Valentine Alexander

Learn More
The dynamics of nitrogen supply was investigated for blue-green and green algae from Smith Lake and other freshwaters of subarctic and arctic Alska. The natural abundance of 15N (defined as δ15N) of six N2-fixing blue-green algae was 1.0±%o(X±SE), indicating supply of metabolic nitrogen from atmospheric N2 (δ15N=0.0). The δ15N of six green algae showed an(More)
Nitrogen cycle studies were carried out on two distinct systems in arctic Alaska. Research on the first, a series of small tundra thaw ponds near Barrow, was carried out as part of the International Biological Program Tundra Biome program. Research on the contrasting system, Toolik Lake, was done during the year following completion of the Barrow pond(More)
Uptake of dissolved nitrogen (NH(4) + NO(3) + urea + N(2)) by a cyanobacterial [Anabaena flos-aquae (Lyngb.)] De Brèb population in Smith Lake, Alaska, was measured every 2 to 4 days during the spring of 1990. Total dissolved nitrogen uptake ranged from 0.34 to 24.75 mumol liter h, with a mean of 5.75 mumol liter h; the euphotic zone accounted for 91% of(More)
We used a combination of N tracer methods and a C(2)H(2) blockage technique to determine the role of sediment nitrification and denitrification in a deep oligotrophic arctic lake. Inorganic nitrogen concentrations ranged between 40 and 600 nmol . cm, increasing with depth below the sediment-water interface. Nitrate concentrations were at least 10 times(More)
The concentrations of metals, Cd, Cu, Fe, Mn, Ni, Pb, and Zn were determined in some lagoons to establish the level of metal pollution. The lagoons studied were Alvarado lagoon, Veracruz; San Andres lagoon, Tamaulipas; and Terminos lagoon, Campeche. The concentrations were determined in water, oyster (Crassostrea virginica), and sediments. Metals were(More)
Twelve 24 h bioassay experiments were conducted in 1980 and 1981 to evaluate seasonal influences of NO 3 - , NH 4 + , PO 4 3- , N+P, vitamins, trace metals, a synthetic chelate and common salts on 14C and 15N primary production in Toolik Lake, Alaska. Addition of N+P, NO 3 - or NH 4 + significantly increased 14C primary production over all other treatments(More)
Nitrogen-fixing shrubby legumes in the Mediterranean area partly overcome nutrient limitations by making use of soil N and atmospheric N₂ sources. Their ability to switch between different sources lets them adjust to the carbon costs pertaining to N acquisition throughout the year. We investigated the utilization of different inorganic N sources by Cytisus(More)
The carbon isotope composition of photosynthetic organisms reflects the (delta)(sup13)C of their inorganic carbon sources. The large difference between (delta)(sup13)C in cyanobacterial surface scums (-25 to -18(permil)) and subsurface blooms (-34 to -30(permil)) in a subarctic lake indicates direct atmospheric CO(inf2) fixation by the scums.
  • 1