Stimulating algal growth through wood ash fertilization in the Åkerberg pit lake, northern Sweden

Abstract

Fifteen microcosms were installed in the Åkerberg pit lake for 15 days in the summer season (July) 2021. To stimulate algal growth, the microcosms were fertilized with two P-rich wood ashes, and KNO3. Chlorophyll-a was used as an indicator of algal growth while filtered (<0.2 μm) and particulate suspended element concentrations (>0.2 μm) were used to estimate algal metal uptake. Water quality measurements and water sampling were conducted on three occasions (every five days) and at the start of the experiment to monitor algal growth. The chlorophyll-a concentration in the microcosms fertilized with wood ash increased from 0.3-0.8 μg/L at the start of the experiment to 53–77 μg/L after 15 days. Algal element uptake of filtered concentrations (<0.2 μm) was observed for many elements including, Ni (33–36%), Zn (22–65%) and Cd (22–54%). This suggests that wood ash could be used to stimulate algal growth in pit lakes by acting as a source for P and potentially also other nutrients. The highest chlorophyll-a concentrations were seen on day 10, indicating that a breakdown of chlorophyll-a impacted the measured concentrations, which otherwise could have been higher.