Native Algae Research Articles

Fractional conversion of microalgae from water blooms

Fractional conversion of natural algae cyanobacteria from Taihu Lake was conducted. The raw Taihu Lake algae (TLA) and pretreated samples were pyrolyzed at 290 °C and 450 °C according to the TGA results. Extraction of lipids or saccharides from the TLA was performed as a pretreatment to obtain lipid extracted algae (LEA) or saccharide extracted algae (SEA). The total yields of bio-oil from fractional pyrolysis were 40.9 wt% from TLA, 42.3 wt% from LEA, and 48.5 wt% from SEA. From TLA, the major components of the bio-oil were fatty acids, amides and hydrocarbons (heptadecane) at 290 °C whereas those at 450 °C were phenols and C10-C15 hydrocarbons. Following the lipid extraction, acids, amides and indoles accounted for a large proportion at 290 °C, while the main products obtained at 450 °C were phenols, indoles and pyrroles. It is worth mentioning that the yield of bio-oil from the LEA had increased, and the composition of the bio-oil was simplified. Moreover, the average molecular weight of the bio-oil obtained from LEA had decreased. Interestingly, the extraction of saccharides inhibited pyrolysis of the lipids, so the distribution of the bio-oil from SEA changed only a little. Fractional pyrolysis of pretreated microalgae not only increased the bio-oil yield but also improved the quality of the bio-oil.

Catalytic pyrolysis of natural algae over Mg-Al layered double oxides/ZSM-5 (MgAl-LDO/ZSM-5) for producing bio-oil with low nitrogen content

Cyanobacteria were catalytically pyrolyzed over Mg-Al layered double oxide/ZSM-5 composites (MgAl-LDO/ZSM-5) to produce bio-oil. MgAl-LDO/ZSM-5 with a Mg/Al molar ratio of four was proved to be the best catalyst. Under the optima condition that the final temperature was 823K, heating rate was 10K/min and catalyst/algae mass ratio was 0.75, a maximum yield of liquid (41.1%) was achieved at 823K with a heating rate of 10K/min and a catalyst/algae mass ratio of 0.75, which was much higher than the one obtained without catalyst. The element analysis results proved that this bio-oil had much lower O/C molar ratio and higher HHV. The GC–MS results showed that the bio-oil had less nitrogenous compounds. MgAl4-LDO/ZSM-5 was proved to be an applicable and effective catalyst to obtain bio-oil from catalytic pyrolysis of water-blooming algae.

Recruitment of the invasive kelp Undaria pinnatifida does not always benefit from disturbance to native algal communities in low-intertidal habitats

Disturbance can play an important role in accelerating the spread of invasive species in coastal habitats by increasing the availability of resources. Thus, reductions in native canopy cover often facilitate the establishment of invasive macroalgae. In this study, the invasion process of the kelp Undaria pinnatifida in disturbed patches on a rocky low-intertidal shore in central New Zealand was investigated. The study site was initially uninvaded, but adjacent to (~150 m) intertidal areas with established U. pinnatifida populations. Experimental plots (0.25 m2) were established that were either partially (50%) or totally (100%) cleared of native algal cover at two different times of year (summer: December 2008 and winter: June 2009). U. pinnatifida recruitment was monitored for 3, 6, 9 and 12 months for the summer run, and for 3 and 6 months for the winter run. Although the timing of first appearance varied across the two runs, we were unable to detect an effect of clearing native algae on U. pinnatifida recruitment. By the end of the experiment, U. pinnatifida had recruited into all plot treatment types at low densities (on average 1.4 plants 0.25 m−2). The fact that U. pinnatifida recruited into the uncleared control plots indicated that invasion of this kelp into new areas does not require disturbance of the native algal community. These results suggest that if light and space are not limiting resources, as is likely the case in this low-intertidal habitat, then disturbance may not facilitate U. pinnatifida recruitment.

정수장 유입조류 전처리를 위한 천연조류제거제(W.H.)의 최적주입농도 결정

정수장 유입조류 전처리를 위한 천연조류제거제(W.H.)의 최적주입농도 결정

The effect of low-temperature pretreatment on the solubilization and biomethane potential of microalgae biomass grown in synthetic and wastewater media

Microalgae have been suggested as a sustainable raw material for biofuel production in the form of methane via anaerobic digestion. Here, pretreatments at 60–80°C were investigated, aiming to study the impact of algae culture media on biomethane potential and pretreatment efficiency. Chlorella vulgaris and mixed culture of native algae species (dominating by Scenedesmus sp.) were grown in synthetic medium, wastewater (sterilized and non-sterilized) and digestate from anaerobic digestion of pulp and paper biosludge (sterilized and non-sterilized). The biomethane potential for native microalgal biomass varied between 154 and 252LCH4kg−1 VS depending on culture media. The efficiency of the low-temperature pretreatment (80°C, 3h) for solubilization (9–12%) of C. vulgaris and native algae biomass was similar for algae grown in sterilized and non-sterilized wastewater media. The pretreatment increased the biomethane potential of native algae biomass by 11–24%.

Enemy release an unlikely explanation for the invasive potential of the brown alga Sargassum muticum: experimental results, literature review and meta-analysis

The enemy release hypothesis predicts that non-indigenous species (NIS) are released from natural enemies (e.g. grazers etc.) in their new environment, thus facilitating invasion. Sargassum muticum is a conspicuous and successful invasive brown alga and several studies have investigated whether native herbivores feed less on S. muticum relative to native algae within the invaded range. Some of these studies have concluded that S. muticum is avoided by native herbivores, but this conclusion may be premature as the results are rather inconclusive and seem to depend partly on the type of algae included in the comparison. We conducted therefore a series of feeding experiments to test if Danish S. muticum is grazed less upon than a range of native algae by the sea urchin Psammechinus miliaris and complemented the experiments with a meta-analysis based on published data. In no-choice trials, P. miliaris consumed S. muticum faster than it consumed slow-growing Fucus vesiculosus and Halidrys siliquosa, whereas Saccharina latissima, Ceramium virgatum and Ulva intestinalis were grazed upon at higher or the same rates as S. muticum. More or less identical patterns were seen in the two- and multiple-choice experiments, where S. muticum was generally consumed faster than F. vesiculosus and H. siliquosa, but slower than S. latissima and the two most fast-growing algal species (U. intestinalis and C. virgatum). We screened the literature for comparable data and found 26 experiments with 27 species of algae and 14 species of invertebrate grazers. Meta-analysis on these data showed the same overall trend as observed in our experiments; S. muticum is generally consumed at the same rate or faster than other thick, leathery and canopy-forming algae, which are assumed to constitute the major competitors to S. muticum, but slower than more fast-growing sub-canopy species and more opportunistic algae. We question therefore that enemy release can explain the invasion success of S. muticum.

Cultivation of a native alga for biomass and biofuel accumulation in coal bed methane production water

Coal bed methane (CBM) production has resulted in thousands of ponds in the Powder River Basin of low-quality water in a water-challenged region. A green alga isolate, PW95, was isolated from a CBM production pond, and analysis of a partial ribosomal gene sequence indicated the isolate belongs to the Chlorococcaceae family. Different combinations of macro- and micronutrients were evaluated for PW95 growth in CBM water compared to a defined medium. A small level of growth was observed in unamended CBM water (0.15g/l), and biomass increased (2-fold) in amended CBM water or defined growth medium. The highest growth rate was observed in CBM water amended with both N and P, and the unamended CBM water displayed the lowest growth rate. The highest lipid content (27%) was observed in CBM water with nitrate, and a significant level of lipid accumulation was not observed in the defined growth medium. Growth analysis indicated that nitrate deprivation coincided with lipid accumulation in CBM production water, and lipid accumulation did not increase with additional phosphorus limitation. The presented results show that CBM production wastewater can be minimally amended and used for the cultivation of a native, lipid-accumulating alga.

Microwave-enhanced pyrolysis of natural algae from water blooms

Microwave-enhanced pyrolysis (MEP) of natural algae under different reaction conditions was carried out. The optimal conditions for bio-oil production were the following: algae particle size of 20-5 mesh, microwave power of 600W, and 10% of activated carbon as microwave absorber and catalyst. The maximum liquid yield obtained under N2, 10% H2/Ar, and CO2 atmosphere was 49.1%, 51.7%, and 54.3% respectively. The energy yield of bio-products was 216.7%, 236.9% and 208.7% respectively. More long chain fatty acids were converted into hydrocarbons by hydrodeoxygenation under 10% H2/Ar atmosphere assisted by microwave over activated carbon containing small amounts of metals. Under CO2 atmosphere, carboxylic acids (66.6%) were the main products in bio-oil because the existence of CO2 vastly inhibited the decarboxylation. The MEP of algae was quick and efficient for bio-oil production, which provided a way to not only ameliorate the environment but also obtain fuel or chemicals at the same time.

Zooplankton grazing on natural algae and bacteria under hypertrophic conditions

Zooplankton grazing on natural algae and bacteria under hypertrophic conditions The grazing characteristics of the natural zooplankton community in the shallow hypertrophic lagoon Albufera de Valencia (Spain) were compared in two periods of the annual cycle: (1) at the onset of spring, after a period of increased water flow through the lagoon that reduces phytoplankton density and cyanobacterial prevalence, and (2) in mid-spring, after the spring phytoplankton bloom and under conditions of cyanobacterial dominance. Clearance, ingestion and assimilation rates were measured by labelling natural seston with 14 C-bicarbonate and 3 H-thymidine/leucine. The clearance rates (CRs) of Daphnia magna, the dominant species at the onset of spring, were low, very likely as a consequence of the high abundance of filamentous cyanobacteria in the lagoon. The CRs were nearly equal for phytoplankton and bacteria, corroborating the unselective feeding of Daphnia. Bacteria supplied ≈40 % of the planktoniccarbon ingested byD. magna, and this carbon was assimilated with the same efficiencyas that provided by phytoplankton.This suggests a strong coupling betweenthe microbial loop and the classic grazer trophic web at the onset of spring in the lagoon and, consequently, an important contribution of the microbial loop to energy transfer to higher trophic levels (fish). The CRs of Bosmina longirostris, the dominant species in mid-spring, were also low probably because the food concentrations were well above the incipient limiting level. The CRs were higher for 1-15 µm algae than for <1 µm bacteria, indicating positive selective feeding towards larger cells. Algae sized 3-15 µm provided most of the planktonic carbon ingested by B. longirostris. Similarly, regarding the total mid-spring zooplankton community, phytoplankton contributed 90 % of the planktonic carbon ingested. This suggests that during the spring phytoplankton bloom when cyanobacteria dominate, the energy transferred to zooplankton via bacteria constitutes a smaller fraction than during periods of lower phytoplanktondensity. Finally, our results indicate that zooplankton consumed a very small proportion of phytoplankton and bacterioplankton production during the two studied periods, which highlights the reduced ability of zooplankton to control phytoplankton growth in hypertrophic systems.

Does cellulose II exist in native alga cell walls? Cellulose structure of Derbesia cell walls studied with SFG, IR and XRD

In nature, algae produce cellulose I where all glucan chains are aligned parallel. However, the presence of cellulose II with anti-parallel glucan chains has been reported for certain Derbesia (Chlorophyceae algae) cell walls; if this is true, it would mean a new biological process for synthesizing cellulose that has not yet been recognized. To answer this question, we examined cellulose structure in Derbesia cell walls, intact as well as treated with cellulose isolation procedures, using sum-frequency-generation spectroscopy, infrared (IR) spectroscopy and X-ray diffraction (XRD). Derbesia walls contain large amounts of mannan and small amounts of crystalline cellulose. Evidence for cellulose II in the intact cell walls was not found, whereas cellulose II in the trifluoroacetic acid (TFA) treated cell wall samples were detected by IR and XRD. A control experiment conducted with ball-milled Avicel cellulose samples showed that cellulose II structure could be formed as a result of TFA treatment and drying of amorphous cellulose. These data suggest that the cellulose II structure detected in the TFA-treated Derbesia gametophyte wall samples is most likely due to reorganization of amorphous cellulose during the sample preparation. Our results contradict the previous report of cellulose II in native alga cell walls. Even if the crystalline cellulose II exists in intact Derbesia gametophyte cell walls, its amount would be very small (below the detection limit) and thus biologically insignificant.

Transient effects of an invasive kelp on the community structure and primary productivity of an intertidal assemblage

Invasive species can have significant impacts on the diversity and productivity of recipient ecological communities. The kelp Undaria pinnatifida (Harvey) Suringar is one of the world’s most successful invasive species but, although its purported impacts are strong, there is little empirical evidence that it displaces native species. Furthermore, as this species naturalises in local communities, its potential effects on community dynamics have not been well tested. Here, we test the ecological impacts of Undaria in intertidal communities in southern New Zealand using a combination of surveys, a 2.5-year press-removal experiment and in situ measures of net primary production to gauge its impact on community structure and productivity. Undaria had transient effects on the composition of communities, affecting two seasonally abundant species in 1 year, but these impacts did not persist into the following year. Overall, there were only small effects of Undaria removal on diversity and abundance of native algae and invertebrates at two sites. However, the presence of Undaria more than doubled net primary production of recipient communities during its annual peak abundance when it increased biomass by 606gDWm–2. We conclude that the invasion of Undaria represents an additional and substantial carbon subsidy to coastal ecosystems with potentially positive effects on nearshore productivity.

The effect of irradiance on the xanthophyll composition of Skeletonema marinoi (Bacillariophycae), Teleaulax sp., Rhodomonas sp. (Cryptophyceae), and Heterocapsa triquetra (Dinophyceae)

Abstract The aim of the research was to determine the effect of irradiance on the content of carotenoids in the natural algae community occurring in the Baltic Sea: diatom Skeletonema marinoi, cryptophytes Teleaulax sp., Rhodomonas sp., and dinoflagellate Heterocapsa triquetra. In the natural population of Skeletonema marinoi, the highest fucoxanthin content was observed in the morning and afternoon, unlike with diatoxanthin+diadinoxanthin, where a mean of 0.008 pg cell-1 was found at dawn and at dusk, whereas maximum values were observed at noon (mean 0.017 pg cell-1). Similar tendencies related to diurnal variations in the content of xanthophylls involved in the xanthophyll cycle occurred also in dinoflagellate Heterocapsa triquetra. In cryptophytes Teleaulax sp. and Rhodomonas sp., no carotenoids of the xanthophyll cycle were detected. The content of alloxanthin showed diurnal variation from 0.048 pg cell-1 to 0.085 pg cell-1 and was not clearly correlated with the irradiance.

The effect of irradiance and temperature on the photosynthesis of a native alga Sargassum fusiforme (Fucales) from Kagoshima, Japan

:The effect of irradiance and temperature on the photosynthesis of a Japanese native brown alga, Sargassum fusiforme, was determined by using dissolved oxygen sensors and pulse amplitude–modulated fluorometry. In situ measurements of the effective quantum yield (ΦPSII) decreased as irradiance increased at 12:00 PM and recovered in the late afternoon, indicating that a proportionately greater number of reaction centres were closed. Likewise, laboratory measurement after 6 hours of exposure to low and high (300 and 1000 μmol photons m−2 s−1, respectively) irradiances showed a relatively greater decrease in ΦPSII at high irradiance, and both treatments recovered overnight. A model of the net photosynthesis–irradiance relationship determined at 20°C revealed that the net photosynthetic rate quickly increased at irradiances below the estimated saturation irradiance of 391 μmol photons m−2 s−1 [95% Bayesian credible interval (BCI) of 299–515 μmol photons m−2 s−1]. Gross photosynthesis and dark respiration rates determined over a range of temperatures (8–36°C) revealed that the maximal gross photosynthetic rate was 24.1 (BCI: 21.5–26.8) μg O2 gww−1 min−1 at 22.9°C (BCI: 19.8–25.8), and the dark respiration rate increased nonlinearly from a low of 0.5 μg O2 gww−1 min−1 at 8°C to a high of 8.9 μg O2 gww−1 min−1 at 36°C. The highest value of the quantum yield (Fv/Fm = ΦPSII at 0 μmol photons m−2 s−1) was estimated to occur at 22.9°C (BCI: 21.8–23.9) and was 0.72 (BCI: 0.70–0.73). The specimens examined appeared to be well adapted to the natural temperature range of the study site. Although the irradiances on the fronds were strong enough to induce a drop in quantum yield, our evidence clearly suggests that full recovery occurs overnight.

Fractional pyrolysis of Cyanobacteria from water blooms over HZSM-5 for high quality bio-oil production

Fractional pyrolysis and one-step pyrolysis of natural algae Cyanobacteria from Taihu Lake were comparatively studied from 200 to 500 °C. One-step pyrolysis produced bio-oil with complex composition and low high heating value (HHV≤30.9 MJ/kg). Fractional pyrolysis separated the degradation of different components in Cyanobacteria and improved the selectivity to products in bio-oil. That is, acids at 200 °C, amides and acids at 300 °C, phenols and nitriles at 400 °C, and phenols at 500 °C, were got as main products, respectively. HZSM-5 could promote the dehydration, cracking and aromatization of pyrolytic intermediates in fractional pyrolysis. At optimal HZSM-5 catalyst dosage of 1.0 g, the selectivity to products and the quality of bio-oil were improved obviously. The main products in bio-oil changed to nitriles (47.2%) at 300 °C, indoles (51.3%) and phenols (36.3%) at 400 °C. The oxygen content was reduced to 7.2 wt% and 9.4 wt%, and the HHV was raised to 38.1 and 37.3 MJ/kg at 300 and 400 °C, respectively. Fractional catalytic pyrolysis was proposed to be an efficient method not only to provide a potential solution for alleviating environmental pressure from water blooms, but also to improve the selectivity to products and obtain high quality bio-oil.

Characterization of a Native Algae Species Chlamydomonas debaryana: Strain Selection, Bioremediation Ability, and Lipid Characterization

Native microalgal species may offer a favorable combination of both wastewater treatment and biofuel production. In this research, a green microalgae, Chlamydomonas debaryana, was isolated from a local lagoon, screened for its lipid content using flow cytometry, and further identified with microscopic observations and DNA sequence analysis. When using swine wastewater as a medium, the biomass yields were between 0.6 and 1.62 g/L, giving a median value of 1.11 g/L. By increasing mass transfer rates and providing sufficient light intensity, the microalgal growth was intrinsically enhanced. The growth of C. debaryana reduced most nutritional contents of the wastewater except iron. When combining the microalgal growth and nutrient removal, C. debaryana was able to utilize 1.3 to 1.6×103 mg COD (chemical oxygen demand)/g biomass, 55 to 90 ppm ammonia/g biomass, and 48 to 89 ppm phosphorous/g biomass, The lipid content of C. debaryana was 19.9 ± 4.3% of cell dry weight. The transesterified microalgal oil mostly consisted of 14 kinds of fatty acids, ranging from C5 to C22, which can be refined into renewable jet fuel or used as sources of omega-3 and omega-6 fatty acids.

Epifaunal community structure and ammonium uptake compared for the invasive algae, Gracilaria salicornia and Acanthophora specifera, and the native alga, Padina thivyi

Macroalgae play an important role in coastal ecosystems by taking up nutrients from the water column and providing habitats and food for epifauna. Invasion of alien macroalgae can displace native macroalgae and cause habitat modification. This may result in changes in the capacity of the community to remove nutrients and the structure of epifaunal assemblages in the area. In Hawaii, the alien invasive macroalgae, Gracilaria salicornia and Acanthophora specifera have been the focus of removal projects in recent years, but little has been studied about their ecological impacts and functions. We investigated potential impacts of these invasive algae by comparing their capacity for ammonium uptake and their epifaunal community structure to those of the native alga Padina thivyi. The capacities of G. salicornia and P. thivyi to remove ammonium as individuals (per gram nitrogen in algae) were comparable. However, ammonium uptake as a community was, on average, higher in the invasive-dominated canopies than in the native-dominated canopies, probably due to a higher density (biomass per sediment surface area) of G. salicornia than P. thivyi. Invasive canopies harbored higher abundances and species richness of epifauna (per sediment surface area) than native canopies. Most of the numerically dominant epifauna in the present study were detritivores and were associated with G. salicornia. Amphipod grazers were the only numerically dominant taxon whose abundance was positively correlated with the biomass of P. thivyi. A. specifera had no apparent effects on epifaunal distributions. This study showed facilitative effects of G. salicornia, but the difference in the epifaunal community structure (i.e. detritivores in the invasive canopies vs. herbivores in the native canopies) should be treated as a potential ecological concern and calls for further investigations of how nutrients taken up by these algal species are cycled in the food web.

Mechanisms influencing the spread of a native marine alga.

Like invasive macrophytes, some native macrophytes are spreading rapidly with consequences for community structure. There is evidence that the native alga Caulerpa filiformis is spreading along intertidal rocky shores in New South Wales, Australia, seemingly at the expense of native Sargassum spp. We experimentally investigated the role physical disturbance plays in the spread of C. filiformis and its possible consequences for Sargassum spp. Cleared patches within beds of C. filiformis (Caulerpa habitat) or Sargassum spp. (Sargassum habitat) at multiple sites showed that C. filiformis had significantly higher recruitment (via propagules) into its own habitat. The recruitment of Sargassum spp. to Caulerpa habitat was rare, possibly due in part to sediment accretion within Caulerpa habitat. Diversity of newly recruited epibiotic assemblages within Caulerpa habitat was significantly less than in Sargassum habitat. In addition, more C. filiformis than Sargassum spp. recruited to Sargassum habitat at some sites. On common boundaries between these two macroalgae, the vegetative growth of adjacent C. filiformis into cleared patches was significantly higher than for adjacent Sargassum spp. In both experiments, results were largely independent of the size of disturbance (clearing). Lastly, we used PAM fluorometry to show that the photosynthetic condition of Sargassum spp. fronds adjacent to C. filiformis was generally suppressed relative to those distant from C. filiformis. Thus, physical disturbance, combined with invasive traits (e.g. high levels of recruitment and vegetative growth) most likely facilitate the spread of C. filiformis, with the ramifications being lower epibiotic diversity and possibly reduced photosynthetic condition of co-occurring native macrophytes.

Neighbourhood competition in coexisting species: The native Cystoseira humilis vs the invasive Sargassum muticum

The introduction of non-indigenous species (NIS) is expected to have negative effects on native competitors, particularly between functionally similar species. Nevertheless, the mechanisms underlying competitive relationships remain poorly studied. Here, a substitutive neighbourhood approach was used to examine the role of assemblage density and neighbour identity in the growth and nutritional strategy of a focal individual. We quantified intra- and interspecific competition between two similar adult macroalgae (Fucales: Sargassaceae), the invasive Sargassum muticum and the native Cystoseira humilis, known to co-occur on intertidal rocky shores from NE Atlantic. Using either S. muticum or C. humilis as the focal species, we monitored the focal individual growth responses and nutrient content in the field, with two different densities of neighbours. Additionally, we quantified the nutrient uptake rates for both species in the laboratory. Our results showed that C. humilis grew at a significantly faster rate showing N accumulation when surrounded by S. muticum, whereas the invader has only showed growth at low density conditions. In addition, C. humilis presented greater uptake rates of nitrate compared to S. muticum suggesting better competitive potential to exploit nitrogen transient pulses of high concentration. Our results suggest a dominant native alga vs the invasive, which is not supported by field observations. This research gives evidence that competition between adult macroalgal individuals may not be the key mechanism linked to the dominance of NIS in introduced habitats.

The effect of irradiance and temperature responses and the phenology of a native alga, Undaria pinnatifida (Laminariales), at the southern limit of its natural distribution in Japan

Phenology, irradiance, and temperature characteristics of an edible brown alga, Undaria pinnatifida (Laminariales), were examined from the southernmost natural population in Japan, both by culturing gametophytes and examining the photosynthetic activity of sporophytes using dissolved oxygen sensors and pulse amplitude-modulated chlorophyll fluorometer (IMAGING-PAM). Our surveys confirmed that sporophytes were present between winter and early summer, but absent by July. IMAGING-PAM experiments were used to measure maximum effective quantum yield (ΦII at 0 μmol photons m−2 s−1) for each of 14 temperatures (8–36 °C). Oxygen production was also determined over a coarser temperature gradient. Net photosynthesis and ΦII (at 0 μmol photons m−2 s−1) were observed to be temperature-dependent; the maximum ΦII was estimated to be 0.67, occurred at 21.2 °C, and was nearly identical to the optimal temperature of the net photosynthetic rate (21.7 °C). A net photosynthesis–irradiance (P–E) model revealed that saturation irradiance (Ek) was 119.5 μmol photons m−1 s−1, and the compensation irradiance (Ec) was 17.4 μmol photons m−1 s−1. Culture experiments on the gametophytes revealed that most individuals could not survive temperatures over 28 °C and that growth rates were severely inhibited. Based on our observations, temperatures greater than 20 °C are likely to influence photosynthetic activity and gametophyte survival, and therefore, it is possible that this species might become locally extinct if seawater temperatures in this region continue to rise.

Nonlinear response of stream ecosystem structure to low‐level phosphorus enrichment

Summary Anthropogenic inputs of nitrogen (N) and phosphorus (P) create environmental conditions that alter biological organisation and ecosystem functioning in fresh waters. We studied 38 wadeable streams spanning a N and P gradient to contrast responses of algal and fish assemblages to nutrient enrichment. Surface water total P (TP) and total N (TN) concentrations represented a wide range (TP: 7–2380 μg L−1; TN: 127–15 860 μg L−1) and were correlated across our study sites. Total P explained significantly more variance in periphyton carbon (C)‐to‐nutrient (C : P, C : N) and N : P ratios than TN. Abrupt, nonlinear declines in these ratios were observed between 20 and 50 μg L−1 TP and 500–1000 μg L−1 TN; beyond these values, ratios exhibited minimal additional decline. Algae assemblage structure was strongly linked to surface water TP, TN and catchment‐scale nutrient sources (wastewater treatment plant (WWTP) discharges and % pasture cover). In particular, there were synchronous declines in frequency and cell densities of many alga species associated with TP concentrations &gt;21 μg L−1 (90% CI of 18–48 μg L−1) as well as simultaneous increases in tolerant species associated with increasing enrichment. Fish assemblage structure was most strongly associated with % pasture, WWTP discharges and fine sediment cover, yet also showed significant but weaker correlations with surface water and periphyton nutrient variables. However, two benthic fish species, Etheostoma spectabile and Campostoma anomalum, significantly declined with TP &gt;28 μg L−1 (90% CI, 24–56 μg L−1) and 34 μg L−1 (90% CI, 21–56 μg L−1), respectively. Conversely, the tolerant minnow Cyprinella lutrensis and invasive carp Cyprinus carpio increased nonlinearly with increasing surface water TP. Our results provide new insights into interpretation and analysis of assemblage‐level responses to nutrient enrichment. Our findings indicate that a numerical criterion for surface water TP of c. 20 μg L−1 would be needed to maintain natural algae assemblages and at least two specialist fishes within our study region. Proliferation of weedy alga species and increased abundance of invasive fishes are also likely when surface water concentrations exceed these thresholds. While many streams likely exceed these thresholds, managers should consider potential low‐level enrichment effects when developing criteria for ecosystems to protect existing nutrient‐limited streams.