Effects of ash from native and alien plants on phytoplankton biomass and mosquito abundances: A mesocosm experiment

Abstract

Wildfires are natural or anthropogenic phenomena increasing at alarming rates globally due to land–use alterations, droughts, climatic warming, hunting and biological invasions. Whereas wildfire effects on terrestrial ecosystems are marked and relatively well–studied, ash depositions into aquatic ecosystems have often remained overlooked, but have the potential to significantly impact bottom–up processes and effects on semi–aquatic taxa such as mosquitoes. This study assessed (i) ash–water–phytoplankton biomass dynamics and (ii) post–colonization mosquito abundances using six plant species [i.e., three natives (apple leaf Philenoptera violacea, Transvaal milk plum Englerophytum magalismontanum, quinine tree Rauvolfia caffra) and three aliens (lantana Lantana camara, gum Eucalyptus camaldulensis, guava Psidium guajava)] based on a six–week mesocosm experiment with different ash concentrations (1 and 2 g L–1). We assessed concentrations of chemical elements, i.e., N, P, K, Ca, Mg, Na, Mn, Fe, Cu, Zn and B from ash collected, and observed significant differences amongst the species. High concentrations of P, K, Mn, Fe, Cu, Zn and B were recorded from Transvaal milk plum ash and low concentrations of P, K, Ca, Mg, Cu and Zn were recorded from apple leaf. An increase in phytoplankton biomass (using chlorophyll–a [chl-a] concentration as a proxy) for all treatments i.e., 1 and 2 g L–1 for all plant species ash was observed a week after, followed by decreases in the following weeks, with the exception of 2 g L–1 for lantana, gum and control. Silicate concentrations (i.e., used as a proxy for diatom abundance) showed increasing patterns among all ash treatments, with exception of controls. However, no clear patterns were observed between native and alien plant ash on both chlorophyll-a (chl–a) and silicate concentrations. We found that ash has notable effects on water chemistry, particularly nitrate, which increased throughout the weeks, whereas, pH and conductivity were high at lower ash concentrations. The impacts of ash on water chemistry, chl–a and silicate concentrations varied with individual species and the amount of ash deposited into the system. Overall, there was no statistically clear difference in colonization between ash from native and alien species. We recorded colonization by two mosquito genera (i.e., Culex spp., Anopheles spp.), with Culex generally much more abundant than Anopheles. Few differences were identified among the plants, with statistically clear effects of ash type and concentration on larval and pupal stages. High Culex egg and larval abundances were shown in lantana and apple leaf treatments compared to controls, and milkplum versus controls for pupae of both genera. Further research is required to elucidate the influence of nutrient inputs from different ash species on vector mosquito population and phytoplankton dynamics.