ALBERT

Description: High‐latitude rivers supply the Arctic Ocean with a disproportionately large share of global riverine discharge and terrigenous dissolved organic matter (DOM). We used the abundance of lignin, a macromolecule unique to vascular plants, and stable carbon isotope ratios (δ13C) to trace the high molecular weight fraction of terrigenous DOM in major water masses of the Arctic Ocean. Lignin oxidation products in ultrafiltered DOM (UDOM; 〉1,000 Da) from Arctic rivers were depleted in syringyl relative to vanillyl phenols (S/V = 0.3–0.5) compared to UDOM in temperate and tropical rivers (S/V = 0.5–1.2), indicating that gymnosperm vegetation is a major source of terrigenous UDOM to the Arctic Ocean. High concentrations of lignin oxidation products (83–320 ng L−1) and a depletion of 13C (δ13C = −23.0 to −21.9) in UDOM throughout the surface Arctic Ocean indicate that terrigenous UDOM accounts for a much greater fraction of the UDOM in the surface Arctic (5–33%) than in the Pacific and Atlantic oceans (0.7–2.4%). In contrast, UDOM in deep water from the Arctic Ocean as well as waters from throughout the Greenland Gyre had relatively low concentrations of lignin oxidation products (24–45 ng L−1 ) and was enriched in 13C (δ13C = −21.0 to −20.8). Terrigenous UDOM has a relatively short residence (∼1–6 yr) in surface polar waters prior to export to the north Atlantic Ocean. Assuming that the bulk of Arctic‐derived DOM is compositionally similar to the UDOM fraction, we estimate that 12–41% of terrigenous DOM (2.9–10.3 Tg C yr−1 ) discharged by rivers to the Arctic Ocean is exported to the North Atlantic via surface waters of the East Greenland Current. It appears very little terrigenous DOM from the Arctic is incorporated into North Atlantic Deep Water and distributed globally via deep thermohaline circulation.

Description: We used variation in algal d13C between river habitats to study the spatial scale of energy flow through river food webs. We found a strong negative relationship between herbivore d13C (which reflects algal d13C) and water velocity in three productive Northern California rivers but not in unproductive streams. The contrast among habitats suggests that water velocity affects algal d13C most strongly when CO2 availability is low relative to photosynthetic rates. Our results help explain the wide variation in published river biota d13C and show that past studies using carbon isotope analyses may have significantly underestimated the importance of algal‐derived carbon to river food webs. While flow‐related variation in d13C complicates this common application of carbon isotope analysis, we show that it provides a natural tracer of the flux of algal production derived from different habitats within rivers to higher trophic levels. Measurements of consumer d13C showed that most invertebrate and vertebrate consumers relied on local production, except for filter‐feeding insects and steelhead trout, which relied on production derived from multiple sources. Stable carbon isotopes may thus be used to spatially delineate the habitats that support river food webs, providing previously unavailable information for understanding and managing river ecosystems.

Description: Marine diatoms generally form large blooms during periods of cool temperature (〈20°C), high NO3− fluxes (〉25 μM‐N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO3− uptake kinetics and direct short‐term temperature manipulation studies on field diatom‐dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO3− uptake by a Rhizoseleni‐dominated population did not appear to saturate even at concentrations as high as 180 µM‐N. We observed contrasting patterns of NO2−, NH+, and urea utilization as a function of experimental temperature (ambient ± 9°C). Over the temperature range of 7–25°C, absolute uptake rates of NO3− (ρNO3−) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while ρNH4+ and ρUREA increased with increasing temperature by an average of 179 and 86% (eight individual experiments), respectively. Based on these observations and the nature of the physical environment, we hypothesize that these diatom‐dominated populations were taking up NO3− in excess of nutritional requirements, the reduction of which may serve as a sink for electrons during transient periods of imbalance between light energy harvesting and utilization. We suggest that the increase in non‐nutritional NO3− uptake increases proportionately with the magnitude of the imbalance between light energy harvesting and imbalance. This hypothesis reconciles previous observations of low C:N uptake ratios, high release rates of dissolved organic nitrogen or NO2− by diatom‐dominated assemblages, other observations of nonsaturating NO3− kinetics in field populations, and the apparent "preference" for NO3− by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative relationship between NO3− uptake and short‐term temperature shifts, have important ecological implications. The hypothesized ability of these diatom‐dominated populations to better modulate the flow of photosynthetic electron energy, via NO3− reduction, in variable environments may provide a competitive advantage to diatoms and could potentially explain why diatoms frequently dominate in regions of cool temperature, high NO3− flux, and turbulent mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO3− uptake. Finally, if a significant fraction of NO3− uptake is regulated by non‐nutritional mechanisms in the cell, and if some fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO2−, NH+, or dissolved organic nitrogen (DON), then estimates of new production based solely on NO3− uptake could be seriously biased.