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 * 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-clay, declined with increasing REI 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^-1, which should be near the initiation of motion current speed for local sand sediments, and an REI of ~3 * 10⁶. 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.

KEY WORDS: Seagrass · Landsape · Pattern · Waves · Current · Water depth · Sediment · Disturbance

Published in MEPS Vol. 171 (1998) on October 1
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1998