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The processes and biomass that characterize any ecosystem are fundamentally constrained by the total amount of energy that is either fixed within or delivered across its boundaries. Ultimately, ecosystems may be understood and classified by their rates of total and net productivity and by the seasonal patterns of photosynthesis and respiration. Such understanding is well developed for terrestrial and lentic ecosystems but our understanding of ecosystem phenology has lagged well behind for rivers. The proliferation of reliable and inexpensive sensors for monitoring dissolved oxygen and carbon dioxide is underpinning a revolution in our understanding of the ecosystem energetics of rivers. Here, we synthesize our current understanding of the drivers and constraints on river metabolism, and set out a research agenda aimed at characterizing, classifying and modeling the current and future metabolic regimes of flowing waters.The fuel that powers almost all of Earth's ecosystems is created by organisms capable of the alchemy of photosynthesis, in which solar energy, water, and carbon dioxide are converted into reduced carbon compounds that are then used to sustain life. We measure this conversion of solar energy into organic energy as the gross primary productivity (GPP) of ecosystems. The collective dissipation of this organic energy through organismal metabolism (of both autotrophs and heterotrophs) is measured as ecosystem respiration (ER). Together, GPP and ER are the fundamental metabolic rates of ecosystems that constrain the energy supply and energy dissipation through food chains, and the balance of these two fluxes, measured as net ecosystem production (NEP), determines whether carbon accumulates or is depleted within an ecosystem. Terrestrial ecosystems often have predictable annual cycles, with both GPP and NEP typically peaking during warmer and wetter months of the year. In many well-studied lakes productivity peaks when warming temperatures, lengthening days, and high nutrient concentrations occur in concert. The life cycles of many consumers are likely synchronized to these seasonal oscillations such that periods of peak energetic demand by consumers coincide with or follow the peak productivity of their preferred plant or prey (e.g., Lampert et al. 1986;Berger et al. 2010). As a result, ecosystem respiration tends to

[1] A growing body of evidence demonstrates the importance of in-stream processing in regulating nutrient export, yet the influence of temporal variability in stream metabolism on net nutrient uptake has not been explicitly addressed. Stream water DIN and SRP concentrations in Walker Branch, a first-order deciduous forest stream in eastern Tennessee, show a repeated pattern of annual maxima in summer and biannual minima in spring and autumn. Temporal variations in catchment hydrologic flow paths result in lower winter and higher summer nutrient concentrations, but do not explain the spring and autumn nutrient minima. Ambient nutrient uptake rates were measured 2-3 times per week over an 18-month period and compared to daily rates of gross primary production (GPP) and ecosystem respiration (ER) to examine the influence of in-stream biotic activity on nutrient export. GPP and ER rates explained 81% of the variation in net DIN retention with high net NO 3 À uptake (and lower net NH 4 + release) rates occurring during spring and autumn and net DIN release in summer. Diel nutrient concentration patterns were examined several times throughout the year to determine the relative importance of autotrophic and heterotrophic activity on net nutrient uptake. High spring GPP corresponded to daily decreases in NO 3 À over the illuminated hours resulting in high diel NO 3 À amplitude which dampened as the canopy closed. GPP explained 91% of the variance in diel NO 3 À amplitude. In contrast, the autumn nutrient minima was largely explained by heterotrophic respiration since GPP remained low and little diel NO 3 À variation was observed during the autumn.

Simultaneous gradients of phosphorus and light were applied in experimental streams to develop quantitative relationships between these two important abiotic variables and the growth and composition of benthic microalgae. Algal biovolume and whole-stream metabolism responded hyperbolically to phosphorus enrichment, increasing approximately two-fold over the 5-300 mg L 21 range of experimental phosphorus concentrations. The saturation threshold for phosphorus effects occurred at 25 mg L 21 of soluble reactive phosphorus (SRP). Light effects were much stronger than those of phosphorus, resulting in a nearly ten-fold increase in algal biovolume over the 10-400 mmol photons m 22 s 21 range of experimental irradiances. Biovolume accrual was light-saturated at 100 mmol photons m 22 s 21 (5 mol photons m 22 d 21 ). Light effects were diminished by low phosphorus concentrations, and phosphorus effects were diminished by low irradiances, but evidence of simultaneous limitation by both phosphorus and light at subsaturating irradiances was weak. Contrary to the light : nutrient hypothesis, algal phosphorus content was not significantly affected by light, even in the lowest SRP treatments. However, algal nitrogen content increased substantially at lower irradiances, and it was very highly correlated with algal chlorophyll a content. Phosphorus enrichment in streams is likely to have its largest effect at concentrations ,25 mg L 21 SRP, but the effect of enrichment is probably minimized when streambed irradiances are kept below 2 mol photons m 22 d 21 by riparian shading or turbidity.

1] Applying ramped pyrolysis radiocarbon analysis to suspended river sediments, we generate radiocarbon ( 14 C) age spectra for particulate organic carbon (POC) from the lower Mississippi-Atchafalaya River system (MARS) to better understand a major river system's role in carbon transport. Ramped pyrolysis 14 C analysis generates age distributions of bulk carbon based on thermochemical stability of different organic components. Our results indicate higher proportions of older material in the POC during higher discharge. Ages increase throughout the high-discharge age spectra, indicating that no single component of the POC is responsible for the overall age increases observed. Instead, older material is contributed across the POC age spectrum and unrelated to increased bedload suspension. In this comparison of 2 spring discharges, less than half of the POC transported during higher discharge is less than 1000 14 C years in age, constraining of the role of the MARS as a flux of atmospheric CO 2 toward longer-term sedimentary sinks in the Mississippi delta and the Gulf of Mexico. The results suggest that delta-building processes benefit disproportionately from high discharge events carrying larger amounts of sediment because these events involve both a higher proportion of millennially-aged carbon from floodplain exchange of POC and a potentially higher proportion of petrogenic carbon (30-530% increase). Overall, an internally consistent picture of PO 14 C age distributions from a major river system emerges, as differences in space and time are small compared to the range of ages of POC sources in such a large basin.