Limnol. Oceanogr., 44(3), 1999, 556–572

Marine diatoms generally form large blooms during periods of cool temperature (,208C), high NO fluxes (.25 3 mM-N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO uptake kinetics and direct short-term temperature manipulation studies 3 on field diatom-dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO uptake by a Rhizoselenia-dominated population did not appear to saturate even at concen3 trations as high as 180 mM-N. We observed contrasting patterns of NO , NH , and urea utilization as a function 2 1 3 4 of experimental temperature (ambient 6 98C). Over the temperature range of 7–258C, absolute uptake rates of NO (rNO32) decreased an average of 46% with increasing temperature from 7 to 258C (nine individual experiments), 3 while rNH41 and rUREA 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 NO in excess of nutritional requirements, the reduction 3 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 NO uptake increases proportionately with the 3 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 NO by diatom-dominated 2 assemblages, other observations of nonsaturating NO kinetics in field populations, and the apparent ‘‘preference’’ 3 for NO by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative 3 relationship between NO uptake and short-term temperature shifts, have important ecological implications. The 3 hypothesized ability of these diatom-dominated populations to better modulate the flow of photosynthetic electron energy, via NO reduction, in variable environments may provide a competitive advantage to diatoms and could 3 potentially explain why diatoms frequently dominate in regions of cool temperature, high NO flux, and turbulent 3 mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO uptake. 3 Finally, if a significant fraction of NO uptake is regulated by non-nutritional mechanisms in the cell, and if some 3 fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO , NH , or dissolved 2 1 2 4 organic nitrogen (DON), then estimates of new production based solely on NO uptake could be seriously biased. 3 A large fraction of the annual autotrophic production in temperate estuaries is typically associated with spring phytoplankton blooms (Malone and Chervin 1979; Sharp et al. 1982; Pennock 1985; Cloern et al. 1989; Glibert et al. 1995; Smith and Kemp 1995). These blooms are commonly dominated by large diatoms (i.e., .20 mm), which appear to be favored by the high nutrient concentrations (Eppley et al. 1969b) and the pulsed nature of nutrient inputs (Turpin and Harrison 1979; Stolte et al. 1994; Stolte and Riegman 1996), as well as the turnover and rapid stabilization of the water column (Malone 1980). During these temperate spring blooms, as well as in coastal upwelling areas and polar regions, NO is the most abundant inorganic nitrogen source 3 (e.g., Malone et al. 1988; Smith 1991; Glibert et al. 1995; Clarke and Leavey 1996) and supports most of the increase