Heterotrophic Streams Research Articles

Integrating Perspectives on Dissolved Organic Carbon Removal and Whole‐Stream Metabolism

AbstractQuantifying organic carbon (OC) removal in streams is needed to integrate the functional role of inland waters into landscape carbon budgets. To illustrate how in‐stream OC removal measurements can be used to characterize ecosystem and landscape carbon fluxes, we compared two common methods: (a) Bioassays measuring water column dissolved organic carbon (DOC) uptake and (b) daily rates of whole‐stream metabolism and OC spiraling calculated from fluorescent dissolved organic matter, oxygen, and discharge measurements. We then assessed how OC removal rates from these two methods, measured in two low‐productivity heterotrophic streams, affected estimates of terrestrial OC loading and export using a mass balance model. OC mineralization velocities calculated from whole‐stream metabolism (0.06 ± 0.03 m d−1 (mean ± SD)) were greater than water column bioassay DOC uptake velocities (0.01 ± 0.01 m d−1), which resulted in higher in‐stream OC removal estimates (0.5%–15.2% and 0.02%–4.2% removal for whole‐stream metabolism and bioassays, respectively). Furthermore, the terrestrial OC inputs needed to sustain in‐stream OC concentrations differ among methods, with simulated inputs ranging from 79 to 1,300 or 3–350 g OC d−1 for whole‐stream metabolism or bioassays, respectively. We show how in‐stream OC removal can be used to quantify terrestrial‐aquatic linkages by estimating OC inputs needed to fuel whole‐stream metabolism in low‐productivity streams, and offer future directions to better link OC removal with whole‐ecosystem OC budgets. Without appropriate conversions to whole‐stream processes, bioassays systematically underestimate whole‐stream carbon cycling. By integrating whole‐stream metabolism with OC transport, we can better elucidate the role of running waters in landscape carbon budgets and the global carbon cycle.

Variable temperature effects between heterotrophic stream processes and organisms

Abstract Temperature is known to stimulate metabolism with cascading effects on multiple biological processes. These effects may, however, vary across processes, types of organisms or levels of biological organisation. They can also vary with nutrient availability, with potentially stronger temperature effects when nutrients are not limiting. This context dependence of temperature effects on processes challenges our ability to anticipate their consequences on ecosystems in a changing world. In headwater streams, the decomposition of allochthonous leaf litter, driven by both microbial decomposers and invertebrates, is known to respond to both temperature and nutrient availability. These food webs are highly tractable and a useful model system to investigate the variations of temperature effects on processes across types of organisms (microbes versus invertebrates), resource availability levels (nutrient concentration), and levels of biological organisation (from individual to ecosystem). In a microcosm experiment, we measured the effects of temperature and nitrogen availability (four levels each) on respiration rates of litter-consuming microbes and invertebrates and their decomposition activity in different contexts of food web complexity. The latter included one treatment without invertebrate detritivore (microbial decomposers only), three single invertebrate taxa (Gammarus, Potamophylax, and Sericostoma) treatments, and one mixed invertebrate taxa treatment (three‐species altogether). Microbial processes increased nearly exponentially with temperature (Arrhenius model, activation energy (± 95% confidence interval) = 0.56 ± 0.53 and 1.00 ± 0.23 eV for litter decomposition and respiration), while invertebrate‐driven processes increased (activation energy from 0.47–1.15 eV) up to a maximal value at an intermediate temperature (c. 11–15°C depending on species and process), above which process rates decreased. By contrast, litter consumption in mixed invertebrate species treatments was not significantly influenced by temperature, because of a negative effect of species mixing occurring above 12°C. Nitrogen had a weaker influence, only slightly stimulating litter consumption by mixed‐species invertebrates, which limited the scope for synergies with temperature effects. Our results raise issues about how aquatic litter consumers meet their energy requirements at high temperature and suggest that a general consequence of warming could be loss of carbon through mineralisation in headwater stream food webs. In several aspects, our results deviate from expectations based on universal relationships between temperature and individual metabolism (e.g. metabolic theory of ecology), suggesting that we may need to develop less simplistic assumptions to predict the consequence of warming on ecosystem processes.

High within‐stream replication is needed to predict litter fluxes in wet–dry tropical streams

Abstract Streams draining forested landscapes are fuelled by terrestrial plant litter, which can be transported downstream or retained and broken down locally. However, fluxes of plant litter in streams can vary at multiple spatio‐temporal scales, affecting the availability of this key resource in heterotrophic stream food webs. To explore this question we quantified several processes related to litter dynamics (i.e. litter inputs, storage, losses by transport and losses by breakdown) by sampling litter at multiple sites in three streams of the Brazilian Cerrado biome (which has a tropical wet–dry climate) for 2 years. We assessed the relative contribution of different spatial (among and within streams) and temporal scales (annual, seasonal and monthly) to total variability of these processes (hereafter fluxes). Spatial and temporal variability of fluxes were both high, but spatial variation was 1.67‐fold greater than temporal variation (61 versus 37%, respectively), especially at the within‐stream scale (50% overall); an exception was litterfall, which varied less spatially than temporally (24 versus 76%). Temporal variation of litter storage (and hence availability to consumers) was mostly seasonal and due to differences in net transport. Inputs and transport were higher in the wet than the dry season (wet versus dry season, 1.45 versus 0.92 and 1.43 versus 0.06 g litter m−2 day−1), while breakdown was similar between both seasons (0.88 versus 0.94 g litter m−2 day−1). Storage (i.e. accumulation) rate was positive and negative in the dry and wet season, respectively, indicating that litter was stored in the dry season and exported in the wet season. The transitional dry–wet season showed the highest inputs, breakdown and storage (3.21, 1.63 g litter m−2 day−1 and 145 g litter m−2), while the wet–dry season showed lower inputs (as in the dry season), higher transport (as in the wet season) and lower breakdown and storage than the other seasons (0.93, 0.65, 0.31 g litter m−2 day−1 and 24 g litter m−2). Our results underscore the role of variation in biophysical drivers of litter fluxes within streams (e.g. pool–riffle configuration, substrate features, biological communities), and suggest that high within‐stream replication is necessary to study litter fluxes at larger scales and over time. The seasonal patterns suggested potential changes in litter dynamics under future climate scenarios in the tropics, including increased storage due to reduced transport in a drier climate.

Efecto del castor en el metabolismo del perifiton y en variables limnológicas de ríos y arroyos fueguinos

El castor norteamericano (Castor canadensis) es una especie invasora en Tierra del Fuego. Es capaz de transformar sistemas loticos en lenticos por medio de la construccion de diques. Influye sobre la hidrologia y la morfologia de los rios, el ciclo de nutrientes, la descomposicion, la zona riberena y sobre las comunidades. El objetivo del presente trabajo fue estudiar el efecto de la actividad del castor sobre variables limnologicas y, en especial, sobre el metabolismo del perifiton (fracciones de masa y clorofila-a) para evaluar su potencial uso como bioindicador de actividad de castor. Durante el verano se estudiaron seis rios y arroyos actualmente impactados, y tres arroyos que fueron impactados, pero que ya no poseen actividad. En cada curso de agua se estimaron, rio arriba y rio abajo de los diques actuales o de donde estuvieron (en el caso de las castoreras abandonadas), las fracciones de masa del perifiton sobre rocas y variables fisicas, quimicas, hidrologicas y morfologicas. La similitud entre sitios se estudio mediante un analisis de agrupamiento en funcion de variables morfologicas, hidrologicas y biologicas. Las fracciones de masa del perifiton se analizaron con modelos lineales generalizados mixtos. Rio arriba de los diques se diferenciaron los sitios impactados de los no impactados por el castor, asi como aquellos que poseian caracteristicas hidrologicas particulares. Estas diferencias no se observaron rio abajo debido a un efecto homogeneizador de la actividad del castor sobre las variables hidrologicas y morfologicas de los tramos impactados. El patron de la biomasa perifitica no mostro diferencias significativas rio abajo de los diques con y sin actividad. Nuestros resultados indican que, en estos arroyos naturalmente heterotroficos, el perifiton no parece ser un buen indicador de la actividad de castor en lo que respecta a las variables de masa y clorofila.https://doi.org/10.25260/EA.18.28.3.0.708

Temperature dependence of daily respiration and reaeration rates during baseflow conditions in a northeastern U.S. stream

Study regionFinger Lakes Region, New York, USA. Study focusThe relationship between stream temperature and daily respiration and reaeration rates was investigated from May 2015 – May 2016 for two sites, one in the headwaters and the other near the mouth of a net heterotrophic stream. Daily respiration and reaeration rates were calculated by applying the night time slope method to open-channel diel oxygen concentrations during baseflow conditions. New hydrological insights for the regionReference values for activation energy (Ea) are presented that could be applicable to lower order streams in temperate climates receiving significant inputs from surface and subsurface flows. There was a sizeable difference in temperature sensitivity between upstream and downstream sites. Ecosystem respiration (ER) exhibited a strong Arrhenius temperature dependence with daily respiration rates of 0.5–9.5 g O2 m−2 d−1 (Ea = 0.61 eV) in the headwaters and 1.8–30.3 g O2 m−2 d−1 (Ea = 0.67 eV) at the downstream location where ER in the headwaters was less sensitive to temperature. Ea for both sites fell within the expected theoretical range for heterotrophic respiration in aquatic ecosystems. Temperature reaeration rate coefficients ranged from 1.011 to 1.020 at the headwaters and from 1.005 to 1.009 downstream suggesting that daily reaeration rates at the headwaters were more sensitive to temperature than the downstream location where reaeration was likely controlled by stream discharge and driven by turbulence.

Fish invasion alters ecosystem function in a small heterotrophic stream

The strength of trophic cascade effects in aquatic ecosystems depend, in part, on the identity of the top predator involved. We examined whether an invasive benthic fish (round goby, Neogobius melanostomus) altered the strength of cascade effects in a heterotrophic stream and in a controlled mesocosm experiment relative to the effects of a functionally similar, native fish. In the stream, the introduced fish had a direct effect on grazer and shredder abundance which led to a significant increase in periphyton chlorophyll a, a significant reduction in leaf breakdown rate, an increase in leaf biomass remaining, but no change in periphyton ash-free dry mass. In mesocosms, native and introduced fish similarly reduced shredder abundance, but this did not lead to an indirect effect on leaf breakdown rates or biomass remaining at the end of the experiment. Indirect effects of introduced fish on periphyton biomass and chlorophyll a in mesocosms were both significant and were stronger than in the field, but were the result of grazer behavioral modification and not reduced grazer abundance. Collectively, these results suggest non-native fish have the ability to initiate trophic cascades in heterotrophic streams, and that both fish identity and environmental context are important in determining the strength of cascades.

Seasonal effects of macrophyte growth on rainbow trout habitat availability and selection in a low‐gradient, groundwater‐dominated river

AbstractRainbow trout habitat use is often described in high‐gradient, runoff‐driven, heterotrophic streams where geomorphic features and overhanging riparian vegetation provide channel complexity and cover. However, many rainbow trout populations thrive in rivers with contrasting aquatic habitat. We describe rainbow trout habitat use in a low‐gradient, groundwater‐dominated tailwater river where river flow management and macrophyte growth and senescence largely govern available trout habitat. In the summers of 2013 and 2014, available aquatic habitat (depth, velocity, macrophyte cover, substrate size) was quantified, while individual trout location was determined by radio telemetry and linked to environmental variables. Detailed habitat surveys indicate that macrophyte cover increases throughout the summer and is a strong determinant of in‐stream habitat characteristics. Paired logistic regression shows that adult rainbow trout prefer greater depths. Water depth increases with macrophyte abundance at both reach and local scales as plants restrict flow, and available trout habitat is linked to this seasonal pattern. When macrophyte abundance is high, adult trout show secondary preference for localised areas of lower macrophyte cover but otherwise show no selectivity for macrophyte cover, velocity or substrate size. Results suggest that submerged aquatic plants increased the quantity and quality of rainbow trout habitat as a source of channel complexity and cover. Macrophytes may play a similar role in other low‐gradient streams and should not be overlooked by fisheries managers considering habitat suitability.

Influence of riparian forests on fish assemblages in temperate lowland streams

The characteristics of riparian vegetation along streams vary with natural and anthropogenic factors. Deforestation for agricultural purposes has consequences for the physical in-stream structure and function, such as the predominance of autotrophic or heterotrophic stream metabolism. Open canopy lowland streams are often dominated by macrophytes, with potential direct and indirect effects on the fish community. We tested for possible differences in the structure (relative abundance of species, mean body size, and density) and composition (species richness, species identity, and different trophic groups) of fish assemblages between open canopy streams (OCS) and riparian forest streams (RFS), including pool and riffle habitats, in temperate lowland Denmark. OCS reaches exhibited higher alpha and beta diversity and frequently hosted rare species. Almost 50 % of the recorded species appeared only in OCS. OCS also had smaller mean body size of fish and tended to have higher fish densities. The relative abundance of the different trophic groups did not differ between the two streams types, but the RFS had a higher abundance and occurrence frequency of intolerant salmonids. Our results suggest that modification of riparian habitats can affect richness patterns and that strong functional changes may occur as a consequence of forest clearance through changes in the relative importance of a keystone species, trout (Salmo trutta).

Whole-stream metabolism in nutrient-poor calcareous streams on Öland, Sweden

We studied whole-stream metabolism in three headwater non-forested stream reaches on the island of Oland, Sweden in order to characterize the metabolism of this unusual ecosystem and to compare it with other stream ecosystems in NW Europe. Gross primary production (GPP) was generally low (<4 g O2 m−2 d−1) with the lowest GPP recorded in the most upstream, shallow reach draining the thin soils of the limestone Alvar plains. Here, completely flooded terrestrial plants could account for the whole primary production at baseflow. Ecosystem respiration (ER) increased several fold with agricultural impact, resulting in heterotrophic stream conditions downstream and higher light requirements for photosynthesis to outweigh respiration. A strong relationship between daily GPP and ER was found at the two most nutrient-poor sites. Temperature corrected instantaneous ER rate was highest in the beginning of the night, but decreased at the end of the night at the same reaches, indicating that dark respiration depleted photosynthetic products and became limited by organic substrates. The broad-scale comparison of open NW European streams showed a 1:1 relationship, indicating a tight link between daily GPP and ER during summer (April–August) but not during winter. This study has extended the range of GPP and ER measurements to include nutrient-poor NW European streams, thereby increasing the knowledge on stream metabolism in this, otherwise, highly agricultural impacted region. It also documented a strong relationship between GPP and ER in streams, ranging from extremely nutrient poor to moderately nutrient rich conditions during spring and summer.

Regulation of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; emissions from temperate streams and reservoirs

Abstract. It has become more and more evident that CO2 emission (FCO2) from freshwater systems is an important part of the global carbon cycle. To date, only a few studies have addressed the different mechanisms that regulate FCO2 in lotic and lentic systems. In a comparative study we investigated how different biogeochemical and physical factors can affect FCO2 values in streams and reservoirs. We examined the seasonal variability in CO2 concentrations and emissions from four streams and two pre-dams of a large drinking water reservoir located in the same catchment, and compared them with environmental factors that were measured concurrently. All the streams were generally supersaturated with CO2 throughout the year, while both reservoirs functioned to a small degree as CO2 sinks during summer stratification and CO2 sources after circulation had set in. FCO2 from streams ranged from 23 to 355 mmol m−2 d−1 and exceeded the fluxes recorded for the reservoirs (−8.9 to 161.1 mmol m−2 d−1). Both the generally high piston velocity (k) and the CO2 oversaturation contributed to the higher FCO2 from streams in comparison to lakes. In both streams and reservoirs FCO2 was mainly governed by the CO2 concentration (r = 0.92, p &lt; 0.001 for dams; r = 0.90, p &lt; 0.001 for streams), which was in turn affected by metabolic processes and nutrients in both systems and also by lateral inflow in the streams. Besides CO2 concentration, physical factors also influence FCO2 in lakes and streams. During stratification, FCO2 in both pre-dams was regulated by primary production in the epilimnion, which led to a decrease of FCO2. During circulation, when CO2 from the hypolimnion was mixed with the epilimnion, FCO2 increased on account of the CO2 input from the hypolimnion. The CO2 from the hypolimnion originates from the mineralisation of organic matter. FCO2 from streams was mainly influenced by geomorphological and hydrological factors affecting k, which is less relevant in low-wind lakes. Under high-wind conditions, however, k regulates FCO2 from lotic systems as well. We developed a theoretical framework describing the role of the different regulation mechanisms for FCO2 from streams and lakes. In summary, the dominant factor affecting FCO2 is the concentration of CO2 in the surface water. Lake stratification has a very important regulatory effect on FCO2 from lakes on account of its influence on CO2 concentrations and metabolic processes. Nevertheless, FCO2 values in heterotrophic streams are generally higher. The higher k values are responsible for the comparatively high degree of FCO2. On a Central European scale, CO2 emission from streams is probably of greater importance than the CO2 flux from standing waters.

Nutrient enrichment of a heterotrophic stream alters leaf litter nutritional quality and shredder physiological condition via the microbial pathway

Streams receiving agricultural runoff are typically enriched with nutrients, which variously impact stream communities. We examined the effects of phosphate and nitrate enrichment on leaf litter breakdown, microbial biomass and the nutrition of an invertebrate shredder to determine how nutrients are transferred through the stream detrital food web. Using artificial streams, individuals of Anisocentropus kirramus (Trichoptera: Calamoceratidae) were fed leaves of Apodytes brachystylus (Icacinaceae) under different nutrient regimes. We measured the amount of leaf material consumed or decomposed and the microbial biomass colonising the leaves. The dry mass, and protein, lipid and carbohydrate composition of A. kirramus larvae were determined after 28-day feeding on the leaves. Supplements of phosphorus, but not nitrogen, enhanced leaf breakdown, microbial growth and growth of larvae. Microbial biomass and dry mass of larvae increased with nutrient enrichment and they were significantly correlated. Thus, the phosphorus supplement was transmitted through the detrital food web via the microbial pathway, resulting in higher nutritional quality of leaves and enhanced physiological condition of the shredder. Understanding such subtle relationships is important in determining the impacts of anthropogenic contaminants on freshwater ecosystems.

Mineralisation of dissolved organic matter by heterotrophic stream biofilm communities in a large boreal catchment

Summary Boreal headwater streams typically receive large contributions of dissolved organic matter (DOM) from wetland and forest soils and can exhibit high CO2 effluxes, but little is known about how stream biofilm respiration of DOM responds to carbon and nutrient substrates in boreal catchments. The aim of this study was to assess (i) the response of heterotrophic biofilms to increased availability of labile carbon (C), nitrogen (N) and phosphorus (P), and (ii) how this response varies across sites differing in the source of catchment organic matter. Normalised biofilm respiration rates (RBiofilm) of dissolved organic carbon (DOC) of DOM sources derived from wetland, deciduous forest and ponds were up to 10 times greater than for coniferous forest DOM. Experimental additions of bog‐ and pond‐derived DOM stimulated RBiofilm in coniferous forest sites (1.5–2.5 times), adding further weight to the evidence for differing responses to stream DOM sources in this boreal catchment. Mineralisation of added glucose by the biofilms was only increased by added N and P, coinciding with a reduction in mineralisation of the extant stream DOM. These findings suggest that increases in labile C, N and P can reduce biofilm mineralisation of stream dissolved organic nitrogen and phosphorus, perhaps due to the greater energy required to access the more complex DOM. In 15 of 19 experiments, the addition of glucose had no effect on biofilm mineralisation of stream DOC (RDOC). In the presence of added N and P, however, RDOC decreased by 30 to 38% with the addition of glucose at sites with the most active biofilms in summer. When glucose was added alone, conversely, RDOC was stimulated (40–50%) at these same sites in autumn, when biofilm respiration was lower and DOM more aromatic and less carbohydrate‐rich. Combined, these results suggest labile C sources, such as algal exudates, may (i) compete as a source of energy and/or stimulate the incorporation rather than mineralisation of the more N‐ and P‐rich stream DOM or (ii) stimulate the mineralisation of stream DOM, potentially depending upon nutrient availability and the composition of stream DOM.

Long-term dynamics of organic matter and elements exported as coarse particulates from two Caribbean montane watersheds

Abstract:In heterotrophic streams the retention and export of coarse particulate organic matter and associated elements are fundamental biogeochemical processes that influence water quality, food webs and the structural complexity of forested headwater streams. Nevertheless, few studies have documented the quantity and quality of exported organic matter over multiple years and under a range of conditions that includes both droughts and hurricanes. This study quantifies the export of coarse particulate organic matter (CPOM, &gt; 12.7 mm), over 18 y in two headwater streams in north-east Puerto Rico. Daily exports ranged from 0 to over 170 g ha−1 d−1 and averaged 7.39 g ha−1 d−1, with similar amounts coming from leaves (3.5 g ha−1 d−1) and wood (3.2 g ha−1 d−1). Export of coarse particulate organic carbon was 3.0 g ha−1 d−1 which constitutes only 1.32% of carbon exports. Most litter falling into the streams was processed in place as only 2.3% of the leaf litter falling directly into these perennial channels was exported as CPOM. On average, 6 wk y−1 had no exports while events transporting more than 10 g ha−1 d−1 occurred every 2.8 mo. Instead of a single annual pulse as observed in deciduous systems, there were annual peaks in CPOM exports during May and September and less export during the drier period from December to February. Ratios of C:N in the exported material were highest in the driest month and lowest during rainy months, while leaf fluxes for nitrogen, phosphorus and calcium were highest in rainy months and lowest during February. Although median daily exports and exports during low- and base-flow periods were similar before and after Hugo, after 16 y exports during moderate- and high-flow periods were still less than those in the 2 y prior to the hurricane. Our observations indicate a system with high rates of internal processing that quickly returns to median daily conditions following hurricanes but requires several decades for storm-flow exports to return to pre-disturbance conditions and indicates that the long-term pattern of CPOM export is associated with the level of maturity of watershed vegetation.

Response of heterotrophic stream biofilm ­communities to a gradient of resources

The metabolism and biogeochemical cycles of aquatic ecosystems are largely mediated by microbial communities, with biofilm assemblages dominating in stream ecosystems. To determine the effects of the availability of resources on the structure and function of heterotrophic stream biofilms, we created an enrichment gradient by amending darkened stream channel mesocosms with a stoichiometrically balanced solution of sucrose, NO3 - and PO4 3- . A total of 1902 bacterial partial 16S rRNA gene sequences yielded 293 unique operational taxonomic units (OTUs, 97% sequence similarity). Significant differences (p < 0.005) were detected between communities from all treat- ments with the exception of the 2 highest enrichment levels, with increasing enrichment resulting in progressive community divergence and declining diversity. Both the productivity and function of the biofilm community increased exponentially with enrichment, with exponents of 1.5 for areal mass, 2.3 for live-cell density, and 2.5 to 3.5 for the activities of 5 extracellular enzymes. The nonlinear increase in functional capacity suggests that heterotrophic biofilms are highly responsive to the avail- ability of resources, probably as a result of the physical structures and synergistic social interactions found in biofilm assemblages.

Experimental shading alters leaf litter breakdown in streams of contrasting riparian canopy cover

Summary 1. Headwater stream ecosystems are primarily heterotrophic, with allochthonous organic matter being the dominant energy. However, sunlight indirectly influences ecosystem structure and functioning, affecting microbial and invertebrate consumers and, ultimately, leaf litter breakdown. We tested the effects of artificial shading on litter breakdown rates in an open-canopy stream (high ambient light) and a closed-canopy stream (low ambient light). We further examined the responses of invertebrate shredders and aquatic hyphomycetes to shading to disentangle the underlying effects of light availability on litter breakdown. 2. Litter breakdown was substantially slower for both fast-decomposing (alder, Alnus glutinosa) and slow-decomposing (beech, Fagus sylvatica) leaf litters in artificially shaded stream reaches relative to control (no artificial shading) reaches, regardless of stream type (open or closed canopy). 3. Shredder densities were higher on A. glutinosa than on F. sylvatica litter, and shading had a greater effect on reducing shredder densities associated with A. glutinosa than those associated with F. sylvatica litter in both stream types. Fungal biomass was also negatively affected by shading. Results suggest that the effects of light availability on litter breakdown rates are mediated by resource quality and consumer density. 4. Results from feeding experiments, where A. glutinosa litter incubated under ambient light or artificial shade was offered to the shredder Gammarus fossarum, suggest that experimental shading and riparian canopy openness influenced litter palatability interactively. Rates of litter consumption by G. fossarum were decreased by experimental shading in the open-canopy stream only. 5. The results suggest that even small variations in light availability in streams can mediate substantial within-stream heterogeneity in litter breakdown. This study provides further evidence that changes in riparian vegetation, and thus light availability, influence organic matter processing in heterotrophic stream ecosystems through multiple trophic levels.

The stoichiometry of nitrogen and phosphorus spiralling in heterotrophic and autotrophic streams

Summary 1. Nutrient spiralling provides a conceptual framework and a whole-system approach to investigate ecosystem responses to environmental changes. We use spiralling metrics to examine how the coupling of nitrogen and phosphorus uptake varies between streams dominated by either heterotrophic (i.e. bacteria-dominated) or autotrophic (algal-dominated) microbial communities. 2. Algae generally exhibit greater capacity to store nutrients than bacteria because of differences in cellular structures. These differences led us to hypothesise that the uptake of N and P in heterotrophic ecosystems should have reduced stoichiometric variation in response to changes in supply N : P compared to autotrophic ecosystems when assimilation dominates nutrient uptake. 3. To test this hypothesis, we used an array of serial nutrient additions in several streams in the South Fork Eel River watershed in Northern California. In one set of experiments, N and P were added alone and simultaneously in separate experiments to two small, heterotrophic streams to assess uptake rates and interactions between nutrient cycles. In a second set of experiments, N and P were added simultaneously at a range of N : P in one heterotrophic and one autotrophic stream to assess differences in uptake responses to changes in supply N : P. 4. Results of these experiments suggest two important conclusions. First, increased N supply significantly shortened P uptake lengths, while P addition had little impact on N uptake in both streams, indicating that uptake of non-limiting nutrients is tightly coupled to the availability of the limiting element. Second, changes in P uptake and uptake ratios (UN : UP) with increased supply N : P supported our hypothesis that heterotrophic streams are more homeostatic in their responses to changes in nutrient supply than autotrophic streams, suggesting that physiological controls on nutrient use scale up to influence ecosystem-scale patterns in nutrient cycling.

Differential effects of nutrients and light on the primary production of stream algae and mosses

Benthic algae and mosses are the main primary producers in headwater streams. In these ecosystems, primary production is mainly determined by the availability of light and nutrients. The present study analyses the different effects of nutrient enrichment (inorganic phosphorus and nitrogen) and light availability on the primary production of both algal and moss communities from a headwater forested stream using laboratory experiments. These laboratory experiments consider four treatments (low light-unenriched, high light-unenriched, low light-enriched and high light-enriched). Photosynthesis-irradiance curves of both communities were also performed. Respiration and net primary production (NPP) of algae and mosses were measured using incubation chambers and oxygen balance measurements. The moss community was dominated by Fontinalis antipyretica and diatom epiphytes, while the algal community consisted of Cladophora glomerata and attached diatoms. Under all treatments, NPP was significantly higher for algae (3-18 mg O 2 mg chl -1 h -1 ) than for mosses (0.4 and 1.8 mg O 2 mg chl -1 h -1 ). Light (as a single factor) significantly affected GPP (gross primary production) and NPP of algae and mosses. Non-inhibitory effects of GPP were observed at saturating light conditions, showing the high plasticity of the stream algal and moss communities. Nutrients produced a significant enhancement of NPP only under high light availability. The greater response of algae to nutrient enrichment was also shown by the highest phosphate uptake rate of this community. Extrapolation of laboratory primary production measurements to the field scale showed the occurrence of significant autotrophic production pulses during periods of light availability (e.g. before leaf emergence and after leaf fall in Mediterranean forested streams), in contrast to the major heterotrophic behaviour, especially in autumn, of these ecosystems. These production pulses in heterotrophic streams could provide high-quality resources to downstream reach, which might be essential for consumers.

Metabolic balance of streams draining urban and agricultural watersheds in central Japan

Empirical data that describe the metabolic balance of stream ecosystems in human-dominated watersheds are scarce. We measured ecosystem metabolism in 23 open-canopied lowland streams draining urban and agricultural areas in the Fuji River Basin, central Japan. Gross primary production (GPP) and community respiration (CR) were estimated using the diurnal dissolved oxygen (DO) change technique, with the reaeration coefficient (K2) determined from seven empirical depth-velocity equations. Because the predicted values of K2 showed variation among the depth-velocity equations, the estimates of stream metabolism also varied according to the equations. However, CR was almost always greater than GPP, resulting in negative net ecosystem production (NEP) and GPP/CR ratios below unity for most of the study reaches. Highly heterotrophic streams were found in intensively farmed watersheds, suggesting that organic matter loading from agricultural lands is likely to be a source of allochthonous carbon fueling excess respiration in the study streams. In contrast, streams draining more urbanized areas were less heterotrophic. The present results suggest that lowland streams in agriculturally developed watersheds are associated strongly with terrestrial ecosystems as a source of organic carbon. The resultant strong respiration might become the dominant process in ecosystem metabolism, as reported for headwater streams, large downstream rivers, and estuaries.

Colonisation of Introduced Timber by Algae and Invertebrates, and its Potential Role in Aquatic Ecosystem Restoration

As part of a habitat restoration experiment wood substrates (red gum) were introduced to two lowland streams of SE Australia in which habitat has been severely degraded by deposition of sand eroded from higher in the catchment. We monitored net primary production (NPP) and community respiration (CR), nutrient concentrations and the succession of algae and invertebrates (abundance and species richness), sampling at 2, 4, 8, 12, 16 and 20 weeks. Colonisation by algae was rapid, and there were distinct changes in the assemblages over the first 4 weeks. Thereafter, changes were much less marked. There were also differences in nutrient concentrations and some measures of algal abundance between the two creeks. As with the algae, invertebrates colonised these substrates extremely rapidly, peaking in abundance and richness in week 8. Invertebrate abundances closely tracked changes in the abundance of algae. By the end of the study both algal and invertebrate communities were in apparent decline, with sharp decreases in invertebrate and algal abundance and invertebrate species richness. Rates of GPP also declined toward the end of the experiment, and this coincided with the detachment of large mats of filamentous algae and the recession of flows over the summer months. However, in both streams the added timber quickly created habitat with high levels of primary production in an otherwise strongly heterotrophic stream system. These hotspots of autotrophic production were quickly colonised by high numbers of macroinvertebrates indicating timber addition may provide an effective means of augmenting habitat for algae and invertebrates in sanded streams.

Mass loss and macroinvertebrate colonisation of fish carcasses in riffles and pools of a NW Italian stream

In this study, we analysed the decomposition of trout carcasses in a low-order Apennine stream, with the aim to investigate the mass loss rate in a Mediterranean lotic system, and to examine the influence of microhabitats on the invertebrates colonising fish carcasses. In May 2003, we put 56 dead rainbow trout (Oncorhynchus mykiss) in the stream, placing seven sets (four trout each) in both riffle and pool habitats. At four dates, we removed one trout per set to measure its dry mass and determine the associated macroinvertebrate assemblage. Fifty-eight macroinvertebrate taxa colonised the carcasses, with significant differences between the erosive and depositional microhabitats. Riffle trouts hosted richer and denser colonist communities than pool trouts. Chironomidae, Serratella ignita, Habrophlebia sp., Dugesia sp. and Protonemura sp. were the five most abundant taxa. Decomposition was initially very rapid in both environments and then tapered off over time. The mass loss rate was higher (k= −0.057 day−1) than that found in other studies. Higher Mediterranean temperatures probably increase the process. Although we found no significant difference between riffles and pools, mass loss was more regular in erosive habitats, underlining the importance of local, small-scale conditions. In small, low-order, heterotrophic streams, fish carcasses represent an important resource and shelter for rich and diversified invertebrate assemblages.