Influence of near-bottom re-suspended sediment on benthic light availability

Increased light attenuation in the water column is a common consequence of the increased organic loading that accompanies anthropogenic eutrophication in coastal systems. Frequently, the best water quality correlate of the light attenuation coefficient is the total suspended solids, even in systems in which nutrient loading occurs primarily by groundwater input, that is, without strong river inputs of sediment. Alteration of bottom sediment texture, organic content, and bulk density by organic loading has been well documented. Here we report the effect of sediment re-suspension on near-bottom light attenuation using an array of in situ light sensors with very close spacing near the sediment–water interface and a radiative transfer (RT) modeling with the software “Hydrolight”. We found that the light attenuation coefficient over 4.5 cm just above the bottom exceeded the attenuation found higher in the water column by a factor ranging from 1.6 to >30. RT modeling indicated that light received at the bottom could be overestimated by a factor 4 or more by extrapolating measurements not taking the near-bottom light attenuation into account. The results may help explain the wide range of seagrass light requirements observed in different systems.