Using remote-sensing products to identify velocity thresholds on submerged aquatic vegetation cover in the Sacramento-San Joaquin Delta

In the Sacramento-San Joaquin River Delta, submerged aquatic vegetation (SAV) alters river hydrodynamics, affecting the transport and fate of sediments, nutrients, and even other vegetation. Establishment and persistence of submerged plants are, in theory, limited by water velocity. Identifying the threshold at which velocities impact submerged plant cover is a critical component in parameterizing spatially distributed models of submerged plant cover and identifying potential niche space. There are velocity thresholds critical to plant cover identified in the literature, however, most are determined experimentally in hydraulic flumes, vary widely depending on methodology, and likely difficult to scale up to the ecosystem level, natural environment. Here, we used submerged aquatic vegetation distribution maps derived from airborne hyperspectral imagery to extract submerged plant cover in a 1 km buffer around velocity monitoring stations deployed throughout the Delta. Since water velocity constrains, but does not necessarily promote, establishment and persistence, we developed and applied an algorithm in which we iterated through different velocity thresholds, performing a t-test at each velocity to determine the significance of the separation in SAV cover above and below each threshold. The velocity position which yielded the most significant separation, then, was determined to be the velocity threshold on SAV establishment and persistence, and the p-value was the significance of this threshold. This analysis resulted in the detection of a significant velocity threshold, the position of which can be used to parameterize spatially distributed models of submerged aquatic plant cover.