Nutrient-diffusing Substrates Research Articles

A long‐established invasive species alters the functioning of benthic biofilms in lakes

Abstract Invasive species often transform environmental conditions, exclude native species and alter ecosystem functioning, including key ecosystem processes underpinning nutrient and energy cycles. However, such impacts have been most documented during periods of invasive species dominance; their influences on functioning at lower relative abundances and after long‐term establishment are less well‐known. We investigated the effects of Elodea canadensis, a macrophyte native to North America with a long invasion history in many regions of the world, on the biomass accrual and metabolism of littoral zone biofilms growing on organic and inorganic substrates. We deployed nutrient diffusing substrates (NDS) in 18 replicate transects distributed across six lakes, comprising three invaded by E. canadensis and three uninvaded reference lakes. NDS were amended with nitrogen (N), phosphorus (P) or N + P together, or were deployed as unamended controls. E. canadensis relative abundance varied widely in the invaded transects, ranging from 13% to 93% of all macrophyte cover. On control substrates, algal biomass, quantified as Chlorophyll‐a, and gross primary production (GPP) were 42% and 78% greater in the invaded compared to uninvaded lakes, respectively. Respiration rates, attributable to responses of both autotrophs and heterotrophs, were 45% greater on control substrates in invaded lakes. By contrast, N‐limitation of both biofilm GPP and respiration was 25% and 35% greater in uninvaded compared with invaded lakes. There was no evidence for differences in nutrients, light availability or grazing pressure between invaded and uninvaded transects. Rather, the observed differences in metabolism suggest that the presence of E. canadensis increases availability of N at local scales, reducing N‐limitation of biofilms and resulting in elevated rates of biofilm productivity. Our results demonstrate that invasive elodeids might have significant impacts on biofilms and processes associated with the cycling of nutrients, even when long‐established and present at lower relative abundances.

Land use differentially alters microbial interactions and detritivore feeding during leaf decomposition in headwater streams

Abstract Human‐driven land conversion from forest to agriculture introduces nutrients, like nitrogen (N) and phosphorus (P), which upon entering nearby streams can stimulate growth and metabolism of microbial autotrophs (algae) and heterotrophs (fungi and bacteria). Recent evidence indicates that algae also can stimulate microbial heterotrophs associated with decaying plant litter by exuding labile carbon (C), defined as the priming effect on decomposition. These autotroph–heterotroph interactions may further alter detritivore feeding ecology, suggesting many complex mechanisms of land‐use change on stream ecosystem functions. In order to investigate the effects of land use on autotroph–heterotroph interactions, we used a full‐factorial design manipulating labile C (glucose and acetate) and nutrients (N and P) to test their effects during red maple litter decomposition within four forested and four pasture agriculture streams in the Point Remove catchment, Arkansas. We deployed nutrient‐diffusing substrates (NDS) containing litter and either agar, agar + labile C, agar + NP, or agar + CNP for 20 days and measured microbial biomass and metabolism, and detrital mass loss. Additionally, we fed litter to Tipula sp. to assess shifts in consumption and egestion rates. Both microbial heterotrophs and autotrophs responded to NDS manipulations, but not land use. Heterotrophic microbes showed evidence of nutrient limitation and negative priming, while algal biomass was negatively associated with heterotrophs. Tipula consumption and egestion rates were both elevated when feeding on litter incubated in agriculture streams, but only egestion responded to NDS manipulations, suggesting a change in assimilation efficiencies. Cumulatively, we suggest that altered stream conditions, such as those resulting from land‐use change, can alter food webs and detritivore feeding more heavily than microbial interactions during litter decomposition in stream ecosystems. These results add to the literature demonstrating shifts in autotroph–heterotroph interactions with anthropogenic nutrient pollution, and the implications for detritivore feeding and overall C cycling in streams.

Spatial and temporal patterns of stream nutrient limitation in an Arctic catchment

Arctic stream biofilm responses to ongoing climate-related changes in physical and chemical conditions have major implications for stream food webs and biogeochemical cycles. Yet, such effects have rarely been studied outside summer months or at sub-catchment scales in the Arctic. We used deployments of nutrient diffusing substrates (NDS) to assess the spatial (20 deployments) and seasonal patterns (10 deployments) and physical and chemical drivers of nutrient limitation within an Arctic stream catchment. Results show that nutrient limitation of autotrophic processes was common during summer, but that light inhibited biomass accrual under the ice in winter. Alongside single N, P and C responses, co-limitation dominated the overall pattern of limitation over time and across the catchment. However, the primary limiting nutrient to autotrophs changed from N to P in parts of the catchment with higher N concentrations. As Arctic studies are often conducted at individual sites during summer, these may miss shifts in the drivers of stream productivity that arise from variable nutrient, temperature, and light regimes. Our results caution against focusing on one single most important limiting nutrient, as we found that this can shift seasonally and over small spatial scales in this Arctic catchment.

Response of nutrient limitation to invasive fish suppression: How carcasses and analog pellets alter periphyton

The native Yellowstone Cutthroat Trout (Oncorhynchus clarkii bouvieri Jordan and Gilbert, 1883) population in Yellowstone Lake, Yellowstone National Park, Wyoming, USA, is in decline because of competition from the introduced, invasive Lake Trout (Salvelinus namaycush Walbaum in Artedi, 1792). Gillnetting is used to suppress adult Lake Trout; however, methods are being developed to suppress embryos, including adding Lake Trout carcasses and carcass-analog pellets to spawning sites. Decomposing carcasses and analog pellets cause decreased dissolved oxygen concentrations thereby leading to Lake Trout embryo mortality, but the effects of these methods on primary producers are unknown. We deployed in-situ nutrient diffusing substrates (NDS) at 3 spawning sites. The 1st site was treated with carcasses, the 2nd site was treated with analog pellets, and a 3rd lacked treatment (control). To estimate how suppression measures may alter nutrient limitation, we measured algal biomass in 6 NDS amendments at each site: nothing (control), N, P, N + P, ground carcasses, or pulverized analog pellets. We deployed 5 replicates of each amendment at each site before and after treating whole sites. N and P co-limited periphyton before carcasses or analog pellets were added to spawning sites (p < 0.01); however, nutrients were not limiting after the treatments were added to spawning sites (p = 0.31–1). Algal biomass was 4× higher after whole-site carcass treatments. In contrast, analog pellets appeared to suppress algal biomass in the amendments (20% of NDS at the control site post-treatment) and in the treatment plot (33% of pre-treatment biomass at analog pellet site). We also measured how individual ingredients in analog pellets altered periphyton biomass, which suggested that vitamin E, estrogen, and soybean oil ingredients reduced the growth of primary producers. Suppression methods may stimulate or reduce algal biomass, depending on the methods used, which could have cascading effects on food webs and potentially reduce the success of the control measures. Estimating how different Lake Trout suppression methods may alter basal resources in the littoral zone of Yellowstone Lake will help natural resource agencies develop effective plans to control invasive predators at early life stages while minimally altering ecosystems.

Nutrient limitation of algae and macrophytes in streams: Integrating laboratory bioassays, field experiments, and field data.

Successful eutrophication control strategies need to address the limiting nutrient. We conducted a battery of laboratory and in situ nutrient-limitation tests with waters collected from 9 streams in an agricultural region of the upper Snake River basin, Idaho, USA. Laboratory tests used the green alga Raphidocelis subcapitata, the macrophyte Lemna minor (duckweed) with native epiphytes, and in situ nutrient-limitation tests of periphyton were conducted with nutrient-diffusing substrates (NDS). In the duckweed/epiphyte test, P saturation occurred when concentrations reached about 100 μg/L. Chlorophyll a in epiphytic periphyton was stimulated at low P additions and by about 100 μg/L P, epiphytic periphyton chlorophyll a appeared to be P saturated. Both duckweed and epiphyte response patterns with total N were weaker but suggested a growth stimulation threshold for duckweed when total N concentrations exceeded about 300 μg/L and approached saturation at the highest N concentration tested, 1300 μg/L. Nutrient uptake by epiphytes and macrophytes removed up to 70 and 90% of the N and P, respectively. The green algae and the NDS nutrient-limitation test results were mostly congruent; N and P co-limitation was the most frequent result for both test series. Across all tests, when N:P molar ratios >30 (mass ratios >14), algae or macrophyte growth was P limited; N limitation was observed at N:P molar ratios up to 23 (mass ratios up to 10). A comparison of ambient periphyton chlorophyll a concentrations with chlorophyll a accrued on control artificial substrates in N-limited streams, suggests that total N concentrations associated with a periphyton chlorophyll a benchmark for desirable or undesirable conditions for recreation would be about 600 to 1000 μg/L total N, respectively. For P-limited streams, the corresponding benchmark concentrations were about 50 to 90 μg/L total P, respectively. Our approach of integrating controlled experiments and matched biomonitoring field surveys was cost effective and more informative than either approach alone.

Spatial Variability in Light Yields Colimitation of Primary Production by Both Light and Nutrients in a Forested Stream Ecosystem

Colimitation of primary production is increasingly recognized as a dominant process across aquatic and terrestrial ecosystems. In streams, both nutrient availability and light availability have been shown to independently limit primary production, but colimitation by both light and nutrients is rarely considered. We used a series of nutrient-diffusing substrates (NDS) bioassays deployed across a range of light availability conditions in a single-study stream over two summers to determine the light level at which the limiting factor for benthic periphyton accrual transitioned from light to nutrients. Stream periphyton accrual was nutrient-limited in high-light patches, and light-limited in low-light patches, with the transition from being predominantly light-limited to being predominantly nutrient-limited occurring when daily light fluxes exceeded 3.5 mol m−2 day−1. We quantified light at each NDS bioassay location and at 5 m intervals throughout our two adjacent 160 m study reaches—one in structurally complex old-growth riparian forest and one bordered by more uniform second-growth forest. Although both reaches were colimited overall, the resource (light or nutrients) dominating limitation differed between the two riparian forest age/structure conditions. In the old-growth section, about three quarters of the reach was predominantly nutrient-limited, whereas in the second-growth reach only about a quarter of the streambed was nutrient-limited. In this stream, colimitation of benthic periphyton accrual by light and nutrients at the reach scale was an emergent property of the ecosystem that manifested as a result of high heterogeneity in riparian forest structure.

Context dependency of top-down and bottom-up effects in a Northern Australian tropical river

AbstractTop-down and bottom-up forces (consumer and resource limitation, respectively) influence biomass of primary producers and primary consumers in natural food webs. Few investigators experimentally examine both in concert, especially in the tropics. Tropical systems probably are more sensitive than temperate systems to eutrophication and other disturbances, such as destruction of riparian canopy cover, because of wide windows of ecological opportunity. We experimentally examined the relative importance of top-down and bottom-up effects at 2 sites in the Edith River in the Australian wet–dry tropics during baseflow conditions. We used large fish-exclusion cages to examine top-down effects of fish, and we used nutrient-diffusing substrates to manipulate bottom-up effects of nutrients. Macroinvertebrates were significantly more abundant in fish-exclusion cages than in open control areas at both sites. At 1 site, chlorophyll a on rock substrate was 1.5× greater and ash-free dry mass was 1.4× greater in f...

Aggregated filter‐feeding consumers alter nutrient limitation: consequences for ecosystem and community dynamics

Nutrient cycling is a key process linking organisms in ecosystems. This is especially apparent in stream environments in which nutrients are taken up readily and cycled through the system in a downstream trajectory. Ecological stoichiometry predicts that biogeochemical cycles of different elements are interdependent because the organisms that drive these cycles require fixed ratios of nutrients. There is growing recognition that animals play an important role in biogeochemical cycling across ecosystems. In particular, dense aggregations of consumers can create biogeochemical hotspots in aquatic ecosystems via nutrient translocation. We predicted that filter-feeding freshwater mussels, which occur as speciose, high-biomass aggregates, would create biogeochemical hotspots in streams by altering nutrient limitation and algal dynamics. In a field study, we manipulated nitrogen and phosphorus using nutrient-diffusing substrates in areas with high and low mussel abundance, recorded algal growth and community composition, and determined in situ mussel excretion stoichiometry at 18 sites in three rivers (Kiamichi, Little, and Mountain Fork Rivers, south-central United States). Our results indicate that mussels greatly influence ecosystem processes by modifying the nutrients that limit primary productivity. Sites without mussels were N-limited with -26% higher relative abundances of N-fixing blue-green algae, while sites with high mussel densities were co-limited (N and P) and dominated by diatoms. These results corroborated the results of our excretion experiments; our path analysis indicated that mussel excretion has a strong influence on stream water column N:P. Due to the high N:P of mussel excretion, strict N-limitation was alleviated, and the system switched to being co-limited by both N and P. This shows that translocation of nutrients by mussel aggregations is important to nutrient dynamics and algal species composition in these rivers. Our study highlights the importance of consumers and this imperiled faunal group on nutrient cycling and community dynamics in aquatic ecosystems.

Bottom-up and top-down effects on algal community dynamics in an aridland river: the Middle Rio Grande, New Mexico*

Abstract Background/Question/Methods In this study, we examined interactive effects of nutrient availability, river flow and food web structure on algal growth and species composition in the Middle Rio Grande, New Mexico. Algal community dynamics can be affected by both bottom-up and top-down factors. Algal production may be limited by low levels of nutrients or low light availability. Conversely, production is expected to increase when nutrient levels are high and light availability is not limited. Top-down factors such as grazing by invertebrates can also alter periphyton growth and community composition. Generally, a reduction in grazers results in periphyton biomass increases and shifts in growth forms (to more upright taxa).Nutrient diffusing substrates (NDS) were used in combination with grazer exclusion to investigate bottom-up and top-down effects on periphyton growth and species composition. Previously, NDS has been used in small, headwater streams and not in large sandy rivers like the Middle Rio Grande. NDS were made from inverted terracotta saucers, with four nutrient treatments – control, N, P and N+P. Large invertebrates were excluded from a subset of the saucers using an electrical field. The saucers were placed in a slow-flowing area of the Middle Rio Grande, and samples were collected weekly for a month. Results/Conclusions River conditions made it challenging to conduct the NDS experiment. Water levels were variable and turbidity levels were extremely high (&gt;4000NTU). Algal growth was extremely low on all treatments, largely due to light limitation caused by consistently high turbidity levels and a series of flooding and drying events which also decimated organismal growth on the substrates. Initial surveys of the diatom communities indicate extremely low densities and chlorophyll biomass was very low (mean ± se = 0.31 ± 0.09mg/m2). While dense periphyton communities did not develop on the NDS saucers, there was evidence of colonization by invertebrate populations. Blackfly larvae (Simuliidae) were most abundant in earlier samples whereas chironomid larvae (Chironomidae) were more abundant later in the experiment. Also, blackfly larvae were significantly more abundant on control saucers than on N+P saucers in the grazer/non-grazer experiment (P = 0.044). However, it was difficult to detect differences in the abundance of invertebrate grazers among nutrient treatments. Relationships among invertebrate population densities and the availability and diversity of periphyton for grazing will be described. These results are not definitive, but there are indications that nutrients, flow and grazers do influence algal community dynamics.

Bottom-up and top-down effects on algal community dynamics in an aridland river: the Middle Rio Grande, New Mexico*

In this study, we examined interactive effects of nutrient availability, river flow and food web structure on algal growth and species composition in the Middle Rio Grande, New Mexico. Algal community dynamics can be affected by both bottom-up and top-down factors. Algal production may be limited by low levels of nutrients or low light availability. Conversely, production is expected to increase when nutrient levels are high and light availability is not limited. Top-down factors such as grazing by invertebrates can also alter periphyton growth and community composition. Generally, a reduction in grazers results in periphyton biomass increases and shifts in growth forms (to more upright taxa).Nutrient diffusing substrates (NDS) were used in combination with grazer exclusion to investigate bottom-up and top-down effects on periphyton growth and species composition. Previously, NDS has been used in small, headwater streams and not in large sandy rivers like the Middle Rio Grande. NDS were made from inverted terracotta saucers, with four nutrient treatments – control, N, P and N+P. Large invertebrates were excluded from a subset of the saucers using an electrical field. The saucers were placed in a slow-flowing area of the Middle Rio Grande, and samples were collected weekly for a month. River conditions made it challenging to conduct the NDS experiment. Water levels were variable and turbidity levels were extremely high (>4000NTU). Algal growth was extremely low on all treatments, largely due to light limitation caused by consistently high turbidity levels and a series of flooding and drying events which also decimated organismal growth on the substrates. Initial surveys of the diatom communities indicate extremely low densities and chlorophyll biomass was very low (mean ± se = 0.31 ± 0.09mg/m2). While dense periphyton communities did not develop on the NDS saucers, there was evidence of colonization by invertebrate populations. Blackfly larvae (Simuliidae) were most abundant in earlier samples whereas chironomid larvae (Chironomidae) were more abundant later in the experiment. Also, blackfly larvae were significantly more abundant on control saucers than on N+P saucers in the grazer/non-grazer experiment (P = 0.044). However, it was difficult to detect differences in the abundance of invertebrate grazers among nutrient treatments. Relationships among invertebrate population densities and the availability and diversity of periphyton for grazing will be described. These results are not definitive, but there are indications that nutrients, flow and grazers do influence algal community dynamics.

Effects of temperature and concentration on nutrient release rates from nutrient diffusing substrates

Nutrient diffusing substrates (NDS) are an important tool for evaluating periphyton nutrient limitation. The rate at which nutrients are released from NDS depends on both the initial nutrient concentration and the length of time that NDS are in place. Whether temperature also affects nutrient release rates from NDS is unclear. However, this information is important because temperature effects on release rates could confound experimental results for NDS-based experiments testing rates of accumulation of periphyton biomass when stream water temperature is variable. We measured N and P release rates from NDS vials with 3 initial concentrations (0.05, 0.1, and 0.5 mol/L) of nutrients at 3 temperatures (4, 15, and 218C) for 21 d. Release rates of both nutrients were greater for vials with higher nutrient concentrations and for vials at warmer temperatures. For all concentrations, release rates decreased log linearly with time, a result that might have important implications for patterns of colonization and subsequent interspecific interactions within the periphyton community. In our opinion, temperature-caused differences in release rates are not biologically important because the differences were much smaller (3%) than expected changes in periphyton maximum growth rates over similar temperature ranges (;300%). Our results suggest that seasonal and site-related differences in temperature will not significantly affect nutrient release rates within the range of temperatures we tested, but researchers should consider nutrient concentration carefully when planning studies using NDS.

Epilithic algal abundance in relation to anthropogenic changes in phosphorus bioavailability and limitation in mountain rivers

Low-level cultural eutrophication (0.1–3.8 µ·L–1 increase in total phosphorus (TP)) of oligotrophic mountain rivers resulted in 4- to 30-fold increases in benthic algal abundance. Because anthropogenic P was more bioavailable than naturally occurring P, there were higher algal abundances downstream relative to upstream of nutrient point sources at a given P concentration. Neither TP nor soluble reactive P concentrations were indicative of P bio availability. Of the measures studied, epilithic alkaline phosphatase activity was most strongly correlated with algal abundance, most indicative of P bioavailability and thus the most precise indicator of P limitation. Although changes in dissolved inorganic nitrogen (DIN) to P ratios in river water and carbon (C) to P ratios in epilithon were consistent with changes in algal abundance and nutrient limitation, published water DIN to TP and tissue C to P ratio thresholds did not always yield accurate predictions of the type or degree of nutrient limitation. Epilithic N to P ratios and algal growth on nutrient-diffusing substrates were also inexact measures of epilithic nutrient limitation but, unlike other measures, were not strongly correlated with algal abundance. Thus, the predictability of the benthic algal response to anthropogenic nutrient additions in oligotrophic rivers will be improved by using measures indicative of both nutrient limitation and bioavailability.

Foodweb Response to Nutrient Enrichment in Rural and Urban Streams

ABSTRACT Nutrient-diffusing substrates were used to examine nutrient limitation in benthic periphyton and invertebrate grazers, in two rural and three urban stream reaches. Substrates released either nitrate, phosphate, both nutrients, or no nutrients, and were colonized during a five week period. Periphyton biomass did not respond to nutrient addition at any of the sites, while the densities of chironomids and other collector-gatherers showed significant responses to nutrient manipulation. At both rural sites and one urban site, the density of collector-gatherers showed a positive response to phosphate addition. Despite a six-fold range of phosphate (from 5 to 36 μg/1) and relatively invariant nitrate levels (ca. 3 mg/l) across sites, average concentrations of dissolved nutrients in streamwater at baseflow did not predict the pattern of response to nutrient addition. The numerical response of invertebrates to nutrient amendment suggests that the invertebrate fauna of both urban and rural streams are effective in controlling algal biomass and that even at moderately high concentrations, nutrients can limit benthic food webs.

INTERACTIONS BETWEEN HERBIVOROUS FISHES AND LIMITING NUTRIENTS IN A TROPICAL STREAM ECOSYSTEM

Ecologists have long been interested in understanding the strengths of con- sumer and resource limitation in influencing communities. Here we ask three questions concerning the relative importance of nutrients and grazing fishes to primary producers of a tropical Andean stream: (1) Are stream algae nutrient limited? (2) Are top-down and bottom-up forces of dual importance in limiting primary producers? (3) Do grazing fishes modulate the degree of resource limitation? We obtained several lines of evidence suggesting that Andean stream algae are nitrogen limited. Addition of nitrogen in flow-through channels resulted in major increases in algal standing crop, whereas there were no measurable effects of phosphorus enrichment. Inter- estingly, the N2-fixing cyanobacteria Anabaena was one of the taxa that responded most dramatically to the addition of nitrogen. Moreover, nutrient uptake rates were significantly higher for inorganic nitrogen (NO3-N and NH4-N) compared to phosphorus (PO4-P). Nutrients and the presence of grazing fishes were manipulated simultaneously in a series of experiments by using nutrient-diffusing substrates in fish exclusions vs. open cages accessible to the natural fish assemblage. We observed strong effects of both nitrogen addition and consumers on algal standing crop, although consumer limitation was found to be of considerably greater magnitude than resource limitation in influencing algal biomass and composition. Finally, the degree of resource limitation varied as a consequence of grazing fishes. Experiments examining nutrient limitation in the presence and absence of fishes showed that the response to nitrogen enrichment was significantly greater on sub- strates accessible to natural fish assemblages compared to substrates where grazing fishes were excluded. These experiments demonstrate simultaneous and interactive effects of top- down and bottom-up factors in limiting primary producers of tropical Andean streams. Whereas other studies have shown that consumers affect nutrient supply in ecosystems, our findings suggest that consumers can play an important role in influencing nutrient

Effects of shade and nutrient manipulation on periphyton growth in a subtropical stream

We used artificial substrates in an open pasture stream in south-east Queensland, Australia, to test (a) the relative importance of shade vs. nutrients on periphyton growth, and (b) whether nitrogen or phosphorus (or both) are limiting. Nutrient-diffusing substrates consisting of agar solutions with three nutrient treatments (N, P, N + P) were positioned in three shade treatments (0, 50%, 90%) and replicated at five sites. Periphyton communities sampled from substrates of all treatments after four and seven weeks were dominated by diatoms. Chlorophyll a and ash-free dry mass values of periphyton in both shaded treatments were similar to those in open sites. However, the addition of nitrogen resulted in significantly higher chlorophyll a levels compared with phosphorus-enriched substrates and control substrates. After seven weeks, lowest chlorophyll a values were found on phosphorus-enriched substrates. Nutrient additions did not have any measurable effects on periphyton ash-free dry mass values. Nitrogen rather than shade appears to be the primary factor limiting periphyton growth in this open pasture stream.

Patterns of nutrient release from nutrient diffusing substrates in flowing water

As nutrient diffusing substrates age, the availability of nutrients to periphyton may decline with time either because of diffusion or dilution of nutrients into the water column or because of the effects of grazing by herbivores. Typically, large amounts of nutrients are added to nutrient diffusing substrates (NDS) to insure continuous enrichment throughout experimental periods of 2 to 8 weeks. This study examined the release of phosphates and nitrates from NDS exposed to three different current velocities (0.07 m s−1, 0.11 m s−1, 0.20 m s−1) in recirculating laboratory flumes. Replicated agar samples from four treatments (control, nitrate (N), phosphate (P), and N+P) were sampled throughout 32 days (day 1, 2, 3, 6, 12, 18, 24, 32). Increasing concentrations of agar were required to solidify the P and N+P treatments.

Responses of chlorophyll-a, organic matter, and macroinvertebrates to nutrient additions in rivers flowing through agricultural and forested land

I conducted field studies to determine if chlorophyll-a, organic matter, and macroinvertebrates would respond to nutrient changes in rivers in agricultural and forested areas of the Eastern Deciduous Forest in southwestern Ontario, Canada. Nutrient manipulations were examined using nutrient diffusing substrates (NDS) modified to permit independent sampling units. Results of a 5-6 week field experiment conducted in three forested and three agricultural rivers during mid-summer showed that concentrations of chlorophyll-a colonizing NDS varied among rivers and between land use types. As expected, concentrations of chlorophyll-a were higher in rivers flowing through agricultural than forested rivers. Variation among rivers within land use type was greater than between land use types in terms of organic matter and invertebrate responses to nutrient amendments. The NDS that were spiked with phosphate contained consistently higher quantities of phosphate than control or nitrate treatments after incubation. There were no significant differences in nitrate concentrations in NDS among treatments after incubation. Chironomids were the most abundant invertebrates that colonized NDS. Of the organisms examined, only one taxon (Tipulidae) appeared to be phosphate limited in forested rivers. Changes in the macroinvertebrate community would be expected after sustained additions of nutrients. The lack of significant differences between macroinvertebrates inhabiting natural rocks and those colonizing control NDS suggests that similarly designed substrata could be used to monitor macroinvertebrates in microhabitats.

Periphyton Patch Preference in Grazing Chironomid Larvae

We determined the feeding preference of chironomid larvae (Paratanytarsus dubius) by observing dispersal of larvae among patches differing in periphyton composition. Nutrient-diffusing substrates were used to generate four algal patch types, which differed in the abundance of several diatom species and the green alga Stigeoclonium. Patch preference of larvae was negatively correlated with abundance of Stigeoclonium, chlorophyll a concentration, and algal biovolume. Preference was positively correlated only with algal diversity. At a density of 0.66 individual/cm² on preferred patches, larvae did not significantly reduce the chlorophyll a content of patches. Chironomid grazing had no effect on taxon diversity of periphyton assemblages. Data on the relationship between periphyton preference and grazer growth and reproduction are necessary to assess the importance of periphyton patch preference in natural populations.

Variation in nutrient limitation of lotic and lentic algal communities in a Texas (USA) river

Nutrient limitation of periphyton and phytoplankton was assessed in the Upper Guadalupe River, Texas USA. Nutrient-diffusing substrates with added nitrogen (N) and phosphorus (P) were used to identify the limiting nutrient for lotic algae at three river sites in summer, fall, and winter. Pots enriched with P had significantly higher chlorophyll a concentrations for 7 of 9 trials. Added N alone did not significantly increase algal standing crops, although it was found to be secondarily limiting on one (and possibly two) occasions. Flow-through enrichment experiments were conducted in order to quantify the concentration of P needed to significantly increase algal standing crops. Response to enrichment was rapid when ambient P concentration was low (< 0.010 mg L−1), but more moderate when ambient P levels were higher (0.015–0.025 mg L−1). Nutrient limitation of phytoplankton in small surface-release reservoirs varied throughout the study, but N was either primarily or secondarily limiting in 6 of 8 trials; shifts in the limiting nutrient were correlated with fluctuations in flow into the reservoirs. Our enrichment studies show that algal response to nutrient addition was unpredictable as phytoplankton tended to be N-limited while periphyton was mainly P-limited. Further, while discharge apparently dictated the nutrient-biomass relationship for phytoplankton in reservoirs, ambient nutrient level is an important determinant of lotic periphyton response to enrichment.

Nitrogen Limitation in a Sonoran Desert Stream

Four nutrient enrichment bioassay experiments were conducted in Sycamore Creek, Arizona, during summer and autumn 1983. In two experiments, nitrogen and phosphorus were added alone and in combination while in the other experiments nitrogen was added singly. In experiments involving enrichment of both nutrient-diffusing substrates (clay flowerpots) and streamwater overlying tile/gravel artificial substrates, nitrogen enrichment significantly enhanced rates of chlorophyll a accrual, primary production, and nitrogen uptake. Addition of phosphorus either singly or in combination with nitrogen did not result in significant responses of these parameters; thus ambient concentrations of phosphorus were above limiting levels, even when excess nitrogen was supplied. Nitrogen additions stimulated periphyton growth when background nitrate-N concentrations were ≤0.055 mg/L. We propose that nitrogen limitation is common in the desert Southwest since concentrations lower than this and atomic nitrogen to phosphorus ratio...