Possible limitation of growth and distribution of freshwater organisms by the availability of potassium, an essential major bio-constituent and plantnutrient, is considered for inland waters. It is interpreted in relation to the range of concentrations normally encountered and experimental work on specific growth rates of algae at low concentrations, minimum cell and biomass quotas of K, biomass yields under graded additions, inhibition at higher concentrations, and response to cation-ratios in the medium.

The range of K concentrations in inland waters is surveyed. Most concentrations exceed 10 µmol L^-1, and are greatly in excess of those (under 1 µmol L^-1) found limiting specific growth rates of test species. They are also in excess of the content in most natural populations of phytoplankton when yields have the experimental minimum or limiting cell/biomass quota, of the order of 1% dry weight. Limiting concentrations for specific growth rate in nature are therefore probably rare. They, and yield limitations, might possibly be reached after extreme depletion by dense stands of aquatic macrophytes; some depletions are recorded for a charophyte and water-cress (Rorippa nasturtium-aquaticum). Retention by seasonal vegetation in the catchment can more than offset delivery in rainfall and result in minimal concentrations of between 1 µmol L^-1 and 5 µmol L^-1. There is evidence that these concentrations, combined with poor nutrition, can limit the distribution of larger Crustacea in some upland streams. Other less substantiated sources of limitation in nature relate to ratios of cationic concentrations, in part for function of algal flagella. There is some experimental evidence for growth- and yieldlimitation of species of the chrysophyte Dinobryon by higher and naturally occurring K concentrations.

While many laboratory and field studies show that zooplankton are negatively affected when exposed to high intensities of ultraviolet radiation (UVR), most studies also indicate that zooplankton are well adapted to cope with large variations in their UVR exposure in the pelagic zone of lakes. The response mechanisms of zooplankton are diverse and efficient and may explain the success and richness of freshwater zooplankton in optically variable waters. While no single behavioural or physiological protection mechanism seems to be superior, and while several unexplained and contradictory patterns exist in zooplankton UVR ecology, recent increases in our understanding are consistent with UVR playing an important role for zooplankton. This review examines the variability in freshwater zooplankton responses to UVR, with a focus on crustacean zooplankton (Cladocera and Copepoda). We present an overview of UVR-induced damages, and the protection and recovery mechanisms freshwater zooplankton use when exposed to UVR. We review the current knowledge of UVR impact on freshwater zooplankton at species and community levels, and discuss briefly how global change over the last three decades has influenced the UVR milieu in lakes.

Appreciation of pH as an ecological index has varied considerably over its past history, influenced by perceptions of chemical rigour, ease or difficulty of measurement, and multiple chemical and biological correlations. These factors, and especially the last, are considered in relation to the extensive range of pH in inland waters. Emphasis is placed upon the role of the CO₂ system, the components of which are subject to biological metabolism (photosynthesis, respiration) and are extensively determined by products from rocks (e.g. limestone) and soils.

Titration alkalinity, or acid neutralising capacity, is a most valuable summarising and reference measure. For this, and CO₂ variables, Potentiometric Gran titration opens new possibilities - including the definition of negative alkalinity (acidity). The relationship of pH and titration alkalinity is close and semi-logarithmic for waters in equilibrium with atmospheric CO₂. Very high pH, above 10, can develop from the photosynthetic depletion of CO₂ and by the evaporative concentration of bicarbonatecarbonate waters in closed basins. Very low pH, below 4.5, results from the introduction of strong acids by volcanic emissions, pyrite oxidation, ‘acid rain’ and cation exchange; here the CO₂ system lacks influence, biological diversity is reduced, and ionic aluminium often exerts toxic biological effects.

Situations of pH excursion are discussed and illustrated; they operate over day—night, seasonal and longterm time-scales. A summer rise of pH is widespread in productive near-surface waters. There is also a seasonal pH rise in the anoxic deep water of many lakes, as a consequence of the interaction of acid-base and oxidation-reduction systems. These can be regarded as two ‘master systems’ of environmental chemistry and — dating from pioneer studies of wetland soils and waters — of much freshwater ecology.

Governmental water quality agencies are faced with identifying water quality goals to protect aquatic biota, applying the best available science in development of water quality standards and associated management principles, implementing water quality laws so that there is consistency with goals, developing guidance documents for applying science to law, monitoring, and enforcement. Deviations in this path from goals to standards to enforcement, however, are common across countries as well as among States within the United States and can result in failure to protect the aquatic biota.

The Clean Water Act (CWA) is the key US law for water quality protection. Its goal is to ‘restore and maintain the chemical, physical, and biological integrity of the Nation's waters’ and to fully protect the most sensitive beneficial uses. The US Environmental Protection Agency (EPA) Gold Book guidance for development of protective water temperature standards, dating from 1973, still recommends the use of MWAT (Maximum Weekly Average Temperature) as an index for assigning protective chronic temperature standards to coldwater fisheries. MWAT, applied according to EPA guidance, is typically used in conjunction with an acute upper limit. Unfortunately, MWAT is a criterion that is not protective, as can be shown by reference to several case studies on salmonids. Use of MWAT at a basin scale can result in considerable reduction in available salmonid rearing area and can, in many cases, be little better than recommending the upper incipient lethal temperature as a standard. Although MWAT is not used by many US States in standards, it is problematic in that it is cited as the official EPA model for a protective standard. The conceptual use of MWAT highlights some critical problems in application of the Clean Water Act and its associated federal regulations for protection of coldwater fishes, such as the concepts of full protection, protection of the most sensitive species, restoration of water quality, and support of species' viability at a basin scale.

Full implementation of the CWA in support of salmonids' thermal requirements has been patchy, with some States taking criteria development, monitoring, listing, and TMDL (Total Maximum Daily Load) development seriously and others virtually ignoring the problem. In addition, Section 316 of the CWA conflicts with the basic goals of the CWA by giving deference to the thermoelectric power industry to discharge heated effluent under a process where variances granted supersede water quality-based limits. This is exacerbated by EPA 316 guidance permitting evaluation of biological trends amidst shifting or uncertain baselines in a limited set of RIS (Representative Important Species), rather than application of best available science to protect both the most sensitive species and the entire aquatic community that is reflective of high quality habitat conditions. Designing water temperature standards to be fully protective and supportive of species viability (abundance, productivity, spatial structure, and diversity) benefits from application of concepts of optimum growth and survival temperatures distributed at a basin scale with reference to the natural thermal potential along a river continuum. Some notable successes in standards development found in the US Pacific Northwest are offered as future national models.