Evaluating seasonal variation in basal resource use and trophic niche space among four prairie river fishes

Authors: Wade Wilson, Jane Rogosch, Scott Collins, Bart Durham

Year: 2024

Abstract

Environmental variation is known to influence the quality, quantity, and availability of instream and riparian food resources for stream consumers. We assessed the effects of seasonal variation on food resource use among an understudied group of southern prairie river fishes of conservation need. Focal species included plains minnow, Hybognathus placitus, prairie chub, Macrhybopsis australis, Red River shiner, Notropis bairdi, and Red River pupfish, Cyprinodon rubrofluviatilis, collected from the upper mainstem Red River during spring, summer, and fall of 2023. We hypothesized that the relative importance of instream and riparian resources would differ seasonally and predicted that fishes would exhibit greater reliance on instream resources during the summer months due to greater diversity and abundance of food items such as algae and aquatic macroinvertebrates. Mean isotope values of δ15N and δ13C varied among species and across seasons. However, the interaction was only significant for δ13C values (F6,544 = 8.12, p < 0.001), suggesting that differences in basal resource use among species are dependent on season. These results were supported by observed differences in δ13C diversity during spring (range = 11.82‰), summer (10.95‰), and fall (range = 12.40‰). Isotope values of δ15N were highest during the summer (mean = 12.75‰) and lowest during the fall (10.67‰), with intraspecific differences observed between these two seasons for each species (p < 0.03). Diet diversity peaked during the summer (H/ = 2.08), though this was likely a result of increased macroinvertebrate abundance and availability (proportion of diets = 0.58). During fall, low flow conditions may have confined fish to lower trophic level items, evidenced by lower δ15N values and diets that consisted of relatively higher proportions of detritus (0.40), filamentous algae (0.04), and diatoms (0.14) when compared to other seasons. Diet diversity was also lowest during the fall (H/ = 1.68). Subsequent trophic richness, breadth, and overlap analyses will provide additional data to aid in interpretation of seasonal patterns (driven by variations in flow and productivity) and how they affect food-web structure. Resulting baseline information about food-web structure may provide a reference from which to compare how prairie fishes may be affected by ongoing river regulation and fragmentation.