Nutrient recycling by fish in streams along a gradient of agricultural land use

Nutrient recycling is an essential ecosystem process provided by animals. In many aquatic systems, fish have been identified as important in ecosystem nutrient recycling; however, this importance can vary widely between systems. The factors controlling intersystem variation in animal‐mediated nutrient cycling have rarely been examined and as such it remains unclear what impact human landscape changes will have upon these processes. Here we examined rates of nutrient recycling for temperate stream fish assemblages along a gradient of agricultural land use (proportion cropland in the watershed: 1–59%). We quantified nutrient excretion rates of both ammonium–N (NH₄⁺–N) and phosphate (as soluble reactive phosphate: SRP) for fish assemblages at eight streams in southern Ontario, Canada with species‐specific excretion measurements and quantitative assemblage sampling. For both nutrients, total assemblage excretion exhibited a strong positive relationship with riparian cropland. The distance required for fish assemblages to turn over ambient nutrient pools was shorter for cropland systems, indicating that the relative importance of excreted nutrients was higher in these systems. Based on measured uptake rates of NH₄⁺–N in two streams (one higher cropland and one low cropland) and on modeled uptake rates for all streams, the proportion of ecosystem demand that can be satisfied by excretion is generally higher in the more agricultural streams. These patterns appear to be driven largely by disproportionate increases in fish assemblage biomass with increasing stream nutrient concentrations.     

Food web pathways for fish communities in small tropical streams

1. It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species.
2. We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow.
3. We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects.
4. Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass.
5. This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high.

     

Effects of nutrient recycling by zooplankton and fish on phytoplankton communities

In laboratory experiments we tested the hypothesis that nutrients supplied by fish and zooplankton affect the structure and dynamics of phytoplankton communities. As expected from their body size differences, fish released nutrients at lower mass-specific rates than Daphnia. On average, these consumers released nutrients at similar N:P ratios, although the ratios released by Daphnia were more variable than those released by fish. Nutrient supply by both fish and Daphnia reduced species richness and diversity of phytoplankton communities and increased algal biomass and dominance. However, nutrient recycling by fish supported a more diverse phytoplankton community than nutrient recycling by Daphnia. We conclude that nutrient recycling by zooplankton and fish have different effects on phytoplankton community structure due to differences in the quality of nutrients released. Received: 21 December 1998 / Accepted: 31 May 1999     

Influence of disturbance on insect communities in Pacific Northwest streams

Coniferous forests of the Pacific Northwest provide a unique setting for stream ecology research because of the great age of the forests and the important role of wood debris in structuring aquatic systems. The composition and diversity of the insect community in Mack Creek, a stream in a 450 yr conifer forest, was compared with that in Grasshopper Creek where it flowed through a recent clearcut, and at Quartz Creek, which had a 40 yr deciduous canopy. Of the 256 taxa identified, Mack Creek had the highest species richness (196) and evenness. The open site had 191 taxa but high dominance of a few grazer taxa. The deciduous-canopy site had 165 taxa with abundant detritivores. Despite differences in density, the biomass of emergence was similar at the three sites, ranging from 1.53 to 1.65 g m^–2 yr^–1.The importance of disturbance in structuring stream communities was demonstrated in phenomenological studies after a debris torrent at Quartz Creek, and by monitoring stream recovery following the eruption of Mt. St. Helens in 1980. At Quartz Creek, the debris torrent eliminated the fauna from a 300 m reach, but there was rapid recovery. Emergence density in the same year was similar to that of the control site. The major shift in populations was a decrease in detritivores and moss associates and an increase in grazers, especially Baetis mayflies.At Ape Creek on Mt. St. Helens, over 200 taxa were recorded by 1987 but most occurred in very low densities. This site is reset by winter freshets and by infilling with glacial fines in the summer so the fauna continues to be dominated by weedy, or early successional species. At Clearwater Creek, the presence of wood debris as a stable substrate and limited inputs of fine sediment after 1980 have hastened population recovery. A decade after the eruption this site can be characterized as being in the mid-stages of succession with high insect productivity from an algal-based food web. With further growth of the riparian vegetation I predict a shift towards a detritus-based food web and fauna more similar to Mack Creek than it is at present.     

Seasonal flow variation allows 'time-sharing' by disparate aquatic insect communities in montane desert streams

1. Flow variation can drive major abiotic changes in stream environments between seasons. Theoretically, disparate biotic communities could be maintained during different seasons at a single site if suitable refuges and colonist sources were available. Using isolated montane desert streams in south‐east Arizona as a model system, we hypothesised that two disparate aquatic insect faunas (montane temperate and neotropical) could be maintained at the same sites through strong seasonal variation in abiotic conditions. 2. We collected aquatic insects representing 59 families from seven streams during high‐flow (March–April) and low‐flow (June) sampling periods across two years. We assessed changes in aquatic insect community and functional feeding group composition by habitat (riffle, pool) and season (high flow, low flow). 3. Within sites, wetted stream area decreased by an average of 97% between high‐flow (predominately riffles) and low‐flow (predominately pools) seasons. Community composition likewise showed strong seasonal patterns; the montane temperate fauna was strongly associated with the high‐flow season while neotropical hemipterans and coleopterans were associated with the low‐flow season. Increased water temperature was significantly associated with this shift from temperate to neotropical assemblages. 4. Functional feeding group composition shifted dramatically by season. The proportion of predators increased from 24.5% (high flow) to 75.2% (low flow) while collector–filterers and shredders declined from 38.4% (high flow) to 1.7% (low flow). 5. We suggest that habitat ‘time‐sharing’ by disparate communities is facilitated via strong seasonal variation in temperature and flow and the presence of high elevation refuges or diapause stages for temperate montane taxa to survive the dry season.     

Barriers to the recovery of aquatic insect communities in urban streams

1. Worldwide, increasing numbers of stream restoration projects are being initiated to rehabilitate waterways modified by urbanisation. However, many of these projects have limited success in restoring stream communities. Prompted by this, we investigated previously unrecognised barriers to aquatic insect colonisation in urban streams. 2. To investigate whether the availability of suitable substrata for oviposition limited the longitudinal distribution of caddisflies, large boulders were added to the upstream reaches of one stream. Prior to the addition, more egg masses were observed downstream and this longitudinal pattern persisted subsequently. 3. Malaise trapping revealed that adult caddisfly diversity and abundance was greater downstream than upstream. Furthermore, in a previous study the authors found larval caddisflies reflected the longitudinal distribution of adults. 4. The only obvious potential obstructions between reaches were roads beneath which the stream flowed through culverted crossings. Malaise trapping was used to examine the effect of road culverts and bridges on caddisfly dispersal. Numbers of caddisflies caught declined upstream and about 2.5 × more individuals were taken in traps immediately below than above five culverts. 4. Bridges, which had a more open structure than culverts, had no significant effect on the size of catches made above and below them. 5. Road culverts could act as partial barriers to upstream flight, with consequences for larval recruitment in urban streams. We recommend that urban planners and designers of restoration projects consider possible synergistic effects of poor oviposition habitat and barriers to aquatic insect dispersal, which may be critical for the colonisation of urban headwater streams and for the maintenance of stream insect populations.     

Morphological traits mediate fish occurrences in oil palm-impacted tropical streams

1. Land-use change is a leading driver of biodiversity loss, especially in tropical fresh waters where the conversion of natural forest to monoculture plantations impacts freshwater fish assemblages. The environmental pathways underpinning shifts in fish assemblages, however, are poorly understood, but could potentially be inferred from trait–environment relationships.
2. We addressed this knowledge gap using eco-morphological traits to explain fish occurrences in oil palm-impacted streams of the Endau drainage in Peninsular Malaysia. We also investigated how traits relate to differences in environmental conditions associated with land-use change. We then integrate findings from the above to test how potential pathways of land-use driven environmental changes can impact species occurrences through effects on life history, feeding habits, and mobility.
3. Mixed-effects models show that fishes with superior (upward-facing) mouths and low body mass were more likely to occur in oil palm streams than forest streams, and these traits were associated with grass-dominated riparian zones and reduced woody debris in oil palm streams, respectively. Structural equation models show that mouth positions statistically mediated the effect of riparian vegetation on fish species occurrences in oil palm streams. Specifically, fishes with superior mouths were more likely to feed on terrestrial invertebrates.
4. Our analysis of easily measurable traits revealed pathways of land-use impact that are potentially more widely applicable than conventional taxa-based approaches. Fishes with superior mouths tended to occur in oil palm streams as they were able to more effectively exploit inputs of terrestrial invertebrates that are potentially associated with grass-dominated riparian vegetation.
5. Moreover, shifts in traits may suggest land-use driven changes in stream ecosystem functioning (e.g. in terms of the role of terrestrial subsidies), thereby informing targeted management actions in land-use impacted habitats (e.g. retention/restoration of riparian trees).

     

Non-interactive fish communities in the coastal streams of North-western France

1. Spatial patterns of freshwater fish species at regional and local scales were investigated to explore the possible role of interspecific interactions in influencing distribution and abundance within communities occupying coastal streams of North-Western France.
2. Nine sites from nine streams situated in the same biogeographical region were sampled annually over the 6-year period from 1990 to 1995.
3. Similar habitats (sites) with richer regional colonization pools exhibited proportionally richer local communities in terms of number of species, total density and total biomass of individuals. Furthermore, no negative relationships were found between density and biomass of each of the most common species and local species richness.
4. Results of dynamic regression models (applied to the above-mentioned species) suggest an absence of strong competition between all pairs of species.
5. The evidence on lack of density compensation for species-poor communities and absence of perceptible interspecific competition between species suggest that the communities studied are non-interactive.
6. Two main explanations can be advanced. First, the local abundance of species in the communities studied could be determined through differential responses to unpredictable environmental changes, rather than through biological interactions. Second, as a result of historical events, the communities studied are reduced in congeneric species which can limit, in turn, the influence of interspecific competition in structuring these communities.
7. These results underline the strong influence of regional processes in shaping local riverine fish communities and minimize the possible influence of species interactions in governing these communities.     

Drivers of beta diversity of macroinvertebrate communities in tropical forest streams

1. There has recently been increasing interest in patterns of beta diversity but we still lack a comprehensive understanding of these patterns in various regions (e.g. the tropics), ecosystems (e.g. streams) and organism groups (e.g. invertebrates). 2. Our aim was to investigate the patterns of beta diversity of stream macroinvertebrates in relation to key environmental (i.e. stream size, pH and habitat degradation) and geographical variables (i.e. latitude, longitude, altitude) in a tropical region. We surveyed a total of 8–10 riffle sites in each of 34 streams (altogether 337 riffle sites were sampled) in Peninsular Malaysia to examine variation in macroinvertebrate community composition at within‐stream and among‐stream scales. 3. Based on test of homogeneity of dispersion, we found that the streams studied differed significantly in within‐stream variation in community composition (i.e. among‐site variation of within stream beta diversity). The patterns were similar based on Bray–Curtis coefficient on abundance data, Sorensen coefficient on presence–absence data and Simpson coefficient on presence–absence data. We also found that within‐stream beta diversity was significantly related to stream size, pH and latitude, with each of these variables individually accounting for around 20% of the variation in beta diversity in simple regressions, while the total variation explained by the three significant variables amounted to around 50% in multiple regressions. By contrast, habitat degradation, longitude and altitude were not significantly related to beta diversity. We also found that the factor drainage basin accounted for much of the variation in beta diversity in general linear models, suppressing the effects of environmental variables. 4. We concluded that within‐stream beta diversity is mainly related to a combination of the identity of a drainage basin and stream environmental factors. Our findings provide important background for stream environmental assessment and conservation planning by emphasising that (i) macroinvertebrate communities within streams are not homogeneous, but show considerable beta diversity, (ii) streams differ in their degree of within‐stream beta diversity, (iii) stream size and water pH should be considered in applied contexts related to within‐stream beta diversity and (iv) historical effects may be different in different drainage basins and may affect present‐day patterns of within‐stream beta diversity.     

The role of indirect interactions in structuring tropical insect communities

Apparent competition is a form of indirect interaction among species that can potentially structure biological communities. In insect communities, parasitoid-mediated apparent competition has been proposed as a particularly important structuring force. We argue that short-term apparent competition may be less important in structuring insect communities in tropical regions, compared with temperate regions. This prediction arises because, compared with temperate insects, tropical insects that share natural enemies are more likely to be isolated in both space and time.     

Identification of key microhabitats for fish assemblages in tropical Brazilian savanna streams

The habitat use by stream fish can provide an understanding of this important component of species niches, and the changes in the availability of certain structures of instream habitats have a strong influence on the occurrence and abundance of fish species. In this study, we identified the interaction networks between fish and microhabitats in 19 streams of three watersheds from the Brazilian savanna, known as the “Cerrado”. We also investigated whether the connectance between fish species and microhabitats depends on either the species abundance or availability of microhabitats and verified whether the use of stream microhabitats vary among watersheds. The data for the fish were obtained using standardized fish collections and underwater (snorkeling) observations to record the association of fish species with 11 stream microhabitats. We used the incidence data to calculate the connectance of species with different microhabitats, a Spearman correlation analysis to test the dependence of the connectance to the species abundance and microhabitat availability, and an Analysis of Similarity to test whether the use of stream microhabitats by fish can vary among watersheds. In all of the watersheds, the tetras of the family Characidae used the largest number of microhabitats. The most‐used microhabitats by the fish watersheds were the lateral surface, column surface, and unconsolidated substrate, which were differentially used by the fish in each watershed, indicating the variable importance of microhabitats. Despite this observation, the fish occurred more frequently in the lateral microhabitats. When the data for all of the streams were combined, the species connectance was correlated with the species abundance. We also discuss the consequences of the elimination of certain bottom structures in the context of siltation, which is the main issue in the studied region. The results of our study allowed the detection of key microhabitat structures and can be used for the aquatic ecosystems restoration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Potential functional redundancy and resource facilitation between tadpoles and insect grazers in tropical headwater streams

1. We quantified production and consumption of stream‐dwelling tadpoles and insect grazers in a headwater stream in the Panamanian uplands for 2 years to assess their effects on basal resources and energy fluxes. At the onset of our study, this region had healthy, diverse amphibian populations, but a catastrophic disease‐driven decline began in September 2004, which greatly reduced amphibian populations. 2. Insect grazer production was 348 mg ash‐free dry mass (AFDM) m^?2 year^?1 during the first year of the study and increased slightly to 402 mg AFDM m^?2 year^?1 during the second year. 3. Prior to amphibian declines, resource consumption by grazers (tadpoles and insects) was estimated at 2.9 g AFDM m^?2 year^?1 of algal primary production, which was nearly twice the estimated amount available. Insect grazers alone accounted for c. 81% of total primary consumption. During the initial stages of the declines, consumption remained at c. 2.9 g AFDM m^?2 year^?1, but only 35% of the available resource was being consumed and insect grazers accounted for c. 94% of total consumption. 4. Production and resource consumption of some insect grazers increased during the second year, as tadpoles declined, indicating a potential for functional redundancy in this system. However, other insect grazer taxa declined or did not respond to tadpole losses, suggesting a potential for facilitation between tadpoles and some insects; differential responses among taxa resulted in the lack of a response by insect grazers as a whole. 5. Our results suggest that before massive population declines, tadpoles exerted strong top‐down control on algal production and interacted in a variety of ways with other primary consumers. 6. As amphibian populations continue to decline around the globe, changes in the structure and function of freshwater habitats should be expected. Although our study was focused on tropical headwater streams, our results suggest that these losses of consumer diversity could influence other aquatic systems as well and may even reach to adjacent terrestrial environments.     

Ecological structure of tropical fish assemblages in wet-zone streams of Sri Lanka

The introduction of four fish species into a depauperate stream system in central Sri Lanka provided a 'natural experiment' that enabled us to determine the ecological structure of wet-zone stream fish assemblages. All the species indigenous to this 'introduction' stream system also co-occur naturally with the introduced species in nearby streams. Analyses of habitat use and dietary requirements revealed that most species in the assemblages were segregated on the basis of macrohabitat, microhabitat and food, regardless of origin of the fishes. Macrohabitat and microhabitat utilizations by species were similar in each stream of the 'introduction' system despite differences in stream conditions. Thus species distributions, relative to each other in ecological space, were consistent among streams, and with the combined data. High overlaps along the resource axes of velocity, depth, substratum and food were few. A major proportion of the high overlaps was due to associations with introduced species; especially Barbus nigrofasciatus and Barbus cumingii. The other two introduced species, Barbus titteya and Rasbora vaterifloris , were more specialized, and interacted relatively less with indigenous species. The indigenous species, however, exhibited pronounced complementarity along three resource axes representing depth, velocity and food. Rasbora vaterifloris and B. nigrofasciatus grew to larger sizes in one stream which had fewer indigenous species, suggesting competitive release. In this stream, the most common indigenous species was a dietary specialist that fed on diatoms, and B. nigrofasciatus , which fed heavily on diatoms in other streams, switched to feeding more on macrophytes. Overall, the data suggest that these assemblages are predictable, co-evolved systems with competition serving as an important structuring force that reinforces species segregation.     

Disturbance,fish, and variation in the predatory insect guild of coastal streams

Species distributions, and thus the composition of communities, are determined by many interacting biotic and abiotic factors. We analyzed the variation in composition of the invertebrate predator guild across eight small, steep coastal streams in eastern Canada, in relation to variation in several broad categories of environmental variables: disturbance-related physical characteristics, top-down effects (fish), and bottom-up effects (prey composition and productivity-related physical/chemical variables). Similarity in composition (relative and absolute abundances of the 19 species belonging to the Rhyacophilidae, Perlodidae, and Chloroperlidae) declined significantly with distance. Streams that were most similar in predator composition were also most similar in physical factors related to disturbance, but were not more similar in prey abundance/composition or in environmental factors associated with productivity (chemistry, canopy cover, and riparian forest type). Similarity in the relative abundance of species within the Rhyacophilidae was linked to variation in the presence/abundance of brook trout. These results suggest that the invertebrate predator guilds of these small coastal streams are structured first and foremost by factors associated with the magnitude and variability of the flow regime, and second by response to top-down factors (predation by brook trout). There was no evidence that bottom-up effects altered the composition or abundance of the predator guild. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Handling editor: Sonja Stendera     

Nutrient recycling by two phosphorus-rich grazing catfish: the potential for phosphorus-limitation of fish growth

In ecosystems where excretion by fish is a major flux of nutrients, the nitrogen (N) to phosphorus (P) ratio released by fish can be important in shaping patterns of algal biomass, community composition, primary production, and nutrient limitation. Demand for N and P as well as energy influences N/P excretion ratios and has broad implications in ecosystems where nutrient recycling by fishes is substantial. Bioenergetics and stoichiometric models predict that natural fish populations are generally energy-limited and therefore N/P recycling by fishes is relatively invariant. Yet, the potential for P limitation of growth has not been examined in herbivorous fishes, which are common in many aquatic habitats. We examined N/P excretion ratios and P demand in two P-rich herbivorous catfishes of the family Loricariidae, Ancistrus triradiatus (hereafter Ancistrus) and Chaetostoma milesi (hereafter Chaetostoma). Both fishes are common grazers in the Andean piedmont region of Venezuela where we conducted this study. Mass balance (MB) models indicate that these fishes have a high P demand. In fact, our Ancistrus’ P MB model predicted negative P excretion rates, indicating that Ancistrus did not consume enough P to meet its P demand for growth. Direct measurement of excretion rates showed positive, but very low P excretion rates and high N/P excretion ratios for both taxa. To obtain measured P excretion rates of Ancistrus from the MB model, gross growth efficiency must be reduced by 90%. Our results suggest that growth rates of both of these herbivorous and P-rich fish are likely P-limited. If P limitation of growth is common among herbivorous fish populations, herbivorous fishes recycle likely at high N/P ratios and act to diminish the quality of their food.     

Modification of benthic insect communities in polluted streams: combined effects of sedimentation and nutrient enrichment

Responses of the benthic insect community of a southern Appalachian trout stream to inorganic sedimentation and nutrient enrichment were monitored over a period of eight months. Entry of pollutants from point sources established differentially polluted zones, allowing an assessment of impacts due to sedimentation alone and in association with elevated nutrient levels. Input of sediment resulted in a significant increase in bed load and decrease of pH at the substrate-water interface (P < 0.05). The zone receiving nutrient runoff from livestock pasture exhibited elevated levels of nitrate and phosphate, but available data indicated such concentrations to be quite low. Species richness, diversity, and total biomass of filter feeding Trichoptera and Diptera, predaceous Plecoptera, and certain Ephemeroptera were significantly reduced in the polluted zones. Inorganic sedimentation, operating indirectly through disruption of feeding and filling of interstitial spaces, was considered to be the primary factor affecting filter feeding taxa. Decomposition of compounds associated with materials in the bed load may depress pH and eliminate acid sensitive species of Plecoptera and Ephemeroptera. Such processes of acidification may be particularly important to Appalachian streams since the pH of regional surface waters is characteristically acidic prior to sedimentation. Accumulation of particles on body surfaces and respiratory structures, perhaps as a function of wax and mucous secretion or surface electrical properties, appears to be the major direct effect of inorganic sedimentation on stream insects. Growths of the filamentous bacterium Sphaerotilus natans were also frequently associated with silted individuals in the zone receiving nutrient addition. Distribution of the bacterium suggested that silted substrates, perhaps as related to the presence of iron compounds, are required for colonization in dilute nutrient solutions. The primary effect of Sphaerotilus colonies appears to be augmentation of particle accumulation through net formation by bacterial filaments. Data indicate that inorganic sedimentation and nutrient addition operate synergistically, eliminating a significantly greater number of taxa than exposure to one pollutant alone.     

A quarter century of change in the fish communities of three small streams modified by anthropogenic activities

Anthropogenic modification of small stream habitats insouthern Ontario has occurred with little concern forindigenous, non-game fish communities. The combinedeffects of urbanization, impoundment, and agricultureon the fish communities of three small streams, fromheadwaters to near-confluence locations, wereevaluated by comparing current fish communitystructure to historical records (species lists)collected over the last quarter century. Comparisonsusing cluster analysis showed that spatial andtemporal shifts in fish community composition did notoccur immediately after stream modifications but tookup to 10 years. Replacement of common species byformerly rare or absent species was evident in twostreams subjected to urbanization and impoundment;stream sections subjected to primarily agriculture land usedemonstrated minimal change during this study period. Urban zones and sections downstream of impoundmentswere dominated by species tolerant of controlledflows, siltation, channelization, homogenous spawningsubstrates, and elevated temperatures. Barrier-freeheadwater and agriculture zones with abundant riparianvegetation supported communities that were intolerantof controlled flows, dependent on lower summer watertemperatures, and require high dissolved oxygenlevels. Greater understanding of the causative forcesshaping fish communities can be used to facilitateintegration of greater biological realism into anyfuture conservation or restoration programs inanthropogenically-modified streams.