Winter Manatee Foraging Behavior and the Decline of Seagrass Beds in the Northern Indian River Lagoon

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With high numbers of manatees using the Florida Power and Light power plant warm water refuge during winter, their impact on the seagrass beds in the Indian River Lagoon is considered an important indicator of the long-term capacity of the area to support the manatees. USGS is working with partners to investigate the spatial extent and intensity of manatee use of seagrass beds in the area. 

Winter Manatee Foraging Behavior and the Decline of Seagrass Beds in the Northern Indian River Lagoon
Manatees use power plant warm water refuge during winter in the Indian River Lagoon, Florida. 

The Science Issue and Relevance: Our initial interest in this study was the impact of winter feeding of Florida manatees on seagrasses in the northern Indian River Lagoon (IRL). With high numbers of manatees (1000+ animals) using the Florida Power and Light (FPL) power plant warm water refuge during winter, their impact on the nearby seagrass beds is considered to be an important indicator of the long-term capacity of the area to support manatees. To accomplish this objective, we are investigating the spatial extent and intensity of manatee use of seagrass beds in the area. Data collection was initiated during the winters of 2009/2010 and conducted annually through 2014/2015. Coincidental with these studies, a major die-off of submerged aquatic vegetation (SAV) occurred in summer 2011 due to an algal bloom that caused historically high turbidity. Data collection was adapted to assist in documenting changes to SAV beds. Changes in manatee foraging strategies are being documented through radio tracking data collected prior to and during the forage plant die-off, and through to the start of recovery.

Methodology for Addressing the Issue: USGS is cooperating with staff from the Florida Fish and Wildlife Conservation Commission and the St. Johns River Water Management District to document fine-scale manatee habitat use within the IRL and the FPL refuge in relation to temperature gradients, depth, and other physical features. Data for this project are collected via field observations and monitoring of individuals tracked with specially designed Global Positioning System (GPS) tags. Physical environmental factors, such as depth, temperature, and substrate, are being characterized for areas associated with manatee use. Seagrass transects and sight characterizations provide insights on specific foraging areas, resources used, and potential forage carrying capacity for manatees in this portion of the IRL. We are correlating environmental data (benthic habitat type, ambient and FPL Canaveral refuge water temperatures) with manatee movement and habitat use. Field efforts include site characterization, temperature monitoring, and documentation of habitat use patterns through radio tracking of tagged manatees. Through analysis of the movement information, we have identified manatee use strata in the IRL that are heavily used, lightly used, or not used by manatees foraging from the winter refuge at the power plant. We then selected transects and monitoring sites to characterize benthic habitats and SAV across these use strata. Variables of interest are shoot count and species composition of seagrasses. 

Winter Manatee Foraging Behavior and the Decline of Seagrass Beds in the Northern Indian River Lagoon
USGS is working with partners to investigate spatial extent and intensity of manatees' use of seagrass beds in the IRL.

Future Steps: SAV and benthic data from transects are being collected and analyzed to determine changes through these periods. Aerial imagery acquired by cooperators is also being used to document changes to the spatial extent of SAV and other benthic habitats. The 2011 seagrass die-off was followed by another major algal bloom in summer 2012 resulting in exceedingly high turbidity. Additional impacts to the IRL ecosystem similar to those mentioned are projected to occur.

Related project(s) or product(s): This study leverages the ongoing manatee tracking and tagging study to study the manatee response related to the repowering of the FPL Canaveral plant. USGS-Sirenia is collaborating with FWC/FWRI for that study. Documentation of changes to SAV and manatee use patterns in the IRL is critical for understanding and managing the historic IRL seagrass die-off. Manatee captures for radio tagging also provided opportunities for manatee health assessments conducted by USGS/WARC staff and cooperators. Tracking activities also enhance the USGS/WARC Manatee Individual Photo-Identification System field efforts by enabling photo-identification of scarred manatees at alternate sites.