Search

Light and nutrients play pivotal roles in determining the growth of autotrophs, yet the potential for synergistic interactions between the two resources in algal communities is poorly understood, especially in stream ecosystems. In this study, light and phosphorus were manipulated in large experimental streams to examine resource colimitation and synergy in stream periphyton. Whole-stream metabolism was simultaneously limited by light and phosphorus. Increasing the supply of either light or phosphorus resulted in significant increases in primary production and the transformation of the streams from heterotrophy to autotrophy. Resource-driven changes in periphyton community structure occurred in concert with changes in production. Algal assemblages in highly shaded streams were composed primarily of small diatoms such as Achnanthidium minutissima, whereas larger diatoms such as Melosira varians predominated at higher irradiances. Phosphorus enrichment had relatively little effect on assemblage structure, but it did substantially diminish the abundance of Meridion circulare, a diatom whose mucilaginous colonies were conspicuously abundant in phosphorus-poor, high-light streams. Bacterial biomass declined relative to algal biomass with increases in primary productivity, regardless of whether the increases were caused by light or phosphorus. Synergistic effects on primary production appeared to occur because the availability of one resource facilitated the utilization of the other. Light increased the abundance of large diatoms, which are known to convert high concentrations of nutrients into primary production more effectively than smaller taxa. Phosphorus enrichment led to the replacement of Meridion circulare by non-mucilaginous taxa in phosphorus-enriched streams, and we hypothesize that this change enabled more efficient use of light in photosynthesis. Higher ratios of chlorophyll a : biomass in phosphorus-enriched streams may have also led to more efficient photon capture and higher photosynthetic rates. Synthesis. Our results underscore the potential for resource colimitation, even in habitats where a single resource is as strongly limiting as is light in shaded streams. The capacity of autotrophic communities to respond to more than one limiting resource suggests that prevailing single-resource models of ecosystem productivity are overly simplistic. [ABSTRACT FROM AUTHOR]

The light nutrient hypothesis posits that herbivore growth is increasingly constrained by low food quality as the ratio of light to nutrients increases in aquatic ecosystems. We tested predictions of this hypothesis by examining the effects of large seasonal cycles in light and nutrients on the mineral content of periphyton and the growth rate of a dominant herbivore (the snail Elimia clavaeformis) in two oligotrophic streams. Streambed irradiances in White Oak Creek and Walker Branch (eastern Tennessee, USA) varied dramatically on a seasonal basis due to leaf phenology in the surrounding deciduous forests and seasonal changes in sun angle. Concentrations of dissolved nutrients varied inversely with light, causing light: nitrate and light: phosphate to range almost 100-fold over the course of any individual year. Periphyton nitrogen and phosphorus concentrations were much lower than the concentrations of these elements in snails, and they bottomed out in early spring when streambed irradiances were highest. Snail growth, however, peaked in early spring when light: nutrient ratios were highest and periphyton nutrient concentrations were lowest. Growth was linearly related to primary production (accounting for up to 85% of growth variance in individual years), which in turn was driven by seasonal variation in light. Conceptual models of herbivore growth indicate that growth should initially increase as increasing light levels stimulate primary production, but then level off, and then decrease as the negative effects of decreasing algal nutrient content override the positive effects of increased food production. Our results showed no, evidence of an inflection point where increasing ratios of light to nutrients negatively affected growth. Snail growth in these intensively grazed streams is probably unaffected by periphyton nutrient content because exploitative competition for food reduces growth rates to levels where the demand for nitrogen and phosphorus is small enough to be satisfied by even low levels of these nutrients in periphyton. Competition for limited food resources in habitats where herbivore densities are uncontrolled by predation or other mortality factors should strongly influence the potential for herbivores to be limited by mineral deficits in their food. [ABSTRACT FROM AUTHOR]

Examines the effects of leaf emergence at multiple levels in two headwater streams in eastern Tennessee. Light; Temperature and discharge; Nutrients; Periphyton biomass and pigments; Primary producers; Herbivores.

Examines the response of periphyton and grazing snails (Elimia clavaeformis) to summer shade in a Tennessee forest. Effects of light limitation and grazing; Shade adaptation; Snail growth versus irradiance; Snail growth; Lipid accumulation; Photosynthesis-irradiance curves.

Distinct seasonal patterns in streamwater nutrient and dissolved organic carbon (DOC) concentrations are evident in the data record from 7 years of weekly sampling in the West Fork of Walker Branch (WB) and 4 years of weekly sampling in a nearby stream, upper White Oak Creek (WOC), both first-order streams in the Ridge and Valley Province of eastern Tennessee. Concentrations of NO3 and soluble reactive phosphorus (SRP) in both streams showed a repeated pattern of annual maxima in summer and biannual minima in autumn and spring. Concentrations of DOC in WB exhibited distinct autumn maxima. To determine whether temporal variations in catchment hydrological processes could explain the seasonal nutrient and DOC variations in WB, we used an end-member mixing analysis involving Ca and SO4 concentrations to separate stream discharge into three catchment flow paths of differing nutrient concentrations. Stream NO3, SRP, and DOC concentrations were predicted solely on the basis of temporal variation in the importance of these flow paths and measurements of nutrient concentrations for the different flow paths. Ratios of observed/predicted concentrations in stream water near 1.0 suggested that catchment effects alone explained streamwater concentrations, whereas ratios substantially different from 1.0 suggested that in-stream processes were important determinants of streamwater concentrations. Observed/predicted NO3 and SRP concentration ratios showed repeated annual patterns with values closer to 1.0 during winter and summer (generally 0.8-1.2) and minima (<0.6) in spring and autumn, suggesting substantial in-stream net uptake at these times. Observed/predicted DOC concentration ratios were more variable and generally ≥1 but did show consistent autumn maxima (>2.5), indicating substantial in-stream DOC generation at this time. Observed/predicted ratios for all nutrients were generally less variable and were closer to 1.0 at high flow compared to low flow, suggesting that in-stream controls on streamwater chemistry are less important at high discharge than at low discharge. Our results indicate two general modes of control of stream nutrient concentrations: (1) catchment control via seasonal variation in the dominant hydrologic pathway (greater proportion of deep groundwater in summer), which produces lower winter and higher summer concentrations, and (2) in-stream control via high rates of net nutrient uptake during the spring (primarily by autotrophs) and autumn (primarily by heterotrophs). [ABSTRACT FROM AUTHOR]

Copyright of Reading Research Quarterly is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Stable isotopes are widely used to infer trophic relationships with little attention paid to temporal variability at the base of the food web. We examined changes in the carbon-stable isotope composition during periphyton development, sampling periphyton that accumulated on ceramic tiles at four stream sites over a 2-month period. Periphyton 13C rose and fell in general concordance with rising and falling biomass at all four sites, resulting in significant correlations between periphyton δ13C and chlorophyll a (Chl a). Mean δ13C values at one site rose from -26% to -20% in 2 weeks, falling back to -24% the next week after a large scouring spate. Periphyton 13C also underwent a smaller, longer-term increase that correlated with a gradual rise in stream temperature. Multiple regression analysis with both Chl a and temperature as independent variables accounted for up to 88% of the temporal variability in δ13C, with Chl a the largest source of variability. Water velocity, measured on each sampling occasion, was unrelated to temporal changes in 13C. Depletion of inorganic carbon within the periphyton matrix is the probable cause of increasing 13C in periphyton as biomass develops. Rising δ13C values during periphyton biomass development suggest the possibility of carbon-limited periphyton growth, even in alkaline waters. The strong link between biomass and periphyton 13C helps explain the large range of δ13C reported for periphyton in streams, where temporal and spatial variability in periphyton biomass are notorious. [ABSTRACT FROM AUTHOR]