Assessing aquatic macrophyte recovery responses following lanthanum-modified bentonite applications

Abstract

Lanthanum – modified bentonite (LMB) has been used successfully in lake restoration projects globally to control phosphorus (P) release from sediments to overlying waters. However, desirable aquatic macrophyte (macrophyte) species recovery following LMB applications, where reported, has been slow or non-existent, despite improvements in water quality. The reasons behind this lack of recovery are unknown. This study is the first comprehensive assessment of macrophyte recovery following sediment capping in lakes and focusses on physical, chemical, and biological constraints which could potentially result in ‘ecological bottlenecks’ to macrophyte reestablishment in lake restoration generally. An assessment of short- (0 – 2 years) and long-term (2 – 10 years) changes in macrophyte composition in lakes following LMB applications revealed that macrophyte communities do not meet European legislative targets, e.g. the Water Framework Directive for good ecological status. Low seedbank viability, dominance of pioneering non-native species and scarcity of external propagule sources may be the main restrictions on macrophyte recovery in LMB treated lakes. A germination trial confirmed that an LMB layer, formed on surface sediments following application, did not impede macrophyte germination success. However, LMB did significantly reduce benthic algal growth which was species-specific. Bioassay experiments revealed that macrophyte species responded differently to LMB under different light conditions. Desirable and non-native invasive species and nationally rare protected species responded in-line with their strategy traits. All species grew when applied with LMB in light conditions, however, all species grew less when applied with LMB in dark conditions. The findings presented demonstrate that additional measures may be required alongside sediment capping to force ecologically recovery, especially where restoration planning is designed to meet ecological targets for desirable vegetation composition. Transplantation of macrophytes may be needed to ensure the establishment of desirable species if viable seedbanks no longer occur following improvements in water quality, or for waterbodies that are isolated from propagule distribution sources, or where pioneering macrophyte species dominate communities.