N<Subscript>2</Subscript>O production by denitrification in an urban river: evidence from isotopes,functional genes,and dissolved organic matter

Rivers are important sources of N₂O emissions into the atmosphere. Nevertheless, N₂O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N₂O, NH₄ ⁺, NO₂ ^?, and NO₃ ^? in surface water to identify the microbial processes of N₂O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N₂O production processes. The observed nitrogen (δ¹⁵N) and oxygen (δ¹⁸O) of N₂O were within the expected isotopic range of N₂O produced by nitrate reduction, indicating that N₂O was dominantly produced by denitrification. The positive significant correlation between N₂O_Net concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N₂O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N₂O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N₂O. Measurement of isotope ratios of N₂O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N₂O production along the Tama River.     

Nitrate and flooding induce N<Subscript>2</Subscript>O emissions from soybean nodules

Nitrous oxide (N₂O) is one of the three main biogenic greenhouse gases (GHGs) and agriculture represents close to 30 % of the total N₂O net emissions. In agricultural soils, N₂O is emitted by two main microbial processes, nitrification and denitrification, both of which can convert synthetic nitrogen fertilizer into N₂O. Legume-rhizobia symbiosis could be an effective and environmental-friendly alternative to nitrogen fertilization and hence, to mitigate soil N₂O emissions. However, legume crops also contribute to N₂O emissions. A better understanding of the environmental factors involved in the emission of N₂O from nodules would be instrumental for mitigating the release of this GHG gas. In this work, in vivo N₂O emissions from nodulated soybean roots in response to nitrate (0, 1, 2 and 4 mM) and flooding have been measured. To investigate the contribution of rhizobial denitrification in N₂O emission from nodules, plants were inoculated with B. japonicum USDA110 and napA and nosZ denitrification mutants. The results showed that nitrate was essential for N₂O emissions and its concentration enhanced N₂O fluxes showing a statistical linear correlation, being the highest N₂O fluxes obtained with 4 mM nitrate. When inoculated plants grown with 4 mM nitrate were subjected to flooding, a 150- and 830-fold induction of N₂O emission rates from USDA110 and nosZ nodulated roots, respectively, was observed compared to non-flooded plants, especially during long-term flooding. Under these conditions, N₂O emissions from detached nodules produced by the napA mutant were significantly lower (p?<?0.05) than those produced by the wild-type strain (382 versus 1120 nmol N₂O h^?1 g^?1 NFW, respectively). In contrast, nodules from plants inoculated with the nosZ mutant accumulated statistically higher levels of N₂O compared to wild-type nodules (2522 versus nmol 1120 N₂O h^?1 g^?1 NFW, p?<?0.05). These results demonstrate that flooding is an important environmental factor for N₂O emissions from soybean nodules and that B. japonicum denitrification is involved in such emission.     

Impacts of varying durations of passive oxygen exposure on freshwater denitrifier community structure and function

Fertilizer use has dramatically increased the availability of nitrate (NO₃ ^?) in aquatic systems. Microbe-mediated denitrification is one of the predominant means of NO₃ ^? removal from freshwaters, yet oxygenation (O₂)-induced disruptions—e.g., extreme precipitation events—can occur, resulting in a disproportional increase in nitrous oxide (N₂O) production and efflux as facultative anaerobic bacterial populations use of O₂ as a terminal electron acceptor increases. We examined the effects of 12- and 24-h passive O₂ exposure on previously anaerobic bacterial communities focusing on denitrification enzyme activity (DEA), N₂O production, and bacterial community 16S rRNA and nitrous oxide reductase gene (nosZ) profiles after 12, 24, and 48 h of anaerobic recovery. Treatments experiencing 24-h O₂ exposure had significantly higher DEA 12 h into anaerobic recovery than treatments undergoing 12-h O₂ exposure. Initial N₂O emissions were significantly lower in the 24-h O₂ exposure treatments although by 24 h a dramatic spike (tenfold relative to the 12-h O₂ exposure treatments) in N₂O concentrations was observed. However, within 6 h (30-h anaerobic recovery) these differences were gone. Community nosZ profiles experiencing 24-h O₂ exposure exhibited reduced diversity after 24-h recovery, which corresponded with an increase in N₂O emissions. However, after 48 h of anaerobic recovery, nosZ diversity had recovered. These observations highlight the effects of short-term aerobic disruption on denitrification, as well as the effects on the denitrifier community profile. Together, these data suggest that recovery to ambient N cycling is exacerbated by disturbance length due to increased lag time and subsequent loss of denitrifier community diversity.     

Nitrification and denitrification by algae-attached and free-living microorganisms during a cyanobacterial bloom in Lake Taihu,a shallow Eutrophic Lake in China

Cyanobacterial blooms may stimulate epiphytic nitrification and denitrification in the water column. To validate this hypothesis, a 4-week floating mesocosms experiment that involved a cyanobacterial decay–growth–decay period was conducted at Lake Taihu. In addition to conventional methods for detecting the physical and chemical properties, quantitative real-time PCR was used to identify the nitrification and denitrification genes (archaeal and bacterial amoA, nirS and nirK). Treatment with cyanobacteria led to removal of about 3.62 mg N L^?1 total nitrogen, 40% of which was organic nitrogen, indicating a nitrogen transformation and removal mechanism was present in the system. Variations in the biogeochemical properties suggested that remineralization and coupling nitrification and denitrification by epiphytic and pelagic microorganisms was the primary pathway through which organic nitrogen was removed. The results of this study revealed that algal blooms can accelerate nitrogen removal efficiency, which may be the primary reason that nitrogen is limited in summer in Lake Taihu.     

Simultaneous nitrification–denitrification and microbial community profile in an oxygen-limiting intermittent aeration SBBR with biodegradable carriers

To enhance the startup and efficient simultaneous nitrification and denitrification for sewage treatment, sequencing batch biofilm reactors (SBBRs) partially coupled with rice husk were established and operated under various intermittent micro-aeration cycles (IMCs) and COD/N ratios under oxygen-limiting intermittent aeration conditions. Experimental results showed that the increase of IMCs with non-aeration/micro-aeration mode of (8 h/4 h)₁ to (2 h/1 h)₄ in a 12 h-cycle accelerated the startup performance and improved NH₄⁺–N and COD removal. NH₄⁺–N, TN and COD removal efficiencies were 98.7?±?0.9, 89.2?±?5.2 and 82.9?±?6.7% at COD/N ratio of 7.6 with the highest IMCs in SBBR, respectively. Higher TN removal efficiencies of 87.2?±?4.0 and 58.1?±?3.5% were also achieved at lower COD/N ratio of 5.6 and 2.8, respectively. In SBBRs with various IMCs, facultative denitrifier like genus Acinetobacter and solid-phase denitrifier belonging to Comamonadaceae family were enriched. However, aerobic denitrifiers with function of heterotrophic nitrification like Paracoccus were favored to enrich under higher IMCs condition, and more anoxic denitrifiers like sulfur-based autotrophic denitrifier Thiothrix and heterotrophic denitrifiers like Pseudomonas and Methyloversatilis were observed at lower IMCs condition. Autotrophic nitrifier (Nitrosomonas and Nitrosipra) and heterotrophic nitrifiers both contributed to the efficient nitrification.     

Effect of the cathode potential and sulfate ions on nitrate reduction in a microbial electrochemical denitrification system

Recently, bioelectrochemical systems have been demonstrated as advantageous for denitrification. Here, we investigated the nitrate reduction rate and bacterial community on cathodes at different cathode potentials [?300, ?500, ?700, and ?900 mV vs. standard hydrogen electrode (SHE)] in a two-chamber microbial electrochemical denitrification system and effects of sulfate, a common nitrate co-contaminant, on denitrification efficiency. The results indicated that the highest nitrate reduction rates (3.5 mg L^?1 days^?1) were obtained at a cathode potential of ?700 mV, regardless of sulfate presence, while a lower rate was observed at a more negative cathode potential (?900 mV). Notably, although sulfate ions generally inhibited nitrate reduction, this effect was absent at a cathode potential of ?700 mV. Polymerase chain reaction–denaturing gradient gel electrophoresis revealed that bacterial communities on the graphite-felt cathode were significantly affected by the cathode potential change and sulfate presence. Shinella-like and Alicycliphilus-like bacterial species were exclusively observed on cathodes in reactors without sulfate. Ochrobactrum-like and Sinorhizobium-like bacterial species, which persisted at different cathode potentials irrespective of sulfate presence, were shown to contribute to bioelectrochemical denitrification. This study suggested that a cathode potential of around ?700 mV versus SHE would ensure optimal nitrate reduction rate and counteract inhibitory effects of sulfate. Additionally, sulfate presence considerably affects denitrification efficiency and microbial community of microbial electrochemical denitrification systems.     

Interactive influences of the marine yabby (<Emphasis Type="Italic">Trypaea australiensis</Emphasis>) and mangrove (<Emphasis Type="Italic">Avicennia marina</Emphasis>) leaf litter on benthic metabolism and nitrogen cycling in sandy estuarine sediment

A previous study has demonstrated that in sandy sediment the marine yabby (Trypaea australiensis) stimulated benthic metabolism, nitrogen regeneration and nitrification, but did not stimulate denitrification, as the intense bioturbation of the yabbies eliminated anoxic microzones amenable to denitrification. It was hypothesised that organic matter additions would alleviate this effect as the buried particles would provide anoxic microniches for denitrifiers. To test this hypothesis a 55-day microcosm (75 cm × 36 cm diameter) experiment, comprising four treatments: sandy sediment (S), sediment + yabbies (S + Y), sediment + A. marina litter (S + OM) and sediment + yabbies + A. marina litter (S + Y + OM), was conducted. Trypaea australiensis significantly stimulated benthic metabolism, nitrogen regeneration, nitrification and nitrate reduction in the presence and the absence of litter additions. In contrast, the effects of litter additions alone were more subtle, developed gradually and were only significant for sediment oxygen demand. However, there was a significant interaction between yabbies and litter with rates of total nitrate reduction and denitrification being significantly greater in the S + Y + OM than all other treatments, presumably due to the decaying buried litter providing anoxic micro-niches suitable to nitrate reduction. In addition, both T. australiensis and litter significantly decreased rates of DNRA and its contribution to nitrate reduction.     

<Emphasis Type="Italic">Methylophilaceae</Emphasis> and <Emphasis Type="Italic">Hyphomicrobium</Emphasis> as target taxonomic groups in monitoring the function of methanol-fed denitrification biofilters in municipal wastewater treatment plants

Molecular monitoring of bacterial communities can explain and predict the stability of bioprocesses in varying physicochemical conditions. To study methanol-fed denitrification biofilters of municipal wastewater treatment plants, bacterial communities of two full-scale biofilters were compared through fingerprinting and sequencing of the 16S rRNA genes. Additionally, 16S rRNA gene fingerprinting was used for 10-week temporal monitoring of the bacterial community in one of the biofilters. Combining the data with previous study results, the family Methylophilaceae and genus Hyphomicrobium were determined as suitable target groups for monitoring. An increase in the relative abundance of Hyphomicrobium-related biomarkers occurred simultaneously with increases in water flow, NO _x ^? load, and methanol addition, as well as a higher denitrification rate, although the dominating biomarkers linked to Methylophilaceae showed an opposite pattern. The results indicate that during increased loading, stability of the bioprocess is maintained by selection of more efficient denitrifier populations, and this progress can be analyzed using simple molecular fingerprinting.     

Effects of solid-phase denitrification on the nitrate removal and bacterial community structure in recirculating aquaculture system

A solid-phase denitrification (SPD) reactor packed with poly (3-hydroxybutyrate-co-3-hydroxyvalerate) as a carbon source was incorporated into a recirculating aquaculture system (RAS) to remove accumulated nitrate. Bacterial community structures in different parts of the RAS, including biofilter unit, SPD reactor, and culture water, were analyzed using Illumina MiSeq sequencing technology. The data showed that nitrate levels decreased remarkably in the RAS connected with SPD reactor (RAS-DR). In contrast, nitrate levels increased continuously in the conventional RAS without SPD reactor (RAS-CK). Biofilter unit and culture water in RAS-DR developed lower species richness and higher bacterial community diversity than that in RAS-CK. The bacterial community structure of RAS was significantly affected by the SPD process and the changes included an increase in the proportion of Proteobacteria and Firmicutes and a decrease in Nitrospira abundance in RAS-DR. Firmicutes was the most abundant phylum (56.9 %) and mainly consisted of Clostridium sensu stricto (48.3 %) in SPD reactor.     

Context-dependent tree species effects on soil nitrogen transformations and related microbial functional genes

Although it is generally accepted that tree species can influence nutrient cycling processes in soils, effects are not consistently found, nor are the mechanisms behind tree species effects well understood. Our objectives were to gain insights into the mechanism(s) underlying the effects of tree species on soil nitrogen cycling processes, and to determine the consistency of tree species effects across sites. We compared N cycling in soils beneath six tree species (ash, sycamore maple, lime, beech, pedunculate oak, Norway spruce) in common garden experiments planted 42 years earlier at three sites in Denmark with distinct land-use histories (forest and agriculture). We measured: (1) net and gross rates of N transformations using the ¹⁵N isotope pool-dilution method, (2) soil microbial community composition through qPCR of fungal ITS, bacterial and archaeal 16S, and (3) abundance of functional genes associated with N cycling processes—for nitrification the archaeal and bacterial ammonia-monooxygenase genes (amoA AOA and amoA AOB, respectively) and for denitrification, the nitrate reductase genes nirK and nirS. Carbon concentrations were higher in soils under spruce than under broadleaves, so N transformation rates were standardized per g soil C. Soil NH₄⁺ parameters (gross ammonification, gross NH₄⁺ consumption, net ammonification (net immobilization in this case), and NH₄⁺ concentrations, per g C) were all lowest in soils under spruce. Soils under spruce also had the lowest gene abundance of bacteria, bacterial:fungal ratio, denitrifying microorganisms, ammonia-oxidizing archaea and ammonia-oxidizing bacteria. Differences in N-cycling processes and organisms among the five broadleaf species were smaller. The ‘spruce effect’ on soil microbes and N transformations appeared to be driven by its acidifying effect on soil and tighter N cycling, which occurred at the previously forested sites but not at the previously agricultural site. We conclude that existing characteristics of soils, including those resulting from previous land use, mediate the effects of tree species on the soil microbial communities and activities that determine rates of N-cycling processes.     

Optimization of continuous-flow solid-phase denitrification via coupling carriers in enhancing simultaneous removal of nitrogen and organics for agricultural runoff purification

Coupling of biodegradable corncob and plastic carrier was optimized in continuous-flow solid-phase denitrification systems for enhancing simultaneously removal of nitrogen and organics in agricultural runoff. In compared with preposition of plastic carriers and mixed distribution method, it was demonstrated that the preposition of corncobs simultaneously enhanced nitrate (6.64 ± 1.35 mg L^?1 day ^?1) and organics removal (6.33 ± 1.44 mg L^?1 day^?1) at a hydraulic retention time (HRT) of 6 h. The operation performance could be further enhanced with extension of HRT to 12 h. The dominant genera found in corncob were denitrifiers for nitrate reduction (Bosea, Simplicispira, Desulfovibrio, Klebsiella, etc.) and fermentative bacteria (Pleomorphomonas, Actinotalea, Opitutus, Cellulomonas, Bacteroides, etc.) responsible for corncob degrading to simple organics for other denitrifiers. However, much lower and different denitrifiers abundances (Bradyrhizobium, Acinetobacter, Bacillus, etc.) exhibited on plastic filler than those of corncob. It well explained that the biofilm on plastic carrier was mainly related with organics removal while the biofilm on corncobs inclined to effectively remove nitrate, and simultaneous removal of nitrogen and organics could be achieved in coupling carriers system with preposition of biodegradable corncob.     

Successional patterns of key genes and processes involved in the microbial nitrogen cycle in a salt marsh chronosequence

Here, we investigated the patterns of microbial nitrogen cycling communities along a chronosequence of soil development in a salt marsh. The focus was on the abundance and structure of genes involved in N fixation (nifH), bacterial and archaeal ammonium oxidation (amoA; AOB and AOA), and the abundances of genes involved in denitrification (nirS, nirK, nosZ). Potential nitrification and denitrification activities were also measured, and increases in nitrification were found in soils towards the end of succession, whereas denitrification became maximal in soils at the intermediate stages. The nifH, nirK and nirS gene markers revealed increases in the sizes of the respective functional groups towards the intermediate stage (35 years), remaining either constant (for nifH) or slightly declining towards the latest stage of succession (for nirK and nirS). Moreover, whereas the AOB abundance peaked in soils at the intermediate stage, that of AOA increased linearly along the chronosequence. The abundance of nosZ was roughly constant, with no significant regression. The drivers of changes in abundance and structure were identified using path analysis; whereas the ammonia oxidizers (AOA and AOB) showed patterns that followed mainly N availability, those of the nitrogen fixers followed plant diversity and soil structure. The patterns of denitrifiers were group-dependent, following the patterns of plant diversity (nirK and nirS) and belowground shifts (nosZ). The variation observed for the microbial groups associated with the same function highlights their differential contribution at different stages of soil development, revealing an interplay of changes in terms of niche complementarity and adaptation to the local environment.     

Tissue-specific turnover rates of the nitrogen stable isotope as functions of time and growth in a cyprinid fish

Ecological applications of stable isotope data require knowledge on the isotopic turnover rate of tissues, usually described as the isotopic half-life in days (T _0.5) or the change in mass (G _0.5). Ecological studies increasingly analyse tissues collected non-destructively, such as fish fin and scales, but there is limited knowledge on their turnover rates. Determining turnover rates in situ is challenging, with ex situ approaches preferred. Correspondingly, T _0.5 and G _0.5 of the nitrogen stable isotope (δ¹⁵N) were determined for juvenile barbel Barbus barbus (5.5 ± 0.6 g starting weight) using a diet-switch experiment. δ¹⁵N data from muscle, fin and scales were taken during a 125 day post diet-switch period. Whilst isotopic equilibrium was not reached in the 125 days, the δ¹⁵N values did approach those of the new diet. The fastest turnover rates were in more metabolically active tissues, from muscle (highest) to scales (lowest). Turnover rates were relatively slow; T _0.5 was 84 (muscle) to 145 (scale) days; G _0.5 was 1.39 × body mass (muscle) to 2.0 × body mass (scales), with this potentially relating to the slow growth of the experimental fish. These turnover estimates across the different tissues emphasise the importance of estimating half-lives for focal taxa at species and tissue levels for ecological studies.     

Association of Novel and Highly Diverse Acid-Tolerant Denitrifiers with N2O Fluxes of an Acidic Fen

Wetlands are sources of denitrification-derived nitrous oxide (N₂O). Thus, the denitrifier community of an N₂O-emitting fen (pH 4.7 to 5.2) was investigated. N₂O was produced and consumed to subatmospheric concentrations in unsupplemented anoxic soil microcosms. Total cell counts and most probable numbers of denitrifiers approximated 10¹¹ cells·g_DW⁻¹ (where DW is dry weight) and 10⁸ cells·g_DW⁻¹, respectively, in both 0- to 10-cm and 30- to 40-cm depths. Despite this uniformity, depth-related maximum reaction rate (v_max) values for denitrification in anoxic microcosms ranged from 1 to 24 and −19 to −105 nmol N₂O h⁻¹· g_DW⁻¹, with maximal values occurring in the upper soil layers. Denitrification was enhanced by substrates that might be formed via fermentation in anoxic microzones of soil. N₂O approximated 40% of total nitrogenous gases produced at in situ pH, which was likewise the optimal pH for denitrification. Gene libraries of narG and nosZ (encoding nitrate reductase and nitrous oxide reductase, respectively) from fen soil DNA yielded 15 and 18 species-level operational taxonomic units, respectively, many of which displayed phylogenetic novelty and were not closely related to cultured organisms. Although statistical analyses of narG and nosZ sequences indicated that the upper 20 cm of soil contained the highest denitrifier diversity and species richness, terminal restriction fragment length polymorphism analyses of narG and nosZ revealed only minor differences in denitrifier community composition from a soil depth of 0 to 40 cm. The collective data indicate that the regional fen harbors novel, highly diverse, acid-tolerant denitrifier communities capable of complete denitrification and consumption of atmospheric N₂O at in situ pH.Nitrous oxide (N₂O) is a potent greenhouse gas with a global warming potential that is 300-fold higher than that of CO₂, and its concentration increased from 270 ppb in 1750 to 319 ppb in 2005 (17). N₂O can be produced in soils during denitrification, nitrification, the dissimilatory reduction of nitrate to nitrite and/or ammonium (hereafter referred to as dissimilatory nitrate reduction), or the chemical transformation of nitrite or hydroxylamine (5, 7, 49). The percentage of N₂O produced in any of these processes is variable, depending mainly on the redox potential, pH, and C/N ratio (49). In anoxic ecosystems such as waterlogged soils, most of the N₂O is considered to be denitrification derived (7, 9). Complete denitrification is the sequential reduction of nitrate to dinitrogen (N₂) via nitrite, nitric oxide (NO), and N₂O (75). The main product of denitrification varies with the organism and in situ conditions and is usually either N₂O or N₂ (68). N₂O can occur as a by-product during dissimilatory nitrate reduction when accumulated nitrite interacts with nitrate reductase to form N₂O (59). The production of N₂O by dissimilatory nitrate reducers is favored in environments with large amounts of readily available organic carbon (65). Thus, their contribution to nitrate-dependent production of N₂O in soils is likely insignificant compared to that of denitrifiers.The oxidoreductases involved in denitrification are termed dissimilatory nitrate reductase (Nar, encoded by narGHJI, or Nap, encoded by napEDABC), nitrite reductase (Nir, encoded by nirK and nirS), NO reductase (cNor and qNor, encoded by norBC and norB, respectively), and N₂O reductase (Nos, encoded by nosZ) (75). Nitrate reductase is also found in dissimilatory nitrate reducers (60). narG can therefore be used as a molecular marker to assess both denitrifiers and dissimilatory nitrate reducers, whereas nosZ is specific for the assessment of denitrifiers (25, 43, 48).Denitrification in soils is regulated by temperature, pH, substrate (i.e., carbon) availability, and water content (10, 24, 66). Although denitrification increases with increasing temperature, it can still occur at temperatures below 0°C (10, 24). Low temperatures appear to limit the activity of N₂O reductase more severely than other enzymes involved in denitrification and thus yield higher relative amounts of denitrification-derived N₂O (24). Although denitrification activity usually decreases under acidic conditions, the relative percentage of N₂O to total denitrification-derived nitrogenous gases increases with increasing acidity, a result attributed to the sensitivity of N₂O reductase to low pH (27, 70). However, denitrifier communities can be adapted to the in situ pH of the system (40, 58, 73).Wetlands are ecosystems in which denitrification is likely a dominant source of emitted N₂O (7, 44, 45). The identification and analysis of main drivers for N₂O production (i.e., the microbiota catalyzing N₂O production and consumption) is thus of major concern in such environments. Fens are specialized wetlands characterized by soil acidity (67). However, information on acid-tolerant denitrifier communities of such wetlands is scarce. It is hypothesized that fens harbor a diverse, hitherto unknown, denitrifier community that is adapted to in situ conditions and associated with N₂O fluxes (i.e., fen denitrifiers are acid tolerant and have a high affinity for nitrate and N₂O). Thus, the main objectives of the present study were to evaluate the capacities of denitrifier communities of an N₂O-emitting fen (20) to produce or consume N₂O and to determine if a novel and diverse denitrifier community was associated with these capacities.     

Patterns of distribution and movement of fishes, <Emphasis Type="Italic">Ophthalmolepis lineolatus</Emphasis> and <Emphasis Type="Italic">Hypoplectrodes maccullochi</Emphasis>, on temperate rocky reefs of south eastern Australia

Current ecological models predict that reef fish assemblages will be strongly influenced by habitat type. Here we test hypotheses about habitat types and abundance patterns of temperate reef fishes from broad spatial scales (100 s of km) to small spatial scales of metres to tens of metres. Habitat preferences are also described over long periods of time (22 years) for two abundant taxa. Patterns of distribution and abundance varied over ～ eight degrees of latitude (29.9–37.5°S) along the coast of New South Wales, Australia. Ophthalmolepis lineolatus (Labridae) preferred kelp and Barrens habitats and juveniles were most abundant in habitats rich in algae. This species also increased in abundance from North to South. In contrast, Hypoplectrodes maccullochi (Serranidae) were usually only found in the Barrens habitat and great variation was found among locations. Both taxa were most abundant on urchin grazed deep reefs (over 10 m deep). Habitat preferences of O. lineolatus and H. maccullochi appeared resistant to major environmental perturbations that included large El Niño events in 1991, 1998 and 2002. Home ranges of O. lineolatus varied from 52 m² to 1,660 m² and often overlapped; fish of all sizes were most abundant in algal dominated habitat. Limited movements and small home ranges (2.1–11.6 m²) combined with a strong affiliation for shelter indicated that most H. maccullochi are strongly site-attached. Habitat type is important to these taxonomically different fishes, but to varying degrees where H. maccullochi was more of a habitat specialist than O. lineolatus and would be more vulnerable to perturbations that alter Barrens. Changes in reef habitats will have a great influence on fish assemblages and this should also be considered in coastal planning (e.g. for Marine Protected Areas, MPAs) and the assessments of resistance and resilience of fishes to climate change.