We investigated the carbon, nitrogen, and phosphorus levels in whole fish and gut samples of several species of cyprinids, relating our findings to nutrient flux models. Some differences in whole-fish nutrient content across species, lakes, and seasons, as well as differences across fish length and mass, were found. N and P contents were highest in fathead minnows and lowest in pearl dace, with northern redbelly dace and finescale dace intermediate. Larger fish had higher percent C and lower percent N and P. However, all differences in whole fish C, N, and P chemistry were small. Cyprinids had the following mean composition: carbon, 46%; nitrogen, 9.7%; and phosphorus, 1.5%. The cyprinid molar C:N:P ratio was 242:16:1. These values make cyprinids relatively low in phosphorus compared to other fish that have been previously studied, especially members of the Percidae and Centrarchidae. Gut contents were lower in N and P than the whole fish, and C:N and C:P ratios were correspondingly higher in gut contents than in the whole fish. Thus, minnows must concentrate both of these nutrients within their biomass compared to what they eat. The N:P ratio of minnows and minnow gut contents had nearly identical means. All chemical variables showed lower variation in the fish than in the gut contents, supporting a homeostatic model of nutrient flux. Stable nitrogen isotope analysis found that minnows were similar to 3 parts per thousand (parts per thousand) heavier than their gut contents, providing evidence that gut contents analyzed were derived mainly from ingested material. A homeostatic nutrient model appears to be an appropriate one for fish.
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