elements of disturbance that might contribute to their decline.     

Spatial variation in host preference in the endangered epiphytic bromeliad Tillandsia carlos-hankii

Although most vascular epiphytes are generalist, some exhibit host preferences. The latter may vary spatiotemporally driven by variation in environmental conditions. However, to our knowledge no previous studies have evaluated this variation. The aim of this study was to analyze variation in host preferences and population structures in the endangered bromeliad Tillandsia carlos-hankii in two pine-oak forests which differ in environmental conditions, tree composition and community structure. Our questions were: i) does Tillandsia carlos-hankii prefer certain host species over others?; ii) does this preference vary between sites?; iii) how does population structure vary between hosts species and sites? The two sites chosen were an east-facing (I_NE) and a west-facing slope (C_NW). The tree community was sampled within an area of 0.1 ha at each site. Preferred host species were identified using two criteria: a) the degree of colonization; and b) the probability of colonization. In addition, population size-structure on each colonized tree was evaluated to compare between sites and host species. The results showed that almost every tree in the community was colonized by this bromeliad. At both sites Quercus rugosa was a preferred host species, while Q. laurina and Mirsyne juergensenii were preferred at I_NE but limiting at C_NW. In turn, Pinus teocote was limiting at both sites. The percentage of small trees that were colonized depended on species identity. Population structures differed between sites and host species. Thus, both host preferences and local population behavior vary spatially given the differential conditions associated with the two sites.