Seagrass growth and senescence exert a strong influence on flow structure and sediment transport processes in coastal environments. However, most previous studies of seasonal seagrass effects either focused on small‐scale field measurements or did not fully resolve the synergistic effects of flow‐wave‐vegetation‐sediment interaction at a meadow scale. In this study, we applied a coupled Delft3D‐FLOW and SWAN model that included effects of seagrass on flow, waves, and sediment resuspension in a shallow coastal bay to quantify seasonal seagrass impacts on bay dynamics. The model was extensively validated using seasonal field hydrodynamic and suspended sediment data within a seagrass meadow and a nearby unvegetated site. Our results show that seagrass meadows significantly attenuated flow (60%) and waves (20%) and reduced suspended sediment concentration (85%) during summer when its density reached a maximum. Probability density distributions of combined wave‐current bed shear stress within the seagrass meadow indicate that significant reductions in sediment resuspension during summer were mainly caused by flow retardation rather than wave attenuation. Although low‐density seagrass in winter resulted in much smaller reductions in flow and waves compared with summer meadows, small changes in winter seagrass density resulted in large differences in the magnitude of attenuation of flow and shear stress. Similarly, while high seagrass densities effectively trapped sediment during summer, small changes in winter density resulted in strong changes in net sediment flux into/out of the meadow. At our study site, low seagrass densities provided significant reductions in wintertime sediment loss compared to losses associated with completely unvegetated conditions.
Acoustic propagation measurements were collected in a seagrass meadow in a shallow lagoon for periods of over 65 h in winter and 93 h in summer. A bottom-deployed sound source transmitted chirps (0.1–100 kHz) every 10 min that were received on a four-receiver horizontal hydrophone array. Oceanographic probes measured various environmental parameters. Daytime broadband acoustic attenuation was 2.4 dB greater in summer than winter, and the median received acoustic energy levels were 8.4 dB lower in summer compared to winter. These differences were attributed in part to seasonal changes in photosynthesis bubble production and above-ground seagrass biomass.
more » « less- Award ID(s):
- 2023211
- NSF-PAR ID:
- 10472024
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- JASA Express Letters
- Volume:
- 3
- Issue:
- 1
- ISSN:
- 2691-1191
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Seagrass meadows are important carbon sinks in the global coastal carbon cycle yet are also among the most rapidly declining marine habitats. Their ability to sequester carbon depends on flow–sediment–vegetation interactions that facilitate net deposition, as well as high rates of primary production. However, the effects of seasonal and episodic variations in seagrass density on net sediment and carbon accumulation have not been well quantified. Understanding these dynamics provides insight into how carbon accumulation in seagrass meadows responds to disturbance events and climate change. Here, we apply a spatially resolved sediment transport model that includes coupling of seagrass effects on flow, waves, and sediment resuspension in a seagrass meadow to quantify seasonal rates of sediment and carbon accumulation in the meadow. Our results show that organic carbon accumulation rates were largely determined by sediment accumulation and that they both changed non‐linearly as a function of seagrass shoot density. While seagrass meadows effectively trapped sediment at meadow edges during spring–summer growth seasons, during winter senescence low‐density meadows (< 160 shoots m−2) were erosional with rates sensitive to density. Small variations in winter densities resulted in large changes in annual sediment and carbon accumulation in the meadow; meadow‐scale (hundreds of square meters) summer seagrass dieback due to marine heatwaves can result in annual erosion and carbon loss. Our findings highlight the strong temporal and spatial variability in sediment accumulation within seagrass meadows and the implications for annual sediment carbon burial rates and the resilience of seagrass carbon stocks under future climate change.
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Seagrasses play an important role in coastal ecosystems and serve as important marine carbon stores. Acoustic monitoring techniques exploit the sensitivity of underwater sound to bubbles, which are produced as a byproduct of photosynthesis and present within the seagrass tissue. To make accurate assessments of seagrass biomass and productivity, a model is needed to describe acoustic propagation through the seagrass meadow that includes the effects of gas contained within the seagrass leaves. For this purpose, a new seagrass leaf model is described for Thalassia testudinum that consists of a comparatively rigid epidermis that composes the outer shell of the leaf and comparatively compliant aerenchyma that surrounds the gas channels on the interior of the leaf. With the bulk modulus and density of the seagrass tissue determined by previous work, this study focused on characterizing the shear moduli of the epidermis and aerenchyma. These properties were determined through a combination of dynamic mechanical analysis and acoustic resonator measurements coupled with microscopic imagery and finite element modeling. The shear moduli varied as a function of length along the leaves with values of 100 and 1.8 MPa at the basal end and 900 and 3.7 MPa at the apical end for the epidermis and aerenchyma, respectively.
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The Atlantic goliath grouper (
Epinephelus itajara ) is the largest grouper species in the Atlantic and exhibits high site fidelity and limited range of movement. By 1990, the goliath grouper population in US waters had declined approximately 95% relative to unfished levels, leading to a harvest ban in 1990. Since then, the south Florida population has grown but the magnitude of recovery remains unknown due to uncertainties about life history characteristics. However, despite these unknowns, the state of Florida approved a limited recreational harvest of goliath grouper. In 2021, fine-scale habitat use of three juvenile goliath grouper was investigated using acoustic telemetry and a positioning solver. All three individuals exhibited high site fidelity as well as a diel habitat use pattern, utilizing seagrass habitat during the night and mangrove habitat during the day. Fine-scale acoustic telemetry provides insight into not only habitat use, but broader habitat preferences as well. This study illustrates the need to consider deep seagrass-dominated channels lined with red mangroves when protecting juvenile goliath grouper populations within Florida Bay, especially as the population is opened to harvest. -
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Shallow coastal ecosystems are threatened by marine heatwaves, but few long-term records exist to quantify these heatwaves. Here, 40-year records of measured water temperature were constructed for a site in a system of shallow bays with documented heatwave impacts and a nearby ocean site; available gridded sea-surface temperature datasets in the region were also examined. Water temperatures at both sites increased significantly though bay temperatures were consistently 3-4°C hotter in summer and colder in winter and were more variable overall, differences not captured in high-resolution gridded sea-surface temperature datasets. There was considerable overlap in heatwave events at the coastal bay and ocean sites. Annual heatwave exposure was similar and significantly increased at both sites while annual heatwave intensity was significantly higher at the bay site owing to the high variance of the daily temperature anomaly there. Event frequency at both sites increased at a rate of about 1 event/decade. Future simulations indicate all heatwave metrics increase, as do days above 28°C, a heat stress threshold for seagrass. Ocean temperatures on the U.S. mid-Atlantic margin have rarely exceeded this threshold, while summer bay temperatures commonly do, allowing ocean exchange with coastal bays to provide thermal relief to bay ecosystems. This will have changed by 2100, creating a thermal environment that threatens seagrass communities in these systems. Documenting such change requires development of long-term water temperature records in more shallow coastal systems.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1121/10.0016752
