Tue, Aug 03, 2021:On Demand
Background/Question/Methods:
Increased water flow velocity (> 2 cm/s) is associated with nutrient loading that may drive changes in Everglades aquatic community dynamics. Flow-induced nutrient loading can change basal resource characteristics and create a gradient of consumer food quality revealed in stoichiometry (decreases in C:P) and fatty acid composition (increases in PUFA: MUFA). However, it is uncertain how these changes are transmitted to aquatic consumer density, community structure, and turnover rate. The Decomp Physical Model (DPM) created a gradient of flow velocity with distance from the S-152 culvert, where low-nutrient water (TP < 10 ppb) has been experimentally discharged in the wet season. In 2019 and 2020, 1-m2 throw-traps and drift fences were used biannually to sample small fishes (standard length < 8 cm) and macroinvertebrates while electrofishing was used to sample large fishes (standard length > 8 cm). The study area has 32 sites located in three transects, one in sloughs impacted by experimentally discharged flow, and two experience ambient flow velocities (< 2 cm/s). Small fish and macroinvertebrate density and biomass, submerged vegetation, emergent vegetation, periphyton volume, water depth and flow velocity were quantified. Three 5- minute electrofishing transects were conducted at each site to capture large fish catch-per-unit-effort (CPUE).
Results/Conclusions: In experimental enclosures, primary consumers have shown the most consistent responses to flow-induced nutrient enrichment, while omnivores and secondary consumers revealed heterogeneous responses. The field data revealed a negative relationship between small fish density, large fish CPUE, and distance from the S152 culvert (P < 0.05). We have observed elevated nutrients in areas near the S-152 structure that decreased sharply with distance from the inflow structure. Flow-induced nutrient loading seems to be driving responses in community composition and species abundances. In 2019, fish density decreased over the first kilometer downstream from the S-152 structure and was generally the lowest at sites furthest from water delivery. In contrast, we detected higher abundances of macroinvertebrates in sites furthest away from the S-152 culvert at flow-impacted sloughs (P < 0.05). In throw-trap samples, community composition differed with distance from S152 culvert and treatment (P < 0.05), and flow impacted sites were dominated by less fish species than other sites. In contrast to results in experimental enclosures, primary consumers revealed little response to flow-induced nutrient enrichment, while secondary consumers displayed a pattern correlated to it. This suggests stronger top down effects with increased flow-induced enrichment and possibly a trophic cascade.
Results/Conclusions: In experimental enclosures, primary consumers have shown the most consistent responses to flow-induced nutrient enrichment, while omnivores and secondary consumers revealed heterogeneous responses. The field data revealed a negative relationship between small fish density, large fish CPUE, and distance from the S152 culvert (P < 0.05). We have observed elevated nutrients in areas near the S-152 structure that decreased sharply with distance from the inflow structure. Flow-induced nutrient loading seems to be driving responses in community composition and species abundances. In 2019, fish density decreased over the first kilometer downstream from the S-152 structure and was generally the lowest at sites furthest from water delivery. In contrast, we detected higher abundances of macroinvertebrates in sites furthest away from the S-152 culvert at flow-impacted sloughs (P < 0.05). In throw-trap samples, community composition differed with distance from S152 culvert and treatment (P < 0.05), and flow impacted sites were dominated by less fish species than other sites. In contrast to results in experimental enclosures, primary consumers revealed little response to flow-induced nutrient enrichment, while secondary consumers displayed a pattern correlated to it. This suggests stronger top down effects with increased flow-induced enrichment and possibly a trophic cascade.