Coarse Benthic Organic Matter Research Articles

Differential responses of macroinvertebrate ionomes across experimental N:P gradients in detritus-based headwater streams

Diverse global change processes are reshaping the biogeochemistry of stream ecosystems. Nutrient enrichment is a common stressor that can modify flows of biologically important elements such as carbon (C), nitrogen (N), and phosphorus (P) through stream foodwebs by altering the stoichiometric composition of stream organisms. However, enrichment effects on concentrations of other important essential and trace elements in stream taxa are less understood. We investigated shifts in macroinvertebrate ionomes in response to changes in coarse benthic organic matter (CBOM) stoichiometry following N and P enrichment of five detritus-based headwater streams. Concentrations of most elements (17/19) differed among three insect genera (Maccaffertium sp., Pycnopsyche spp., and Tallaperla spp.) prior to enrichment. Genus-specific changes in the body content of: P, magnesium, and sodium (Na) in Tallaperla; P, Na, and cadmium in Pycnopsyche; and P in Maccaffertium were also found across CBOM N:P gradients. These elements increased in Tallaperla but decreased in the other two taxa due to growth dilution at larger body sizes. Multivariate elemental differences were found across all taxa, and ionome-wide shifts with dietary N and P enrichment were also observed in Tallaperla and Pycnopsyche. Our results show that macroinvertebrates exhibit distinct differences in elemental composition beyond C, N, and P and that the ionomic composition of common stream taxa can vary with body size and N and P enrichment. Thus, bottom-up changes in N and P supplies could potentially influence the cycling of lesser studied biologically essential elements in aquatic environments by altering their relative proportions in animal tissues.

Fatty acids elucidate sub-Antarctic stream benthic food web dynamics invaded by the North American beaver (Castor canadensis)

Despite being remote, polar and sub-polar regions are increasingly threatened by global ecological change. For instance, South America’s sub-Antarctic forest ecoregion is considered one of the world’s last wilderness areas and a global reference site for pre-Industrial Revolution nutrient cycles. Nonetheless, the North American beaver (Castor canadensis) was introduced to Tierra del Fuego in 1946 and, as an invasive ecosystem engineer, has transformed the ecology of regional watersheds. Beavers’ engineering activities transform forested streams (FS) into beaver ponds (BP), where there is greater light and primary production (allochthonous organic matter) and, consequently, increased basal resource quality. To investigate this, we analyzed algal, diatom, fungal and bacterial fatty acid (FA) biomarkers in three basal resource categories (biofilm, very fine benthic organic matter, coarse benthic organic matter) and benthic consumers from four functional feeding groups (FFG). The amphipod Hyalella spp. was chosen as an indicator species due to its abundance and biomass in both habitats. Hyalella spp. had higher proportions of algal and bacterial FA in BP than FS. In FS, Hyalella spp. (gatherer) and Gigantodax spp. (filterer, Diptera) had greater contributions of higher quality FA (higher in polyunsaturated FA), while Rheochorema magallanicum (predator, Trichoptera) and Meridialaris spp. (scraper, Ephemeroptera) showed lower quality monounsaturated and saturated FA. All FFGs showed evidence of microbial FA and had higher levels of autochthonous FA biomarkers than their food resources. Scrapers had the greatest proportion of autochthonous FA. These data provide new insights into the utilization of basal resources by stream consumers in sub-Antarctic streams and how beavers modify these ecosystems.

Inter‐regional comparison of land‐use effects on stream metabolism

Summary1. Rates of whole‐system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first‐order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions.2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban‐influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input.3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams.4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land‐use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land‐use categories in the catchment as regulators of ER.5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land‐use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER.6. Overall, consideration of the data separated by land‐use categories showed reduced inter‐regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.

Leaf Litter Dynamics and Its Use by Invertebrates in a High‐Altitude Tropical Andean Stream

AbstractThis study evaluates litter dynamics and its relation to macroinvertebrate communities (assemblages and feeding habits) at Piburja stream (3300 m, Ecuador). Annual litterfall (216 g AFDM/m2) was not related to rainfall, but differed significantly between months. Transport of Coarse Particulate Organic Matter (CPOM) did not differ between seasons, but retention was higher during the dry season. Thus, CPOM standing stock was higher in the dry (125.28 g AFDM/m2) compared to the wet season (12.27 g AFDM/m2). Macroinvertebrate richness and diversity were positively related to Coarse Benthic Organic Matter (CBOM) especially during the wet season. Gut content analysis revealed that, regardless of season, CPOM and Fine Particulate Organic Matter (FPOM) were the most important food items in the diet of most invertebrate taxa, including those that are not considered shredders or gathering collectors in the literature. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Nitrate retention and removal in Mediterranean streams bordered by contrasting land uses: a <sup>15</sup>N tracer study

Abstract. We used 15N-labelled nitrate (NO3−) additions to investigate pathways of nitrogen (N) cycling at the whole-reach scale in three stream reaches with adjacent forested, urban and agricultural land areas. Our aim was to explore among-stream differences in: (i) the magnitude and relative importance of NO3− retention (i.e. assimilatory uptake) and removal (i.e. denitrification), (ii) the relative contribution of the different primary uptake compartments to NO3− retention, and (iii) the regeneration, transformation and export pathways of the retained N. Streams varied strongly in NO3− concentration, which was highest in the agricultural stream and lowest in the forested stream. The agricultural stream also showed the lowest dissolved oxygen (DO) concentration and discharge. Standing stocks of primary uptake compartments were similar among streams and dominated by detritus compartments (i.e. fine and coarse benthic organic matter). Metabolism was net heterotrophic in all streams, although the degree of heterotrophy was highest in the agricultural stream. The NO3− uptake length was shortest in the agricultural stream, intermediate in the urban stream, and longest in the forested stream. Conversely, the NO3− mass-transfer velocity and the areal NO3− uptake rate were highest in the urban stream. Denitrification was not detectable in the forested stream, but accounted for 9% and 68% of total NO3− uptake in the urban and the agricultural stream, respectively. The relative contribution of detritus compartments to NO3− assimilatory uptake was greatest in the forested and lowest in the agricultural stream. In all streams, the retained N was rapidly regenerated back to the water column. Due to a strong coupling between regeneration and nitrification, most retained N was exported from the experimental reaches in the form of NO3−. This study provides evidence of fast in-stream N cycling, although the relative importance of N retention and removal varied considerably among streams. Results suggest that permanent NO3− removal via denitrification may be enhanced over temporary NO3− retention via assimilatory uptake in heterotrophic human-altered streams characterized by high NO3− and low DO concentrations.

Responses in organic matter accumulation and processing to an experimental wood addition in three headwater streams

Summary1. Additions of large wood are being used to restore streams that have been subjected to channelization, wood removal or riparian timber harvest. This added wood potentially increases channel stability, habitat complexity and organic matter retention and improves habitat and productivity of higher trophic levels. However, few stream restorations monitor restoration effectiveness after project completion.2. We added 25 aspen logs (each 2.5 m length × 0.5 m diameter) to 100‐m reaches of each of three forested headwater streams in the Upper Peninsula of Michigan, U.S.A. These wood‐poor streams drain forests that were completely harvested of timber over a century ago and have been selectively logged for the past 50–60 years. An upstream unmanipulateds 100‐m reach in each stream served as the control.3. We evaluated responses in organic matter processing by measuring red maple leaf decomposition 1 year before and 2 years after wood addition. We also quantified coarse organic matter standing stocks in the main channel and in debris accumulations associated with large wood. In response to wood addition, we predicted both organic matter standing stocks and leaf decomposition rates would increase, thereby enhancing resource availability to higher trophic levels.4. Leaf decomposition rates did not change following wood addition. Temporal variation in rates among streams was mostly explained by differences in degree days, water velocity, scour/burial and water column inorganic nitrogen concentrations, but not large wood. Variation within streams across years was explained by differences in degree days, water velocity and shredder biomass.5. Contrary to our prediction, organic matter standing stocks did not increase significantly at the reach scale. However, the experimentally added wood retained c. 4% of total annual coarse benthic organic matter (CBOM) in the first year and an additional c. 15% in the second year, suggesting accumulation over time in the manipulated reaches. The CBOM held by the new logs may be more biologically available because it is less susceptible to burial and transport than material in the streambed.6. Some shredding macroinvertebrates responded to changes caused by the wood additions. In particular, the common caddisfly shredder, Lepidostoma sp., increased in abundance in leaf bags following wood addition, whereas the biomass of the winter stoneflies, Capniidae, declined in the first year.7. Considerable funds are spent to restore in‐stream habitat, but few restorations are monitored, particularly over long periods (>5 years). Our results show that longer‐term monitoring is needed to determine the efficacy of these restorations on ecosystem function; organic matter decomposition in our low‐gradient streams did not respond to a substantial increase in large wood after 2 years.

Spiraling down the river continuum: stream ecology and the U-shaped curve

The spiraling concept provides an explicit approach to modeling the longitudinal linkages within a river continuum. I developed a spiraling-based model for particulate organic C dynamics in the Little Tennessee River to synthesize existing data and to illustrate our current understanding of ecosystem processes in river ecosystems. The Little Tennessee River is a medium-sized river flowing ∼100 km through the southern Appalachian Mountains of northern Georgia and western North Carolina (USA). Across this distance, allochthonous inputs decrease and autochthonous production increases, resulting in a U-shaped curve of energy input. The model was set up as an advecting seston compartment interacting with 3 benthic compartments: coarse benthic organic matter, fine benthic organic matter, and autotrophs. Model-estimated ecosystem respiration was consistently lower than measured values, suggesting a need to evaluate our measurements of whole-stream metabolism. Also, model-predicted seston concentrations were generally lower than measured values, reflecting a need to consider additional sources of organic C in the model. For the whole river system, leaves accounted for 19% of inputs, primarily near the headwaters, and the remaining input was from instream primary production in the lower reaches of the river. Almost ½ of the input was respired, 28% by autotrophic respiration and 21% by heterotrophic respiration, and the remaining 51% was transported downstream. Ecosystem efficiency was ∼50% along the length of the river, and turnover length increased from several hundred meters at the headwaters to >100 km downstream. Based on various measures, the transition from heterotrophy to autotrophy ranged from 25 to >100 km downstream from the headwaters. As this model illustrates, a consequence of downstream transport is that much of the particulate C in streams is metabolized a considerable distance downstream from where it enters the stream. This longitudinal linkage is essential to our understanding of stream ecosystems.

Influence of organic matter on invertebrate colonization of sand substrata in a northern Michigan stream

AbstractSand is a common substratum in many streams, especially in lacustrine geologies, but has been less studied as a habitat for invertebrates than other substrata such as gravel and cobble. We hypothesized that benthic organic matter (BOM) content would influence the abundance and community structure of macroinvertebrates in sand habitats. Levels of coarse BOM (no = 0, low = 1%, high = 5% organic matter, as dry mass) were manipulated in 45 colonization chambers (volume = 539 cm3) implanted in a sand-dominated reach of Shane Creek in the Ottawa National Forest, Michigan. Chironomidae, Tipulidae, and Trichoptera were common colonizers of chambers. At the end of the 32-d experiment, invertebrate abundance (F2,12 = 7.5, p = 0.015) and biomass (F2,12 = 8.7, p = 0.010) were significantly higher in chambers with low BOM than with no BOM, but the high-BOM treatment did not differ from the no-BOM treatment. Throughout the experiment, functional feeding groups (FFG) were dominated numerically by gathering colle...

Effects of a waste water treatment plant on organic matter dynamics and ecosystem functioning in a Mediterranean stream

We studied the effects of a waste water treatment plant on organic matter dynamics in Fosso Bagnatore, a Mediterranean stream near Rome (Italy). Concentrations of dissolved organic carbon and seston, along with the standing crops of coarse benthic organic matter and fine benthic organic matter, were monitored monthly during 2002 in two reaches situated upstream and downstream of a waste water treatment plant effluent. Additionally, we examined two stream functional characteristics: whole stream metabolism, which was measured from diel changes in oxygen concentrations, and nutrient removal, which was studied by experimental additions of NH4-N, NO3-N and PO4-P. The possible influence of WWTP organic matter alterations on ecosystem functioning was addressed by measuring epilithon characteristics (standing crop, C:N:P ratios and chlorophyll a content). The WWTP effluent caused an increase (several-fold) of DOC and seston concentrations, and FBOM standing crop, moreover it modified their temporal variation. Downstream of the wastewater treatment plant we detected an higher heterotrophic microbial biomass and a correspondent higher community respiration. The uptake of NH4-N, NO3-N and PO4-P was highly variable probably as consequence of the high variability of the compartments involved (e.g. DOC, FBOM, community respiration). We showed that WWTP effluent altered the organic matter dynamics of this Mediterranean-type headwater stream and that these effects were reflected in its ecosystem functioning. Because of the linkage of headwaters to downstream ecosystems and due to their significant role in landscape and catchment processes, the alterations of organic matter dynamics that we detected at a local scale may have important consequences on a broader scale.

Macroinvertebrate and zooplankton responses to emergent plant production in upper Mississippi River floodplain wetlands

The upper Mississippi River is managed as a multiple use system, balancing the needs of industry, recreation, and navigation. Through water-level management (WLM), the U. S. Army Corps of Engineers manipulates water levels in some navigation pools to promote emergent vegetation for fish and wildlife in off-channel habitats. A manipulative experiment was conducted to assess responses of invertebrates to vegetation and associated organic matter associated with WLM in floodplain wetlands of Mississippi River navigation pool 25. Macroinvertebrate and zooplankton communities and benthic organic matter were compared in replicated paired plots consisting of experimentally de-vegetated and adjacent control plots. Hydrology was variable during the study, resulting in a strong vegetation response year (1999), a no response year (2000), and a moderate response year (2001). Differences in benthic organic matter between plot types were most pronounced in fall 2000, following the strong vegetation response year, where residual vegetation resulted in significantly higher total and coarse benthic organic matter in vegetated plots. Of the 2 years (2000, 2001) they were sampled, differences in macroinvertebrate communities were only evident in 2001 when there was a moderate vegetation response. Total macroinvertebrate densities during 2001 were similar between devegetated and vegetated plots, but responses of individual taxa varied. Total macroinvertebrate biomass was significantly higher in the vegetated plots during 2001. Of dominant groups, Oligochaeta biomass was significantly higher in vegetated plots, whereas total Chironomidae abundance and biomass were both significantly higher in devegetated plots. Community metrics reflected higher macroinvertebrate diversity in vegetated plots. Zooplankton were not abundant or diverse in plots, but total densities were significantly higher in vegetated plots during the 1999 strongest vegetation response year. Results demonstrate that WLM to enhance emergent vegetation in off-channel habitats of this large river influences organic matter dynamics and invertebrates. Although many groups responded positively to vegetation, responses varied with the degree of vegetation response and the taxonomic resolution of analyses, suggesting that hydrologic variability, and associated spatial and temporal variability in emergent vegetation, can enhance invertebrate diversity in floodplain wetland habitats.

The influence of ammonium, nitrate, and dissolved oxygen concentrations on uptake, nitrification, and denitrification rates associated with prairie stream substrata

Substrata samples were collected from Kings Creek on Konza Prairie Biological Station (Manhattan, Kansas) and incubated with varying levels of ammonium (NH4+), nitrate (NO3−), and dissolved oxygen (O2) to examine the response of nitrogen (N) uptake and transformation rates. Substrata collected were fine benthic organic matter (FBOM), coarse benthic organic matter, filamentous green algae, bryophytes, suspended particulate organic matter, and epilithic diatoms. Nitrification and denitrification were estimated by use of the nitrapyrin and acetylene inhibition methods, respectively. Ammonium uptake demonstrated Michaelis‐Menten kinetics, with the highest maximum rates (Vmax) associated with filamentous green algae (5.90 mg N gdm−1 d−1) and epilithic diatoms (4.96 mg N gdm−1 d−1). Nitrate uptake did not saturate at the highest NO3− addition (25 µg N L−1) above ambient when associated with FBOM. Overall, maximum uptake rates of NH4+ were 10‐fold higher than for NO3−. Nitrification response to increasing NH4+ concentrations was highly variable, depending on the substrata type. Nitrification was lowest under low O2 conditions, being undetectable when NO3− was added but not when NH4+ was added. Denitrification increased linearly with NO3− concentration when associated with epilithic diatoms and FBOM but became saturated at ~20 µg N L−1 above ambient concentrations when associated with filamentous green algae. Samples purged with N2 gas had the highest rates of denitrification. We predicted stream ecosystem rates using equations derived from the experimental data and substrata mass estimates measured in the field. Substantial temporal variability was predicted in uptake (0–1,300 mg NH4+−N m−2 d−1; 0–5.2 mg NO3−−N m−2 d−1), nitrification (0–35 mg NH4+−N m−2 d−1), and denitrification (0–130 µg N2O‐N m−2 d−1) as due to natural variation in water column NH4+, NO3−, and O2 concentrations.

COMPARISONS OF NITRIFICATION AND DENITRIFICATION IN PRAIRIE AND AGRICULTURALLY INFLUENCED STREAMS

Whole-stream nitrification and denitrification rates were calculated from field incubation studies of representative substrata in Kings Creek and Shane Creek watersheds at the Konza Prairie Biological Station, Kansas, USA. Substrata from a relatively pristine upstream and an agriculturally influenced downstream reach were measured in both watersheds. Rates were scaled to the whole stream by the mass of each substratum in the stream reach. Substrata sampled included epilithon, leaf packs and wood (coarse benthic organic matter, CBOM), filamentous green algae, bryophytes, fine benthic organic matter (FBOM), and suspended particulate organic matter. Upstream sites had significantly lower nitrification and denitrification rates than downstream sites. Nitrification rates were dominated by nitrification associated with epilithon and filamentous green algae. Downstream sites had a higher proportion of nitrification occurring in the water column relative to upstream sites. Whole-stream denitrification was dominated by denitrification associated with FBOM and CBOM and was higher downstream. Water column ammonium (NH4+) and nitrate (NO3−) concentrations were correlated with whole-stream nitrification rates but not whole-stream denitrification rates. Whole-stream nitrification rates were positively correlated with whole-stream denitrification rates, being coupled at the whole-stream level, but not at the substratum level. Relative to the amount of NO3− produced through nitrification, the proportion of NO3− denitrified was 1.4–4.6 times greater in the downstream sites compared to the upstream sites, but the proportion of the in-stream NO3− load denitrified was ∼10 times greater in the upstream sites. These data indicate that changes in stream nitrogen cycling related to increased dissolved inorganic nitrogen concentrations do not result in a large enough increase in denitrification (the stream's capacity to retain nitrogen) to compensate for the increased nitrogen loading. Data also suggest that whole-stream nitrification and denitrification may be decoupled at the reach level by factors that decrease habitat heterogeneity (the variety of substrata types) in stream channels. This indicates that anthropogenic disturbances of small streams, such as channelization and removal of riparian vegetation, would be expected to decrease the capacity of streams to retain nitrogen.

Comparisons of Nitrification and Denitrification in Prairie and Agriculturally Influenced Streams

Whole-stream nitrification and denitrification rates were calculated from field incubation studies of representative substrata in Kings Creek and Shane Creek watersheds at the Konza Prairie Biological Station, Kansas, USA. Substrata from a relatively pristine upstream and an agriculturally influenced downstream reach were measured in both watersheds. Rates were scaled to the whole stream by the mass of each substratum in the stream reach. Substrata sampled included epilithon, leaf packs and wood (coarse benthic organic matter, CBOM), filamentous green algae, bryophytes, fine benthic organic matter (FBOM), and suspended particulate organic matter. Upstream sites had significantly lower nitrification and denitrification rates than downstream sites. Nitrification rates were dominated by nitrification associated with epilithon and filamentous green algae. Downstream sites had a higher proportion of nitrification occurring in the water column relative to upstream sites. Whole-stream denitrification was dominated by denitrification associated with FBOM and CBOM and was higher downstream. Water column ammonium (NH4+) and nitrate (NO3−) concentrations were correlated with whole-stream nitrification rates but not whole-stream denitrification rates. Whole-stream nitrification rates were positively correlated with whole-stream denitrification rates, being coupled at the whole-stream level, but not at the substratum level. Relative to the amount of NO3− produced through nitrification, the proportion of NO3− denitrified was 1.4–4.6 times greater in the downstream sites compared to the upstream sites, but the proportion of the in-stream NO3− load denitrified was ∼10 times greater in the upstream sites. These data indicate that changes in stream nitrogen cycling related to increased dissolved inorganic nitrogen concentrations do not result in a large enough increase in denitrification (the stream's capacity to retain nitrogen) to compensate for the increased nitrogen loading. Data also suggest that whole-stream nitrification and denitrification may be decoupled at the reach level by factors that decrease habitat heterogeneity (the variety of substrata types) in stream channels. This indicates that anthropogenic disturbances of small streams, such as channelization and removal of riparian vegetation, would be expected to decrease the capacity of streams to retain nitrogen.

Centimeter-scale patterns in dissolved oxygen and nitrification rates in a prairie stream

Dissolved oxygen (O2) was measured with microelectrodes in shallow subsurface microsites in a prairie stream and related to rates of nitrification determined in the laboratory using the nitrapyrin method. Substrata sampled included diatom mats, leaves and wood (coarse benthic organic matter, CBOM), filamentous green algae, bryophytes, and fine benthic organic matter (FBOM). Significant differences in O2 concentrations were found among the substrata, with anoxic zones occurring primarily in FBOM from deep pool sediments and CBOM from litter accumulations. Filamentous green algae and bryophytes had average O2 concentrations near saturation and intermediate rates of nitrification. Diatom mats had the highest concentrations of O2 (up to several times saturation) and the highest rates of nitrification. In the summer, O2 concentrations were above saturation in epilithon and filamentous green algal mats. Nitrification rates were highest in epilithon and filamentous green algae samples taken in the spring and autumn. A significant positive relationship between nitrification rates and O2 concentration was observed in all seasons except summer. These data suggest that O2 concentration could control nitrification in prairie streams.

A comparison of the trophic ecology of the crayfishes (Orconectes nais (Faxon) and Orconectes neglectus (Faxon)) and the central stoneroller minnow (Campostoma anomalum (Rafinesque)): omnivory in a tallgrass prairie stream

Omnivorous fish, such as the central stoneroller minnow (Campostoma anomalum(Rafinesque)), and crayfish often play important roles in the trophic dynamics of streams. The trophic role of these two omnivores has not been compared within a system even though they both consume algae, detritus and invertebrates and often co-occur in streams in the Midwestern United States. Natural abundance of 15N and 13C isotopes and a whole stream 15N-labeled ammonium chloride release were used to compare the trophic ecology of the central stoneroller minnow (Campostoma anomalum (Rafinesque)) and two species of crayfish (Orconectes neglectus (Faxon) and Orconectes nais (Faxon)) in a tallgrass prairie stream. The δ15N and δ13C values of Orconectes spp. were more similar to coarse benthic organic matter (CBOM) and filamentous green algae than to invertebrates, fine benthic organic matter (FBOM), and periphyton. Values for δ15N and δ13C in C. anomalum were more similar to grazer and collector invertebrates and filamentous green algae than to FBOM and periphyton. Results from a 15N tracer release also indicated a portion of algae and/or invertebrates were a component of nitrogen assimilated in Orconectes spp. and C. anomalum diets. Gut contents of C. anomalum were also analyzed. In contrast to stable isotope data, amorphous detritus was a significant component of C. anomalum guts, followed by diatoms and filamentous green algae. A significant percentage of invertebrate material was found in C. anomalum guts sampled in the spring. Experiments were conducted in artificial streams to determine if Orconectes spp. and C. anomalum could reduce epilithic algal biomass in small streams. Algal biomass on clay tile substrata was decreased relative to controls in artificial stream channels containing O. neglectus (3.4 fold, p=0.0002), C. anomalum (2.1 fold, p=0.0012), and both species combined (3.0 fold, p=0.0003). Results indicate that Orconectes spp. are functioning more as algal and detrital processors than as predators in Kings Creek. Isotope and gut content data show that C. anomalum includes invertebrates as well as algae and detritus in its diet. Both species have the potential to affect algal biomass and are important omnivores in the stream food web.

Colonization and development of stream communities across a 200-year gradient in Glacier Bay National Park, Alaska, U.S.A.

In May 1997, physical and biological variables were studied in 16 streams of different ages and contrasting stages of development following glacial recession in Glacier Bay National Park, southeast Alaska. The number of microcrustacean and macroinvertebrate taxa and juvenile fish abundance and diversity were significantly greater in older streams. Microcrustacean diversity was related to the amount of instream wood and percent pool habitat, while the number of macroinvertebrate taxa was related to bed stability, amount of instream wood, and percent pool habitat. The percent contribution of Ephemeroptera to stream benthic communities increased significantly with stream age and the amount of coarse benthic organic matter. Juvenile Dolly Varden (Salvelinus malma) were dominant in the younger streams, but juvenile coho salmon (Oncorhynchus kisutch) abundance was greater in older streams associated with increased pool habitat. Upstream lakes significantly influenced channel stability, percent Chironomidae, total macroinvertebrate and meiofaunal abundance, and percent fish cover. Stable isotope analyses indicated nitrogen enrichment from marine sources in macroinvertebrates and juvenile fish in older streams with established salmon runs. The findings are encapsulated in a conceptual summary of stream development that proposes stream assemblages to be determined by direct interactions with the terrestrial, marine, and lake ecosystems.

Colonization and development of stream communities across a 200-year gradient in Glacier Bay National Park, Alaska, U.S.A.

In May 1997, physical and biological variables were studied in 16 streams of different ages and contrasting stages of development following glacial recession in Glacier Bay National Park, southeast Alaska. The number of microcrustacean and macroinvertebrate taxa and juvenile fish abundance and diversity were significantly greater in older streams. Microcrustacean diversity was related to the amount of instream wood and percent pool habitat, while the number of macroinvertebrate taxa was related to bed stability, amount of instream wood, and percent pool habitat. The percent contribution of Ephemeroptera to stream benthic communities increased significantly with stream age and the amount of coarse benthic organic matter. Juvenile Dolly Varden (Salvelinus malma) were dominant in the younger streams, but juvenile coho salmon (Oncorhynchus kisutch) abundance was greater in older streams associated with increased pool habitat. Upstream lakes significantly influenced channel stability, percent Chironomidae, total macroinvertebrate and meiofaunal abundance, and percent fish cover. Stable isotope analyses indicated nitrogen enrichment from marine sources in macroinvertebrates and juvenile fish in older streams with established salmon runs. The findings are encapsulated in a conceptual summary of stream development that proposes stream assemblages to be determined by direct interactions with the terrestrial, marine, and lake ecosystems.

Isotopic analysis of three food web theories in constricted and floodplain regions of a large river.

Analyses of stable isotope (δ13C and δ15N) and C:N ratios of food webs within a floodplain and a constricted-channel region of the Ohio River during October 1993 and July 1994 indicate that the increasingly influential flood pulse concept (FPC) does not, for either location, adequately address food web structure for this very large river. Furthermore, results of this study suggest that the riverine productivity model (RPM) is more appropriate than the widely known river continuum concept (RCC) for the constricted region of this river. These␣conclusions are based on stable isotope analyses of potential sources of organic matter (riparian C3 trees, riparian C4 grasses and agricultural crops, submerged macrophytes, benthic filamentous algae, benthic particulate organic matter, and transported organic matter containing detritus and phytoplankton) and various functional feeding groups of invertebrate and fish consumers. The FPC, which stresses the key contribution of organic matter, particularly terrestrial organic matter, originating from the floodplain to riverine food webs, was judged inappropriate for the floodplain region of the Ohio River for hydrodynamic and biotic reasons. The rising limb and peak period of discharge typically occur in November through March when temperatures are low (generally much less than 10°C) and greater than bank-full conditions are relatively unpredictable and short-lived. The major food potentially available to riverine organisms migrating into the floodplain would be decaying vegetation because autotrophic production is temperature and light limited and terrestrial insect production is minimal at that time. It is clear from our data that terrestrial C4 plants contribute little, if anything, to the consumer food web (based on δ13C values), and δ15N values for C3 plants, coarse benthic organic matter, and fine benthic organic matter were too depleted (∼7-12‰ lower than most invertebrate consumer values) for this organic matter to be supporting the food web. The RPM, which emphasizes the primary role of autotrophic production in large rivers, is the most viable of the remaining two ecosystem models for the constricted-channel region of the Ohio based on stable isotope linkage between sources and consumers of organic matter in the food web. The most important form of food web organic matter is apparently transported (suspended) fine (FTOM) and ultra-fine particulate organic matter. We propose that phytoplankton and detritus of an autochthonous origin in the seston would represent a more usable energy source for benthic (bivalve molluscs, hydropsychid caddisflies) and planktonic (microcrustaceans) suspension feeders than the more refractory allochthonous materials derived from upstream processing of terrestrial organic matter. Benthic grazers depend heavily on nonfilamentous benthic algae (based on gut analysis from a separate study), but filamentous benthic algae have no apparent connection to invertebrate consumers (based on δ13C values). Amphipod and crayfish show a strong relationship to aquatic macrophytes (possibly through detrital organic matter rather than living plant tissue). These observations contrast with the prediction of the RCC that food webs in large rivers are based principally on refractory FTOM and dissolved organic matter from upstream inefficiencies in organic-matter processing and the bacteria growing upon these suspended or dissolved detrital compounds. The conclusions drawn here for the Ohio River cannot yet be extended to other floodplain and constricted-channel rivers in temperate and tropical latitudes until more comparable data are available on relatively pristine and moderately regulated rivers.

Particulate organic matter in a mountain stream in the south-western Cape, South Africa

The quality and quantity of allochthonous inputs and of benthic organic matter were investigated in a second-order, perennial mountain stream in the south-west Cape, South Africa, between April 1983 and January 1986. Although the endemic, riparian vegetation is sclerophyllous, low and evergreen, inputs of allochthonous detritus to the stream (434 to 500 g m−2y−1) were similar to those recorded for riparian communities worldwide, as were calorific values of these inputs (9548 to 10 032 KJ m−2y−1). Leaf fall of the riparian vegetation is seasonal, occurring in spring (November) as discharge decreases, resulting in retention of benthic organic matter (BOM) on the stream bed during summer and early autumn (maximum 224 g m−2). Early winter rains (May) scoured the stream almost clean of benthic detritus (winter minimum 8 g m−2). Therefore, BOM was predictably plentiful for about half of each year and predictably scarce for the other half. Coarse BOM (CBOM) and fine BOM (FBOM) constituted 46–64% of BOM standing stock, ultra-fine BOM (UBOM) 16–33% and leaf packs 13–24%. The mean annual calorific value of total BOM standing stock was 1709 KJ m−2. Both standing stocks and total calorific values of BOM were lower than those reported for streams in other biogeographical regions. Values of C:N ratios decreased with decrease in BOM particle size (CBOM 27–100; FBOM 25–27; UBOM 13–19) with no seasonal trends. The stream is erosive with a poor ability to retain organic detritus. Its character appears to be dictated by abiotic factors, the most important of which is winter spates.