Light attenuation by periphyton on Vallisneria americana

Abstract

Human activities continue to alter the world’s aquatic systems, with increases in nuisance algae being a common problem arising from a suite of anthropogenic activities. In particular, increases in epiphytic algae and associated periphyton on the leaves of submersed macrophytes cause shading that is implicated in the loss of these higher-order plants, along with the refuge, foraging habitat and other ecosystem services they provide. For example, increases in periphytic loads were temporally concordant with losses of important macrophytes, such as Vallisneria americana, in the Chassahowitzka River, a spring-fed system in Florida. For these reasons, we explored the relationship between periphytic loads and light attenuation by measuring light transmission through different loads found on V. americana leaves. Periphytic loads dominated by filamentous algae ranged from 0.21 mg dry mass (DM) cm−2 of leaf to 16.66 mg DM cm−2 of leaf, resulting in light being attenuated by 12.8–95.5%. An exponential decay model with two parameters explained over 80% of the variation in data documenting light attenuation at different periphytic loads. The results suggested that relatively low loads of periphyton reduce available light to a level below reported light requirements for a variety of submersed macrophytes. The model predicted a critical threshold of approximately 6 mg DM cm−2 of leaf for periphytic loads that will cause detrimental ecological effects on V. americana, with this load reducing incident light by 85%. This threshold provides an early warning of degradation, a measure of successful restoration, and a complement to metrics currently used to document the health of Florida’s spring-fed systems and other systems where V. americana is important.