We investigated whether rates of net primary production (NPP) and biomass turnover of floating grasses in a central Amazon floodplain lake (Lake Calado) are consistent with published evidence that CO 2 emissions from Amazon rivers and floodplains are largely supplied by carbon from C4 plants. Ground-based measurements of species composition, plant growth rates, plant densities, and areal biomass were combined with low altitude videography to estimate community NPP and compare expected versus observed biomass at monthly intervals during the aquatic growth phase (January-August). Principal species at the site were Oryza perennis (a C3 grass), Echinochloa polystachya, and Paspalum repens (both C4 grasses). Monthly mean daily NPP of the mixed species community varied from 50 to 96 g dry mass m À2 day À1 , with a seasonal average (AE1SD) of 64 AE 12 g dry mass m À2 day À1 . Mean daily NPP (AE1SE) for P. repens and E. polystachya was 77 AE 3 and 34 AE 2 g dry mass m À2 day À1 , respectively. Monthly loss rates of combined above-and below-water biomass ranged from 31% to 75%, and averaged 49%. Organic carbon losses from aquatic grasses ranged from 30 to 34 g C m À2 day À1 from February to August. A regional extrapolation indicated that respiration of this carbon potentially accounts for about half (46%) of annual CO 2 emissions from surface waters in the central Amazon, or about 44% of gaseous carbon emissions, if methane flux is included.

Algal biomass and the chemical composition of seston and floating meadow periphyton were measured in an Amazon floodplain lake during a period of channelized river inflow and a period of overbank flow. During both periods, sedimentation of riverine particulates and loss of dissolved nutrients created strong spatial chemical gradients from the river onto the floodplain which were stable for months at a time. The molar N : P of dissolved inorganic nutrients supplied by river water was very low (NO, + NH,+ : SRP < 5) and decreased further as water entered the floodplain. Initial loss of sediment in surface waters increased the organic content of seston and periphyton and decreased the C : N of seston within 2 km of the river. More gradual loss of finer P-rich particles caused 2-4-fold increases in N : P and C : P ratios inside the floodplain. During channelized river inflow, epiphytic algal biomass peaked where inorganic turbidity was high (>50 mg liter') and phytoplankton biomass was low. Later in the year, overbank flow elevated dissolved nutrient levels inside the lake and disrupted thermal stratification. This transition from lentic to lotic conditions coincided with the seasonal high for epiphytic algal biomass and the seasonal low for phytoplankton biomass. Overall, epiphytic algal biomass was 4-10 times higher than phytoplankton biomass on a per unit area basis. In situ bag experiments revealed that inorganic turbidity regulates the ability of attached algae to utilize nutrients supplied by river water. Epiphytic algal biomass decreased after contact with undiluted river water. Epiphytic algal biomass increased 5-fold in river water diluted by 90%.

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[1] The effects of prescription burning on watershed balances of major ions in mixed conifer forest were examined in a 16-year paired catchment study in Sequoia National Park, California. The objective was to determine whether fire-related changes in watershed balances persist as long as estimated low-end natural fire-return intervals ( 10 years), and whether cumulative net export caused by fire could deplete nutrient stocks between successive fires. Inputs (wet + dry deposition) and outputs (stream export) of N, S, Cl À , HCO 3 À , Ca 2+ , Mg 2+ , Na + , K + , H + , and SiO 2 were measured for 7 years preceding, and 9 years following, a prescribed burn of one of the catchments. After fire, runoff coefficients increased by 7% (in dry years) to 35% (in wet years). Inorganic N was elevated in stream water for 3 years after fire. Increased export of water, SO 4 2À , Cl À , SiO 2 , and base cations continued through the end of the study. Pools and processes attributed to fire led to the cumulative loss, per hectare, of 1.2 kg N, 16 kg S, 25 kg Cl À , 130 kg Ca 2+ , 19 kg Mg 2+ , 71 kg Na + , 29 kg K + and 192 kg Si, above that predicted by prefire regression equations relating export in the paired catchments. This additional export equaled <1% of the N, up to one-third of the Ca and Mg, and up to three-fourths of the K, contained in the forest floor prior to combustion. Changes in watershed balances indicated that low-end natural fire-return intervals may prevent complete reaccumulation of several elements between fires.