Relative effects of physical and small-scale nutrient factors on the distribution of tropical seagrasses in the Great White Heron National Wildlife Refuge, Lower Florida Keys

• We evaluated physical factors and nutrients on % cover of seagrasses within plots.
• Physical and small scale factors were important in affecting seagrass distribution.
• Localized nutrient availability may enhance the habitat suitability for Halodule.
• Erosion by wind driven currents may allow the dominance of Thalassia not Halodule.
• Exposure time at low tide explained the predominance of Syringodium in deep channels.

We tested the relative effects of physical factors such as exposure time and water depth as well as nutrient availability on Thalassia testudinum, Halodule wrightii and Syringodium filiforme distribution within the Great White Heron National Wildlife Refuge, Florida Keys. We quantified the percent cover of each seagrass species in 1-m2 plots (n = 325) along intertidal and shallow subtidal flats adjacent to Upper Harbor Key Water Keys and Howe Key. We used model selection to evaluate the effects of physical parameters and water column nutrients on the percent cover and composition of seagrass species within plots. Best models were selected based on lowest Akaike’s information criteria (AIC) values and maximum model weights (ωi). We found that the presence of the other species, distance to nearest island and time of exposure during diurnal low tides best explained the distribution of T. testudinum (ωi = 0.44). Model averaged parameter estimates (β) showed that H. wrightii and S. filiforme had the greatest negative influence on T. testudinum (β = −0.396, −0.278, respectively). H. wrightii distribution was affected strongly by the presence of the other species, distance to Pine Channel, exposure time and mean lower low water (MLLW) (ωi = 0.56) with T. testudinum and S. filiforme exerting the greatest negative influences (β = −0.450, −0.184, respectively). The best model indicated that S. filiforme was strongly influenced by the other species, distance to Pine Channel and MLLW (ωi = 0.5). Model averaging indicated that S. filiforme was associated with deep water (βMLLW = −28.0.018). Our study showcased that small scale (<100 m) habitat heterogeneity influenced the composition of seagrass communities.