Small reductions in forest cover weaken terrestrial-aquatic linkages in headwater streams

1. We assessed the impacts of deforestation on the energy base of headwater food webs in seven headwater streams in the Upper Chattahoochee basin, GA, U.S.A where percentage forest in catchments ranged from 82 to 96%. We measured terrestrial organic matter standing crop and determined consumer (crayfish and insectivorous fish) dependence on terrestrial versus aquatic energy sources via gut content and stable isotope analyses. 2. Standing crop of coarse particulate organic matter (CPOM) declined with deforestation at large scales (i.e. catchment deforestation and riparian deforestation at the entire stream network scale). Terrestrial plant matter, the dominant component of crayfish guts, declined in crayfish guts with reductions in CPOM standing crop and with deforestation. 3. Crayfish and insectivorous fish δ¹³C showed enrichment trends with deforestation, indicating isotopic divergence from CPOM, the most ¹³C‐depleted basal resource, with reductions in catchment and riparian forest cover. Crayfish δ¹³C also exhibited enrichment with decreased instream CPOM standing crop. 4. A concentration‐dependent mixing model was used to calculate the relative dependence of crayfish and fish on terrestrial versus aquatic basal resources. Results suggested that both allochthonous CPOM and autochthonous production were important basal resources. Consumer dependence on CPOM decreased with reductions in canopy cover. 5. Our data suggest the importance of forest cover to headwater food webs at multiple scales, and that relatively low levels of riparian deforestation along headwater streams can lead to reductions in stream food web dependence on terrestrial subsidies.     

Biotic interactions enhance survival and fitness in the caddisfly <Emphasis Type="Italic">Micropterna sequax</Emphasis> (Trichoptera: Limnephilidae)

Fish-based index of biotic integrity (F-IBI) is widely used to assess river ecosystems. With survey data from the Yellow River fishery resources in the 1980s and 2008, fish composition and abundance, vertical distribution, trophic structure, reproductive guilds and tolerance in the river’s upstream, midstream, downstream, and estuary were examined, and F-IBI systems were established for each reach to assess river ecosystem health. Results showed that compared to the 1980s, the number of fish species in 2008 sharply declined in the midstream and downstream reaches, percentage of benthic fish species decreased in upstream and estuary, the number and percentage of omnivorous species decreased in all reaches, and percentage of tolerant fish species increased 15 times in upstream but decreased in midstream and downstream. The F-IBI scores in the four reaches in the 1980s were all higher than those in 2008 and decreased from upstream to estuary; the healthy conditions indicated by F-IBI scores in the 1980s were “good,” “fair,” “poor,” and “fair” from upstream to estuary and “degraded” to “poor” in all the reaches in 2008. This indicated that the river ecosystem has degraded from the 1980s to 2008. This was also shown by variations in water chemistry.     

Evidence of spatial and temporal changes in sources of organic matter in estuarine sediments: stable isotope and fatty acid analyses

We investigated spatial and temporal changes in sources of organic matter in sediments within an estuarine environment in South Africa using fatty acids (FA) and stable isotopes (SI). Samples of sediments and sources of organic matter [i.e., particulate organic matter, microphytobenthos (MPB), macrophytes, salt marsh plants, and terrestrial leaves] were collected during spring and summer 2012, and autumn and winter 2013 from the upper, middle, and lower reaches. A Stable Isotope Analysis in R (SIAR) mixing model was used to identify the organic matter sources contributing to sediments in each estuarine reach and season. We found that diatom-associated fatty acids (20:5ω3; 16:1ω7) increased toward the upper reaches, while long-chained terrigenous fatty acids (24:0) tended to be more prevalent in lower reach sediments. In support of the FA results, the SI mixing model showed a substantial contribution from the marsh grass Spartina maritima in sediments of the lower estuary during periods of low-freshwater discharge (autumn and winter), while MPB was the main component in sediments from the upper and middle reaches during all seasons. Our results have implications for evaluating estuarine food webs since the spatial and seasonal variability in the organic matter deposited can influence estuarine community structure.     

Longitudinal changes and indicator species of the fish fauna in the subtropical Sinos River, Brazil

This study investigated the fish fauna in the subtropical Sinos River, considering species addition and replacement and the occurrence of indicator species along a longitudinal gradient. Patterns in fish assemblage ordination evaluated by correspondence analysis reflected the headwater and lowland river reaches. Uppermost sampling stations were characterized by species with restricted occurrence and medium abundance, whereas in the lowland most species were captured only once during the surveyed period and in low abundance. Species addition prevailed between the headwater sampling stations and both addition and replacement occurred between the lowland sampling stations. Nine species were characteristic for the headwater reach and 14 species for the lowland reach.     

Tracing Mississippi River influences in estuarine food webs of coastal Louisiana

The Breton Sound estuary in southern Louisiana receives large amounts of Mississippi River water via a controlled diversion structure at the upstream end of the estuary. We used stable isotopes to trace spatial and seasonal responses of the downstream food web to winter and spring introductions of river water. Analysis of δ¹³C, δ¹⁵N, and δ³⁴S in the common local consumers such as grass shrimp (Palaemonetes sp.), barnacles (Balanus sp.), and small plankton-feeding fish (bay anchovies, Anchoa mitchilli) showed that the diversion was associated with two of the five major source regimes that were supporting food webs: a river regime near the diversion and a river-influenced productive marsh regime farther away from the diversion. Mixing models identified a third river-influenced source regime at the marine end of the estuary where major natural discharge from the Bird’s Foot Delta wraps around into estuarine waters. The remaining two source regimes represented typical estuarine conditions: local freshwater sources especially from precipitation and a brackish source regime representing higher salinity marine influences. Overall, the Mississippi River diversion accounted for 75% of food web support in the upper estuary and 25% in the middle estuary, with influence strongest along known flow pathways and closest to the diversion. Isotopes also traced seasonal changes in river contributions, and indicated increased plant community productivity along the major flow path of diversion water. In the Breton Sound estuary, bottom–up forcing of food webs is strongly linked to river introductions and discharge, occurring in spatial and temporal patterns predictable from known river input regimes and known hydrologic circulation patterns.     

Structure and Spatial‐Temporal Dynamics of Chironomidae Fauna (Diptera) in Upland and Lowland Fluvial Habitats of the Chocancharava River Basin (Argentina)

Structure of benthic Chironomidae assemblages and their spatial‐temporal dynamic were analyzed in upland and lowland habitats from the Chocancharava River basin (Córdoba, Argentina). Sampling was performed in three tributary streams and in three lowland reaches of the river during high and low rainfall periods. Characteristic taxa of upland and lowland reaches and of the different habitats in these reaches were identified using the IndVal method. Chironomidae assemblages were different between upland and lowland reaches and among habitats in each reach, as assessed by Multiresponse Permutation Procedure and Canonical Correspondence Analyses. Substrate type and current velocity were the major explanatory variables structuring the assemblages in upland reaches whereas in lowland reaches current velocity and aquatic vegetation were the most important variables. The highest richness was found in the most complex habitat units in both upland and lowland stretches as assessed by Analyses of Variance. Chironomidae larvae responded to longitudinal changes of hydraulic variables and to local variations of fluvial habitats at different reaches. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The populations, characterization and activity of suspended bacteria in the Welsh River Dee

In a 29 month study of bacterial populations at three sites on the Welsh River Dee, the aerobic heterotrophic bacteria increased from an average basal level of about 1.2 times 10⁴ colony-forming units (cfu)/ml near the source of the river, to 2 times 10⁵ cfu/ml in the lower reaches. The ratio, total bacterial cell count: viable count, decreased from 70 in the upland reaches to 10 in the lower parts of the river. There was no apparent seasonal variation in bacterial numbers but on occasions the bacterial populations in both upland and lowland reaches of the river increased simultaneously. Fluctuations in bacterial numbers over a 50-fold range were observed in this study. Bacterial isolates from both upland and lowland sites were predominated by two groupings of bacteria, the Pseudomonas—Agrobacterium—Alcaligenes group and the Flavobacterium—Cytophaga—Flexibacter group. Results suggested that the latter group may have been part of the autochthonous population. Seasonal variation in heterotrophic potential ( V _max) for acetate uptake was shown to occur over a 30-fold range in the lowland reaches of the River Dee. Peaks in activity at the lowland site occurred during the summer months, the range of V _max values for acetate ranged from 0.2 to 30 μg/1/h. Fluctuations in V _max values from the upland site were not seasonal but were instead linked to faecal pollution, V _max values from this site range from 0.04 to 3 μg/1/h.     

Flow intermittency influences the trophic base,but not the overall diversity of alpine stream food webs

Alpine streams can exhibit naturally high levels of flow intermittency. However, how flow intermittency in alpine streams affects ecosystem functions such as food web trophic structure is virtually unknown. Here, we characterized the trophic diversity of aquatic food webs in 28 headwater streams of the Val Roseg, a glacierized alpine catchment. We compared stable isotope (δ¹³C and δ¹⁵N) trophic indices to high temporal resolution data on flow intermittency. Overall trophic diversity, food chain length and diversity of basal resource use did not differ to a large extent across streams. In contrast, gradient and mixing model analysis indicated that primary consumers assimilated proportionally more periphyton and less allochthonous organic matter in more intermittent streams. Higher coarse particulate organic matter (CPOM) C:N ratios were an additional driver of changes in macroinvertebrate diets. These results indicate that the trophic base of stream food webs shifts away from terrestrial organic matter to autochthonous organic matter as flow intermittency increases, most likely due to reduced CPOM conditioning in dry streams. This study highlights the significant, yet gradual shifts in ecosystem function that occur as streamflow becomes more intermittent in alpine streams. As alpine streams become more intermittent, identifying which functional changes occur via gradual as opposed to threshold responses is likely to be vitally important to their management and conservation.     

Invertebrate food webs along a stream resource gradient

1. The flow of energy through food webs with similar species can vary with both space and time. The river continuum concept (RCC) provides a useful framework for predicting variability in the biota and food availability along streams. We estimated the flow of organic matter (g m^?2 year^?1) through food webs, arrayed along a stream, that had different resource inputs. Four sites were sampled along the Little Tennessee River, North Carolina, U.S.A.: two fifth order sites, one sixth and one seventh order site. The dominant resource is leaf detritus in the upstream reach (the upstream fifth order site), algae in the mid‐reaches (the downstream fifth and sixth order sites), and suspended material downstream (seventh order site). 2. Eleven genera, contributing from 50 to 66% of the total macroinvertebrate secondary production of each site, were studied. We estimated organic matter flow from resource to consumer by combining previously measured rates of invertebrate secondary production with gut content analyses and assimilation efficiencies. 3. Organic matter flow through food webs increased in a downstream direction, while the structure of the food webs remained constant. The total food consumed by the taxa analysed increased from 34 g m^?2 year^?1 at the upstream site to 730 g m^?2 year^?1 at the most downstream site. We estimate that the organic matter consumed by the entire macroinvertebrate community ranged from 66 to 1164 g m^?2 year^?1. These results indicate that there is variation in the magnitude of organic matter flow through the food webs along this river continuum. 4. The dominant food resource consumed also changed along the gradient. Leaf detritus consumption decreased from 58% of the total consumption upstream to 6% downstream, whereas consumption of amorphous detritus increased from 18 to 64%. The proportion of animal material consumed also increased from 3 to 27%. The total consumption of autochthonous resources (diatoms and filamentous algae) increased along the continuum (from 6.41 to 34.05 g m^?2 year^?1). We conclude that these results are related to variation in resource availability, dietary shifts and invertebrate secondary production. These results link resource availability to energy flow, a relationship originally suggested by the RCC.     

Spatial and temporal patterns of carbon flow in a temperate,large river food web

Using natural abundances of stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes, we quantified spatial and temporal patterns of carbon flow through the main channel food web in the lowland section of New Zealand’s longest river, the Waikato River. The study was undertaken with the objective of determining whether the Waikato River conforms to contemporary theoretical concepts regarding carbon flow in large river food webs. Potential organic carbon sources and invertebrate and fish consumers were sampled from three different hydrogeomorphic zones on six occasions, representing a range of seasonal and flow conditions. In line with the predictions of the riverine productivity model and riverine ecosystem synthesis, autochthonous algae and biofilms were the most important basal carbon source contributing to consumer biomass. These were often supported by C₃ aquatic macrophytes and allochthonous C₃ riparian plants. The relative importance of organic carbon sources differed between zones and appeared to change depending on season, presumably in response to water temperature and flow, particularly in the unconstrained zone of the lower river. We also demonstrate that to draw robust conclusions, consideration must be given to quantifying the isotopic signatures of organisms lower in the food web, as these can change significantly between sampling times and hydrogeomorphic zones.     

Trophic network of aquatic macroinvertebrates along an altitudinal gradient in a Neotropical mountain river

Studies of trophic networks and the evaluation of processes that occur along altitudinal gradients in river systems are of great importance because they allow an understanding of energy flow dynamics and provide scientific tools for the planning and management of river ecosystems. This research describes the trophic network of aquatic macroinvertebrates along an altitudinal gradient of the Gaira River, a mountain Neotropical watercourse located in the Sierra Nevada de Santa Marta in northern Colombia. The organisms were collected in the upper, middle and lower reaches of the river during the rainy and dry seasons (between 2010 and 2013). Trophic relationships were evaluated through gut content analysis. The contents were determined and quantified using photographs and expert review, and a binary consumption matrix was used to determine the characteristics of the trophic network. We characterized the diet composition at each site for each season using discriminant analysis. Trophic networks during the dry seasons showed higher trophic species richness and linkage density, and the predominance of coarse particulate organic matter (CPOM) and fine particulate organic matter (FPOM) in the high and medium sections of the river. During the dry seasons the diets had a lower number of basal species, but in the low river section there was a high percentage of fungi and microalgae. During the rainy seasons, no patterns were observed for the percentage of resources. Results indicated a direct relation between periods of hydrologic stability and an increase of CPOM during dry seasons and an increase of resource diversity consumed by macroinvertebrates at all sites during the rainy season, showing that changes in trophic networks of the Gaira River were more important during seasonal periods than along the altitudinal gradient.     

Terrestrial Contributions to the Aquatic Food Web in the Middle Yangtze River

Understanding the carbon sources supporting aquatic consumers in large rivers is essential for the protection of ecological integrity and for wildlife management. The relative importance of terrestrial and algal carbon to the aquatic food webs is still under intensive debate. The Yangtze River is the largest river in China and the third longest river in the world. The completion of the Three Gorges Dam (TGD) in 2003 has significantly altered the hydrological regime of the middle Yangtze River, but its immediate impact on carbon sources supporting the river food web is unknown. In this study, potential production sources from riparian and the main river channel, and selected aquatic consumers (invertebrates and fish) at an upstream constricted-channel site (Luoqi), a midstream estuarine site (Huanghua) and a near dam limnetic site (Maoping) of the TGD were collected for stable isotope (δ¹³C and δ¹⁵N) and IsoSource analyses. Model estimates indicated that terrestrial plants were the dominant carbon sources supporting the consumer taxa at the three study sites. Algal production appeared to play a supplemental role in supporting consumer production. The contribution from C₄ plants was more important than that of C₃ plants at the upstream site while C₃ plants were the more important carbon source to the consumers at the two impacted sites (Huanghua and Maoping), particularly at the midstream site. There was no trend of increase in the contribution of autochthonous production from the upstream to the downstream sites as the flow rate decreased dramatically along the main river channel due to the construction of TGD. Our findings, along with recent studies in rivers and lakes, are contradictory to studies that demonstrate the importance of algal carbon in the aquatic food web. Differences in system geomorphology, hydrology, habitat heterogeneity, and land use may account for these contradictory findings reported in various studies.     

Allochthonous and autochthonous carbon sources for fish in floodplain lagoons of an Australian dryland river

Dryland rivers associated with arid and semi-arid land areas offer an opportunity to explore food web concepts and models of energy sources in systems that experience unpredictable flooding and long dry spells. This study investigated the sources of energy supporting three species of fish feeding at different trophic levels within floodplain lagoons of the Macintyre River in the headwaters of the Murray-Darling river system, Australia. Stable isotope analyses revealed that fish consumers derived, on average, 46.9% of their biomass from zooplankton, 38.1% from Coarse Particulate Organic Matter (CPOM) and 24.0% from algae. Ambassis agassizii derived on average 57.6% of its biomass carbon from zooplankton and 20.4–27.8% from algae or CPOM. Leiopotherapon unicolor derived most of its carbon from zooplankton and CPOM (38.3–39.5%), with relatively high contributions from algae compared to the other species (33.3%). An average of 48.4% of the biomass of Nematalosa erebi was derived from zooplankton, with CPOM contributing another 38.1%. Zooplankton was the most important source of organic carbon supporting all three fish species in floodplain lagoons. Phytoplankton, and possibly, particulate organic matter in the seston, are the most likely energy sources for the planktonic suspension feeders (zooplankton) and, consequently, the fish that feed on them. These results indicate a stronger dependence of consumers on autochthonous sources and on locally produced organic matter from the riparian zone (i.e., the Riverine Productivity Model), than on other resources.     

Effects of hydrological forcing on the structure of a tropical estuarine food web

River flow can impact which sources of particulate organic matter (POM) fuel estuarine food webs. Here, we used stable carbon (C) and nitrogen (N) isotope analyses to compare how contributions of different POM sources (terrestrial, estuarine, and marine) to the diets of zooplankton and juvenile fishes differed between low and high river flow conditions, as well as spatially across a tropical estuary, Hilo Bay, Hawaii, USA. Diets of zooplankton and juvenile fishes were affected by river flow conditions, but the magnitude and the change in the basal resources depended on the location of the station in the estuary relative to the ocean and the river mouths. Consumers from the station most isolated from the ocean and with groundwater and overland flow inputs, utilized a combination of estuarine and terrestrial POM during both low and high river flow conditions and exhibited less variability in their basal resources than stations with direct ocean exchange. Consumers from stations in the Bay most affected by ocean exchange and river inputs utilized a combination of estuarine, terrestrial, and marine POM during low flow conditions, but shifted to marine and terrestrial POM during high river flow conditions. This shift to using terrestrial POM during high river flow conditions was substantial and up to 40% higher than values measured in other estuaries. Factors suspected to be affecting which POM source(s) consumers use in Hilo Bay are gross primary production, biological availability of exported terrestrial OM, and estuarine bacteria biomass, all of which are affected by river flow. Overall, our results suggest that Hilo Bay's food web and possibly those from other tropical estuaries are vulnerable to changes in hydrology, which may be further enhanced by global climate change.     

Reach-scale geomorphology affects organic matter and consumer δ13C in a forested Piedmont stream

1. We investigated the spatial (longitudinal position and reach geomorphology) and seasonal (spring and autumn) influences on the variation of δ¹³C among organic matter sources and consumers in a forested Piedmont river, South Carolina, U.S.A. 2. Six sites were sampled along a continuum and varied in basin area from approximately 30 to 300 km². Sites fell into two geomorphic categories (i) high‐gradient, rock bed (‘rock’) or (ii) low‐gradient, sand bed (‘sand’) sites. 3. Variation in δ¹³C was more strongly related to reach geomorphology than longitudinal position. δ¹³C of biofilm and consumers was consistently enriched at rock sites. Leaf litter (i.e. coarse particulate organic matter, CPOM) δ¹³C did not vary with bed type. There was significant δ¹³C enrichment at rock sites for biofilm, seston, fine benthic organic matter (FBOM), and eight of nine consumer trophic guilds (e.g. grazing invertebrates, insectivorous fishes). δ¹³C of biofilm and four trophic guilds was also positively correlated with drainage area, but the magnitude of enrichment was less than between bed types. 4. δ¹³C was generally enriched in spring, but this varied among organic matter types, consumers, and by bed type. CPOM and seston were enriched in spring, FBOM was enriched in autumn, and biofilm showed no trend. Five consumer guilds were enriched in spring, and only one fish guild, generalised carnivores, showed enrichment of muscle tissue in autumn. 5. Consumer δ¹³C enrichment at rock sites suggests greater reliance on algal carbon than for consumers at sand sites, but we also found δ¹³C enrichment of biofilm at rock sites. Thus, differences in consumer δ¹³C between bed types could be related to (i) increased consumption of biofilm at rock compared with sand sites, or (ii) consumption of biofilm at rock sites that is enriched relative to biofilm at sand sites or (iii) both mechanisms. 6. δ¹³C signatures in local food webs appear to respond to processes operating at multiple spatial scales. Overall downstream enrichment of biofilm and consumers was disrupted by strong local effects related to bed morphology. These results suggest that human alteration of channel habitat will have corresponding effects on stream food webs, as assessed by changes in δ¹³C.     

Lateral input of particulate organic matter from bank slopes surpasses direct litter fall in the uppermost reaches of a headwater stream in Hokkaido, Japan

In forested streams, surrounding riparian forests provide essential supplies of organic matter to aquatic ecosystems. We focused on two pathways of particulate organic matter inputs: direct input from upper riparian forests and indirect lateral input from bank slopes, for which there are limited quantitative data. We investigated the inputs of coarse particulate organic matter (CPOM) and carbon and nitrogen in the CPOM into the uppermost reaches of a headwater stream with steep bank slopes in Hokkaido, Japan. CPOM collected by litter traps was divided into categories (e.g., leaves, twigs) and weighed. Monthly nitrogen and carbon inputs were also estimated. The annual direct input of CPOM (ash-free dry mass) was 472 g m⁻², a common value for temperate riparian forests. The annual lateral CPOM input was 353 g m⁻¹ and 941 g m⁻² when they were converted to area base. This value surpassed the direct input. Organic matter that we could not separate from inorganic sediments contributed to the total lateral input from the bank slopes (124 g m⁻¹); this organic matter contained relatively high amounts of nitrogen and carbon. At uppermost stream reaches, the bank slope would be a key factor to understanding the carbon and nitrogen pathways from the surrounding terrestrial ecosystem to the aquatic ecosystem.     

拉萨河中下游纤毛虫群落时空分布模式及其驱动机制

为了探究拉萨河中下游纤毛虫群落的组成模式、时空多样性格局及其维持机制, 本文于2015年5月和8月以及2016年10月在拉萨河中下游17个样点进行采样, 采用活体观察、鲁哥氏碘液固定染色以及Wilbert蛋白银法相结合的物种鉴定方法, 对纤毛虫群落结构进行了研究。通过Shannon多样性指数、Margalef丰富度指数、物种数分析群落结构时空上的差异性; 通过共现网络分析纤毛虫类群之间的相互作用; 通过冗余分析(redundancy analysis, RDA)探讨水体理化因子对纤毛虫群落结构的影响。结果表明, Shannon多样性指数在季节和河段间没有显著性差异; Margalef丰富度指数、物种数在河段间存在极显著性差异; 中游和下游河段共现网络节点间的相关关系均以正相关为主; 溶解氧(DO)、总氮(TN)、总磷(TP)、总溶解盐(TDS)是影响纤毛虫群落结构的关键因子。综上所述, 拉萨河中下游纤毛虫群落结构在季节间没有显著差异, 在空间上具有显著差异; 纤毛虫在纲级水平上类群间的相互作用以协同作用为主导, 不同类群间存在复杂的相互作用, 整体上互作关系在春季较为复杂、夏季较为简单; 影响拉萨河中下游纤毛虫群落结构是多个环境因子共同作用的结果。     

Development and ecological importance of phytoplankton in a large lowland river (River Meuse,Belgium)

The ecological importance of the River Meuse phytoplankton with regard to carbon and nutrient transport has been examined in two reaches of the Belgian course of the river.Field measurements of total particulate organic carbon (POC), particulate organic nitrogen (PON) and particulate phosphorus (PP) show that the large autochtonous production of organic matter strongly affects the carbon and nutrient budget of the aquatic system. During the growing season, phytoplankton accounts for nearly 60% of the POC and dominates the PON. Calculations of the carbon and oxygen budget in the upper reach of the Belgian Meuse demonstrates that the ecosystem is autotrophic, i.e. that autochtonous FPOM (fine particulate organic matter) production is the major carbon input. This suggests that in large lowland rivers, primary production (P) may exceed community respiration (R), i.e. P:R>1, whereas they are assumed to be heterotrophic (P:R<1) in the River Continuum concept.The question of maintenance of phytoplankton in turbid mixed water columns is also addressed, and the case of the River Meuse is treated on the basis of studies of photosynthesis and respiration (ETS measurements). The results suggest that the potamoplankton may show some low-light acclimation, through an increase of chlorophyll a relative to biomass, when it comes to deep downstream reaches, and that algal respiration rate may be reduced. A simulation of the longitudinal development of the algal biomass shows the different phases of algal growth and decline along the river and brings support to the importation hypothesis for explaining maintenance of potamoplankton in the downstream reaches.     

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 (K ₂) determined from seven empirical depth-velocity equations. Because the predicted values of K ₂ 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.     

Macroinvertebrate diversity in headwater streams: a review

1. Headwater streams are ubiquitous in the landscape and are important sources of water, sediments and biota for downstream reaches. They are critical sites for organic matter processing and nutrient cycling, and may be vital for maintaining the 'health' of whole river networks.
2. Macroinvertebrates are an important component of biodiversity in stream ecosystems and studies of macroinvertebrate diversity in headwater streams have mostly viewed stream systems as linear reaches rather than as networks, although the latter may be more appropriate to the study of diversity patterns in headwater systems.
3. Studies of macroinvertebrate diversity in headwater streams from around the world illustrated that taxonomic richness is highly variable among continents and regions, and studies addressing longitudinal changes in taxonomic richness of macroinvertebrates generally found highest richness in mid-order streams.
4. When stream systems are viewed as networks at the landscape-scale, α-diversity may be low in individual headwater streams but high β-diversity among headwater streams within catchments and among catchments may generate high γ-diversity.
5. Differing ability and opportunity for dispersal of macroinvertebrates, great physical habitat heterogeneity in headwater streams, and a wide range in local environmental conditions may all contribute to high β-diversity among headwater streams both within and among catchments.
6. Moving beyond linear conceptual models of stream ecosystems to consider the role that spatial structure of river networks might play in determining diversity patterns at the landscape scale is a promising avenue for future research.