Seagrass in a Changing Estuary, the Indian River Lagoon, Florida, United States

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.

The Abundance and Persistence of Temperate and Tropical Seagrasses at Their Edge-of-Range in the Western Atlantic Ocean

“…As coastal development and population growth continue to proceed at a rapid pace (Mallin et al, 2000;Neumann et al, 2015) more direct and indirect delivery of nutrients to polyhaline waters could further contribute to an accelerated enrichment of chlorophyll a and the impairment of optical water quality (Paerl et al, 2018). In this future scenario we hypothesize that there would be a higher probability that stressful summer temperatures would then begin to significantly impact Z. marina comparable to what has been occurring in other thermally stressed temperate and tropical seagrass systems (Shields et al, 2018;Morris et al, 2022).…”

“…However, when the area and biomass data are examined concurrently the significantly lower total seagrass biomass observed post 2008 and the decline in meadow area extent between 2013 and 2019 may indicate a reduction in the resilience of the meadows over the last ten years of our dataset. During this period, the cumulative effect of increasing summer heat stress coupled with periodic and shorter term fluctuations in optical water quality may be "thinning" seagrass biomass (Morris et al, 2022).…”

“…As one of the largest seagrass ecosystems in the western Atlantic (Green and Short, 2003;Field et al, 2021), it is widely recognized as critically important fish and wildlife habitat for the mid-and south Atlantic regions of the U.S. (Thayer et al, 1975;Havel, 2018), while also providing important ecological structure, functions, and economic value for local and regional coastal communities (Thayer et al, 1984;NCDEQ (North Carolina Department of Environmental Quality), 2016). Given the severe declines in neighboring seagrass ecosystems in Chesapeake Bay (Lefcheck et al, 2018) and further south along the east coast of Florida (Morris et al, 2022), the local and more extensive regional implications of climate related effects on the North Carolina seagrass ecosystem are not trivial.…”

Population decline of Gulf pipefish (Syngnathus scovelli) in a subtropical estuary: ecosystem changes and habitat loss

“…Dramatic long-term declines in the S. scovelli population were observed in the IRL during the study period. While the population declines were concurrent with large-scale declines in seagrass coverage, habitat, and water-quality degradation (Morris et al 2018(Morris et al , 2022Lapointe et al 2020), moderate declines in S. scovelli were observed with moderate downward trends in seagrasses before the onset of the precipitous seagrass reductions observed during the middle of the study period (2010)(2011)(2012). The declines observed in this sentinel species indicate chronic impairment to the estuary that may have pushed the system beyond a functional threshold for many fish species, including those that support regional fisheries, despite ongoing efforts to restore lost habitat and reduce excess nutrient inputs (Adams et al 2019).…”

Aquatic Vegetation, an Understudied Depot for PFAS

“…Growing anthropogenic pressures, such as increased coastal development, and climate change have led to an ongoing decline of global vegetation, seagrasses in particular . For example, the Indian River Lagoon (IRL) in Florida experienced a 60% reduction in total seagrass extent from 1943 to 1999, with additional loss of ∼19,000 ha between 2011 and 2019, attributed to, among several proposed causes, eutrophication-induced light limitation . However, apart from macronutrients, − the extent of environmental pollutant uptake in AV has remained unexplored in many vital aquatic systems, especially those that are currently experiencing unprecedented loss of seagrass.…”