Trophic dynamic interactions in a temperate river: a comparison between high and lower karst flow reaches

Background/Question/Methods

Surface streams in karst landscapes are often characterized by high nutrient levels due to incomplete filtration through series of innumerable, below-ground conduits. These conduits can transport this nutrient-rich groundwater into surface streams, potentially leading to higher nutrient levels in the surface water bodies.  Prolific growth of the filamentous alga, Cladophora, is typically associated with nutrient-rich waters. Our research compared foodweb structure between riverine reaches with contrasting underlying karst topography, nutrient levels, and Cladophora cover. Recent work in these reaches has shown a high correlation between Cladophora cover and nutrient content, particularly nitrate. Two interrelated questions regarding the role of Cladophora in riverine food webs were addressed, (1) are consumers’ assimilated dietary contributions from Cladophora higher in the reaches with well-developed karst topography as compared to weakly-developed karst reaches?, and (2) are niche breadths for primary consumers narrower in the reaches with developed karst topography due to the high Cladophora growth compared to weakly-developed karst reaches with sparse Cladophora growth?

Results/Conclusions

The range of δ¹³C values of potential food resources was identical between upstream and downstream reaches. Primary consumer niche breadth, measured by the range of δ¹³C of primary consumer taxa, is also similar between the upstream and the downstream reaches. Even though niche breadth was similar, Cladophora cover was markedly higher in downstream reaches, suggesting that this food resource was more available. Stable isotopic analyses using IsoSource revealed that Cladophora comprised a higher proportion of primary consumers’ diets in the downstream reaches compared to the upstream reaches with less Cladophora cover. The estimated assimilated dietary contribution of Cladophora for four abundant insect taxa (Isonychia, Cheumatopsyche, Pycnopsyche, Stenelmis) was much higher in the downstream, high Cladophora cover reaches. In contrast, estimated dietary contributions of Cladophora for Lepidostoma and Maccaffertium were very similar between reaches. Our results indicate that high nutrient levels associated with well-developed karst landscapes may be a prominent factor shaping primary consumer diets, and subsequently, foodweb structure of whole riverine communities.