Changes in Phytoplankton structure due to prematurely limited restoration treatments

Abstract

Sustainable restoration using three methods – water aeration, phosphorus inactivation, and biomanipulation – was applied in Swarzędzkie Lake to slow progressive eutrophication. The treatments were limited to wind aeration alone after three years of restoration as a result of lack of funding. The research was carried out in a period of limited restoration. Phytoplankton was sampled monthly from the surface layer. Samples were preserved with Lugol’s solution and counted using a Sedgewick-Rafter chamber with a light microscope. The fluctuation of phytoplankton abundance and biodiversity were determined, comparing data before (2011), during (2012-14), and after the main restoration measures (2015-16). Mean phytoplankton abundance ranged from ca. 31.0×103 spec. ml-1 (2011) by 23.6×103 spec. ml-1 (2012-2014) to 17.5×103 spec. ml-1 (in 2016). Although cyanobacterial bloom occurred in 2013, the average amount of cyanobacteria decreased at least three times during the restoration and one year after the limitation of treatments. The abundance of cyanobacteria again increased sharply in 2016 (ca. 18×103 spec. ml-1). H’ index was rising from 1.29 in 2011 to 2.63 in 2015 and started to decrease in 2016. Despite the fact that the total phytoplankton abundance decreased during the restoration processes (2012-16), the abundance of cyanobacteria, which was the main problem of Swarzędzkie Lake, increased when the restoration was limited to wind-aeration (2015-16). An increase of biodiversity and low abundance of cyanobacteria prognosticated the stabilization of the lake’s ecosystem. However, a drastic limitation of restoration treatments resulted in another increase of cyanobacteria abundance, which ruined the lake repair. Restoration should not be ceased before the end of the stabilization phase of the ecosystem.