Abstract
Seagrass is a major structural habitat in the Indian River Lagoon. Maps documented locations and areal extents of beds periodically since the 1940s, and surveys of fixed transects yielded changes in percent cover and depths at the end of the canopy since 1994. Areal extent increased by ∼7,000 ha from 1994 to 2009, mean percent cover within beds decreased from ∼40 to 20%, and mean percent cover standardized to maximum transect length remained near 20%. Thus, conditions supported a consistent biomass because cover decreased as areal extent increased. Between 2011 and 2019, ∼19,000 ha or ∼58% of seagrasses were lost, with offshore ends of canopies moving shoreward and shallower, and standardized mean percent cover decreased to ∼4%. These changes coincided with blooms of phytoplankton, and ≤ 27% of incident subsurface irradiance at 0.9 m was stressful. Decreases in mean percent cover per month of stress became larger when initial mean cover per transect was < 20%, which suggested that the ratio of aboveground to belowground tissues in the expanded and sparser beds led to respiratory demand that was not met by photosynthesis. Despite intermittent improvements in light penetration, widespread recovery of seagrasses has not occurred potentially due to detrimental feedbacks. For example, loss of seagrass exposed sediments to waves, and the resulting disturbance may have hampered recruitment of new shoots. The same decreases also made 58–88% of the carbon, nitrogen, and phosphorus in seagrass tissue available to other primary producers. These nutrients did not enhance growth of epiphytes, whose biomass decreased by ∼42%, but they apparently fueled blooms of phytoplankton, with mean chlorophyll-a concentrations increasing by > 900%. Such intense blooms increased shading and loss of seagrasses. Fortunately, data showed that patches of seagrasses at depths of 0.5–0.9 m persisted for 22–24 years, which suggested that this depth zone could hold the key to recovery. Nevertheless, optimistic estimates predict recovery could take 12–17 years. Such a long-term, widespread loss of a key structural habitat may generate multiple adverse effects in the system, and mitigating such effects may entail planting seagrasses to accelerate recovery.
Highlights
Seagrasses have been termed “coastal canaries” because they integrate and respond visibly and relatively rapidly to a variety of anthropogenic stressors (Orth et al, 2006)
We examine how the distribution and abundance of seagrasses in the Indian River Lagoon (IRL) responded to spatiotemporal patterns in salinity, temperature, and the availability of light
Based on the mean of the environmental limits established for H. wrightii, S. filiforme, and T. testudinum (Morris et al, 2021) and the persistence of seagrass at depths of 0.5–0.9 m, limits for stress were set at salinities below 23 psu, water temperatures below 20◦C, and percent of subsurface irradiance reaching 0.9 m below 27%
Summary
Seagrasses have been termed “coastal canaries” because they integrate and respond visibly and relatively rapidly to a variety of anthropogenic stressors (Orth et al, 2006). Signs of stress include loss of species richness (Hemminga and Duarte, 2000; Morris et al, 2021) and loss of areal extent (Orth et al, 2010; Lefcheck et al, 2017; Morris et al, 2018) Such changes represent a loss of refuge and food for fauna, including commercially and recreationally important species, manatees, and sea turtles (Lewis, 1984; Unsworth et al, 2018). Reduced availability of light is not the only stressor affecting seagrass, with salinity and temperature being two primary factors not related to physical disturbance, increased sedimentation, or excessive loads of nutrients and subsequent eutrophication (Dennison et al, 1993; Duarte, 2002; Fourqurean et al, 2003; Greve and Binzer, 2004; Lee et al, 2007). The resulting understanding is translated into effects on nutrient cycling, recommendations for management, and questions for further investigation
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
