Aquatic community structure and biomass response to experimental water flow in an oligotrophic wetland

Marco Fernandez, Biological Sciences, Florida International University, Miami, FL, Joel C. Trexler, Biological Science, Florida State University, St. Teresa, FL and Erik Tate-Boldt, Everglades Systems Assessment, South Florida Water Management District, West Palm Beach, FL

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.