Epilithic Algal Biomass Research Articles

Interactions between light and wave exposure differentially affect epilithic algal biomass and productivity in two large lakes of different trophy

Light and nutrient availability determine the depth distribution of littoral attached algae, but erosional forces, such as wind waves, structure shallow, nearshore habitats. We measured the biomass and productivity of algae on cobbles (epilithon) in the wave-influenced erosional zones of two large lakes with contrasting trophic status, eutrophic Lake Erie and oligotrophic Lake Huron. We used fetch and wind speed to create an index of wave exposure for 10 sample sites and used regression models to determine the variation in algal biomass and productivity explained by physicochemical parameters. Lake Erie epilithon had greater biomass (average = 59–249 mg chlorophyll a m−2) and higher productivity (76–92 mg C m−2 hr−1) than Lake Huron epilithon (1–147 mg m−2 and 14–57 mg C m−2 hr−1, respectively). In Lake Erie, light and wave exposure had significant and opposite effects on algal biomass and productivity. Maximum algal biomass occurred between 1.5 and 3 m in Lake Erie, where the effects of waves begin to diminish and light was still relatively abundant. Conversely, Lake Huron biomass was uniformly low along the depth gradient. Although depth or wave exposure did not explain variation in biomass, Lake Huron epilithic productivity was best explained by the interaction between depth and wave exposure. Rocky nearshore areas in large lakes are dynamic habitats for attached algae, leading to high spatial variation in biomass and productivity. Understanding how physicochemical dynamics influence benthic algal biomass and productivity are necessary to predict their distributions in a changing world.

Spatial and seasonal changes in a naturally acidified river (Río Agrio, Argentina) and their effect on epilithic algae

ABSTRACT This study investigated seasonal changes in the longitudinal pH gradient of a naturally acidified river and how the attributes of epilithic algae (biomass, abundance, diversity) react to these variations. Five sites were selected on the Río Agrio river for in situ measurements, water sampling, and epilithic algae collection in different seasons from 2007 to 2018. Water pH increased along the length of the river, the average increase being highest in summer (from 2.5 to 7.0, acidity to neutrality) and lowest in spring (from 2.8 to 4.0, acidity to acidity). Conductivity and nutrient and ion concentrations decreased along the length of the river, showing a seasonal pattern of variation mainly influenced by the hydrological cycle. Epilithic algal biomass varied seasonally along the river, generally increasing in summer and decreasing in spring, with seasonal maximum values at different locations mainly associated with the dynamics of iron precipitation and nutrient availability. Species diversity and numbers were low at the most acidic site (regardless of season), but in the remaining sites these attributes tended to increase during summer–autumn and decrease during winter–spring. Statistical analysis showed that differences between sites responded mainly to the chemical gradient of the river and seasonality. Therefore, seasonal changes in the longitudinal chemical gradient of the river imposed differential, changing conditions on the epilithic algae present in each site, allowing identification of characteristic zones and their respective epilithic algal attributes. In the context of proposed models of ecosystem response to stress, similarities with systems acidified by anthropic activities are discussed.

The use of epilithic diatoms grown on artificial substrata to indicate water quality changes in the lower reaches of the St Lucia Estuary, South Africa

In shallow estuaries, artificial substrates provide a means of assessing the response of the diatom community to water chemistry. The shallow St Lucia Estuary was historically connected to the Mfolozi River at the mouth. This connection was severed during the early 1950s due to sediment input from the agriculturally impacted Mfolozi River. A connection has recently been re-established and the potential impact of dissolved inorganic nutrients from the Mfolozi River needs to be determined, as it may alter the ecological integrity of the St Lucia Estuary which forms part of a UNESCO World Heritage Site. This study found that the epilithic algal biomass and diatom species composition grown on glass slides were good indicators of nutrient enrichment in the estuary. Over a 28-day study period an above-average rainfall event resulted in an increase of nutrient-enriched freshwater flow into the estuary. No significant biomass differences were recorded in either the natural phytoplankton or microphytobenthos communities because of high variability. By contrast, the epilithic algal biomass accumulation on the glass slides was highest following the freshwater input. Statistical analyses indicated that the accumulation of the epilithic community on the glass slides was mostly affected by DIN and salinity. Diversity index scores for both the natural phytoplankton and microphytobenthos were variable, while the epilithic diatom index scores consistently declined from Day 7 (H’ = 1.2) to Day 28 (H’ = 0.7). These data indicate that epilithic algae grown on glass slides can be used as an effective monitoring tool to detect nutrient-induced changes as a supplementary method in this highly variable estuary.

Inverse relationship of epilithic algae and pelagic phosphorus in unproductive lakes: Roles of N2 fixers and light

Abstract Phosphorus (P) often limits the biomass of primary producers in freshwater lakes. However, in unproductive northern lakes, where anthropogenic nitrogen (N) deposition is low, N instead of P can limit primary producers. In addition, light can be limiting to primary producers at high concentrations of coloured dissolved organic matter (cDOM), as cDOM is the major determinant of light penetration in these lakes. To address resource limitation of epilithic algal biomass, we repeatedly sampled epilithon (periphyton on stony substrata) in 20 lakes covering a large, correlated cDOM and N‐deposition gradient across boreal and subarctic Sweden. Across these lakes, pelagic total N (TN) and total P (TP) were positively correlated, and benthic light supply was negatively correlated, with cDOM. Microscopically determined algal biovolume and epilithic carbon (C), N and P were subsequently regressed against benthic light supply and pelagic TN and TP. Patterns in epilithic biovolume were driven by N2‐fixing cyanobacteria, which accounted for 2%–90% of total epilithic biovolume. Averaged over the growing season, epilithic algal biovolume, C and N were negatively related to TP and positively to TN, and were highest in the clearest, most phosphorus‐poor lakes, where epilithon was heavily dominated by potentially N2‐fixing cyanobacteria. A structural equation model supports the hypothesis that cDOM had two counteracting effects on total epilithic algal biovolume: a positive one by providing N to algae that depend on dissolved N for growth, and a negative one by shading N2‐fixing cyanobacteria, with the negative effect being somewhat stronger. Together, these findings suggest that (1) light and N are the main resources limiting epilithic algal biomass in boreal to subarctic Swedish lakes, (2) epilithic cyanobacteria are more competitive in high‐light and low‐nitrogen environments, where their N2‐fixing ability allows them to reach high biomass, and (3) epilithic N increases with N2 fixer biomass and is—seemingly paradoxically—highest in the most oligotrophic lakes.

Factors Controlling Changes in Epilithic Algal Biomass in the Mountain Streams of Subtropical Taiwan.

In upstream reaches, epilithic algae are one of the major primary producers and their biomass may alter the energy flow of food webs in stream ecosystems. However, the overgrowth of epilithic algae may deteriorate water quality. In this study, the effects of environmental variables on epilithic algal biomass were examined at 5 monitoring sites in mountain streams of the Wuling basin of subtropical Taiwan over a 5-year period (2006–2011) by using a generalized additive model (GAM). Epilithic algal biomass and some variables observed at pristine sites obviously differed from those at the channelized stream with intensive agricultural activity. The results of the optimal GAM showed that water temperature, turbidity, current velocity, dissolved oxygen (DO), pH, and ammonium–N (NH4–N) were the main factors explaining seasonal variations of epilithic algal biomass in the streams. The change points of smoothing curves for velocity, DO, NH4–N, pH, turbidity, and water temperature were approximately 0.40 m s-1, 8.0 mg L-1, 0.01 mg L-1, 8.5, 0.60 NTU, and 15°C, respectively. When aforementioned variables were greater than relevant change points, epilithic algal biomass was increased with pH and water temperature, and decreased with water velocity, DO, turbidity, and NH4–N. These change points may serve as a framework for managing the growth of epilithic algae. Understanding the relationship between environmental variables and epilithic algal biomass can provide a useful approach for maintaining the functioning in stream ecosystems.

Bayesian Modeling of the Effects of Extreme Flooding and the Grazer Community on Algal Biomass Dynamics in a Monsoonal Taiwan Stream.

The effects of grazing and climate change on primary production have been studied widely, but seldom with mechanistic models. We used a Bayesian model to examine the effects of extreme weather and the invertebrate grazer community on epilithic algal biomass dynamics over 10years (from January 2004 to August 2013). Algal biomass and the invertebrate grazer community were monitored in the upstream drainage of the Dajia River in Taiwan, where extreme floods have been becoming more frequent. The biomass of epilithic algae changed, both seasonally and annually, and extreme flooding changed the growth and resistance to flow detachment of the algae. Invertebrate grazing pressure changes with the structure of the invertebrate grazer community, which, in turn, is affected by the flow regime. Invertebrate grazer community structure and extreme flooding both affected the dynamics of epilithic algae, but in different ways. Awareness of the interactions between algal communities and grazers/abiotic factors can help with the design of future studies and could facilitate the development of management programs for stream ecosystems.

The effects of storm-induced events on the seasonal dynamics of epilithic algal biomass in subtropical mountain streams

Information concerning the drivers of seasonal variation in algal biomass in subtropical mountain streams is limited. To identify the drivers of biomass dynamics for epilithic algae, a 20-month study was conducted in mountain streams in Taiwan, an area characterised by different levels of riparian vegetation coverage and agricultural activity, in which a process-based model was optimally fit to field data. We found that episodic typhoon-induced floods were the major drivers shaping the seasonal variations in algal biomass. Flow-induced detachment was frequently observed in periods of higher algal biomass. In contrast, an increased flow stimulated algal growth during periods with slower flow rates. Increased temperature stimulated algal growth at sites with an open canopy cover and higher light availability but constrained biomass at sites with dense canopy shading. Overall, scraper biomass exerted less influence on algal biomass than did environmental factors. The effects of grazing were visible only at the pristine, low-stream-order site in winter. The effects of minimal algal biomass required for recovery was comparable to environmental factors only at sites with intermediate canopy cover, moderate discharge, and higher nutrient concentrations. We suggest that agricultural activity and riparian vegetation can affect epilithic algal biomass in subtropical mountain streams.

Long-term moderate nutrient inputs enhance autotrophy in a forested Mediterranean stream

Summary 1. The effects of long-term nutrient addition at moderate levels were examined in the food web of a forested Mediterranean stream. Basal concentrations of N and P were increased twofold (to c. 750 μg N) and threefold (to c. 30 μg P) from ambient concentrations in an experimental reach. Variations in the abundance of microbes (bacteria and algae), meiofauna and macrofauna, microbial processing of organic matter (extracellular enzyme activities) and stoichiometry of biofilms and invertebrates were compared to an upstream control reach during 4 years of artificial nutrient enhancement. 2. Effects were faster in the bacterial compartment but more substantial in the algal compartment. Epilithic algal biomass doubled in the enriched section jointly triggered by nutrients and increased light irradiance in winter and early spring. Only a few animal groups reacted to the enrichment, including the meiofaunal Copepoda, linked to their high use of enriched FPOM, and macrofaunal grazers (Ancylus), which followed the large algal biomass increase. 3. The enrichment caused biofilm phosphatase activity to decrease, while activities related to the use of algal-related materials (peptidase, β-glucosidase) increased. Enzymatic activities related to the use of allochthonous organic matter showed only minor and episodic increases. 4. Changes in stoichiometric ratios were apparent in the epilithic compartment, but not in the sand sediment or in the FPOM. Increases in P content were delayed for 9 months in epilithic biofilms and for nearly 2 years in the case of N. 5. After 2 years of enrichment, the flatworm Schmidtea polychroa (predator), oligochaetes (detritivore) and tadpoles of Bufo bufo (grazer) showed higher per cent N. 6. Enrichment effects were produced in spite of flow cessations that occurred commonly in summer. The results show that forested streams subjected to sustained (though minor) nutrient enrichment changed aspects of their biological structure and metabolism and that changes were especially favoured by periods when light was not limiting.

Experimental evaluation of evolution and coevolution as agents of ecosystem change in Trinidadian streams

Evolution has been shown to be a critical determinant of ecological processes in some systems, but its importance relative to traditional ecological effects is not well known. In addition, almost nothing is known about the role of coevolution in shaping ecosystem function. Here, we experimentally evaluated the relative effects of species invasion (a traditional ecological effect), evolution and coevolution on ecosystem processes in Trinidadian streams. We manipulated the presence and population-of-origin of two common fish species, the guppy (Poecilia reticulata) and the killifish (Rivulus hartii). We measured epilithic algal biomass and accrual, aquatic invertebrate biomass, and detrital decomposition. Our results show that, for some ecosystem responses, the effects of evolution and coevolution were larger than the effects of species invasion. Guppy evolution in response to alternative predation regimes significantly influenced algal biomass and accrual rates. Guppies from a high-predation site caused an increase in algae relative to guppies from a low-predation site; algae effects were probably shaped by observed divergence in rates of nutrient excretion and algae consumption. Rivulus-guppy coevolution significantly influenced the biomass of aquatic invertebrates. Locally coevolved populations reduced invertebrate biomass relative to non-coevolved populations. These results challenge the general assumption that intraspecific diversity is a less critical determinant of ecosystem function than is interspecific diversity. Given existing evidence for contemporary evolution in these fish species, our findings suggest considerable potential for eco-evolutionary feedbacks to operate as populations adapt to natural or anthropogenic perturbations.

Epilithic chironomid larvae and water enrichment: is larval distribution explained by epilithon quantity or quality?

The effects of nutrient enrichment of stream water on epilithic chironomid larval assemblages were examined during spring in a Mediterranean catchment. Three reaches that differed in degree of enrichment were selected for comparison. Water chemistry, epilithon biomass (as ash-free dry mass (AFDM)), epilithic algal biomass (as chlorophyll a (chl a)), algal biovolumes, and algal composition were analyzed to determine their association with density and taxonomic composition of epilithic chironomids. Kendall's s coefficient of rank correlations and canonical correlation analysis were used to evaluate correlations between the chironomid and algal assemblages. Significant correlations were found between chironomid larval densities and total algal cell densities and biovolumes. Chironomid larval densities were not significantly correlated with chl a or AFDM. Several contrasting trends were identified in the relationships between chironomid and algal assemblages. Densities of fixed-tube scrapers, such as Eukiefferiella claripennis and Cricotopus bicinctus, were strongly correlated with biovolumes of Rhodophyceae and Diatomophyceae, whereas densities of free- living scrapers, such as Thienemaniella, were slightly correlated with Chlorophyceae biovolume. Therefore, nutrient enrichment strongly influenced epilithic chironomid assemblage structure through its effects on biovolumes and cell densities of the epilithic algal assemblages.

Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment

Summary1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood‐prone German stream.2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m2) to a depth of 15–20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near‐bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM).3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study.4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near‐bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters.5. Our results suggest that several ‘direct’ initial effects of local disturbance history on the invertebrates were subsequently replaced by ‘indirect’ effects of disturbance history (via disturbance‐induced changes in habitat parameters such as current velocity or food).

Indirect effects of brook trout (Salvelinus fontinalis) on the structure of epilithic algal communities in an oligotrophic boreal forest stream

The impact of fish on epilithic algae community structure was studied in a replicated series of artificial outdoor channels fed by a boreal forest stream (Canada) to test the hypotheses that brook trout (Salvelinus fontinalis) induce a three-trophic chain top-down change in epilithic community structure, at both taxonomic and physiognomic levels. Fish were introduced into five of ten channels and the epilithic algae growing on tiles were sampled four times during summer to determine biomass, algal physiognomy and species composition in relation to presence of fish and invertebrate community structure. Diatom biomass showed distinctive development patterns, increasing at the beginning of fish introduction and decreasing towards the end of the experiment. The responses were the reverse in the absence of fish. Chrysophyceae increased in the absence of fish, particularly at the end of the experiment. From the seven algal physiognomic types considered, epilithon overstory, attached erected algae and loosely attached algae were generally more abundant in the presence of fish. Species' biomass was markedly different in both treatments and followed contrasted temporal patterns. In the presence of fish, attached erected diatoms species rapidly reached a complex spatial overstory structure, while in the absence of fish the epilithic community remained at a younger successional stage. Partial redundancy analysis showed that those changes were mediated by large grazing invertebrates, which had lower abundance over the bottom surfaces in the presence of fish. Variance partitioning analysis reinforced the hypothesis that most of the variability in epilithic community structure was explained by fish-invertebrate interactions and that the effect of fish presence alone was not significant. The results show that reduction in total epilithic algal biomass was a transient top-down effect, but the three-trophic level control on major taxa, species composition and community physiognomy remained along the summer experiment.

Stream Communities Along a Catchment Land-Use Gradient: Subsidy-Stress Responses to Pastoral Development

When native grassland catchments are converted to pasture, the main effects on stream physicochemistry are usually related to increased nutrient concentrations and fine-sediment input. We predicted that increasing nutrient concentrations would produce a subsidy-stress response (where several ecological metrics first increase and then decrease at higher concentrations) and that increasing sediment cover of the streambed would produce a linear decline in stream health. We predicted that the net effect of agricultural development, estimated as percentage pastoral land cover, would have a nonlinear subsidy-stress or threshold pattern. In our suite of 21 New Zealand streams, epilithic algal biomass and invertebrate density and biomass were higher in catchments with a higher proportion of pastoral land cover, responding mainly to increased nutrient concentration. Invertebrate species richness had a linear, negative relationship with fine-sediment cover but was unrelated to nutrients or pastoral land cover. In accord with our predictions, several invertebrate stream health metrics (Ephemeroptera-Plecoptera-Trichoptera density and richness, New Zealand Macroinvertebrate Community Index, and percent abundance of noninsect taxa) had nonlinear relationships with pastoral land cover and nutrients. Most invertebrate health metrics usually had linear negative relationships with fine-sediment cover. In this region, stream health, as indicated by macroinvertebrates, primarily followed a subsidy-stress pattern with increasing pastoral development; management of these streams should focus on limiting development beyond the point where negative effects are seen.

Limitations on the effects of ultraviolet radiation on benthic algae in a clear boreal forest lake

We examined the effects of ultraviolet radiation (UVR) (280–400 nm) on epilithic algal metabolism, biomass, and taxonomy from early June to mid August 1999 (66 d) in Lake 224, an oligotrophic lake in the Experimental Lakes Area, northwestern Ontario, Canada. Epilithon colonized bare clay tiles placed under UVR-transparent (UVR+) and UVR-shielding (UVR−) acrylic filters, at depths of 0.5, 0.8, and 1.4 m. Filamentous chlorophytes and cyanophytes were the dominant algal taxa at 0.5 and 1.4 m depths, respectively, on day 28. Biovolumes did not differ between UVR treatments for any major algal class. We measured epilithic metabolism on days 28, 45, and 66 after the tiles were deployed. Photosynthesis was not affected by UVR on any date. On day 28, dark respiration was 33 to 49% greater in UVR+ than in UVR− treatments at all depths. On day 45, respiration was 11% greater in UVR+ than in UVR− treatments only at 0.5 m, and by day 66, respiration did not differ between treatments at any depth. Thus, the effects of UVR on respiration diminished with depth and over time and were strongest when epilithic biofilms were least developed. UVR probably has little impact on naturally colonized, fully developed epilithon deeper than 0.5 m in Lake 224, i.e., UVR affects <6% of the littoral zone. Comparison of our findings with those of an earlier study in Lake 224 suggests that the intensity and specific nature of UVR effects depend on the taxonomic composition of the benthic algal assemblage.

Examining primary producer–consumer interactions in a Lake Superior tributary using 15N-tracer, grazer-reduction, and nutrient-bioassay experiments

AbstractPrimary producer–consumer interactions and N dynamics were studied in a 3rd-order reach of Amity Creek, a Lake Superior tributary in northeastern Minnesota (mean discharge = 0.08 m3/s). A nutrient-limitation-bioassay experiment showed that primary production was co-limited by N and P. To evaluate the importance of grazers in controlling epilithic algal biomass, a grazer-reduction experiment was conducted using ceramic tile epilithon substrates (ambient grazing and low grazing treatments). Grazers significantly reduced epilithic biomass as measured by chlorophyll a, demonstrating that grazers constrain algal primary producers in this system. 15N-enriched NH4Cl was added to the stream for 6 wk, as a stable isotope tracer of N flow through producer and grazer compartments. A comparison of δ15N of tile epilithon, incubated for 4 wk during the 15N addition, to that of bulk epilithon collected from rock scrapings, showed that the thinner epilithon associated with tiles was much more enriched in 15N than...

Longitudinal distribution of macroinvertebrates in two glacier‐fed New Zealand rivers

1. Environmental variables, benthic algal biomass and macroinvertebrate fauna were examined from September 1999 to January 2000 (austral summer) along two glacier‐fed rivers in South Island, New Zealand.2. The rivers were characterized by high flow variability, high turbidity and physically disturbed beds. Water temperature ranged from &lt;1 °C near the glacier margin to 10 °C further downstream.3. Epilithic algal biomass was very low (&lt;0.1 mg m–2) in months characterized by heavy rainfall, but ranged from 1.1 to 14.4 mg m–2 following an extended period with negligible precipitation.4. Abundance and diversity of invertebrates in both rivers was low. Dominant taxa were Chironomidae (Orthocladiinae, Podonominae, Diamesinae), although mayfly species (Deleatidium: Leptophlebiidae) also occurred at most sites. A species of Eukiefferiella (Orthocladiinae) was collected at all sites and was the most abundant invertebrate close to the glacier margins. No meiofauna were found in either river.5. Faunal diversity increased at the lowermost stations where species of Plecoptera, Trichoptera, Coleoptera and non‐chironomid Diptera also occurred.6. The faunas of the two New Zealand rivers conformed to the conceptual model of Milner &amp; Petts (1994) in that taxon richness increased downstream with water temperature. However, invertebrate abundance increased downstream in only one of the two rivers. Also in contrast to the model predictions, Leptophlebiidae and Orthocladiinae, rather than Diamesinae, dominated the fauna at the coldest sites.

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

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

Trophic structure of three streams with contrasting riparian vegetation and geomorphology

The relative influence of riparian vegetation and geomorphology on trophic structure was examined at three streams in adjacent catchments of the French Massif Central mountains. The study sites differed mainly by the degree of anthropogenic alteration of riparian and watershed vegetation, and by valley geomorphology. Fishes, benthic macroinvertebrates (grouped into functional feeding groups: FFG), macrophytes and periphyton were sampled seasonally between July 1991 and April 1992. At the riffle-pool (10 0 m) scale, instream morphological units appear to control the spatial partitioning of trophic resources and their consumers. For example, depositional zones supported a specialized trophic structure based on coarse organic matter consumption, whereas in erosional zones (subdivided into lotic and lentic units) periphyton and fine organic matter were the main food source. At the reach-segment scale (10 1 to 10 2 m), valley morphology was the primary factor controlling the stream ecosystem, but anthropogenic alteration of riparian vegetation seems to override geomorphological controls on the trophic structure. For example, the open-canopied Triouzoune Creek (gentle plateau valley), characterized by scrapers (>60% by density) and a greater epilithic algal biomass, had a autotrophic-based structure. At this site, 20 % of total fish biomass was represented by brown trout (Salmo trutta) dominated by fish 50% by density) and fish communities dominated by brown trout with high densities of age 1+ individuals. Overall, the results emphasize the importance of riparian vegetation on the trophic structure of streams, especially when anthropogenic alteration of riparian vegetation is severe. We conclude that there is a need to better integrate riparian vegetation is severe. We conclude that there is a need to better integrate riparian vegetation into European stream management practices in order to maintain the vitality of these systems over the long-term.

Comparisons of Phytoplankton and Epilithic Algae of Selby Lake – Gates of the Arctic National Park and Preserve, Alaska

ABSTRACT Selby Lake in Gates of the Arctic National Park and Preserve was assessed as oligotrophic due to measurements of low nutrient concentrations and resultant low planktonic biomass as total chlorophyll. An orthograde oxygen profile in early summer also indicated an oligotrophic condition. Total nitrogen averaged about 0.4 μg·L−1 and total phosphorus about 4 μg·L−1, resulting in a TN:TP ratio of about 100, which indicated the planktonic algae were probably limited by phosphorus. Nutrient stimulation bioassays confirmed this. Nitrogen alone was not stimulating to plankton algal growth, but nitrogen and phosphorus added together in the bioassays stimulated growth more than just phosphorus, as is often found in oligotrophic lakes. Aerial observation of Selby Lake and similar nearby lakes had indicated that epilithic benthic algal biomass was probably relatively high because the extensive shallows were visibly green. When recent growth in the ice-scour zone (< 2 m) was sampled in early July, the whole lake average standing crop of epilithic algae was estimated as approximately equal to the whole lake standing crop of phytoplankton. Biomass was greater near inlets than away from inlets. Epilithic algal biomass varied inversely with the temperature of inlets and directly with the inlets' total nitrogen concentration. Root zone fixation of atmospheric nitrogen by alders may be the source of N in the enriched inlets, and cold inlets probably plunge to the lake bottom, bathing the epilithicalgae with nutrients. The importance of benthicalgae in clear-oligotrophic lakes with extensive shallows need to be recognized.

Benthic communities of forest streams in the South Island, New Zealand: effects of forest type and location

Algal biomass, standing stocks of particulate organic matter (POM) and benthic invertebrate communities of 20 streams draining native forest, exotic conifer forest and tussock grassland catchments in the South Island, New Zealand, were examined in spring/summer 1994-95. Sixteen were forest streams, 8 in native and 8 in exotic forest, with 4 of each kind located on each side of the Main Alpine Divide and therefore in different climatic (including rainfall) zones. Chemistry of stream water reflected geographic location more strongly than forest type with western streams being slightly more acid and having lower ionic concentrations than those in the east. Algal biomass on stones in forest streams ranged from 7-51mg chl.m -2 , and regardless of forest type, appeared to be limited primarily by light and substrate stability. POM standing stocks were low (22-254g AFDW m -2 ) and dominated by coarse materials (>13 mm) in both native and exotic forest streams. Benthic faunas of all stream groups were dominated numerically by Chironomidae and Ephemeroptera and contained similar numbers of species (35-40). However, differences in community composition were found between eastern and western streams, and these were greater than between forest types. Total invertebrate abundances were moderately low (2008- -6083 m -2 ) and were significantly correlated with epilithic algal biomass. In contrast, correlations between invertebrate numbers and detrital biomass were weak, perhaps reflecting poor retention of riparian inputs. Our results support the view that forest type has little effect on stream benthic communities in the South Island, New Zealand, and that geographical factors (climate, location) are of greater importance.