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Most previous systems for quantifying ventilatory flow in fish involve prior anesthesia and difficult surgery to sew or glue membranes to the animal, which are undoubtedly stressful. By modification of the original 'van Dam box' design and incorporation of an electromagnetic blood flow probe, we have developed a less invasive system that avoids these problems and provides breath-to-breath measurements of ventilatory flow in real time. The fish can be quickly moved in and out of the apparatus, facilitating repeated measurements on the same animal after different treatments. We have used the system to document the hyperventilatory and hypoventilatory responses to environmental hypoxia and hyperoxia, respectively, in both ~400-g trout (Oncorhynchus mykiss) and 10-g goldfish (Carassius auratus); the method is easily adaptable to fish of other sizes. Separate experiments on trout have demonstrated that responses to these treatments in buccal pressure amplitude, breathing frequency, and ventilation index are not altered by the attachments used in the apparatus. This less invasive methodology may prove more acceptable to animal ethics committees. La plupart des systemes existants utilises pour quantifier le debit ventilatoire de poissons necessitent une anesthesie prealable et une intervention chirurgicale difficile pour coudre ou coller des membranes au specimen et entrainent assurement un stress. En modifiant le schema de << boite de van Dam >> original et en y integrant une sonde electromagnetique de debit sanguin, nous avons mis au point un systeme moins effractif qui evite ces ecueils et produit des mesures en temps reel du debit ventilatoire entre respirations. Les poissons peuvent etre places dans l'appareil et en etre retires rapidement, ce qui facilite des mesures repetees sur le meme specimen apres differents traitements. Nous avons utilise le systeme pour documenter les reactions hyperventilatoire et hypoventilatoire a une hypoxie et une hyperoxie ambiantes, respectivement, chez des truites (Oncorhynchus mykiss) de ~400 g et des carassins dores (Carassius auratus) de ~10 g; la methode peut facilement etre adaptee a des poissons d'autres tailles. D'autres experiences sur des truites ont demontre que les reponses a ces traitements en ce qui concerne l'amplitude de la pression buccale, la frequence des respirations et l'indice de ventilation ne sont pas modifiees par les pieces fixees au specimen qui font partie du dispositif. Des comites d'ethique animale pourraient trouver plus acceptable cette methode moins effractive. [Traduit par la Redaction], Introduction The measurement of [??]w, the total ventilatory flow of water (i.e., ventilation volume) through the gills, is fundamental to understanding the respiratory physiology of fish but is not easy [...]

Environmental context: Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health. Rationale: Dissolved organic carbon (DOC) is both ubiquitous and heterogeneous in freshwater. Freshwaters are browning universally and pH values are decreasing with climate change. DOCs influence water pH, whereas changes in water pH potentially alter the conformation and function of DOCs. The physicochemical properties of DOCs can be characterised by optical and chemical indices, but the inter-relationships among them, and the effects of low pH, are not well understood. Methodology: We characterised five naturally sourced DOCs, spanning large differences in origin and composition, at pH 7 and 4, using multiple indices: specific absorbance coefficient at 340 nm, molecular weight index, fluorescence index, octanol–water partition coefficient, molecular charge, proton binding index, size-fractionation, and percentage humic-acid-like, percentage fulvic-acid-like and percentage protein-like components by fluorescence-based parallel factor analysis. Results: Many of the indices changed between pH 7 and 4 as reflected in the corresponding absorbance and fluorescence profiles. Generally, apparent aromaticity, apparent molecular weight and molecular charge all decreased with low pH, while lipophilicity increased. Key positive correlations occurred between aromaticity and apparent molecular weight, chemical reactivity and apparent molecular weight, and aromaticity and chemical reactivity, and a negative correlation between lipophilicity and molecular charge. These relationships were pH dependent. Discussion: Our results highlight that physicochemical indices used to characterise DOCs from distinct sources should consider pH and be interpreted carefully. The pH-dependent changes in many of the indices are likely alterations in the conformation, reflected in the optical signatures, rather than changes in the composition of DOCs. In contrast, increased lipophilicity and reduced charge at lower pH are due to actual changes in DOC molecules, resulting from proton binding. The ecological functions of DOCs are dependent on source and will likely change with natural acidification events such as increasing atmospheric CO2. Environmental context. Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health. (Image credit: Carolyn Morris.) [ABSTRACT FROM AUTHOR]