[1] We analyzed 13 years (1992À2004) of CO 2 flux data, biometry, and meteorology from a mixed deciduous forest in central Massachusetts. Annual net uptake of CO 2 ranged from 1.0 to 4.7 Mg-C ha À1 yr À1 , with an average of 2.5 Mg-C ha À1 yr À1 . Uptake rates increased systematically, nearly doubling over the period despite forest age of 75-110 years; there were parallel increases in midsummer photosynthetic capacity at high light level (21.5À31.5 mmole m À2 s À1 ), woody biomass (101À115 Mg-C ha À1 from 1993À2005, mostly due to growth of one species, red oak), and peak leaf area index (4.5À5.5 from 1998-2005). The long-term trends were interrupted in 1998 by sharp declines in photosynthetic capacity, net ecosystem exchange (NEE) of CO 2 , and other parameters, with recovery over the next 3 years. The observations were compared to empirical functions giving the mean responses to temperature and light, and to a terrestrial ecosystem model (IBIS2). Variations in gross ecosystem exchange of CO 2 (GEE) and NEE on hourly to monthly timescales were represented well as prompt responses to the environment, but interannual variations and long-term trends were not. IBIS2 simulated mean annual NEE, but greatly overpredicted the amplitude of the seasonal cycle and did not predict the decadal trend. The drivers of interannual and decadal changes in NEE are long-term increases in tree biomass, successional change in forest composition, and disturbance events, processes not well represented in current models.

Turbulence, quantified as the rate of dissipation of turbulent kinetic energy (e), was measured with 1400 temperature-gradient microstructure profiles obtained concurrently with time series measurements of temperature and current profiles, meteorology, and lake-atmosphere fluxes using eddy covariance in a 4 km 2 temperate lake during fall cooling. Ãw /kz, where u Ãw is the water friction velocity computed from wind shear stress, k is von Karman's constant, z is depth, and J B0 is surface buoyancy flux. Below a depth equal to |L MO | during cooling, dissipation was uniform with depth and controlled by buoyancy flux. Departures from similarity scaling enabled identification of additional processes that moderate near-surface turbulence including mixed layer deepening at the onset of cooling, high-frequency internal waves when the diurnal thermocline was adjacent to the air-water interface, and horizontal advection caused by differential cooling. The similarity scaling enables prediction of near-surface e as required for estimating the gas transfer coefficient using the surface renewal model and for understanding controls on scalar transport.

[1] Do the influences of river breezes or other mesoscale effects lead to a systematic river proximity bias in Amazon rainfall data? We analyzed rainfall for a network of 38 rain gauges located near the confluence of the Tapajós and Amazon rivers in the eastern Amazon Basin. Tipping bucket rain gauges worked adequately in the Amazon rainfall regime, but careful field calibration and comparison with collocated conventional rain gauges were essential to incorporate daily totals from our array into regional maps. Stations very near the large rivers miss the afternoon convective rain, as expected if a river breeze promotes subsidence over the river, but paradoxically, this deficiency is more than compensated by additional nocturnal rainfall at these locations. The NOAA Climate Prediction Center (CPC) Morphing technique (CMORPH) passive infrared inferred rainfall data do an adequate job of describing medium scale variability in this region, but some localized breeze effects are not resolved at 0.25°resolution. For areas inland from the rivers, nocturnal rainfall contributes less than half of total precipitation. A large-scale rainfall increase just to the west of Santarém manifests itself locally as a 'tongue' of enhanced rain from along the wide area of open water at the Tapajós-Amazon confluence. The Amazon River breeze circulation affects rainfall more than does the Tapajós breeze, which moves contrary to the predominant wind. East of the riverbank, the effects of the Tapajós breeze extend only a few kilometers inland. Rainfall increases to the north of the Amazon, possibly the result of uplift over elevated terrain. Dry season rainfall increases by up to 30% going away from the Amazon River, as would be expected given breeze-induced subsidence over the river.

To study how changing agricultural practices in the eastern Amazon affect carbon, heat and water exchanges, a 20 m tower was installed in a field in August 2000. Measurements include turbulent fluxes (momentum, heat, water vapor, and CO 2 ) using the eddy covariance (EC) approach, soil heat flux, wind, and scalar profiles (T, q, and CO 2 ), soil moisture content, terrestrial, total solar radiation, and photosynthetically active radiation (PAR, 400-700 nm). At the beginning of the measurements, in September 2000, the field was a pasture. On November 2001, the pasture was burned, plowed, and planted in upland (nonirrigated) rice.Calm nights were the norm in this site. Anomalously low values of net ecosystem exchange (NEE) were found using the EC method, even when the common criterion u à o0.2 m s À1 was used to identify and exclude poor performance nights. We observed more plausible values of NEE using criterion u à o0.08 m s À1 , indicating that the criterion must be revised downward for flow over surfaces smoother than forests. However, even using the lower threshold, u à was lower than this limit for 82% of nights, and this led to nocturnal respiration underestimates. We compensate for this difficulty by estimating the respiration rate using the nocturnal boundary layer budget method.Land-use change from pasture to rice cultivation strongly affected both diurnal rates of turbulent exchange but also the pattern of seasonal variation. Seasonal wet and dry season differences in vegetation state were clearly detected in the albedo and PARalbedo. These reflectivity changes were accompanied by modified net radiative flux, turbulent heat flux and evaporation rates. The highest evaporation rate was observed during the rice crop, when the field had total evaporation approximately half the precipitation input, less than that of the surrounding forest. Effects of the land-cover changes were also detected in the carbon budget. For the pasture, the maximum CO 2 uptake occurred in May, appreciably delayed from the start of the rainy season. After the field was plowed and the soil was exposed and there was efflux of CO 2 to the atmosphere day and night for an extended period. Highest values of carbon uptake occurred during the rice plantation. Although the upland rice took up carbon at double the rate of the pasture that it replaced, the field was left fallow for much of the year, during the dry season.

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