Seasonal patterns of dissolved inorganic nitrogen and inorganic carbon uptake by the sublittoral epilithic periphyton community in N-deficient, oligotrophic Lake Tahoe were examined. The biomass dominants of this community, N,-fixing blue-green algae (e.g. Calothrix, Tolypothrix, and Nostoc) were persistent and retained their nitrogenase activity throughout the year. Seasonal rates of N, fixation exhibited considerable variation, with a distinct summer maximum and winter minimum. Uptake of both N03- and NH4+ followed Michaelis-Menten kinetics. K, values were typically extremely high (> 100 pg N liter-l) compared to the ambient concentrations of these forms of nitrogen (< 10 pg N liter-l). N, fixation was the most important source of inorganic N to the yearly N budget of this benthic community. Low ambient substrate concentrations coupled with a low physiological affinity for these substrates at ambient levels were responsible for the relative unimportance of N03- and NH,+ uptake. Dark uptake of N03-, NH4+, and N, fixation were all significant and could not be neglected in determining rates of daily inorganic nitrogen utilization. This blue-green algal community is not adapted for efficient use of N03- or NH,+ and can survive in the N-deficient environment because of its ability to use N,. In striking contrast,

Patterns of phosphorus supply and utilization were investigated for 2 years. In Lake Wash-ington, the phytoplankton bloom began in February or early March, reaching peaks of 2,165 ( 1974) and 1,885 mg Cm rnd2 *d-l (1975). Most of the P utilized was from phosphate accumulated in the water, resulting in depletions of soluble reactive phosphorus in the euphotic zone of 260-12 mg POrP.rne2. Zooplankton excretion supplied phosphate during the later stages of the bloom and in summer. In Findley Lake, the blooms were of shorter duration and reached maxima of 400-711 mg C *m-2*d-1. Phosphate was supplied from fluvial inputs and zooplank-ton excretion, with little accumulation (lo-50 mg P0,-P*m-2). Comparison of the measured rate constants for turnover (k,) with the constants expected for photosynthesis (k,) showed P flux patterns. In Lake Washington, k, varied from <0.05.h ’ in winter to 5-lO*h- ’ during summer stratification, whereas k, was comparable during prebloom and early-bloom periods and only about 0.1-h- ’ in summer. In Findley Lake, ki (2.9-5.3*h-‘) was always>k, (O.OS.h-l). 0 1 ft n y a er phosphate depletion did the usual pattern of rapid uptake and release occur. Nutrient addition and isotope partitioning experiments suggested that the divergence

In wetland ecosystems a considerable part of the or-ganic material production is formed by emergent ma-crophytes (Wetzel l990, Gessner et al. 1996), which play an important part in the detritus food chain (Wet-

The interstitial water chemistry of Lake Washington sediments collected with an in situ sampler was studied. The depth where the sulfate concentration reaches a low background value appears to delineate a transition within the sediments above and below which profiles oftotal C02, alkalinity, methane, and ammonium are linear, suggesting zones dominated by diffusion separated by a narrow zone of reaction. A stoichiometric model is used to identify the increase of alkalinity with depth in the interstitial waters. Ammonium production, followed by iron reduction, makes the largest contribution to this increase; about 30-40 % is due to ammonia that has been produced but subsequently adsorbed by the sediments. The measured and calculated ammonia exchange constants agree well. The C:N ratio of the decomposing organic matter increases systematically, from 3.9 at 5 cm to over 14 at 50 cm. The organic matter being decomposed becomes nitrogen-poor with age. Flux calculations for the lake-sediment system show good agreement between the flux of carbon to the sediments and the sum of the diffusive loss from the sediments of CH, and total CO, and the permanent burial in the sediments. About a fourth of the detrital rain of carbon is recycled to