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511.
512.
The magnitude and physiological characteristics of biological nitrogen fixation have been studied in the oligotrophic waters of the North pacific gyre. The filamentous blue-green algae Trichodesmium spp. and Richelia intracellularis were the important nitrogen-fixing phytoplankton. Most of the nitrogen fixation occurs in the upper 40 m of the water column, with detectable fixation as deep as 90 m, which corresponds to about the 1 % light depth. There was no evidence of photoinhibition of nitrogen fixation, although CO2 reduction was depressed slightly at the highest light levels. The rate of nitrogen fixation in the water column varied throughout the day, being highest in mid-morning and in late afternoon. Relatively high fixation rates were also found during periods of darkness. Elevated oxygen concentrations had a marked inhibitory effect on rates of nitrogen fixation, a pO2 of 0.4 atm causing a 75% inhibition. Data from studies of nitrogen fixation and assimilation rates of 15N-labelled nitrate, ammonium, and urea indicate that nitrogen fixation furnished about 3% of the total daily fixed nitrogen requirement for phytoplankton growth. Studies with isolated colonies of Trichodesmium spp. indicated that 100% of their nitrogen requirement was met by nitrogen fixation. Chemical composition of the Trichodesmium colonies showed that the C:N ratio was 4.1 and that their phosphorus content relative to carbon or nitrogen was much lower than that of the total particulate material in the water column. Elevated ratios of carbon: adenosine triphosphate (ATP) also suggest that phosphorus deficiency may be limiting the growth of Trichodesmium. The magnitude of nitrogen fixation in the gyre is seasonally dependent, with high rates in late summer and autumn. At these times the water column is stratified, with phosphate and nitrate barely detectable in the upper 100 m. Our data suggest that during these months of stratification, biological fixation of nitrogen amounts to about 33 g-at N/m2/day. 相似文献
513.
A.I. Kuznetsova O.V. Zarubina G.A. Leonova 《Environmental geochemistry and health》2002,24(3):205-212
Data on Zn, Cu, Pb, Ni, Cr, Sn, Mo concentrations in some food fish in Baikal Lake and Bratsk reservoir have been compared. The muscles, skin and gills of roach and perch (Rutilus rutilus and Perca fluviatis L.) were analysed. The conditions of atomic-emission analysis, detection limits, reproducibility, coefficients of transformation from the wet to dry mass and to an ash residue are reported. To verify accuracy, the results of reference samples OK-1 (Baikal perch) and OM-1 (Baikal roach) were used along with atomic-absorption and neutron-activation results of ashed and dried samples. The observed range of concentrations in tissues exceeds the probable analytical errors and reflects natural variability. The content of Zn, Cu, Pb, Ni and Cr in tissues of fish collected in the Bratsk reservoir from the conventionally uncontaminated and polluted sites correspond to the concentrations in tissues of similar fish from Lake Baikal. The latter are normal for fish of weakly mineralised oligotrophic reservoirs. The ranges of concentration in studied fish (roach and perch) consist of mg kg–1 dry wt: in muscles-Zn(14–28), Cu(2.2–4.7), Pb(0.30–0.61); in skin-Zn(94–119), Cu(1.2–1.4), Pb(0.16–0.08); in gills-Zn(70–280), Cu(2.0–7.4), Pb(0.19–1.2). 相似文献
514.
We used microsatellite genetic markers to investigate adult population structure and the formation of a new year-class in
Sebastes mystinus (blue rockfish). Since S. mystinus may live as long as 45 years and reach reproductive age at approximately 5 years, the adult population may contain as many
as eight generations of reproductive adults. We investigated whether the juveniles of the 2000 year-class and the adult population
were genetically homogeneous along the California coast. We sampled approximately 100 juveniles from three sites, two sites
along the Monterey Peninsula (Carmel and Monterey) in central California and one at Fort Ross in northern California, and
approximately 50 adult S. mystinus from five sites throughout the population center. The adult sampling spanned approximately 700 km from the northern Channel
Islands to Fort Bragg. The juveniles showed significant heterogeneity in allele frequencies among distant locations and genetic
homogeneity among adjacent locations. In contrast, the adults showed genetic homogeneity over large distances (San Miguel
Island to Fort Bragg), indicating little limitation of gene flow in this region. Allele frequencies of juveniles differed
from adult samples and in some cases reduced genetic diversity indicative of sweepstakes recruitment (small sample of the
adult reproductive potential). The genetic structure of the 2000 year-class suggests that despite a genetically homogenous
adult population, settled juveniles can be genetically heterogeneous along the California coast. The results also suggest
that the adults, with several year-classes, are capable of maintaining a panmictic population despite the genetic distinctiveness
of individual year-classes. 相似文献
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517.
O. S. Astthorsson 《Marine Biology》1990,107(1):147-157
The seasonal abundance, distribution, maturity, growth and population dynamics of the euphausiidsThysanoessa raschi (M. Sars, 1864),T. inermis (Krøyer, 1846) andMeganyctiphanes norvegica (M. Sars, 1857) were studied in Ísafjord-deep, a fjord in northwest Iceland, from February 1987 to February 1988. Sampling was made at nine stations along the length of the fjord at approximately monthly intervals, along with hydrographic measurements and water sampling for nutrient analysis and measurements of chlorophylla concentrations. Spring warming of the water began in late May and maximum temperatures (8° to 10°C) were observed in late July–September. The phytoplankton spring-bloom started in early April, and the highest chlorophylla levels were measured in early May (7.0 mg m–3). A small increase was observed in the chlorophylla content in August. The greatest abundance of juveniles and males and females of all three species was observed during January and February 1988, during which period the euphausiids were concentrated in the middle and inner parts of the fjord. Euphausiid eggs were first recorded in the plankton in mid-May, and the greatest abundance ofThysanoessa spp. larvae occurred at the end of May. Larvae ofM. norvegica were not observed in Ísafjord-deep, indicating that recruitment of this species was occurring from outside the fjord.T. raschi andT. inermis had a life span of just over 2 yr; the life span ofM. norvegica was more difficult to determine. Almost all femaleT. raschi were mature at the age of 1 yr, while mostT. inermis females appeared not to mature until 2 yr of age. Most males of both species took part in breeding at 1 yr of age. The maximum carapace length ofT. raschi andT. inermis was 8 to 9 and 9 to 10 mm, respectively. The largestM. norvegica had a carapace length of 9 to 10 mm. The spawning of the euphausiids in Ísafjord-deep appeared to be closely related to the phytoplankton spring bloom; water temperature appeared to have no influence on spawning. 相似文献
518.
A widely accepted view of intertidal community organizatiton in the NW Atlantic proposes that fucoid vegetation is maintained by the actions of predators which remove species competitively superior toFucus species. Herbivory is an important component of these predatory interactions, but has been studied largely with reference to the interaction betweenF. vesiculosus andLittorina littorea. There are many species of fucoids and herbivorous invertebrates on the shores of the NW Atlantic and this paper reports field and laboratory experiments performed in 1987 (in or near Halifax, Canada) on the effects of grazing by three species ofLittorina on adults and juveniles of four species ofFucus. In laboratory experiments, portions ofFucus species were presented singly (no choice) or together (multiple choice) to single species ofLittorina. AdultF. distichus was grazed to only a small extent.F. evanescens andF. vesiculosus adult tissues were heavily grazed in most laboratory experiments.F. spiralis adults were heavily grazed in no choice experiments, but were grazed only slightly in multiple choice tests. When adult fucoid tissues were exposed to a mixed suite of grazers in the field, onlyF. vesiculosus was grazed heavily. Tests done on adult tissues showed clearly thatFucus species are highly variable in their vunerability to grazers. Moreover, we found variability among periwinkle species in grazing rates on adultFucus. We also found a great disparity in the feeding rates of different periwinkle species on juveniles ofFucus. Therefore, it is not possible to extrapolate from a single periwinkle species/single fucoid species interaction when attempting to identify vegetation structuring processes. 相似文献
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520.