Seasonal differences in nutrient uptake dynamics in coastal spring-fed ecosystems: the potential effects of Florida manatees and humans

Florida manatees (Trichechus manatus latirostris) may affect nutrient cycling through excretion or sediment bioturbation when they migrate from coastal environments to spring-fed ecosystems in the winter for thermal refugia. The combination of exceedingly clear water and migratory manatees associated with these spring-fed ecosystems also attract tourists, which may alter nutrient cycling via bioturbation. We assessed the effects of manatees and humans on benthic and water column nutrient uptake rates (NO₃⁻, NH₄⁺, PO₄³⁻) at Three Sisters Springs and effects of humans alone on uptake rates at Hunters Springs, two spring-fed ecosystems in Kings Bay, FL during October, January, and April. Manatees congregate in high abundances at Three Sisters Springs from November to March when they migrate into Kings Bay, but manatee abundances remain low at Hunters Springs year-round. Manatees are largely absent from Kings Bay by mid-March, but human presence is high at both sites beginning in March due to warming temperatures. We found three overall patterns: (1) large decreases in chl a and nutrient uptake were evident when manatees were present, likely from bioturbation and subsequent sedimentation; (2) humans alone decreased water column nutrient uptake rates and benthic chl a, but changes to benthic nutrient uptake were only evident at one site; (3) the water column could account for a large proportion of nutrient uptake despite having low chl a and suspended material. These results suggest that manatees and humans may function similarly by decreasing benthic chl a, but sediment bioturbation effects of manatees coupled with humans were greater than the effects of humans alone.