Stoichiometric relationships in vernal pond plankton communities

1. The light-nutrient hypothesis (LNH) predicts that changes in light supply can alter the balance of nutrient and energy limitation in primary producers. We tested this prediction by examining temporal changes in vernal forest ponds, which are highly dynamic systems with respect to seasonal change in light and nutrient supply. In three vernal ponds that differ in productivity, we measured changes in light, total and seston nitrogen and phosphorus, and seston carbon and chlorophyll during the spring, before and after tree leaf-out. We also quantified changes in the population dynamics of the major zooplankton grazers in these systems.
2. In each pond, nutrient levels increased and light levels declined, creating a temporal shift in light-nutrient supply to the plankton. Results generally supported predictions of stoichiometric theory and the LNH, but there were notable exceptions.
3. Seston C : N : P ratios rapidly changed in response to dramatic increases in N and P supply rates. However, seston N : P was typically lower than values for total N : P in the water. Furthermore, as predicted, we observed a decline in seston C : P as the light : nutrient ratio declined, but seston C : N simultaneously increased. These results suggest an unexpected shift towards potential nitrogen limitation. Alternatively, this change in nutrient ratios may be driven by a seasonal change in phytoplankton composition or nutritional mode.
4. Seston carbon concentrations remained stable despite seasonal changes in grazing intensity associated with the phenology of large-bodied Daphnia grazers. However, chlorophyll concentrations declined dramatically as the season progressed, resulting in a simultaneous decline in the C : Chlorophyll ratio of seston. Both pond shading and increased grazing probably contributed to the decline in chlorophyll.     

Effect of a Zooplankton Community on Seston Elimination in a Restored Pond in Japan

The rates of seston elimination by zooplankton and primary production were measured in Funada-ike Pond, typical of human-made impoundments in Japan, from April to September in order to evaluate various treatments of the pond aimed at improving water quality by reducing seston abundance. The treatments included draining the pond water, dredging the bottom mud, eliminating the wastewater inflow, and biomanipulation through removal of all fish. After the treatment, seston abundance was reduced from more than 10 to 0.4–2.5 mg C/liter, and large daphnid species, Daphnia similis and D. magna, occurred and predominated in the zooplankton community. Seston abundance remained at a relatively low level from June to August but increased markedly in late August, while the biomass of zooplankton became high from June to mid-August and then decreased. A decrease in seston abundance was found when the elimination rate exceeded the primary production rate. The results indicate that the development of daphnid populations was effective in keeping seston abundance at a low level. The relationship between the rate of primary production and the zoo-plankton biomass required to offset this rate, however, suggests that biomanipulation aimed at increasing zooplankton biomass alone is less effective in a pond with a high primary production. The success in improving water quality in this pond seems to depend not only on the increase in biomass of large daphnid species that resulted mainly from the removal of fish, but also on the decrease in nutrient load that was realized by the other treatments.     

Elemental stoichiometry of lower food web components in arctic and temperate lakes

Lakes were surveyed to assess the potential patterns of latitudinalvariation in carbon:nitrogen:phosphorus (C:N:P) stoichiometryof lower food web components. Thirty-four lakes were surveyedat an arctic latitude (68°38'N, 149°38'W) and 10 lakesat a temperate latitude (46°13'N, 89°32'W) during 1997.The temperate data set was augmented with earlier survey resultsemploying similar methods. It was hypothesized that differencesin environmental variables across latitude would cause differencesin community C:N:P ratios, leading to differences in trophicinteractions. Physical measurements (light, temperature), sestonand zooplankton were collected from each lake. Seston and zooplanktonwere analyzed for C, N and P content, and zooplankton were countedand measured for biomass estimates. The degree of trophic interactionbetween seston and zooplankton was assessed by (i) measuringelemental imbalances between seston and zooplankton and (ii)calculating the potential recycling ratio by the zooplanktoncommunity available for seston. Seston C:nutrient, but not N:P,ratios were higher in temperate than arctic lakes. Conversely,arctic zooplankton had higher C:nutrient, but not N:P, ratiosthan zooplankton in temperate lakes. Elemental imbalances weregreater in temperate than in arctic lakes, but N:P stoichiometryof potential zooplankton recycling was nearly identical betweenthe two latitudes. Zooplankton community C:N:P ratios were notrelated to either latitude or seston C:N:P. In accordance withstoichiometric theory, relative abundances of calanoid copepodswere positively correlated with seston C:N in temperate lakes.Additionally, relative abundances of Daphnia were negativelycorrelated with seston C:N ratios in temperate and arctic lakes,and positively correlated with N:P ratios in the arctic. Ingeneral, these results suggest that seston and zooplankton communitystoichiometry differ across latitude, and these differenceshave the potential to affect trophic interactions.     

Abundance patterns of filter-feeding Caddisflies (Trichoptera : Hydropsychidae) and seston in a Montana (U.S.A.) lake outlet

The abundance patterns of four species of net-spinning caddisflies (Trichoptera : Hydropsychidae) were studied in a Montana (U.S.A.) lake outlet stream. Total Hydropsychidae reached very high densities (X > 1400 / 0.2 m²) near the lake outfall but declined precipitously downstream. Abundance patterns generally agree with a published model of filter-feeder / seston interactions in lake outlets. Larval densities were greater on moss-covered substrates and one species occupied habitats of lower water velocity compared to other species. Seston concentrations increased downstream contrary to expectation. Nutrient quality of seston may be important in predicting abundance of stream filter-feeders.     

Landscape controls on seston stoichiometry in urban stormwater management ponds

1. Stormwater management ponds (SWMPs) are taking the place of natural ponds and wetlands in urban areas. SWMPs have the potential to serve as hotspots for nutrient cycling, yet little is known about how urban catchments affect nutrient chemistry and stoichiometry within these ponds. 2. We sampled 50 SWMPs in Southern Ontario, Canada, to characterise their seston stoichiometry and make comparisons with published lake and pond data and models of seston stoichiometry. We tested (i) whether C : N : P ratios were similar to natural ponds and small lakes, (ii) whether seston stoichiometry was scale dependent and (iii) whether variability in seston chemistry could be explained by landscape and pond characteristics, such as catchment imperviousness and hydrological condition (based on recently received rainfall). 3. Seston C : N and C : P ratios were significantly lower in SWMPs than published ratios for small lakes, likely because of high nitrogen and phosphorus concentrations in SWMPs. Our results also showed no dependency of stoichiometric ratios on pond size. Analyses of ratios versus landscape and pond characteristics revealed significant relationships only when ponds were grouped based on the hydrological condition of the catchments. 4. It is likely that SWMPs function very differently during wet and dry periods. When SWMPs are disconnected from the landscape after a lengthy dry period, internal processes become increasingly important for seston stoichiometry.     

Influence of seston quantity and quality on growth of tropical cladocerans.

The seston of the small, shallow, and tropical lake Monte Alegre was tested for quantity and quality for cladocerans by growth bioassays, which were carried out in spring (Daphnia gessneri and Moina micrura), summer (D. gessneri, M. micrura, Ceriodaphnia cornuta, and Simocephalus mixtus), and winter (D. gessneri and D. ambigua). Cohorts of newborns originating from ovigerous females collected in the lake or from laboratory cultures were submitted, at a room temperature of 23 degrees C to the following treatments: (1) the chlorophytes Ankistrodesmus falcatus and/or Scenedesmus spinosus; (2) lake seston; and (3) lake seston + chlorophytes. Growth rate, clutch size, and fecundity were evaluated. Seston alone was not the best food for promoting cladoceran growth. There were seasonal differences in food quantity and quality with spring and summer seston being better for growth than that of the winter. Adding chlorophytes to the seston increased clutch size and fecundity for most species in summer and winter, but not in spring. Energy limitation seems to be the most important factor influencing cladoceran growth in summer and especially in winter.     

Enzymatic release of phosphate in water from subarctic lakes in Northern Sweden

Enzymatic phosphate release was studied in water from three small lakes in the subarctic region of northern Sweden. Free dissolved hydrolytic enzymes were shown to release considerable amounts of phosphate from autoclaved lake water. It was possible to show that phosphate was enzymatically released from particulate matter by exposure of autoclaved, ³²P-labelled plankton to lake enzymes followed by characterization of degradation products with gel chromatography. Seston turned out to be vitally important as a phosphorus source for planktic organisms since plankton inoculated in lake water freed from seston were shown to produce high activities of phosphatase.     

The relationship of bioenergetics and the environment to the field growth of cultured bivalves

The field culture of bivalve molluscs is dependent on the production and supply of phytoplankton and other food sources, its consumption by these suspension feeders, and its transformation into bivalve tissue. An understanding of the processes by which food is captured and utilized is fundamental to prediction of bivalve growth and management of shellfish aquaculture. The following perspective considers the role of food quality and quantity in the ingestion and absorption of food particles, as well as the influence of temperature, water circulation and other environmental variables on bivalve energy budgets and growth rate. A particular focus is the role of simulation modelling in quantifying the influence of these variables on scope for growth. Seston depletion is a major influence on cultured animals such that bivalves are limited by both food quality and quantity. Seston depletion can be managed by taking advantage of particle maxima (e.g. resuspension), matching culture species to turbidity gradients, and arranging hanging culture to take advantage of seston renewal in oblique flow. Food limitation interacts with temperature in cold temperate areas where high primary production occurs at low temperatures. Model simulations of mussel growth demonstrate the importance of temperature acclimation of filtration and its relationship to the timing and magnitude of the spring bloom. In addition to the energetics of individual animals, energy flow through culture ecosystems is considered in terms of bivalve processing of organic matter and competition with Zooplankton and fish. Because bioenergetics is at the root of all of these issues, simulation modelling of scope for growth and related components of the ecosystem is a vital tool in prediction and management of commercial bivalve aquaculture.     

Suspended particulate matter dynamics in the shallow mesotidal urdaibai estuary (Bay of Biscay,Spain)

Seston quantity and quality was measured in the Urdaibai Estuary in summer, when river discharge is low and tidal flushing is the main driving force to transport and resuspend particulate matter in the estuary. The highest seston concentrations are found in the upper estuary, where more than 90% of total suspended particulate matter is made up of particles <20 μm. There is a temporal ebb-flood asymmetry and turbulent mixing is stronger at flood tide. During dry calm periods sediment resuspension is observed for a short period of time at floods during spring tides, decreasing drastically before slack waters. In spite of this, particulate organic matter mainly originates from anthropogenic sources (domestic wastewater discharge) and plankton production.     

Daphnia population growth but not moulting is a substantial phosphorus drain for phytoplankton

SUMMARY 1. Negative effects of zooplankton on the availability of phosphorus (P) for phytoplankton as a result of the retention of nutrients in zooplankton biomass and the sedimentation of exoskeletal remains after moulting, have been recently proposed. 2. In a mesocosm study, the relative importance of these mechanisms was tested for the freshwater cladoceran Daphnia hyalina×galeata. A total of 13 mesocosm bags was suspended in a mesotrophic German lake during summer 2000 and fertilised with inorganic P in order to obtain a total nitrogen to total P ratio closer to the Redfield ratio. D. hyalina×galeata was then added at a logarithmically scaled density gradient of up to 40 ind. L^?1. Zooplankton densities, dissolved inorganic, particulate organic (seston <100 μm), as well as total nutrient concentrations were monitored. Additionally, nutrient concentrations of sediment water removed from the bottom of the mesocosm bags via a manual pump were determined. 3. Seston carbon (C), seston P and total P were significantly negatively correlated with Daphnia densities. The amount of particulate P (～5–6 μg P L^?1) sequestered from the seston compartment by Daphnia corresponded roughly to the increase of zooplankton biomass (population growth). Soluble reactive phosphorous (SRP) was at all times high (～25–35 μg P L^?1) and possibly unavailable to phytoplankton as a result of P adsorption to calcite during a calcite precipitation event (whiting). P concentrations determined in sediment water were generally <60 μg P m^?2 and thus never exceeded 1% of the total amount of P bound in particulate matter of the overlying water column. 4. Seston C : P ratios followed a polynomial second‐order function: At Daphnia densities <40 ind. L^?1 a positive linear relationship was evident, which is explained by the stronger reduction of P compared with C in seston, and transfer of seston P to zooplankton. Highest seston C : P ratios of ～300 : 1 were observed at Daphnia densities of ～30–50 ind. L^?1, which is in agreement with proposed threshold values limiting Daphnia reproductive growth. At Daphnia densities >40–50 ind. L^?1 C : P ratios were decreased because of the strong reduction of seston C at close to constantly low seston P‐values of ～3–4 μg P L^?1. 5. At least for Daphnia, it may be concluded that – unlike population growth – the sedimentation of faecal pellets and carapaces after moulting seem negligible processes in pelagic phosphorus dynamics.     

Linking major nutrients (C,H, N,and P) to trace metals (Fe,Mn, and Cu) in lake seston in southern Brazil

We investigated the relationships between major nutrients (C, H, N, and P) and trace metals (Cu, Fe, and Mn) in seston samples from ten lake/lagoon systems in southern Brazil. The systems were characterized by a diverse set of limnological features, including surface areas from 10⁻¹ to 10² km², water color, a _CDOM(440), from 1.4 to 12.9 m⁻¹, and electrical conductivity from 50 to 100 000 μS cm⁻¹. Seston concentrations also varied a great deal, 32-fold. The elemental (C: N, C: P, and N: P) and C: Chl-a ratios in the seston samples indicated, however, common features; i.e., most of the lakes were N-and/or P-limited, and the seston organic fraction was composed of nonvascular plants (e.g., phytoplankton). Our intersystem comparison revealed that the relative content of organic matter in seston and seston concentrations in lake water tended to correlate positively and negatively, respectively, with trace metal concentrations across the seston samples. Possible influences of elemental and C: Chl-a ratios on the association of metals with seston matrices, although theoretically important, were only partially evidenced here; positive correlations were found between C: N and also Org-H: N ratios with trace metal concentrations. We speculate that such results could be circumstantial, as the nature of the seston matrices appeared to be very similar among them. This hypothesis should thus be the theme of further research. In short, our findings suggest that C: N and Org-H: N ratios as well as the relative content of major nutrients in seston and seston concentrations can be importantly related to trace metal concentrations in seston samples. In discussing the results, however, we consider that metal-seston relationships depend on a variety of physical, chemical, and biological factors and/or variables other than those measured in this study, which could also contribute for defining and explaining variations in metal-seston concentrations in lake ecosystems.     

Quantity and quality of seston in an impounded and a free-flowing river in Virginia,U.S.A.

Seston in the impounded North Anna River (NAR) was analyzed and compared to that in the free-flowing South Anna River (SAR) in Virginia, U.S.A. A wet filtration technique was used to separate seston into five size classes. The overall quantity of organic seston was much lower in the NAR than in the SAR. The seston in the NAR was composed of more living organisms, in particular zooplankters in the medium large (234–864 µm) and small (105–234 µm) size classes and diatoms and other algae in the fine (43–105 µm) and very fine (25–43 µm) size classes. The percentage of zooplankton declined sharply before reaching the downstream study site (32 km). The seston in the NAR tended to consist of slightly larger particles, but 80–85% of the seston in both rivers was in the ultrafine (0.45–25 µm) size class, which was almost entirely composed of detritus. One indication of seston food quality, the organic/inorganic ratio, was considerably higher for the medium large and small size classes in the NAR immediately below the dam and for the fine and very fine size classes farther downstream. Another indication of food quality, the usable caloric content, showed that the seston in the NAR contained considerably less total assimilable energy as far as 32 km downstream from the dam, but that the medium large and small size classes were rich in assimilable energy immediately below the dam because of zooplankton released from the reservoir. Thus, the overall quantity and quality of seston was lower in the NAR, but the quality of the seston, for at least a short distance below the dam, was higher in a well-defined size range that can be effectively utilized by certain filter feeders.     

Food quality for Daphnia in humic and clear water lakes

1. Growth and reproduction of Daphnia fed lake seston were measured in two categories of meso‐ to eutrophic lakes differing with respect to terrestrial organic matter influence (humic and clear water lakes). The content of highly unsaturated fatty acids (HUFA), P and N, as well as the taxonomical composition of seston were analysed. 2. Seston HUFA and C : P ratios were similar between lake categories, whereas C : N ratios were lower in the clear water lakes in both spring and summer. Despite the similarity in HUFA and P content of seston, Daphnia growth rate, clutch size and the proportion of gravid females were, respectively, about 1.5, 3 and 6 times higher in the clear water lakes. 3. Differences in growth and reproduction were related to a combination of higher N content and good fatty acid quality of the seston in the clear water lakes. Relatively high biomass of edible algae, such as Rhodomonas sp. and Cryptomonas sp., in the clear water lakes, and differences in water pH likely contributed to the observed differences in Daphnia growth and reproduction between lake categories. Additionally, it is possible that Daphnia was energy limited in the humic lakes despite high particulate organic carbon (POC) concentrations, as the contribution of non‐algal and detrital C to the POC pool was high. 4. Our results suggest that dietary HUFA content has the potential to improve herbivore growth and reproduction if N and P are not limiting. N merits more attention in studies of zooplankton nutrition.     

Ecosystem development in different types of littoral enclosures

Macrophyte growth was studied in two enclosure types (gauze and polythene) in a homogeneousPotamogeton pectinatus bed in Lake Veluwe (The Netherlands). The gauze was expected to allow for sufficient exchange with the lake to maintain similar seston densities, the polythene was expected to exclude fish activity and most water exchange. Polythene enclosures held higher totalP. pectinatus biomass (ash-free dry weight, AFDW) than the lake, gauze enclosures were intermediate. The enclosures had a higher abundance of other macrophyte species (Chara sp.,Potamogeton pusillus) than the lake. Seston ash content was not but seston AFDW, periphyton ash content and AFDW were lower in polythene than in gauze enclosures. The difference in plant biomass between gauze and polythene may be attributed to a difference in periphyton density and in seston AFDW due to zooplankton grazing (Rotatoria andDaphnia densities were higher in polythene enclosures). Since seston and periphyton AFDW and ash content were similar in lake and gauze enclosures, the intermediate macrophyte biomass in the gauze enclosures may be explained by reduced wave action and mechanical stress. Alternatively, phytoplankton inhibition by allelopathic excretions from the macrophytes may have caused the high macrophyte biomass in the polythene, and an absence of sediment-disturbing fish the intermediate biomass in the gauze enclosures. Creation of sheltered areas may favour macrophyte growth through both mechanisms and we conclude that this can be an important tool in littoral biomanipulation.     

Determination of seston alkalinity in natural waters

Seston alkalinity is a term introduced with the purpose of considering the acid base neutral capacity of the particles integrating it. It can be determined by a simple method, based on a volumetric titration. In alkaline waters, this variable is associated with the insoluble forms of carbonates of the seston.Contribution No. 476, Instituto de Limnología, Dr Raúl A. Ringuelet.     

Microbial colonization of seston and free bacteria in an impounded river

A headwater river-reservoir system was studied from March–November, 1982 to determine effects of impoundment on seston quality. We used epifluorescent microscopy to compate and contrast microbial colonization of seston and abundance of free microbes. There was a significant relationship between colonization density and particle size at all sites. Smaller particles were colonized more densely by bacteria than larger particles. Total counts of bacteria (free plus attached) did not differ significantly beween sites. However there were significantly more free bacteria (# ml^–1) in the reservoir and 5 km below the dam than upstream of the impoundment. Density of attached bacteria (# µm^–2) was similar upstream and downstream of the reservoir but slightly higher at lentic sites. Proportionally, attached bacteria were more important upstream (mean 42–45% of total counts) than in the reservoir and downstream (19–31%). We concluded that reservoir seston was of higher quality than riverine seston and major downstream effects included decreases in seston concentration (due to sedimentation) and shifts in proportional abundance of free and attached bacteria. Seston consumers capable of using ultrafine particles (<25 µm), including free bacteria, had higher quality food resources below the dam than in upstream areas.     

Are zooplankton food resources poor in the vegetated littoral zone of shallow lakes?

1. The distribution of zooplankton in shallow lakes is negatively related to macrophyte density. However, the abundance of their food along density gradients of macrophytes is unknown. A common but untested assumption is that food quantity and quality for pelagic zooplankton is poor in the littoral zone owing to the deleterious influence of macrophytes on phytoplankton. 2. We tested this assumption with a combination of a field survey and laboratory experiments. We collected seston samples from the littoral and pelagic zones of four shallow temperate lakes and related food quantity (phytoplankton biovolume) and quality to macrophyte abundance (per cent volume infested). Seston food quality was assessed in three ways: N/C and P/C ratios, polyunsaturated fatty acid content and phytoplankton community composition. In the laboratory, we measured the growth and reproduction of Daphnia pulicaria on diets consisting of seston from the littoral and pelagic zones in one lake. 3. In our four study lakes, food quantity was not significantly influenced by macrophyte abundance, and food quality was generally high. Laboratory experiments showed increased juvenile growth, but no significant change in D. pulicaria reproduction, when feeding on littoral resources compared to pelagic resources. 4. Our results suggest that there is no nutritional cost to pelagic zooplankton inhabiting the littoral zone. Therefore, it is likely that other factors (e.g. predation, abiotic factors) are involved in determining zooplankton habitat use.     

Atomabsorptionsmessungen von Blei,Cadmium, Kupfer,Eisen und Zink im Seston der Ostsee

Zusammenfassung 1. Auf einer Forschungsfahrt in die Ostsee im April/Mai 1973 wurden 100 Sestonproben entnommen und auf ihren Gehalt an Blei, Cadmium, Zink, Kupfer und Eisen mit Hilfe der flammenlosen Atom-Absorptions-Spektrometrie analysiert.2. Die mittlere Konzentration im Seston der Ostsee beträgt für Blei 123 ppm, für Cadmium 5,8 ppm, für Zink 300 ppm, für Kupfer 61 ppm und für Eisen 3535 ppm.3. Im Vergleich zu Metallanalysen ozeanischen Sestons müssen die Blei- und Cadmiumkonzentrationen als stark überhöht angesehen werden.4. Die Konzentrationen des partikulär gebundenen Eisens, Zinks und Kupfers können bis auf lokal stark erhöhte Eisen- und Zinkkonzentrationen als natürlich angesehen werden.5. Arbeitsweise und Meßgenauigkeit der Metallanalytik werden beschrieben.

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Atomic absorption measurements of lead, cadmium, copper, iron and zinc in seston of the Baltic Sea

During a cruise with R. V. Alkor from Kiel Bight to Finnish Bay, nearly 100 seston samples were taken during April/May 1973. Samples from the 23 stations were analyzed for lead, cadmium, copper, iron and zinc by flameless atomic absorption spectrometry. The mean values for lead and cadmium were 123 and 5.8 ppm, respectively; for zinc, 733 ppm, for copper, 61 ppm, and for iron, 3535 ppm. In contrast to trace-metal analyses of off shore seston, the concentrations for lead and cadmium in Baltic-Sea seston are very high; these results are in good agreement with analyses of the dissolved metals. The concentrations of zinc, copper and iron in the particulate matter from the Baltic-Sea, apart from some local influences, agree well with values published from the concentrations in off shore seston. The distribution and variation of the individual values are discussed. The method for analysing the metals and its precision are briefly described.

     

Does the seston of Lake Biwa release dissolved inorganic nitrogen and phosphorus during aerobic decomposition?: Its implication for eutrophication

Seston was collected from depths of 5 and 60 m in the north basin and a depth of 0 m in the south basin of Lake Biwa in the summer of 1985 and decomposed in the laboratory under aerobic conditions for three months. There was no net release of dissolved inorganic phosphorus from any seston, in contrast with the liberation of dissolved inorganic nitrogen. From this result and other available data, it is suggested that almost all phosphorus entering Lake Biwa and remaining within it is finally transformed into particulate phosphorous, transported to the bottom sediments, and fixed there without being recycled. Thus, this mechanism seems to play an important role in the prevention of rapid eutrophication of Lake Biwa, in spite of high external phosphorus loading.     

Factors influencing the longitudinal distribution of larval Hydropsychidae (Trichoptera) in a southern Appalachian stream system (U.S.A.)

The influence of physical habitat variables and suspended particulate organic matter (seston) on the distribution and production of eight species of larval Hydropsychidae was studied along a 6.4 km section of a southern Appalachian stream. Samples were collected at six stations encompassing stream orders 1–4 and an elevation range of 610 m. Multivariate analysis of covariance (using time as the covariable) and discriminant function analysis were used to examine habitat differences between the sampling stations due to the following variables: current velocity; coarse benthic detritus; substrate composition (by particle size); substrate heterogeneity; degree-days; and diel temperature fluctuation. The associations of these variables with the abundance (and production) of larval hydropsychids was also examined using the same statistical procedures. The six sampling stations represented three or four distinct habitats based on patterns of change in the variables along the stream continuum. Diel temperature fluctuation, median substrate particle size, the proportion of sand substrate, and substrate heterogeneity were most closely associated with the overall difference between the sampling stations. Hydropsychid species distribution along the stream system followed subfamily lines, i.e., Arctopsychinae and Diplectroninae were more abundant and productive in the upper 4.5 km of the stream, while Hydropsychinae were dominant in the lower 1.9 km. Diel temperature fluctuation was the habitat variable most highly correlated with patterns of hydropsychid abundance and production. The longitudinal pattern of species distribution, i.e., larger particle feeding Arctopsychinae being replaced downstream by smaller particle feeding Hydropsychinae, also coincided with the distribution of seston particle size classes along the stream. Mean seston particle size generally declined downstream, as particles <42 µm increased in relative abundance while those between 43 µm and 5 mm decreased. Temperature, seston, and substrate composition all undoubtedly exerted an important influence on the distribution and production of Hydropsychidae and other filter feeding insects in this stream. The validity and generality of these results depend upon the scope of the sampling effort. Conclusions drawn from data collected over the entire range (and limits) of a species' distribution are more sound than those based on data from a limited area.