Experimental sublethal exposures to nitrite and nitrate were conducted on adult G. holbrooki from six native populations in Florida and North Carolina (USA) and three introduced in Extremadura, Spain. All nine populations were selected based on the historic baseline of nitrogen pollution. Hemoglobin, respiratory rates, life history traits, and gene expression of exposed fish have been analyzed. The results revealed distinct patterns based on the biological complexity of the endpoints. Early-response endpoints, such as the gene expression in the gills, was primarily influenced by historic nitrogen pollution and by laboratory nitrite exposures. In contrast, endpoints of higher organizational complexity, such as respiratory rates and life history traits, were more strongly affected by the region of origin and nitrite exposure, with interactions observed in respiratory rates. These findings highlight the impact of nitrogen pollution on fish health and reveal that physiological responses to nitrite vary between populations of the same species.
This suggests that contemporary evolution has enabled native and introduced populations to better adapt to eutrophic conditions. However, regional characteristics appear to be conserved in life history traits in closely related populations, such as the invasive populations in Spain and their source population in NC. Both produce smaller embryos, which may explain the superior colonization abilities of fish from the expanding ranges of the species' distribution.
During the return phase we investigated the impact of nitrite pollution on three species of amphipods (Gammarus fossarum, G. pulex and G. roeselii) that play key roles in the rivers of the Main catchment area and have distinct colonization histories. Initially, lethal concentration values (LC50) were determined, serving as a basis for selecting sublethal nitrite concentration ranges. These ranges were then used to assess sublethal effects on gene expression, leaf litter consumption, and behavioral activity during 10-day experimental trials. The results revealed significant interspecies differences in responses to nitrite exposure. Notably, the native species, particularly G. fossarum, displayed much higher sensitivity compared to the more resilient G. roeselii. This heightened sensitivity among native species offers insights into ongoing shifts in species composition due to nitrogen pollution. These findings suggest that increasing nitrite pollution in aquatic ecosystems could drive further expansion of G. roeselii in headwaters. This has profound ecological implications, as the decline of sensitive native species may disrupt ecosystem functions and reduce biodiversity.
These results have been disseminated so far in two conferences (43th North American SETAC Meeting in Pittsburgh PA and the Southeast SETAC Meeting in Auburn AL), two internal seminars at the Goethe Universität Frankfurt, one at the University of Florida, one for professional management stakeholders (UF/IFAS CISMA series) and one as outreach dissemination in Mataró (Spain). The researcher has also been invited in November 2024 to give a lecture related to this project for the students of BSc Biology at the Universitat de Barcelona, and two online seminars the Universidad Nacional de Colombia. The data are presently in the final stages of analysis, and manuscripts for publication in ISI-indexed journals are being prepared.
