Canadian Journal of Fisheries and Aquatic Sciences

From headwaters to mouth, the physical variables within a river system present a continuous gradient of physical conditions. This gradient should elicit a series of responses within the constituent populations resulting in a continuum of biotic adjustments and consistent patterns of loading, transport, utilization, and storage of organic matter along the length of a river. Based on the energy equilibrium theory of fluvial geomorphologists, we hypothesize that the structural and functional characteristics of stream communities are adapted to conform to the most probable position or mean state of the physical system. We reason that producer and consumer communities characteristic of a given river reach become established in harmony with the dynamic physical conditions of the channel. In natural stream systems, biological communities can be characterized as forming a temporal continuum of synchronized species replacements. This continuous replacement functions to distribute the utilization of energy inputs over time. Thus, the biological system moves towards a balance between a tendency for efficient use of energy inputs through resource partitioning (food, substrate, etc.) and an opposing tendency for a uniform rate of energy processing throughout the year. We theorize that biological communities developed in natural streams assume processing strategies involving minimum energy loss. Downstream communities are fashioned to capitalize on upstream processing inefficiencies. Both the upstream inefficiency (leakage) and the downstream adjustments seem predictable. We propose that this River Continuum Concept provides a framework for integrating predictable and observable biological features of lotic systems. Implications of the concept in the areas of structure, function, and stability of riverine ecosystems are discussed.Key words: river continuum; stream ecosystems; ecosystem structure, function; resource partitioning; ecosystem stability; community succession; river zonation; stream geomorphology

The most appropriate strategy to be used to create a permutation distribution for tests of individual terms in complex experimental designs is currently unclear. There are often many possibilities, including restricted permutation or permutation of some form of residuals. This paper provides a summary of recent empirical and theoretical results concerning available methods and gives recommendations for their use in univariate and multivariate applications. The focus of the paper is on complex designs in analysis of variance and multiple regression (i.e., linear models). The assumption of exchangeability required for a permutation test is assured by random allocation of treatments to units in experimental work. For observational data, exchangeability is tantamount to the assumption of independent and identically distributed errors under a null hypothesis. For partial regression, the method of permutation of residuals under a reduced model has been shown to provide the best test. For analysis of variance, one must first identify exchangeable units by considering expected mean squares. Then, one may generally produce either (i) an exact test by restricting permutations or (ii) an approximate test by permuting raw data or some form of residuals. The latter can provide a more powerful test in many situations.

The accurate measurement of ammonium concentrations is fundamental to understanding nitrogen biogeochemistry in aquatic ecosystems. Unfortunately, the commonly used indophenol blue method often yields inconsistent results, particularly when ammonium concentrations are low. Here, we present a fluorometric method that gives precise measurements of ammonium over a wide range of concentrations and salinities emphasizing submicromolar levels. The procedure not only solves analytical problems but also substantially simplifies sample collection and preservation. It uses a single working reagent (consisting of orthophthaldialdehyde, sodium sulfite, and sodium borate) that is stable for months when stored in the dark. The working reagent and sample can be mixed immediately after sample collection and the reaction proceeds to completion within 3 h at room temperature. Matrix effects and background fluorescence can be corrected without introducing substantial error. This simple method produces highly reproducible results even at very low ammonium concentrations.

Here I quantitatively review the literature on how fish gill morphology is affected by chemical and physical irritants in the surrounding water (e.g. various toxicants, extremes of temperature or pH). I catalogued histopathological gill lesions that were reported, and used statistics to explore how such lesions relate to the irritant-exposure conditions under which they occurred (specifically, to dose and class of irritant, to temperature, and to salinity of the surrounding water). Frequently recorded histopathologic lesions include changes in gill epithelium (lifting, necrosis, hyperplasia, hypertrophy, rupture), bulbing or fusing of gill lamellae, hypersecretion and proliferation of mucocytes, and changes in chloride cells and gill vasculature. I conclude that these lesions are largely nonspecific in nature, as each was detected under many different exposure conditions. The lesions are not entirely independent of exposure conditions, however, as my statistical analysis discerns these trends: (1) Most gill lesion types have been reported more frequently after lethal than after sublethal exposure to irritants. (2) Some lesions were more frequently detected in studies employing heavy metals than in studies using organic toxicants or other irritants; such lesions include necrosis and hypertrophy of gill epithelial cells, plus mucous hypersecretion. (3) Lifting of the branchial epithelium, the most commonly reported lesion, was reported more often in freshwater than in marine fish, suggesting that osmolarity of the ambient water influences this lesion. Little relation was found between recorded lesion frequencies and temperature. Following my statistical analysis, the etiology of irritant-induced gill lesions is considered. The nonspecificity of branchial alterations suggests that they primarily represent stereotyped physiological reactions of gills to stress, and many of them are logically considered defense responses. Some branchial alterations have been considered inflammatory, but I conclude that the literature cannot support that hypothesis. Ultrastructural studies have detected irritant-induced disruptions of branchial epithelial cells, including cytoplasmic vacuolization, autophagosomes and inclusions, loss of microvilli, and abnormal mitochondria and nuclei.

A gill biopsy, in which a small portion of gill tissue was removed from anesthetized fish, was shown to have no detrimental effect on subsequent survival, growth, and salinity tolerance of juvenile Atlantic salmon (Salmo salar). A method for measurement of Na⁺, K⁺ -ATPase activity in this small amount of gill tissue is presented. These methods are useful for nonlethal monitoring of physiological smolt characteristics in salmonids and may have applications in the study of disease, toxicology, and physiological ecology of many fish species.

A technique is presented, which allows the rapid and precise determination of methylmercury in aqueous samples. The sample is first reacted with sodium tetraethylborate, to convert the nonvolatile monomethyl mercury to gaseous methylethylmercury. The volatile adduct is then purged from solution, and recollected on a graphitic carbon column at room temperature. The methylethylmercury is then thermally desorbed from the column, and analyzed by cryogenic gas chromatography with cold vapour atomic fluorescence detection. The method allows the simultaneous determination of labile Hg(II) species, through the formation of diethylmercury, and of dimethylmercury, which is not ethylated. The methylmercury detection limit is about 0.6 pg Hg, or 0.003 ng∙L⁻¹ for a 200-mL sample. The technique has been successfully applied directly to a wide variety of freshwater samples and alkaline tissue digestates. Seawater is analyzed following a simple extraction step to separate the methylmercury from the interfering chloride matrix. Analyses of natural surface waters have shown methylmercury levels typically in the range of 0.02–0.10 ng∙L⁻¹, with values as high as 0.64 ng∙L⁻¹ in a polluted urban lake. Waters collected from the anoxic bottom waters of a stratified remote lake have shown methylmercury levels as high as 4 ng∙L⁻¹ as Hg.

We monitored the change in the isotope composition of sulfur, carbon, and nitrogen in broad whitefish (Coregonus nasus) tissues in response to a change in the isotope composition of their food. One of two batches of 2.5-yr-old fish raised in the laboratory were given a new food source with different δ³⁴S, δ¹³C, and δ¹⁵N, which were monitored in muscle and liver tissue for 1 yr. A model including change due to tissue accumulation (growth) and metabolic replacement was developed. For all three isotopes, most of the change could be attributed to growth. Metabolic replacement expressed as a turnover rate was only 0.1 −0.2%∙d⁻¹ and was similar for the three isotopes. Although liver tissue was −4.4 and −4.1‰, respectively, for δ³⁴S and δ¹³C relative to muscle tissue, the response over time to the new food was the same as for muscle. We expect that the complete change in the isotope composition of fish tissue in response to a change in food could take years in slow-growing wild populations. The stable isotope composition would represent a long-term average of the food. In fast-growing fish the rate of change would directly reflect the growth rate.

Autocorrelation in fish recruitment and environmental data can complicate statistical inference in correlation analyses. To address this problem, researchers often either adjust hypothesis testing procedures (e.g., adjust degrees of freedom) to account for autocorrelation or remove the autocorrelation using prewhitening or first-differencing before analysis. However, the effectiveness of methods that adjust hypothesis testing procedures has not yet been fully explored quantitatively. We therefore compared several adjustment methods via Monte Carlo simulation and found that a modified version of these methods kept Type I error rates near . In contrast, methods that remove autocorrelation control Type I error rates well but may in some circumstances increase Type II error rates (probability of failing to detect some environmental effect) and hence reduce statistical power, in comparison with adjusting the test procedure. Specifically, our Monte Carlo simulations show that prewhitening and especially first-differencing decrease power in the common situations where low-frequency (slowly changing) processes are important sources of covariation in fish recruitment or in environmental variables. Conversely, removing autocorrelation can increase power when low-frequency processes account for only some of the covariation. We therefore recommend that researchers carefully consider the importance of different time scales of variability when analyzing autocorrelated data.

Range extensions of aquatic Ponto-Caspian macroinvertebrate species in Europe have mainly been facilitated by the interconnection of river basins through man-made canals and intentional introductions. Three inland migration corridors can be distinguished: (i) a northern corridor: Volga [Formula: see text] Lake Beloye [Formula: see text] Lake Onega [Formula: see text] Lake Ladoga [Formula: see text] Neva [Formula: see text] Baltic Sea, (ii) a central corridor connecting the rivers Dnieper [Formula: see text] Vistula [Formula: see text] Oder [Formula: see text] Elbe [Formula: see text] Rhine, and (iii) a southern corridor connecting the Danube and Rhine rivers. Important trade harbours in Europe were connected via these corridors allowing further range extensions of macroinvertebrate species attached to a vessel's hull or in ballast water. The central corridor was the main migration route before 1992, after which the southern corridor became the most important migration route for the range expansions to the west because of the reopening of the Main-Danube Canal, connecting the Rhine and Danube basins. Especially the water level maintenance in the upper part of the canal, with water supply from the Danube basin, facilitated migration of mobile animals (e.g., crustaceans) from the Danube basin towards the Rhine basin; however, contribution of other transport mechanisms (e.g., shipping) is expected in the near future.

A controversial precept of aquatic ecology asserts that low ratios of nitrogen to phosphorus (N:P) lead to noxious and sometimes toxic blooms of Cyanobacteria. Cyanobacteria dominance is a major risk to human and ecosystem health. The stoichiometric control of Cyanobacteria therefore has become central to freshwater resource management. This controversial concept is based on observed Cyanobacteria dominance in lakes with low N:P and the results of lab and field experiments. Here we analyze data from 99 of the temperate zone's most studied lakes and show that this model is flawed. We show that the risk of water quality degradation by Cyanobacteria blooms is more strongly correlated with variation in total P, total N, or standing algae biomass than the ratio of N:P. Risks associated with Cyanobacteria are therefore less associated with N:P ratios than a simple increase in nutrient concentrations and algal biomass.