Influence of physical setting on seagrass landscapes near Beaufort, North Carolina, USA

Abstract

Field surveys were conducted in Core and Back Sounds, North Carolina. USA, to relate the physical setting of seagrass beds, as measured by a wave exposure index (REI), tidal current speed and water depth, to various measures of the sedimentary environment, spatial heterogeneity of seagrass distribution and measures of seagrass abundance. Seagrass beds in this area form patterns ranging from continuous to semi-continuous to widely dispersed, discrete patches across a gradient of increasing hydrodynamic activity. Tidal current speeds, exposure to waves and relative water depths revealed strong correlative evidence that physical processes influenced landscape-scale (50 X 50 m range with 1 m resolution) features of seagrass beds. Some habitat attributes, such as percent cover of seagrasses, seagrass bed perimeter to area ratio, sediment organic content and percent silt-cldy, declined with increasing REJ and current speed. Increased aggregation of these data was observed above and below the 50% seagrass cover value, and, supported by principle components analysis, signaled an abrupt transition of environmental setting correlated with this coverage level. The 50% cover value also occurred at current speeds of -25 cm S-', which should be near the initiation of motion current speed for local sand sediments, and an RE1 of -3 X 10h This coverage is also near to the 59.28% coverage value, previously determined to be where landscape elements join and below which they tend to exist as discrete patches. We hypothesize that a rapid loss of seagrass habitat structural integrity may occur as the habitat fragments, and seagrass landscape elements become isolated, contributing to the observed transition and accompanying data aggregation above and below the -50% coverage level. We discuss the interaction of physical setting, disturbance, landscape contiguity and growth response by these modular plants in the production of the observed landscape patterns across these hydrodynamic gradients.

Seagrass–Sediment Feedback: An Exploration Using a Non-recursive Structural Equation Model

“…Hydrodynamic stress, caused by currents and waves, negatively impacts on seagrass density because it inflicts direct damage to the plants or causes uprooting due to erosion of sediment (Fonseca and Bell 1998;Koch 2001). Because we had no information on currents and wave exposure, we used seagrass cover surrounding a particular observation point (irrespective of the density at the sample point) as a proxy.…”

“…First, by decreasing water currents and waves, seagrass meadows reduce the constant movement of sediment and hydrodynamic drag which negatively affect shoots (Fonseca and Bell 1998;Koch 2001;Madsen and others 2001). Second, by retaining receding water, seagrasses may reduce negative effects of desiccation (Powell and Schaffner 1991;Boese and others 2005) which benefits photosynthesis and growth.…”

Effect of environmental factors (wave exposure and depth) and anthropogenic pressure in the C sink capacity of Posidonia oceanica meadows

“…Among them, the exposure to waves, which determines the patterns of sedimentation and may lead to sediment resuspension and erosion during high-energy events (Van Keulen and Borowitzka 2003;Bradley and Houser 2009;Hansen and Reidenbach 2012). Indices of wave exposure, such as effective fetch, have been successfully used as predictors of sediment grain size and percentage of sediment organic carbon (% C org ) in seagrass meadows, where those with smaller sediment grain size and higher C org concentration usually coincide with low-energy environments (Murphey and Fonseca 1995;Fonseca and Bell 1998). Because of this close coupling between sediment grain size and coastal hydrodynamics, grain size may be used as an indicator of hydrodynamic conditions (Cabaço et al 2010).…”

Contributions of the direct supply of belowground seagrass detritus and trapping of suspended organic matter to the sedimentary organic carbon stock in seagrass meadows

“…This value is also among the highest among globally compiled data (Duarte and Chiscano, 1999). Possible reasons for the exceptional development of belowground biomass include (1) morphological plasticity for resistance to high wave energy (Fonseca and Bell, 1998), which is supported by the low mud content at our sites compared to that reported by previous studies (Koch, 2001;Serrano et al, 2016a), and (2) nutrient limitation, 25 which can lead to more allocation of biomass to belowground parts to enable the plant to acquire nutrients in deeper sediment layers (Lee et al, 2007). The low OCsed may be attributable to (1) high wave energy in association with increased OC lability due to the low specific surface area of sediments (Miyajima et al, 2017) and (2) the low gross primary production/respiration (P/R) ratio in this geographical region (Duarte et al, 2010).…”