Does eutrophication affect the ability of biofilms to stabilize muddy sediments?

Abstract

Benthic biofilms, matrixes of microbial cells and their secretions, have been shown to stabilize sediment in coastal environments. While there have been numerous studies on the effects of nutrients on the ability of vascular plants to stabilize sediment, few studies have investigated how nutrients affect biofilm growth and their ability to stabilize sediment. In these experiments, diatom-dominated biofilms were experimentally grown on a settled bed of bentonite clay under a saline water column. Biofilms developed with a 12-h light-dark cycle and given either no, low, or high nutrient treatments. Erodibility at different stages of biofilm growth was measured using a Gust Erosion Microcosm System (UGEMS), which applied shear stresses from 0.05 to 0.6 Pa. Incipient biofilms (1–2 weeks of growth) consistently decreased the erodibility of muddy sediments for low to moderate stresses (0.05–0.3 Pa), while mature biofilms (3–5 weeks of growth) stabilized sediments for moderate to high stresses (0.45 Pa or higher). Nutrients affected the texture of the biofilm and sediment surface but did not change the ability of mature biofilms to stabilize muddy sediments. Increases in critical shear stress and decreases in mass eroded occurred faster in the high nutrient treatment compared to the low and no added nutrient treatments. As such, high nutrient concentrations allow biofilms to sustain high shear stresses during their incipient stage, during which they are more susceptible to erosion. Under most naturally occurring flow conditions, incipient biofilms, such as those found on mudflats and drowned marshes, can reduce sediment erosion regardless of nutrient concentrations.