Rise and fall of toxic benthic freshwater cyanobacteria (Anabaena spp.) in the Eel river: Buoyancy and dispersal

Benthic cyanobacteria in rivers produce cyanotoxins and affect aquatic food webs, but knowledge of their ecology lags behind planktonic cyanobacteria. The buoyancy of benthic Anabaena spp. mats was studied to understand implications for Anabaena dispersal in the Eel River, California. Field experiments were used to investigate the effects of oxygen bubble production and dissolution on the buoyancy of Anabaena dominated benthic mats in response to light exposure. Samples of Anabaena dominated mats were harvested from the South Fork Eel River and placed in settling columns to measure floating and sinking velocities, or deployed into in situ ambient and low light treatments to measure the effect of light on flotation. Floating and sinking occurred within minutes and were driven by oxygen bubbles produced during photosynthesis, rather than intracellular changes in carbohydrates or gas vesicles. Light experiment results showed that in a natural ambient light regime, mats remained floating for at least 4 days, while in low light mats begin to sink in <24 h. Floating Anabaena samples were collected from five sites in the watershed and found to contain the cyanotoxins anatoxin-a and microcystin, with higher concentrations of anatoxin-a (median 560, max 30,693 ng/g DW) than microcystin (median 30, max 37 ng/g DW). The ability of Anabaena mats to maintain their buoyancy will markedly increase their downstream dispersal distances. Increased buoyancy also allows toxin-containing mats to collect along shorelines, increasing threats to human and animal public health.     

Operation of partial nitrification to nitrite of landfill leachate and its performance with respect to different oxygen conditions

The coupled system of partial nitrification and anaerobic ammonium oxidation (Anammox) is efficient in nitrogen removal from wastewater. In this study, the effect of different oxygen concentrations on partial nitrification performance with a sequencing batch reactor (SBR) was investigated. Results indicate that, partial nitrification of landfill leachate could be successfully achieved under the 1.0–2.0 mg L⁻¹ dissolved oxygen (DO) condition after 118 d long-term operation, and that the effluent is suitable for an Anammox reactor. Further decreasing or increasing the DO concentration, however, would lead to a decay of nitrification performance. Additionally, the MLSS concentration in the reactor increased with increasing DO concentration. Respirometric assays suggest that low DO conditions (<2 mg L⁻¹) favor the ammonia-oxidizing bacteria (AOB) and significantly inhibit nitrite oxidizing bacteria (NOB) and aerobic heterotrophic bacteria (AHB); whereas high DO conditions (>3 mg L⁻¹) allow AHB to dominate and significantly inhibit AOB. Therefore, the optimal condition for partial nitrification of landfill leachate is 1.0–2.0 mg L⁻¹ DO concentration.     

Wastewater treatment by using kenaf in paddy soil and effect of dissolved oxygen concentration on efficiency

We previously reported that kenaf (Hibiscus cannabinus L.) planted in a zeolite-bed filter-ditch system provided highly effective treatment of wastewater. Here we compared that system with treatment in fallow paddy fields irrigated in different ways in a greenhouse. Paddy soil was a useful alternative to zeolite as the bed filter material. The efficiency of removal of N and P under furrow irrigation and flooding was 82–92% of that of the zeolite system. Most kenaf roots were distributed in water with a high dissolved oxygen (DO) concentration and a high redox potential; few roots grew in reducing soil under water. The roots distributed in the water contributed most to wastewater treatment. A low DO concentration (0.3 mg L⁻¹) decreased the efficiency of N and P removal. However, nightly low DO concentration (near 0 mg L⁻¹) alternating with daily high DO concentration did not seriously restrict the efficiency. An increase of alpha-naphthylamine oxidation activity in kenaf roots at low DO concentration is discussed in regard to induction of an oxygen-protective enzyme.     

Nitrification rates of algal–bacterial biofilms in wastewater stabilization ponds under light and dark conditions

The objective of this study was to investigate nitrification rates in algal–bacterial biofilms of waste stabilization ponds (WSP) under different conditions of light, oxygen and pH. Biofilms were grown on wooden plates of 6.0 cm by 8.0 cm by 0.4 cm in a PVC tray continuously fed with synthetic wastewater with initial NH₄-N and Chemical Oxygen Demand (COD) concentrations of 40 mg l^?1 and 100 mg l^?1, respectively, under light intensity of 85–95 μE m^?2 s^?1. Batch activity tests were carried out by exposure of the plates to light conditions as above (to simulate day time), dim light of 1.8–2.2 μE m^?2 s^?1 (to simulate reduced light as in deeper locations in WSP) and dark conditions (to simulate night time). Dissolved oxygen (DO) concentration and pH were controlled. At some experiments, both parameters were kept constant, and at others they were left to vary as in WSP. Results show biofilm nitrification rates of 945–1817 mg-N m^?2 d^?1 and 1124–1615 mg-N m^?2 d^?1 for light and dark experiments. When the minimum DO was 4.1 mg l^?1, the biofilm nitrification rates under light and dark conditions did not differ significantly at 95% confidence. When the minimum DO in the dim light experiment was 3.2 mg l^?1, the nitrification rates under light and dim light conditions were 945 mg-N m^?2 d^?1 and 563 mg-N m^?2 d^?1 and these significantly differed. Further decrease of DO to 1.1 mg l^?1 under dark conditions resulted in more decrease of the nitrification rates to 156 mg-N m^?2 d^?1. It therefore seems that under these experimental conditions, biofilm nitrification rates are significantly reduced at a certain point when bulk water DO is between 3.2 mg l^?1 and 4.1 mg l^?1. As long as bulk water DO under dark is high, light is not important in influencing the process of nitrification.     

Potential impact of an exceptional bloom of Karenia mikimotoi on dissolved oxygen levels in waters off western Ireland

In the summer of 2005 an exceptional bloom of the dinoflagellate Karenia mikimotoi occurred along Ireland's Atlantic seaboard and was associated with the mass mortality of both benthic and pelagic marine life. Oxygen depletion, cellular toxicity and physical smothering, are considered to be the main factors involved in mortality. In this paper we use a theoretical approach based on stoichiometry (the Anderson ratio) and an average K. mikimotoi cellular carbon content of 329 pg C cell⁻¹ (n = 20) to calculate the carbonaceous and nitrogenous oxygen demand following bloom collapse. The method was validated against measurements of biochemical oxygen demand and K. mikimotoi cell concentration. The estimated potential oxygen utilisation (POU) was in good agreement with field observations across a range of cell concentrations. The magnitude of POU following bloom collapse, with the exception of three coastal areas, was considered insufficient to cause harm to most marine organisms. This indicates that the widespread occurrence of mortality was primarily due to other factors such as cellular toxicity and/or mucilage production, and not oxygen depletion or related phenomena. In Donegal Bay, Kilkieran Bay and inner Dingle Bay, where cell densities were in the order of 10⁶ cells L⁻¹, estimated POU was sufficient to cause hypoxia. Of the three areas, Donegal Bay is considered to be the most vulnerable due to its hydrographic characteristics (seasonally stratified, weak residual flow) and hypoxic conditions (2.2 mg L⁻¹ O₂) were directly observed in the Bay post bloom collapse. Here, depending on the time of bloom collapse, depressed DO levels could persist for weeks and continue to have a potentially chronic impact on the Bay.     

Partial nitrification under limited dissolved oxygen conditions

Partial nitrification to nitrite is technically feasible and economically favourable, especially when wastewaters contained high ammonium concentrations or low C/N ratios. Partial nitrification can be obtained by selectively inhibiting nitrite-oxidizing bacteria (NOB) through appropriate regulation of the pH, temperature and dissolved oxygen (DO) concentrations. The effect of pH, DO levels and temperature on ammonia oxidation rate and nitrite accumulation was investigated in order to determine the optimal conditions for partial nitrification of synthetic wastewater with high ammonia concentration. The experiments performed at low DO levels to lower the total oxygen needed in the nitrification step, which means great saving in aeration. During the start-up stage pH and DO were set at 7.0–7.4 and 0.5 mg/l, respectively. The reactor was operated until complete partial nitrification was achieved. The effect of pH, DO on partial nitrification was studied, as pH was kept at 6.5, 7.5, 8.5, 9.5 and DO at 0.5±0.2, 1.5±0.2 and 2.5±0.2 mg/l, and temperature at 30 °C. The influence of temperature on k_a value was studied by keeping pH=7.5, DO=1.5 mg/l and temperature was controlled at 12, 20 and 30 °C, respectively. The results showed that partial nitrification to nitrite was steadily obtained and the optimal operational parameters were pH=7.5, DO=1.5 mg/l, T=30 °C based on ammonia oxidation rate and nitrite accumulation rate. The maximum k_a was achieved and to be 115.1×10⁻³ mg NH₄⁺–N (mg VSS h)⁻¹ under this condition.     

Effect Factors for marine eutrophication in LCIA based on species sensitivity to hypoxia

Hypoxia is an important environmental stressor to marine species, especially in benthic coastal waters. Increasing anthropogenic emissions of nutrients and organic matter contribute to the depletion of dissolved oxygen (DO). Biotic sensitivity to low levels of DO is determined by the organisms’ ability to use DO as a respiratory gas, a process depending on oxygen partial pressure. A method is proposed to estimate an indicator of the intensity of the effects caused by hypoxia on exposed marine species. Sensitivity thresholds to hypoxia of an exposed ecological community, modelled as lowest-observed-effect-concentrations (LOEC), were compiled from literature for 91 benthic, demersal and benthopelagic species of fish, crustaceans, molluscs, echinoderms, annelids, and cnidarians, and converted to temperature-specific benthic (100 m depth) LOEC values. Species distribution and LOEC values were combined using a species sensitivity distribution (SSD) methodology to estimate the DO concentration at which the potentially affected fraction (PAF) of the community's species having their LOEC exceeded is 50% (HC50_LOEC). For the purpose of effect modelling in Life Cycle Impact Assessment (LCIA), Effect Factors (EF [(PAF) m³ kgO₂⁻¹]) were derived for five climate zones (CZ) to represent the change in effect due to a variation of the stressor intensity, or EF = ΔPAF/ΔDO = 0.5/HC50_LOEC. Results range from 218 (PAF) m³ kgO₂⁻¹ (polar CZ) to 306 (PAF) m³ kgO₂⁻¹ (tropical CZ). Variation between CZs was modest so a site-generic global EF of 264 (PAF) m³ kgO₂⁻¹ was also estimated and may be used to represent the average impact on a global ecological community of marine species exposed to hypoxia. The EF indicator is not significantly affected by the major sources of uncertainty in the underlying data suggesting valid applicability in characterisation modelling of marine eutrophication in LCIA.     

Dynamic simulation of a WWTP operated at low dissolved oxygen condition by integrating activated sludge model and a floc model

While an aeration tank in an activated sludge process is often operated with high dissolved oxygen (DO) concentration to ensure organic degradation and nitrification, it may be operated at low DO concentration to reduce energy consumption and achieve desired denitrification. The ASM1 (Activated Sludge Model No. 1) can be used to describe the activated sludge process if the nitrification and denitrification occur either during different phases or in different tanks, but it may encounter problems in simulating the denitrification phenomenon caused by low DO concentration in the aeration tank. In the present work, we developed a model integrating the ASM1 kinetics and a biofloc model to account for the actual anoxic and aerobic rates. Oxygen was assumed the only substrate of both bio-kinetically and flux limiting in the flocs and its dispersion coefficient was estimated as 1.2 × 10⁻⁴ m² day⁻¹ by using a set of measured effluent qualities of a full-scale wastewater treatment plant (WWTP) operating at low DO concentration (∼0.80 mg L⁻¹) for 60 days. Simulation studies predicted the optimal DO level of 0.36 mg L⁻¹ which would lead to minimum total nitrogen of 15.7 mg N L⁻¹ and also showed the insignificance of the addition of carbon source for nitrogen removal for the operation under study. The developed model may be helpful for process engineers to predict the plant behaviors under various configurations or operating strategies.     

Microprofiles of photosynthesis and oxygen concentration in Microcystis sp. scums

Abstract Oxygen microelectrodes were used to monitor oxygen concentration and rates of gross photosynthetic activity in Microcystis sp. scums which were formed and incubated under laboratory conditions. The depth of the photic layer, rate of photosynthesis, oxygen concentration and the location of the transition to anoxia in the scum depended on irradiance levels and colony size. Gross photosynthetic activity never extended below 2.5 mm depth in the scum. At high irradiance levels oxygen concentration in the upper 1.5 mm of the scum decreased and the oxygen concentration peak shifted to greater depth. Oxygen concentrations in scums composed of small colonies (<55 μm) were higher than concentrations in large colonies scums (> 300 μm) but small colonies showed stronger indications of photoinhibition. In a natural scum small colonies are presumably shielded from inhibitory intensities by larger colonies which will dominate the upper layers. Accumulation of low-light adapted, smaller colonies in deeper layers likely yielded a second peak in photosynthetic activity. In order to systematically discuss scums and scum formation a distinction is made in three different scum types.     

Potential strategies for process control and monitoring of stabilization of dairy wastewaters in batch aerobic treatment systems

This research was conducted to study the relationships between pH, dissolved oxygen (DO), and oxidation–reduction potential (ORP) during low-intensity aeration of dairy wastewaters and to determine potential strategies for monitoring and/or control of this treatment process. The results of this study ascertained that, close to the detection limit of commercially available DO probes (0.1 mg/l), DO is a poor indicator of the oxidation–reduction status of the dairy wastewater during this treatment processes. All the three parameters (ORP, DO, and pH) displayed features defining stabilization of the wastewater and hence all three can be used singly or in combination to monitor and/or to control this treatment process. The study also established strong linear relations between ORP and the log of DO; manifest in the high-correlation coefficients of 0.98 and 0.95 at the aeration rates of 0.067 and 0.034-l [air] l⁻¹ [manure] min⁻¹, respectively. The latter observation confirms the higher sensitivity of ORP over DO at very low-oxygen levels; a fact which indicates the superiority of ORP in the monitoring and control of oxidation–reduction status of the wastewater close to DO detection limit. Finally, both total volatile solids (TVS) and chemical oxygen demand (COD), which are common measures of wastewater stabilization; correlated well with pH, DO, and ORP during the entire treatment process. However, because the measurements of DO are erratic close to the DO detection limit, and because ORP and pH measurements are much more consistent than the DO in the entire range of treatment, these two parameters will be more suitable for monitoring and control especially of extended aeration treatments.     

Fed-batch fermentation of recombinant Citrobacter freundii with expression of a violacein-synthesizing gene cluster for efficient violacein production from glycerol

The extensive prospects of violacein in the pharmaceutical industry have attracted increasing interest. However, the fermentation levels of violacein are currently inadequate to meet the demands of industrial production. This study was undertaken to develop an efficient process for the production of violacein by recombinant Citrobacter freundii. The effects of dissolved oxygen (DO) and pH on cell growth and violacein production in batch cultures were investigated first. When the DO and pH of the medium were controlled at around 25% and 7.0, respectively, the biomass and concentration of violacein were maximized. Based on the consumption of nutrients in the medium observed during batch culture, a fed-batch fermentation strategy with controlled DO and pH was implemented. By continuously feeding glycerol, NH₄Cl, and l-tryptophan at a constant feeding rate of 16 mL h⁻¹, the final concentration of violacein reached 4.13 g L⁻¹, which was 4.09-fold higher than the corresponding batch culture, and the maximal dry cell weight (DCW) and average violacein productivity obtained for the fed-batch culture were 3.34 g DCW L⁻¹ and 82.6 mg L⁻¹ h⁻¹, respectively. To date, this is the first report on the efficient production of violacein by genetically engineered strains in a fermentor.     

Effect of surface contaminants on oxygen transfer in bubble column reactors

Gas hold-up (ɛ_g), sauter mean bubble diameter (d₃₂) and oxygen transfer coefficient (k_La) were evaluated at four different alkane concentrations (0.05, 0.1, 0.3 and 0.5 vol.%) in water over the range of superficial gas velocity (u_g) of (1.18–23.52) × 10⁻³ m/s at 25 °C in a laboratory-scale bubble column bioreactor. Immiscible hydrocarbons (n-decane, n-tridecane and n-hexadecane) were utilized in the experiments as impurity. A type of anionic surfactant was also employed in order to investigate the effect of addition of surfactant to organic-aqueous systems on sauter mean bubble diameter, gas hold-up and oxygen transfer coefficient. Influence of addition of alkanes on oxygen transfer coefficient and gas hold-up, was shown to be dependent on the superficial gas velocity. At superficial gas velocity below 0.5 × 10⁻³ m/s, addition of alkane in air–water medium has low influence on oxygen transfer coefficient and also gas hold-up, whereas; at higher gas velocities slight addition of alkane increases oxygen transfer coefficient and also gas hold-up. Increase in concentration of alkane resulted in increase in oxygen transfer coefficient and gas hold-up and roughly decrease in sauter mean bubble diameter, which was attributed to an increase in the coalescence-inhibiting tendency in the presence of surface contaminant molecules. Bubbles tend to become smaller with decreasing surface tension of hydrocarbon, thus, oxygen transfer coefficient increases due to increasing of specific gas–liquid interfacial area (a). Empirical correlations were proposed for evaluating gas hold-up as a function of sauter mean bubble diameter, superficial gas velocity and interfacial surface tension as well as evaluating Sherwood number as a function of Schmidt, Reynolds and Bond numbers.     

Evaluation of sediment profile imagery as a tool for assessing water quality in Greenwich Bay,Rhode Island,USA

The Benthic Habitat Quality (BHQ) index was used to assess habitat visible in sediment profile images (SPI) following hypoxia disturbance in a shallow (<10 m) estuarine embayment in Rhode Island, USA. We tested for associations between the BHQ, SPI features and water quality over several assessment windows (1, 3, 7, 14, and 28 days prior to imaging) and at multiple dissolved oxygen (DO) thresholds (2.0 mg l^?1, 2.9 mg l^?1, and 4.8 mg l^?1). Using categorical data analysis, we established empirical relationships between hypoxia prevalence and presence/absence of biogenic features visible in SPI. Fecal pellets, tubes, feeding pits, voids, mounds, and BHQ score were good affirmative features, meaning that their presence (or score greater than 5) indicated a high probability of good water quality. However, low sensitivity to hypoxia precluded their usefulness as indicators, and was attributed to rarity in images and to factors acting on time intervals longer than those examined, e.g. long-term organic enrichment or hypoxia. Burrow structures and the apparent redox potential discontinuity (aRPD), or oxidized layer of surface sediment, were good discriminatory features, with high sensitivity and specificity for both hypoxia and normoxia. Both were strong surrogates for water quality over multiple assessment windows and DO thresholds, and had the highest overall predictive values. We conclude that SPI images can be used to widen the spatial extent of water quality monitoring efforts by utilizing the relationships between aRPD, burrows and hypoxia prevalence.     

Impact of oxygen mass transfer on nitrification reactions in suspended carrier reactor biofilms

Biofilm-internal and external mass transfer resistance was investigated in laboratory-scale nitrifying suspended carrier reactors (SCR), demonstrating the importance of these factors for these increasingly popular reactor systems. Controlled respirometric experiments revealed that oxygen mass transfer resistance regulated the process performance up to a DO concentration of 20 mg L^?1. External mass transfer exerts significant control over the overall reaction rate, thus biofilm models must adequately account for this resistance. Whilst carrier type and characteristics have some influence, biofilm structure seems primarily responsible for differences in mass transfer and nitrification performance. Heterogeneous biofilms grown under high ammonium loadings had much greater area-specific rates than the gel-like biofilms sourced from low loaded systems.Being a mass-transfer controlled process, the overall reaction rate of these SCR systems could be immediately increased by elevating the DO above normal operating levels (up to 20 mg L^?1). Long-term oxygen deficiency in the lower biofilm sections does not negatively affect the biomass activity.     

Cyanotoxin diversity and food web bioaccumulation in a reservoir with decreasing phosphorus concentrations and perennial cyanobacterial blooms

In a shallow multifunction dam reservoir, perennial water blooms formed by several toxin-producing cyanobacteria (Anabaena spp., Aphanizomenon spp., Planktothrix agardhii and Microcystis spp.) were observed. Over a seven-year period, concomitantly with a gradual decrease in phosphate and total phosphorus concentrations in the water and an increase in the DIN to DIP ratio, a reduced biomass of cyanobacteria was noted. Simultaneously, a twofold increase in cyanobacterial species richness was found. The concentration of intracellular anatoxin-a was positively correlated with the total cyanobacterial biomass, but the concentration of intracellular microcystins was significantly negatively correlated with the level of phosphorus in the water. Therefore, in a period with a very low (2.3–3.6) DIN:DIP ratio, intracellular ANTX prevailed in the reservoir, while in the following years (at DIN:DIP = 23–36) much higher MC levels were noted. The highest total concentrations (22.2 μg L⁻¹) of intracellular MCs (MC-LF > -LY > -LR > -LA = -LW) and ANTX (14.4 μg L⁻¹) were found in 2010. In the following year, eight MC iso-forms were detected (MC-LF > -LY > -LA > -LR > -LW > -WR > -YR > -RR). The number of MC variants was positively correlated with the increased contribution of Anabaena planctonica/A. affinis and Microcystis spp. to cyanobacteria biomass. The indigenous bentho-pelagic fish Abramis brama L. accumulated in their tissues relatively high amounts of both ANTX (e.g. 6.2–18.4 μg g⁻¹ FW of liver) and different variants of MCs (up to 4.4 μg g⁻¹ FW of liver). Cyanotoxin tissue contents decreased in the following order: gills > liver > muscles. These observed strong changes in the species structure of cyanobacteria assemblages, even at their considerably smaller biomass, appeared to be an undesirable phenomenon due to the predominance of the efficient MC and ANTX producers, such as Anabaena spp., which is easily digested by fish. The variability of the profile of cyanobacterial blooms that depends on nutrient fluctuations and may account for the diverse toxin accumulation and tissue distribution in freshwater ichthyofauna is noteworthy, especially in water bodies used for fishery.     

Start-up performance and granular sludge features of an improved external circulating anaerobic reactor for algae-laden water treatment

The microbial characteristics of granular sludge during the rapid start of an enhanced external circulating anaerobic reactor were studied to improve algae-laden water treatment efficiency. Results showed that algae laden water was effectively removed after about 35 d, and the removal rates of chemical oxygen demand (COD) and algal toxin were around 85% and 92%, respectively. Simultaneously, the gas generation rate was around 380 mL/gCOD. The microbial community structure in the granular sludge of the reactor was complicated, and dominated by coccus and filamentous bacteria. Methanosphaera, Methanolinea, Thermogymnomonas, Methanoregula, Methanomethylovorans, and Methanosaeta were the major microorganisms in the granular sludge. The activities of protease and coenzyme F₄₂₀ were high in the granular sludge. The intermittent stirring device and the reverse-flow system were further found to overcome the disadvantage of the floating and crusting of cyanobacteria inside the reactor. Meanwhile, the effect of mass transfer inside the reactor can be accelerated to help give the reactor a rapid start.     

Oxygen enhances the recovery of Potamogeton maackianus from prolonged exposure to very low irradiance

Recovery ability in relation to carbohydrate content of Potamogeton maackianus growing in two dissolved oxygen concentrations (8 and 2 mg L⁻¹) was investigated during 28 days exposure to very low irradiance (about 0.06 μmol m⁻² s⁻¹). Plant weight remained relatively constant (0.19 g dry wt plant⁻¹) within the initial 21 days in the high oxygen treatment, but decreased to 0.14 g dry wt plant⁻¹ at the end of the experiment. In low oxygen environments, plant weight was similar within the initial 14 days, but decreased to 0.08 g dry wt plant⁻¹ at 21 day. During the experimental period, both soluble sugar and starch contents in shoots decreased with time. Compared to high oxygen treatment, plants in the low oxygen treatment depleted starch more quickly (25 versus 18 mg g⁻¹ at 28 day) but remained a relatively high soluble sugar content (0.9 versus 1.8 mg g⁻¹ at 28 day). After recovery in high light and high dissolved oxygen conditions for 1 week, plant growth rate, new branch number, stem elongation rate and leaf recruitment number were significantly higher in high oxygen than in the low oxygen treatments. These data suggest that the ability of the plant to recover from prolonged exposure to very low irradiance is related to the depletion level of carbohydrate stored in plant tissues, which is regulated by oxygen availability in the water.     

Growth performance and carcass traits of forage-fed hair sheep wethers

The objective of the present study was to compare live animal performance and carcass characteristics of 3/4 or 7/8 Dorper (DO; n = 30), purebred Katahdin (KA; n = 20), purebred St. Croix (SC; n = 17) and purebred Suffolk (SU; n = 10) lambs born in the spring and fall of 2001. After weaning, lambs were supplemented with up to 680 g of a corn-soybean meal supplement while grazing bermudagrass pastures overseeded with ryegrass. Lambs were slaughtered at approximately 210 d of age. From birth to weaning, DO lambs gained faster (P < 0.001) than KA or SC lambs, whereas KA lambs had higher (P < 0.001) ADG than SC lambs. Additionally, DO and SU wethers had greater (P < 0.02) ADG from weaning to slaughter than SC or KA wethers. Suffolk lambs were heavier (P < 0.001) at slaughter and produced heavier (P < 0.001) carcasses than lambs from hair-sheep breeds. Carcasses of KA lambs were fatter (actual fat thickness; P < 0.02) resulting in higher yield grades (P < 0.03) than carcasses of DO, SC, or SU lambs. Carcasses of DO and SU had larger (P < 0.001) longissimus muscle (LM) areas than those of KA or SC carcasses, whereas kidney fat weight and percentage were greater (P < 0.001) in carcasses from KA and SC than DO and SU lambs. Lean maturity was similar (P = 0.32) among breed-types. However, skeletal maturity was greater (P < 0.001) in SU than hair-sheep carcasses. Flank-streaking scores were similar (P = 0.19) among the breed-types, but conformation scores were higher (P < 0.001) for DO and SU carcasses and resulted in higher (P < 0.001) quality grades than SC carcasses. The LM of SU lambs was lighter (higher L^* values; P < 0.05) than that of KA and SC lambs, whereas the LM from DO lambs was redder (higher a^* values; P < 0.001) than SC and SU and more (P < 0.001) yellow than that of the other breed-types. Chops from SU lambs were tougher (higher shear force values; P < 0.007) than chops from the hair-sheep breeds. Results of this study indicate that ADG, carcass muscularity and meat quality were similar between DO and SU lambs, and, although fatter, carcass muscularity of KA was similar to that of DO lambs.     

The silk cocoon of the silkworm,Bombyx mori: Macro structure and its influence on transmural diffusion of oxygen and water vapor

The cocoon of insect larvae is thought to help conserve water while affording mechanical protection. If the cocoon is a barrier to water loss, then it must also impose a barrier to inward oxygen diffusion. We tested this hypothesis in pupae of the silkworm, Bombyx mori. The rate of water loss and oxygen uptake (V?O₂) at 25 °C was measured in control pupae in their naturally spun cocoon and in exposed pupae experimentally removed from their cocoon. Additional measurements included the oxygen diffusion coefficient, DO₂, of the cocoon wall and dimensions and density of the cocoon fibers. Water loss (as % body mass loss) in both control and exposed pupae was ～ 1%^.day^? 1, and was not significantly different between populations. Similarly, V?O₂ was statistically identical in both control and exposed pupae, at 0.22 ± 0.01 and 0.21 ± 0.02 mL g^? 1 · h^? 1, respectively. The silk fiber diameter was significantly different in the outer fibers, 26 ± 1 µm, compared with 16 ± 1 µm for the inner fibers lining the cocoon. Inner fibers were also spun significantly more densely (20.8 ± 1.2 mm^? 1 transect) than outer fibers (8.3 ± 0.2). Mean DO₂ at 25 °C was 0.298 ± 0.002 cm² · s^? 1, approximately the same as unstirred air. These data indicate that the cocoon, while creating a tough barrier offering mechanical protection to the pupa, imposes no barrier to the diffusion of oxygen or water vapor.     

Flow velocity affects internal oxygen conditions in the seagrass Cymodocea nodosa

The internal oxygen status of seagrass tissues, which is believed to play an important role in events of seagrass die-off, is partly determined by the rates of gas exchange between leaves and water column. In this study, we examined whether water column flow velocity has an effect on gas exchange, and hence on internal oxygen partial pressures (pO₂) in the Mediterranean seagrass, Cymodocea nodosa. We measured the internal pO₂ in the horizontal rhizomes of C. nodosa in darkness at different mainstream flow velocities, combined with different levels of water column oxygen pO₂ using an experimental flume in the laboratory. Flow velocity clearly had an effect on the internal oxygen status. In stagnant, but fully aerated water the mean internal pO₂ was 6.9 kPa, corresponding to about 30% of air saturation. The internal pO₂ increased with increasing flow velocity reaching saturation of around 12.2 kPa (60% of air saturation) at flow velocities ≥7 cm s⁻¹. Flow had a relatively larger influence on internal pO₂ at lower water column oxygen concentrations. By extrapolating linear relationships between internal and water column pO₂ in this experimental setup, rhizomes would become anoxic at a water column oxygen pO₂ of 4–4.5 kPa (∼20% of air saturation) in flowing water, but already at 6.4 kPa (∼30% of air saturation) in stagnant water. Water flow may play an important role for seagrass performance and survival in areas with poor water column oxygen conditions and may, in general, be of importance for the distribution of submerged rooted plants.