Trophic dynamic interactions in a temperate river: A comparison between high and lower karst flow reaches
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. Seasonal growth of the filamentous alga, Cladophora, is typically associated with nutrient-rich waters. Our research compared food web structure between riverine reaches with contrasting underlying karst topography, nutrient levels, and Cladophora cover during late spring. 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 the assimilated dietary contributions to primary consumers 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?
Primary consumer niche breadth, measured by the range of d13C of primary consumer taxa, was similar between the upstream and downstream reaches. Even though niche breadth was similar, Cladophora cover was significantly higher in downstream reaches, suggesting that this food resource was markedly more available. Overall, the δ13C stable isotopes of consumers decreased longitudinally, with over half the consumer taxa becoming increasingly δ13C-depleted downstream. Some in-stream food resources also displayed a similar longitudinal trend, becoming more δ13C-depleted downstream. While primary consumer stable isotopic signatures differed between upstream and downstream reaches, this may have reflected isotopic changes in in-stream producer isotopes rather than actual differences in consumer diets. Linear mixing models suggested that consumers assimilated low to moderate amounts of filamentous algae in all study reaches, likely due to the low availability of filamentous algae a few weeks before the study period in mid-spring. Stable isotopic results in this study may be more representative of primary consumer diets during pre-Cladophora bloom periods.