2003. Variation in trophic shift for stable isotope ratios of carbon, nitrogen, and sulfur. -Oikos 102: 378-390.Use of stable isotope ratios to trace pathways of organic matter among consumers requires knowledge of the isotopic shift between diet and consumer. Variation in trophic shift among consumers can be substantial. For data from the published literature and supplementary original data (excluding fluid-feeding consumers), the mean isotopic shift for C was + 0.5 9 0.13‰ rather than 0.0‰, as commonly assumed. The shift for C was higher for consumers analyzed as muscle ( + 1.3 9 0.30‰) than for consumers analyzed whole ( + 0.3 9 0.14‰). Among consumers analyzed whole, the trophic shift for C was lower for consumers acidified prior to analysis ( −0.2 90.21‰) than for unacidified samples ( + 0.59 0.17‰). For N, trophic shift was lower for consumers raised on invertebrate diets ( + 1.4 90.21‰) than for consumers raised on other high-protein diets ( + 3.39 0.26‰) and was intermediate for consumers raised on plant and algal diets ( + 2.29 0.30‰). The trophic shift for S differed between high-protein ( +2.0 9 0.65‰) and low-protein diets (-0.5 90.56‰). Thus, methods of analysis and dietary differences can affect trophic shift for consumers; the utility of stable isotope methods can be improved if this information is incorporated into studies of trophic relationships. Although few studies of stable isotope ratios have considered variation in the trophic shift, such variation is important because small errors in estimates of trophic shift can result in large errors in estimates of the contribution of sources to consumers or in estimates of trophic position.

Nitrite is an intermediate in the oxidation of ammonium to nitrate. An elevated ambient nitrite concentration is a potential problem for freshwater fish since nitrite is actively taken up across the gills in competition with chloride. Nitrite is a well-known toxicant for fish as well as a disrupter of multiple physiological functions including ion regulatory, respiratory, cardiovascular, endocrine and excretory processes. One critical consequence of nitrite accumulation is the oxidation of haemoglobin to methaemoglobin, compromising blood oxygen transport. Nitrite toxicity to fish varies considerably and depends on a large number of external and internal factors. Among the most important ones are water quality (e.g. pH, temperature, cation, anion and oxygen concentration), length of exposure, fish species, fish size and age, and individual fish susceptibility. Chloride concentration in water is considered one of the most important factors influencing nitrite toxicity to fish. The importance of individual factors is assessed and re-evaluated continuously.

Control of lacustrine phytoplankton biomass by phosphorus is one of the oldest and most stable paradigms in modern limnology. Even so, evidence from bioassays conducted by multiple investigators at numerous sites over the last three decades shows that N is at least as likely as P to be limiting to phytoplankton growth. A number of important flaws in the evidence supporting the phosphorus paradigm have contributed to an unrealistic degree of focus on phosphorus as a controlling element. These include insufficient skeptism in interpretation of: 1) the phosphorus : chlorophyll correlation in lakes, 2) the results of whole-lake fertilization experiments, and 3) stoichiometric arguments based on total N:total P ratios for inland waters. A new paradigm based on parity between N and P control of phytoplankton biomass in lakes seems more viable than the P paradigm. The new paradigm renews interest in the degree to which plankton communities are molded in composition by small differences in relative availability of N and P, the mechanisms that lead to a high frequency of N limitation in oligotrophic lakes, and the failure of aquatic N-fixers to compensate significantly for N deficiency under most conditions. A new N/P paradigm still must acknowledge that suppression of P loading often will be the most effective means of reducing phytoplankton biomass in eutrophic lakes, even if N is initially limiting. From error to error one discovers the entire truthSIGMUND FREUD

SUMMARY. 1. Limiting nutrients for phytoplankton were studied experimentally in eight mountain lakes of central Colorado between May and November of 1984. 2. Five categories of phytoplankton limitation were identified: no limitation, N limitation, P limitation, concurrent limitation (stimulation only by simultaneous additions of N and P), and reciprocal limitation (stimulation by addition of either N or P). The phytoplankton communities of three lakes were primarily N‐limited, one was primarily phosphorus‐limited, and four showed primarily combined limitation (concurrent or reciprocal). Switching between categories of limitation was also observed within lakes. Nitrogen was the most frequently limiting nutrient; N, either alone or in combination with P, accounted for 79% of all observed instances of limitation. 3. Nine indices were tested for effectiveness in predicting phytoplankton limitation by N and P. The best indices for discriminating all limitations were ratios of dissolved inorganic N: total P (84% accuracy) and dissolved inorganic N:total dissolved P (80% accuracy). The effectiveness of these indices may be explained by the degree to which they represent N and P fractions actually available to the phytoplankton.