Dominant Size Class Research Articles

A review of the biology and ecology of Calanus agulhensis off South Africa

Calanus agulhensis is the dominant large copepod on South Africa's Agulhas Bank, and is an important food item for pelagic fish. This paper reviews current knowledge and understanding of its biology and ecology and includes comparisons with other Calanus species. Its distribution is influenced by the prevailing hydrography, being advected from the eastern to the western Agulhas Bank and then north along the edge of the west coast shelf. Highest biomass of C. agulhensis is on the central Agulhas Bank in the vicinity of a cool ridge of upwelled water that is thought to enhance local retention. Daily vertical migration on the Agulhas Bank is linked to food concentration, with a strong correlation between the extent of migration of large stages (C4-female) and food abundance in the chlorophyll-rich layer. A diel feeding rhythm has been observed, independent of whether or not animals are migrating vertically. Females offered natural assemblages show a preference for the larger particles of the dominant size classes. Development time from egg to adult (20.3 d at 15.5°C) is fast compared with other Calanus species. The long N3 stage duration suggests that it is the first feeding stage. Isochronal growth is approximated, but the equiproportional rule is not adhered to. In the field, growth rate is influenced more by food than by temperature, particularly larger stages that are more frequently food-limited. This is probably because the small phytoplankton cells that dominate at warm temperatures are generally at low concentrations (<2 mg chlorophyll a m−3), insufficient for fast growth of large stages. Egg production following short periods (1–3 d) of starvation returns quickly to normal on the reintroduction of food, an adaptation that may be beneficial in the relatively stable food environment of the Agulhas Bank. Following long periods (9 d) without food, however, many females are unable to regain normal levels of egg production. At present, there is no information on the extent of omnivory in C. agulhensis, and little on the ecology of the naupliar and young copepodite stages. These may be fruitful areas for future research.

Zooplankton assemblages and biomass during a 4-period survey in a northern Mediterranean coastal lagoon

The authors proposed to examine zooplankton biomass at three stations inside (T and Z) and outside (M) a coastal lagoon of the north-western Mediterranean Sea. Station T represented the lagoon central area, and station Z was positioned in a shellfish farming sector, while the seaside station (M) served as a reference of marine environment. Analyses were designed to outline the net zooplankton assemblages (taxonomic structures and length distributions) in different environmental conditions, including the farming activity. A discriminant analysis of environmental variables determined that temperature, salinity and phytoplankton implied mainly in spatial pattern of the samples. An ordination of taxa biomasses showed two main factors which might contribute to the organisation of the zooplankton assemblages: the geographical position and the thermal period. The geographical position integrated the lagoon–sea water exchange under forcing parameters (habitat, tides and winds). The thermal period reflected both the populations development cycles and the environmental constraints (temperature, salinity, trophic resources). The resulting effects appeared in structured zooplankton assemblages in space and time. The number of 50 μm interval length classes and of taxa decreased from the seaside and the lagoon central area free of farming activity to the shallower farming zone. But the biomass-length distribution profiles did not closely follow such an expected opposition between opened and confined areas: more extended profiles were observed at station Z. Biomass dominant size classes concerned the range up to 300 μm. This size category appeared to collapse in terms of biomass from the seaside or central area of the lagoon towards the farming area, similarly to zooplankton global biomass fluctuations. Difference between biomass levels and between biomass structures suggested that net zooplankton partly acted as food competitors of macro-filtering organisms, and as preys for farming shellfish and associated epifauna. This impact mainly concerned microzooplankton populations.

Differences in leaf gas exchange and water relations among species and tree sizes in an Arizona pine-oak forest

We compared leaf gas exchange and water potential among the dominant tree species and major size classes of trees in an upland, pine-oak forest in northern Arizona. The study included old-growth Gambel oak (Quercus gambelii Nutt.), and sapling, pole, and old-growth ponderosa pines (Pinus ponderosa var. scopulorum Dougl. ex Laws.). Old-growth oak had higher predawn leaf water potential (Psi(leaf)) than old-growth pine, indicating greater avoidance of soil water stress by oak. Old-growth oak had higher stomatal conductance (G(w)), net photosynthetic rate (P(n)), and leaf nitrogen concentration, and lower daytime Psi(leaf) than old-growth pine. Stomatal closure started at a daytime Psi(leaf) of about -1.9 MPa for pine, whereas old-growth oak showed no obvious reduction in G(w) at Psi(leaf) values greater than -2.5 MPa. In ponderosa pine, P(n) and G(w) were highly sensitive to seasonal and diurnal variations in vapor pressure deficit (VPD), with similar sensitivity for sapling, pole, and old-growth trees. In contrast, P(n) and G(w) were less sensitive to VPD in Gambel oak than in ponderosa pine, suggesting greater tolerance of oak to atmospheric water stress. Compared with sapling pine, old-growth pine had lower morning and afternoon P(n) and G(w), predawn Psi(leaf), daytime Psi(leaf), and soil-to-leaf hydraulic conductance (K(l)), and higher foliar nitrogen concentration. Pole pine values were intermediate between sapling and old-growth pine values for morning G(w) and daytime Psi(leaf), similar to sapling pine for predawn Psi(leaf), and similar to old-growth pine for morning and afternoon P(n), afternoon G(w), K(l), and foliar nitrogen concentration. For the pines, low predawn Psi(leaf), daytime Psi(leaf), and K(l) were associated with low P(n) and G(w). Our data suggest that hydraulic limitations are important in reducing P(n) in old-growth ponderosa pine in northern Arizona, and indicate greater avoidance of soil water stress and greater tolerance of atmospheric water stress by old-growth Gambel oak than by old-growth ponderosa pine.

Changes in cell volume of bacteria and heterotrophic nanoflagellates in a hypereutrophic pond

Changes in cell volume of planktonic bacteria and heterotrophic nanoflagellates (HNF) were examined in a hypereutrophic pond from April to October, 1997. There were marked changes in the abundance of bacteria, HNF and ciliates and in protistan bacterivory during this period. The cell volume of free-living bacteria (0.121 ± 0.031 μm3, mean ± SD) was large relative to that reported in the literature. The cell volumes of HNF was 71.1 ± 24.8 μm3. Both cell volumes did not follow a seasonal trend. The dominant size class of bacteria was seasonally variable, whereas density of filamentous bacteria was relatively high between August and September. Biomass of filamentous bacteria accounted for up to 33.6% of total bacterial biomass. A correlation analysis for cell volume of bacteria and HNF, density of filamentous bacteria and some microbial variates was performed. The positive correlations detected (p<0.05) were between density of bacteria and cell volume of HNF, and between density of filamentous bacteria and cell volume of HNF.

Multiple Mechanisms Support Oligoclonal T Cell Expansion in Rheumatoid Synovitis

The synovial T cell infiltrate in rheumatoid arthritis (RA) is diverse but contains clonally expanded CD4+ populations. Recent reports have emphasized that RA patients have a tendency to develop CD4+ T cell oligoclonality which also manifests in the peripheral blood. Clonal dominance in the tissue may thus result from antigen specific stimulation in the synovial membrane or may reflect the infiltration of expanded clonotypes present throughout the lymphoid system. We have explored to what extent clonal populations amongst tissue CD4+ T cells display joint specificity as defined by their restriction to the joint, their persistence over time, and their expression of markers indicative for local activation. Matched samples of peripheral blood and synovial fluid or synovial tissue were collected from 14 patients with active RA and CD4+ IL-2R+ and CD4+ IL-2R- T cells from both compartments were purified. Clonal populations of CD4+ T cells were detected by RT-PCR amplification of T cell receptor (TCR) transcripts with BV and BJ specific primers followed by size fractionation and direct sequencing of dominant size classes of TCR transcripts. Clonal CD4+ T cells were detected in the synovial fluid and synovial tissue of all patients. All patients carried synovial clonotypes that were undetectable in the blood but were present in independent joints or at several non-adjacent areas of the same joint. These joint restricted CD4+ clonotypes were generally small in size, were preferentially found in the IL-2R+ subpopulation, and persisted over time. A second type of clonogenic T cells in the synovial infiltrate had an unrestricted tissue distribution and was present at similar frequencies amongst activated and nonactivated T cells in the blood and affected joints. Ubiquitous clonotypes isolated from two different patients expressed sequence homologies of the TCR beta chain. Two types of expanded CD4+ clonotypes contribute to the T cell infiltrate in rheumatoid synovitis. Differences in the distribution pattern and in molecular features suggest that distinct mechanisms are supporting the clonal outgrowth of these two groups of clonotypes. Clonally expanded T cells restricted to the joint but present in several independent joints appear to respond to locally residing antigens. Clonogenic cells with an unrestricted distribution pattern and widespread activation in the blood and tissue may react to a different class of antigens which appear to be shared by multiple patients. T cell recognition in RA may be involved at several different levels and may be related to more than one pathomechanism.

Effects of nitrogen source on the physiology and metal nutrition of Emiliania huxleyi grown under different iron and light conditions

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 130:255-267 (1996) - doi:10.3354/meps130255 Effects of nitrogen source on the physiology and metal nutrition of Emiliania huxleyi grown under different iron and light conditions Muggli DL, Harrison PJ Emilianiahuxleyi, a small oceanic coccolithophore, was isolated from the NE subarctic Pacific and maintained under oceanic conditions. Coccolith-forming cultures were grown with either NO3- or NH4+ as the primary nitrogen source. Fe-stress was induced experimentally, and physiological parameters including metal quotas (Fe, Mn, Zn, Cu) were measured for both NO3-- and NH4+-grown cells to determine whether it was advantageous for the cells to grow on NH4+ rather than NO3- under Fe-stressed conditions. The parameters used to observe the cell's physiological status were specific growth rate (mu), cell volume (CV), carbon (C), chlorophyll a (chl a), and nitrogen (N) per cell volume. Under Fe-replete conditions (100 nM Fe), no physiological parameters were significantly different (p &LT 0.05) between NO3-- and NH4+-grown cells. However, under Fe-stressed conditions, CV (NH4+ &GT NO3-), chl a CV-1 (NH4+ &LT NO3-), Mn CV-1 (NH4+ &LT NO3), and Mn:C (NH4+ &LT NO3-) were all significantly different (p &LT 0.05) for NO3-- and NH4+-grown cells. Under Fe-stressed conditions, NO3--grown cells of E. huxleyi maintained the same or greater levels of chl a, N, Fe, Mn, and Cu as NH4+-grown cells, largely due to the drastic decrease in CV of NO3--grown cells under Fe-stress. Although NO3--grown cells substantially decreased their CVs under Fe-stressed conditions (whereas NH4+-grown cells did not), both NO3-- and NH4+-grown cells reduced their CVs equally under limiting-irradiance, Fe-replete conditions. A reduction in CV by the NO3--grown cells is particularly advantageous for cells living in a low Fe environment, since it reduces the cellular requirements for photosynthate, N, and Fe. The fact that these already small cells become smaller, along with their unique ability to maintain chlorophyll synthesis at Fe levels limiting to cell division, may help explain why E. huxleyi is a member of the numerically dominant size class in the NE subarctic Pacific. Nitrogen . Ammonium . Nitrate . Coccolithophore . Emilianiahuxleyi . Iron . Iron limitation . Metal quotas . Oceanic . Subarctic Pacific Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 130. Publication date: January 11, 1996 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1996 Inter-Research.

Sea-bird aggregation at a deep North Pacific seamount

During June 1991, we studied sea birds at a mid-ocean seamount (Fieberling Guyot) in the eastern North Pacific Ocean. Avifaunal composition changed from small Procellariiformes [a storm-petrel; Oceanodroma leucorhoa (Vieillot)] away from the seamount to an assemblage dominated by larger tubenoses [mostly black-footed albatross Diomedea nigripes Audubon and Cook's petrel Pterodroma cookii (Gray)]. Compared to adjacent waters, sea-bird density and biomass within a 30-km radius centered on the seamount summit were 2.4 and 8 times higher, respectively. Individual sea-bird taxa were 2 to 40 times more abundant at the seamount relative to values reported previously from large-scale surveys of deep-ocean regions in the central North Pacific. In September 1991 we studied potential prey of sea birds in the upper water column using a neuston net and multiple opening-closing net system (MOCNESS) tows. Most potential prey types in the neuston exhibited no significant enhancement over the seamount. MOCNESS samples at 10 m depth, however, showed several prey types to be more abundant over the seamount, and the dominant size class of fish was slightly larger. We attribute the sea-bird aggregation observed at this seamount to changes in the abundance and/or behavior of pelagic organisms in the deep scattering layer (not adequately sampled in this study), perhaps augmented by migrations of seamount residents into the surface layers. Processes on and in the vicinity of seamounts may provide spatially-predictable prey to wide-ranging aerial sea birds foraging in this relatively austere environment.

Planktonic microbial community oxygen consumption rate in Cananéia waters (25°S 48°W), Brazil

Water samples were taken monthly in 1976 at three sites along the Cananeia estuarine-lagoon system (25°S 48°W), on the southern coast of Sao Paulo State, Brazil. After screening through a 100 μm mesh silk screen, samples from two sites were size fractionated in classes between 100–50 μm, 50–10 μm and 10–0.45 μm and, concentrated. Oxygen consumption rates (OCR) of the fractionated and unfractionated surface samples were estimated after incubating in dark bottles at ‘in situ’ temperature. OCR varied from 0.26±0.95 to 41.61±2.56 μl O2 l−1 h−1. Highest OCR was observed in summer, at the site under fresh-water influence. At this site, for most of the year, the smallest size category contributed mostly to total OCR. Bacteria attached to particulate matter appear to be responsible for most of the oxygen consumption measured. At the more saline waters, OCR was found to be associated with organisms in the 10–100 μm size class. At this site, size analysis of chlorophylla showed that the 0.45–10 μm size category comprised the major portion of total chlorophyll. The lack of association between the OCR dominant size class and the prevailing phytoplankton and bacteria size categories, suggests the oxygen consumption to be due to other organisms, probably nano- and microzooplankton. OCR measured in the unfractionated samples, at site 2, showed the same order of magnitude to those of the fractionated ones. At this site the importance of the bacterial metabolism in relation to the total OCR measured is discussed. It is concluded that in the surface waters of the Cananeia estuarine lagoon system the contribution of bacterial metabolism to total oxygen consumption differs along the system, being significant at the upper reaches of the estuary, where the inputs of organic matter are larger. In saline waters, in spite of high bacterial densities, oxygen consumption seems to reflect the metabolism of larger organisms.

Maize root-derived soil organic carbon estimated by natural 15c abundance

The natural enrichment of 13C in maize plant material, compared to that of indigenous soil organic carbon, was used in the field to estimate the quantity of underground maize-derived C at harvest in October. Maize-derived C in the soil fractions coarser than 200 μm accounted for 12% of the C of the aboveground plant parts. Half of it was situated in the 20 cm wide × 20 cm deep soil portion situated beneath the plant. The soil fraction size-class 200–2000 μm contained 25% of the total and was the dominant size-class in the deeper layers. Because of the low resolution of the 13C tracer, only a 95% confidence interval could be obtained for the quantity of solid material finer than 200 μm: between 1 and 9% of the aerial parts of maize. Contribution of maize-derived C to soil organic matter in early August was not significantly different from that at the harvest date.

Size-selective grazing on bacteria by natural assemblages of estuarine flagellates and ciliates

The small average cell size of in situ bacterioplankton, relative to cultured cells, has been suggested to be at least partly a result of selection of larger-sized cells by bacterivorous protozoa. In this study, we determined the relative rates of uptake of fluorescence-labeled bacteria (FLB), of various cell sizes and cell types, by natural assemblages of flagellates and ciliates in estuarine water. Calculated clearance rates of bacterivorous flagellates had a highly significant, positive relationship with size of FLB, over a range of average biovolume of FLB of 0.03 to 0.08 microns3. Bacterial cell type or cell shape per se did not appear to affect flagellate clearance rates. The dominant size classes of flagellates which ingested all types of FLB were 3- to 4-microns cells. Ciliates also showed a general preference for larger-sized bacteria. However, ciliates ingested a gram-positive enteric bacterium and a marine bacterial isolate at higher rates than they did a similarly sized, gram-negative enteric bacterium or natural bacterioplankton, respectively. From the results of an experiment designed to test whether the addition of a preferentially grazed bacterial strain stimulated clearance rates of natural bacterioplankton FLB by the ciliates, we hypothesized that measured differences in rates of FLB uptake were due instead to differences in effective retention of bacteria by the ciliates. In general, clearance rates for different FLB varied by a factor of 2 to 4. Selective grazing by protozoa of larger bacterioplankton cells, which are generally the cells actively growing or dividing, may in part explain the small average cell size, low frequency of dividing cells, and low growth rates generally observed for assemblages of suspended bacteria.

Feeding of copepods on natural suspended particles in Tokyo Bay

Community grazing rates of copepods were estimated from data taken during three cruises in Tokyo Bay, based on bottle incubations and a temporal variation of gut fluorescence. Special attention was paid to the feeding selectivity in the estimations. Differential grazing was observed in the copepod communities:Acartia omorii, abundant in February, selectively fed on the particles of dominant size classes, whileOithona davisae, dominant throughout the year, andCentropages abdominalis selected large particles (>20µm). The maximum filtering rates on certain size classes were several times the average. In addition, a 34-hr investigation of the gut fluorescence of copepods revealed nocturnal feeding inParacalanus spp.,Pseudodiaptomus marinus andOithona davisae.

Nothofagus(Fagaceae) forest on Mt Giluwe, New Guinea

Abstract Populations of Nothofagus spp. (Fagaceae) were studied on Mt Giluwe, New Guinea (6°S, 144°E, 2300–2800 m alt.). Nothofagus spp. formed monospecific stands transitional with a taxonomically diverse forest, replaced abruptly by gymnosperm forest above 2800 m altitude. Extensive stands of N. pullei had a single dominant size class and, at 30–79 cm d.b.h., many cohorts were subject to dieback associated with pathogenic Phytophthora spp. and pinhole borers. Regeneration in dieback patches was dominated by N. pullei suckers, forming a new cohort. In the absence of dieback, all Nothofagus species reached a maximum d.b.h of 100–150 cm and regenerated, especially in canopy gaps, forming a mixed size structure. A variety of canopy tree taxa were established in stands containing large N. grandis trees, forming a transition or succession to the taxonomically diverse forest. Regular growth rings of parenchyma and, less clearly, vessels were apparently annual. Dieback typically occurred in trees with 100–180 r...