While phosphorus is generally considered to be the primary nutrient limiting algal growth in lakes, limitation of algal growth by nitrogen has been observed in freshwater. It is also commonly observed that the most pronounced phytoplankton responses to enrichment occur when both N and P are added together. This led us to re-evaluate nitrogen's status as a secondary nutrient in freshwater through a systematic literature search. In our survey of enrichment bioassays, we found considerable deficiencies in the degree to which investigators have applied sufficient replication, performed and reported statistical tests, and assessed seasonal and spatial differences in algal nutrient limitation. Given these limitations, however, we found that combined N + P enrichment enhanced algal growth much more frequently and more substantially than did addition of N or P singly. On average, the frequency and degree of algal response did not differ for P vs. N enrichment. From our review of whole-lake fertilizations, we concluded that the roles of N and P in constraining algal growth at the whole-lake scale have not been completely separated. However, examination of the distribution between treatments (+N, +P, +N, and P) of lake-years in which substantial algal growth response occurred indicated that combined N and P enrichment was required to consistently produce substantial algal growth response. A more important role than has previously been recognized for N as a limiting nutrient in freshwaters is indicated. We suggest that greater attention should be given to both P and N in the future.

Whole-stream metabolism in a first-order stream was measured using upstream–downstream changes in dissolved oxygen (DO) concentration measured at 1-min intervals over a 40-h period. The measured change in DO was corrected for reaeration flux using a reaeration coefficient determined from injections of conservative and volatile tracers. The whole-stream metabolism measurement was compared in the spring with in situ chamber measurements performed a few days later in the same stream reach. Chamber measurements of community respiration extrapolated to a 24-h period (CR24) were about one third the whole-stream measurements, while gross primary production (GPP) measured at midday in the chambers was roughly 20% less than the whole-stream estimate. Whole-stream GPP was higher during the spring just prior to forest canopy closure than in summer or autumn. Community respiration exceeded whole-stream GPP on all dates and was greatest during the summer. Our results suggest that this whole-stream approach provides a measure of total stream metabolism that is relevant to other stream ecosystem processes measured on reach scales, such as nutrient spiralling.

We used nondisruptive, whole-stream methods to measure hydraulic characteristics, ecosystem metabolism, and phosphorus cycling in the west fork of Walker Branch (WB), Tennessee and in Hugh White Creek (HWC), North Carolina. Although similar in many of their hydrological and chemical characteristics, transient storage zone volume in HWC was relatively large (-1.5 times that of the flowing water zone), whereas transient storage zone volume in WB was small (-0.1 times that of the flowing water zone). Both streams were highly heterotrophic (gross primary production : total respiration ratios

Streams in deciduous forests undergo marked transitions from light‐replete to light‐limited ecosystems every spring when leaves emerge on streamside trees. During the course of leaf emergence and enlargement, shade from leaves on streamside trees can reduce photosynthetically active radiation (PAR) falling on the streambed from >1000 to <30 μmol·m−2·s−1. In this study, we examined the effects of leaf emergence at multiple levels in two headwater streams in eastern Tennessee. Primary production estimated from both photosynthesis–irradiance measurements of periphyton in the laboratory and whole‐steam diurnal oxygen measurements decreased dramatically over the course of canopy closure. Monthly carbon fixation estimates for periphyton in White Oak Creek declined from 354 μg C/cm2 in April to 66 μg C/cm2 in June, while carbon fixation in Walker Branch declined from 495 to 168 μg C/cm2. Periphyton photosynthesis became increasingly efficient at low irradiances (α increased more than threefold) as ambient streambed irradiances declined, but this increase in efficiency only partially compensated for the photon scarcity caused by riparian shade. Ecological photosynthetic efficiency (percentage of incident PAR energy fixed by photosynthesis) estimated from static models, whole‐stream measurements, and ambient PAR was a negative exponential function of incident PAR, increasing from <0.3% to 2% during canopy closure. This increase was attributable to (1) inefficient use of the relatively high irradiances before leaf emergence, and (2) greater photoefficiency (increased α) at low irradiances after leaf emergence. Nutrient concentrations (dissolved nitrate and phosphate) in both streams increased coincident with leaf emergence, implying a cascade of shade effects through primary producers to abiotic components of the ecosystem. Shade effects also propagated to higher trophic levels: growth rates of grazing snails (Elimia clavaeformis) in both streams decreased substantially from April to June, consistent with modeled decreases in the productivity of their food resource (periphyton). Snail growth rates were almost zero in White Oak Creek and were negative in Walker Branch during summer when streambed PAR was lowest. The multilevel effects of leaf emergence reported in this study accentuate the importance of light variation in aquatic ecosystems and illustrate the close coupling between streams and their surrounding terrestrial ecosystems.

The effect of periphyton biomass on hydraulic characteristics and nutrient cycling was studied in laboratory streams with and without snail herbivores. Hydraulic characteristics, such as average water velocity, dispersion coefficients, and relative volume of transient storage zones (zones of stationary water), were quantified by performing short-term injections of a conservative tracer and fitting an advection-dispersion model to the conservative tracer concentration profile downstream from the injection site. Nutrient cycling was quantified by measuring two indices: (1) uptake rate of phosphorus from stream water normalized to gross primary production (GPP), a surrogate measure of total P demand, and (2) turnover rate of phosphorus in the periphyton matrix. These measures indicate the importance of internal cycling (within the periphyton matrix) in meeting the P demands of periphyton. Dense growths of filamentous diatoms and blue-green algae accumulated in the streams with no snails (high-biomass streams), whereas the periphyton communities in streams with snails consisted almost entirely of a thin layer of basal cells of Stigeoclonium sp. (low-biomass streams). Dispersion coefficients were significantly greater and transient storage zones were significantly larger in the high-biomass streams compared to the low-biomass streams. Rates of GPP-normalized P uptake from water and rates of P turnover in periphyton were significantly lower in high biomass than in low biomass periphyton communities, suggesting that a greater fraction of the P demand was met by recycling in the high biomass communities. Increases in streamwater P concentration significantly increased GPP-normalized P uptake in high biomass communities, suggesting diffusion limitation of nutrient transfer from stream water to algal cells in these communities. Our results demonstrate that accumulations of periphyton biomass can alter the hydraulic characteristics of streams, particularly by increasing transient storage zones, and can increase internal nutrient cycling. They suggest a close coupling of hydraulic characteristics and nutrient cycling processes in stream ecosystems.