Characteristics of softwater streams in Rhode Island II. Composition and seasonal dynamics of macroalgal communities

Forty stream segments in Rhode Island, U.S.A., were examined seasonally from June 1979 to March 1982. Thirty-nine species of macroalgae were collected, respresenting 25 genera. The composition of the lotic flora was 54% green algae, 31% red algae, 5% blue-green algae, 5% xanthophytes, 3% chrysophytes and 3% diatoms. The majority of these taxa (85%) were filamentous. From a biweekly examination of five stream segments, macroalgal communities could be grouped according to light regime. Species in unshaded streams exhibited little seasonality, whereas in streams shaded by one or more layers of riparian canopy, maxima in species numbers and abundance occurred during colder seasons. The most widespread and abundant species were the blue-green alga Phormidium retzii, the green alga Draparnaldia acuta, and the diatom Eunotia pectinalis. P. retzii and E. pectinalis were aseasonal annuals, while D. acuta was primarily a winter-spring form. It appears that pH is a major factor affecting broad geographic distribution patterns of stream macroalgae, whereas the light regime established by overhanging canopy is an important factor which influences localized abundance and seasonality of lotic macroalgal communities. Niche pre-emption appears to be a common mode of resource space division among stream macroalgae in Rhode Island. E. pectinalis is the strongly developed dominant in this drainage system.     

Seasonality of macroalgae and epilithic diatoms in spring-fed streams in Texas, USA

A seasonal study of two spring-fed stream systems in south-central Texas was undertaken over a 15-month period from June 1996 to September 1997. Relative abundance of the epilithic diatom flora, percent cover of macroalgae, and several physical and chemical conditions were monitored in one 20-m stretch in each of the streams at approximately two-month intervals. Six additional spring-fed stream segments were sampled in May 1996 for macroalgae and measured for the same conditions, in order to determine how representative the macroalgal floras of the study sites were of south-central and western Texas. Conditions in the two stream segments were relatively constant over the 15 months, except for maximum depth; this finding is similar those reported for other spring-fed systems worldwide. Sixty-eight diatom taxa and nine macroalgae taxa were identified from the San Marcos River, which compares to 46 diatom taxa and six macroalgae taxa from the Comal River. Several diatom and macroalgae species were significantly correlated to stream conditions (e.g. Dichotomosiphon tuberosus and Hildenbrandia angolensis with daylength, and Amphora pediculus with maximum depth). Twenty-one macroalgal taxa were identified from the six additional spring-fed steam segments. Conditions in these streams were very similar to those measured in the San Marcos River and Comal River, except that they had a higher average pH _x= 8.1 versus _x= 7.6). The epilithic diatom flora of the San Marcos River and the Comal River contains many of the diatom and some macroalgal taxa commonly reported from springs globally. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrodynamic control of filamentous macroalgae in a sub-tropical spring-fed river in Florida,USA

Hydrodynamic properties of rivers and streams are primary controls on the abundance and structure of algal communities. In Florida, USA, filamentous macroalgal proliferation in spring-fed rivers has largely been attributed to nitrogen enrichment; however, changes in hydrodynamics and other potential mechanisms have not been thoroughly evaluated. This study tested whether macroalgal abundance (primarily Lyngbya wollei) was related to hydrodynamics in Gum Slough, a sub-tropical Florida spring-fed river. Flow velocities and macroalgal cover were measured at three transects bimonthly between June 2010 and January 2013, a period during which river discharge fluctuated in response to a drought and a tropical storm. Temporal variation in macroalgal cover was significantly inversely related to flow velocity, excluding fall/winter sampling events when light limitation was likely. The results indicate a velocity threshold below which macroalgal cover increased substantially, and a lower threshold below which complete cover frequently occurred. Local differences in channel morphology and substrate type (i.e., aquatic plant beds) seemed to affect the relationship between macroalgal cover and velocity at each transect. Nitrogen and phosphorus concentrations and other water chemistry parameters were not associated with variation in macroalgal cover. Overall, this study suggests that hydrodynamics can strongly control filamentous macroalgae in Florida spring-fed rivers.     

Seasonality and distribution of epilithic diatoms,macroalgae and macrophytes in a spring-fed stream system in Ontario,Canada

A study of the epilithic diatom, macroalgal and macrophyte communities from a spring-fed stream in Ontario, Canada was undertaken from September 1996 to July 1997. The relative abundance of the epilithic diatom flora, percent cover of macroalgal and macrophyte taxa, and several physical and chemical stream conditions were monitored along a 20-m stretch at each of four sites, approximately every 2 months. Several stream conditions were relatively constant over the sampling period (pH, maximum width and maximum depth), while others exhibited a distinct seasonal pattern (water temperature, specific conductance and daylength) and some fluctuated strongly with no discernable seasonal pattern (turbidity, current velocity). A total of 124 taxa were identified from the four sites, including 79 epilithic diatoms, three macroalgal diatom species (large gelatinous masses), one cyanobacterium, two red algae, eight green algae, one chrysophyte alga, one tribophyte alga, three mosses, three horsetails and 23 angiosperm taxa. Species richness was positively correlated to stream channel maximum width and depth, indicating that the total number of species tends to increase in a downstream direction. Distribution of several diatom and macroalgal species was significantly correlated to stream conditions (e.g. Gomphonema parvulum and Phormidium subfuscum with current velocity); however, the vast majority of species did not display seasonal variation in abundance that could be explained by changes in stream conditions. Many of the taxa identified from Blue Springs Creek are common elsewhere in North America.     

Estimating the susceptibility of tree species to attack by the gypsy moth, Lymantria dispar

ABSTRACT.

• 1 Numbers of gypsy moth larvae feeding on each of 922 randomly sampled trees in a Quercus—Acer—Fraxinus forest in southwestern Quebec, Canada were counted in 1979 and in 1980 to quantify the larval feeding preferences as observed in the field for eighteen deciduous and one coniferous tree species at the northern range limit of the gypsy moth.
• 2 Both the diameter at breast height (dbh) and the estimated foliage biomass of the sampled trees were used to calculate the relative proportions of foliage represented by each of the nineteen tree species in the forest canopy. With these data on availability and utilization of the tree species by the gypsy moth larvae an Ivlev-type electivity index was used to quantify the larval feeding preferences. These preferences observed in the field define the susceptibility of a tree species to attack by the gypsy moth.
• 3 The feeding preferences calculated using estimated foliage biomass were comparable to the simpler calculation based on dbh (Spearman's rho = 0.79; P= 0.0001). The dbh-based feeding preferences remained almost unchanged in 1979 and 1980 (Spearman's rho = 0.83; P= 0.0001).
• 4 The composite 1979—80, dbh-based feeding preferences show Quercus rubra, Populus grandidentata, Ostrya virginiana, Amelanchier spp. and Acer saccharum were preferentially attacked by gypsy moth. Prunus serotina, Betula lutea, Acer rubrum, A. pensylvanicum, Fraxinus americana, Ulmus rubra, P. pensylvanicum and B. papyrifera were avoided. All nineteen tree species were, however, utilized to at least some degree by gypsy moth larvae.
• 5 These results quantitatively affirm and clarify earlier reports of gypsy moth feeding preferences in North America and Eurasia. The advantages and limitations of using an electivity index to estimate the susceptibility of different tree species to attack by folivores like the gypsy moth are discussed.

     

POPULATION CHARACTERISTICS AND MIGRATION OF SUMMER AND LATE-SEASON HUMPBACK WHALES (MEGAPTERA NOVAEANGLIAE) IN SOUTHEASTERN ALASKA

A total of 326 humpback whales (Megaptera novaeangliae) were individually identified in southeastern Alaska during five summer seasons (July to September) and four late seasons (November to February) spanning the years 1979 to 1983. Peak numbers of whales were found late in August or early in September. Whales arrived 1–2 wk later in 1982 than in 1981. Whales sighted in both the summer and late seasons of 1981 and 1982 remained about 3.7 mo and one whale remained for at least 4.9 mo. Humpback whales from southeastern Alaska wintered in Hawaiian or Mexican waters, but generally did not travel to other feeding regions. The most rapid migratory transit between Hawaii and southeastern Alaska was 79 d. Based on mark-recapture analyses of the photographic data, we estimate a population of 270–372 whales in the southeastern Alaska feeding herd.     

Retardation of larval development in the salamander <Emphasis Type="Italic">Hynobius retardatus</Emphasis> in a permanent pond with abundant spring water

Plasticity in the larval life history of the Ezo salamander Hynobius retardatus has been reported. In the present study, we monitored larvae of this salamander species in a fragmented forest in the environs of Sapporo, Japan. Overwintering larvae were detected in one of the two ponds examined, i.e., in the pond that was permanent, with water supplied from several spring-fed points. Four seasonal transition phases in water temperature were observed because of the abundant spring-fed groundwater supply and canopy cover from May to October. These phases included a warming period, a constant-high period, a cooling period, and a constant-low period. During the constant-low period, premetamorphic larvae that had already emerged during the cooling period were continuously detected; however, the composition of the developmental stages remained unchanged, with larval growth progressing slowly. There is apparently a critical temperature that represents the threshold for metamorphosis initiation. The critical temperature is expected to be slightly higher than the groundwater temperature at the spring-fed points. Emigration of overwintered larvae resumed during the warming period and continued during the constant-high period in the year following hatching. In the nearby temporary pond, the one-year-old cohort completed metamorphosis during the summer of the hatch year.     

Response of zooplankton to the reduction and elimination of submerged vegetation by grass carp and herbicide in four Florida lakes

Thirteen physical and chemical characteristics of five softwater streams in Rhode Island, U.S.A. were examined biweekly to monthly for seventeen months. One first-order, two second-order and two third-order streams were included in the study. The mean annual temperature ranged from 9.4 °C in the spring-fed headwater stream to 14 °C in an open, third-order stream, with seasonal fluctuations of 14.5 ° to 28.0 °C. The heavily shaded first-order stream generally received less than 30% incident light at its surface throughout the year. By contrast, the other streams either were unshaded or were associated with distinct periodicity of incident light quantity due to seasonality of the tree canopy. The mean annual current velocity ranged from 22 to 100 cm s^–1 among the streams, pH ranged from 3.7 to 6.4, and specific conductance was generally less than 50 µS cm^–1. The first-order stream was associated with lowest mean annual temperature, current velocity, light penetration and nitrate, as well as relatively high and constant silica concentrations. Temperature was negatively correlated with current speed in second- and third-order streams, and temperature was also negatively correlated with light in shaded streams. There was a general pattern in all streams for decreasing pH following precipitation events. Concentrations of total phophorus, nitrate-nitrogen, ammonium-nitrogen and silica were among the lowest reported for lotic systems.     

Effects of Stream Restoration on Woody Riparian Vegetation of Southern Appalachian Mountain Streams,North Carolina,U.S.A

Riparian revegetation, such as planting woody seedlings or live stakes, is a nearly ubiquitous component of stream restoration projects in the United States. Though evaluations of restoration success usually focus on in‐stream ecosystems, in order to understand the full impacts of restoration the effects on riparian ecosystems themselves must be considered. We examined the effects of stream restoration revegetation measures on riparian ecosystems of headwater mountain streams in forested watersheds by comparing riparian vegetation structure and composition at reference, restored, and degraded sites on nine streams. According to mixed model analysis of variance (ANOVA), there was a significant effect of site treatment on riparian species richness, basal area, and canopy cover, but no effect on stem density. Vegetation characteristics at restored sites differed from those of reference sites according to all metrics (i.e. basal area, canopy cover, and species composition) except species richness and stem density. Restored and degraded sites were structurally similar, with some overlap in species composition. Restored sites were dominated by Salix sericea and Cornus amomum (species commonly planted for revegetation) and a suite of disturbance‐adapted species also dominant at degraded sites. Differences between reference and restored sites might be due to the young age of restored sites (average 4 years since restoration), to reassembly of degraded site species composition at restored sites, or to the creation of a novel anthropogenic ecosystem on these headwater streams. Additional research is needed to determine if this anthropogenic riparian community type persists as a resilient novel ecosystem and provides valued riparian functions.     

Forest canopy cover determines invertebrate diversity and ecosystem process rates in depositional zones of headwater streams

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Effect of diflubenzuron on flight of adult aquatic insects (Plecoptera, Trichoptera) following emergence during the second year after aerial application

With Malaise traps, we monitored the flight of adult Plecoptera and Trichoptera following emergence from headwater streams in the Fernow Experimental Forest, WV, during the second year after application of diflubenzuron. We placed five traps at various distances from each stream during May through September of 1991, 1992, and 1993. We collected pretreatment samples during the first year. In May 1992, diflubenzuron was applied to two watersheds, and the other two watersheds were used as untreated references. The 1992 study tested the effects of diflubenzuron that fell directly into the streams or were washed into the stream during the first year. For 1993, we tested the hypothesis that diflubenzuron affected adult flight following emergence during the year following abscission and possible ingestion of the treated leaves. The analysis compared the regressions of the number of adults caught in each trap versus distance of the trap from the stream among years and between treatments for each species. The flight of the stonefly Leuctra ferruginea (Walker) was reduced in the treatment watersheds compared with the reference watersheds during the year following abscission of the treated leaves. Adult flight of other species did not decrease in the treatment watersheds during 1993. These results show a relatively small effect of diflubenzuron on these aquatic insects; however, our study involved only a single application of diflubenzuron. Additional research may be needed to predict the possible effects of multiple applications of diflubenzuron over several years as often occurs during actual efforts to suppress gypsy moth, Lymantria dispar (L.).     

Diversity of carbon use strategies in a kelp forest community: implications for a high CO2 ocean

Mechanisms for inorganic carbon acquisition in macroalgal assemblages today could indicate how coastal ecosystems will respond to predicted changes in ocean chemistry due to elevated carbon dioxide (CO₂). We identified the proportion of noncalcifying macroalgae with particular carbon use strategies using the natural abundance of carbon isotopes and pH drift experiments in a kelp forest. We also identified all calcifying macroalgae in this system; these were the dominant component of the benthos (by % cover) at all depths and seasons while cover of noncalcareous macroalgae increased at shallower depths and during summer. All large canopy‐forming macroalgae had attributes suggestive of active uptake of inorganic carbon and the presence of a CO₂ concentration mechanism (CCM). CCM species covered, on average, 15–45% of the benthos and were most common at shallow depths and during summer. There was a high level of variability in carbon isotope discrimination within CCM species, probably a result of energetic constraints on active carbon uptake in a low light environment. Over 50% of red noncalcifying species exhibited values below ?30‰ suggesting a reliance on diffusive CO₂ uptake and no functional CCM. Non‐CCM macroalgae covered on average 0–8.9% of rock surfaces and were most common in deep, low light habitats. Elevated CO₂ has the potential to influence competition between dominant coralline species (that will be negatively affected by increased CO₂) and noncalcareous CCM macroalgae (neutral or positive effects) and relatively rare (on a % cover basis) non‐CCM species (positive effects). Responses of macroalgae to elevated CO₂ will be strongly modified by light and any responses are likely to be different at times or locations where energy constrains photosynthesis. Increased growth and competitive ability of noncalcareous macroalgae alongside negative impacts of acidification on calcifying species could have major implications for the functioning of coastal reef systems at elevated CO₂ concentrations.     

Phytoplankton population dynamics of a small reservoir: effect of intermittent mixing on phytoplankton succession and the growth of blue-green algae

The seasonal succession of the phytoplankton of a small reservoir(Guelph Lake, Ontario) in the spring-summer of 1982 was comparedto that in 1981. Mixing of the deeper waters occurred severaltimes throughout the summer in 1982 but not in 1981. The waterat 10 m became anoxic for only 2 weeks in late July in 1982.In contrast, in 1981, the water at 10 m became anoxic at thebeginning of July and remained so until mid-September. The phytoplanktondynamics observed in 1982 did not follow the typical progressionfrom spring diatoms to summer species adapted to survive understratified conditions, as in 1981. Diatoms were present throughoutthe summer in higher amounts in 1982 than in 1981. The mostobvious difference in the two summers was the much greater abundanceof Aphanizomenon flow-aquae in 1982. Other blue-green algaeincluding Microcystis aeruginosa, Gomphosphasria lacustris,and Lyngbya birgei appeared earlier on in 1982, but did notimmediately increase in abundance as in 1981. In 1982, ratesof phytoplankton community change were low in May and June andincreases were observed in mid-July, early August, late Augustand late September. In contrast, in 1981, the rate of communitychange increased in late May, mid-June, early July and lateJuly and remained low throughout August and September. ¹Present address: Department of Zoology, University of Alberta,Edmonton, Alberta T6G 2E1, Canada. ²Present address: CSIRO Division of Fisheries Research, G.P.O.Box 1538, Hobart, Tasmania 7001, Australia     

Distinct effects of N and P addition on soil enzyme activities and C distribution in aggregates in a subalpine spruce plantation

We assessed stream ecosystem-level response to a drought-induced defoliation event by gypsy moth caterpillars (Lymantria dispar) with high-frequency water quality sensors. The defoliation event was compared to the prior year of data. Based on long-term records of precipitation and drought indices, the drought of 2015–2016 in Rhode Island, USA was an extreme climatic event that preceded and likely precipitated the defoliation from insect infestation. Canopy cover in the riparian area was reduced by over 50% increasing light availability which warmed the stream and stimulated autotrophic activity. Frass and leaf detritus contributed particulate carbon and organic nutrients to the stream. Based on locally calibrated s::can spectro::lyser data, nitrate concentration and flux did not significantly increase during defoliation while orthophosphate concentration and flux did significantly increase during part of the defoliation period. Lower mean daily dissolved oxygen (DO) levels and wider diel cycles of DO indicated higher biological activity during the defoliation event. Stream metabolism metrics were also significantly higher during defoliation and pointed to heterotrophic activity dominating in the stream. The increases in stream metabolism were low compared to other studies; in streams with higher nutrient levels (e.g., in agricultural or urban watersheds) the increase in light and temperature could have a stronger influence on stream metabolism. The in-stream metabolic processes and nutrient fluxes observed in response to the drought-driven defoliation event resulted from the long-term deployment of high-frequency water sensors. The proliferation of these water sensors now enable studies that assess ecosystem responses to stochastic, unusual disturbances.     

Comparison of methods for estimating the spread of a non-indigenous species

Aim  To compare different quantitative approaches for estimating rates of spread in the exotic species gypsy moth, Lymantria dispar L., using county-level presence/absence data and spatially extensive trapping grids.
Location  USA
Methods  We used county-level presence/absence records of the gypsy moth's distribution in the USA, which are available beginning in 1900, and extensive grids of pheromone-baited traps, which are available in selected areas beginning in 1981. We compared a regression approach and a boundary displacement approach for estimating gypsy moth spread based on these sources of data.
Results  We observed relative congruence between methods and data sources in estimating overall rates of gypsy moth spread through time, and among regions.
Main conclusions  The ability to estimate spread in exotic invasive species is a primary concern in management programmes and one for which there is a lack of information on the reliability of methods. Also, in most invading species, there is generally a lack of data to explore methods of estimating spread. Extensive data available on gypsy moth in the USA allowed for such a comparison. We show that, even with spatially crude records of presence/absence, overall rates of spread do not differ substantially from estimates obtained from the more costly deployment of extensive trapping grids. Moreover, these methods can also be applied to the general study of species distributional changes, such as range expansion or retraction, in response to climate change or other environmental effects.     

Discrimination and production of disparlure enantiomers by the gypsy moth and the nun moth

ABSTRACT. Two odour receptor cells were physiologically identified within male antennal hair sensillae of the gypsy moth, Lymantria dispar L, and the nun moth, L. monacha L. In the gypsy moth, one cell responded to (+)-disparlure, while a neighbouring cell responded to (-)-disparlure. In the nun moth both cells responded to (+)-disparlure. The lack of sensitivity to (-)-disparlure in the nun moth was corroborated by electroantennogram (EAG) recordings, which indicated no affinity to this enantiomer. Single cell responses of male gypsy moth to different concentrations of the synthetic enantiomers of disparlure were then compared to responses elicited by hexane extracts of female glands of both species. The gypsy moth's extracts stimulated almost exclusively the receptor cell specialized for (+)-disparlure, while both cells were simultaneously stimulated by the extracts of the nun moths. From the response characteristic of the cells it is estimated that pheromone production of the nun moth is about 10% (+) and 90% (-)-disparlure, and that of the gypsy moth is almost 100% (+)-disparlure. Stimulation of the antenna of each species by female gland extracts of both species did not indicate the presence of receptors for other hexane elutable pheromone components in either species.     

Gypsy moth feeding in the canopy of a CO2-enriched mature forest

Rising atmospheric carbon dioxide (CO₂) concentration is expected to change plant tissue quality with important implications for plant–insect interactions. Taking advantage of canopy access by a crane and long‐term CO₂ enrichment (530 μ mol mol^?1) of a natural old‐growth forest (web‐free air carbon dioxide enrichment), we studied the responses of a generalist insect herbivore feeding in the canopy of tall trees. We found that relative growth rates (RGR) of gypsy moth (Lymantria dispar) were reduced by 30% in larvae fed on high CO₂‐exposed Quercus petraea, but increased by 29% when fed on high CO₂‐grown Carpinus betulus compared with control trees at ambient CO₂ (370 μ mol mol^?1). In Fagus sylvatica, there was a nonsignificant trend for reduced RGR under elevated CO₂. Tree species‐specific changes in starch to nitrogen ratio, water, and the concentrations of proteins, condensed and hydrolyzable tannins in response to elevated CO₂ were identified to correlate with altered RGR of gypsy moth larvae. Our data suggest that rising atmospheric CO₂ will have strong species‐specific effects on leaf chemical composition of canopy trees in natural forests leading to contrasting responses of herbivores such as those reported here. A future change in host tree preference seems likely with far‐ranging consequences for forest community dynamics.     

Highly predictable photosynthetic production in natural macroalgal communities from incoming and absorbed light

Photosynthesis–irradiance relationships of macroalgal communities and thalli of dominant species in shallow coastal Danish waters were measured over a full year to test how well community production can be predicted from environmental (incident irradiance and temperature) and community variables (canopy absorptance, species number and thallus metabolism). Detached thalli of dominant species performed optimally at different times of the year, but showed no general seasonal changes in photosynthetic features. Production capacity of communities at high light varied only 1.8-fold over the year and was unrelated to incident irradiance, temperature and mean thallus photosynthesis, while community absorptance was a highly significant predictor. Actual rates of community photosynthesis were closely related to incident and absorbed irradiance alone. Community absorptance in turn was correlated to canopy height and species richness. The close relationship of community photosynthesis to irradiance is due to the fact that (1) large differences in thallus photosynthesis of individual species are averaged out in communities composed of several species, (2) seasonal replacement of species keeps communities metabolically active, and (3) maximum possible absorptance at 100% constrains the total photosynthesis of all species. Our results imply that the photosynthetic production of macroalgal communities is more predictable than their complex and dynamic nature suggest and that predictions are possible over wide spatial scales in coastal waters by measurements of vegetation cover, incoming irradiance and canopy absorptance.     

Impact of seasonal changes in stream metabolism on nitrate concentrations in an urban stream

Nitrate (NO₃ ^?) dynamics in urban streams differ from many natural streams due to stormwater runoff, sewage inputs, decreased groundwater discharge, often limited hyporheic exchange, increased primary productivity, and limited carbon input. We investigated NO₃ ^? dynamics in a first-order urban stream in Syracuse, NY, which has urbanized headwaters and a geomorphologically natural downstream section. Twice-monthly water sampling, NO₃ ^? injection tests, NO₃ ^? isotopic analysis, filamentous algae mat density, and riparian shading were used to identify processes regulating NO₃ ^? dynamics in the stream over a 12-month period. The urban headwater reach had low NO₃ ^? (0.006–0.2 mg N/L) in the spring through fall, with a minimum uptake length of 900 m, no canopy cover, and high algae mat density. The downstream natural reach (100% canopy cover during the summer and low algae mat density) had nitrate concentrations between 0.6 and 1.2 mg N/L from winter to summer, which decreased during autumn leaf-off. In the urban reach, autotrophic uptake by filamentous green algae is a major NO₃ ^? sink in summer. In the natural reach, the addition of organic matter to the stream at leaf-off led to a decrease in NO₃ ^? concentration followed by an increase in NO₃ ^? concentration in winter as gross primary productivity decreased. This study shows that the balance between autotrophy and heterotrophy in urban streams is variable and depends on an interplay of drivers such as temperature, light, and carbon inputs that are mediated by the riparian ecosystem.     

Preliminary observations on nitrogen transport during summer in a small spring-fed Ontario stream

Nitrogen transport in a 2 km-long, spring-fed stream was studied during the summer months by analyzing weekly water samples from four stations. The water at the spring had a consistently high level of nitrate-N ranging from about 7 mg/l in late spring to about 3 mg/l in early fall. However, over the length of the stream, 60% (about 97 kg) of the incoming nitrate-N is lost from the water during the summer period. The loss, which does not appear to be attributable to the uptake by aquatic macrophytes or to immobilization, is thought to result from denitrification.