From green to transparent waters : Managing eutrophication and cyanobacterial blooms by geo-engineering

Abstract

Managing eutrophication remains a challenge to water managers. Currently, the manipulation of biogeochemical processes (i.e., geo-engineering) by using phosphorus-adsorptive techniques has been recognized as an appropriate tool to manage the problem. The first step in finding potential mitigating materials is conducting a sequence of upscaling studies that commence with controlled laboratory experiments. Here, the abilities of 10 possible solid-phase-sorbents (SPS) to adsorb P were examined. Four materials adsorbed P, and two of these materials were modified, i.e., a lanthanum-modified-bentonite (LMB) and an aluminummodified-zeolite (AMZ), and had the highest adsorption capacities of 11.4 and 8.9 mg P g-1 , respectively. Two natural materials, a red soil (RS) and a bauxite (BAU), were less efficient with adsorption capacities of 2.9 and 3.4 mg P g-1 , respectively. Elemental composition was not related to P adsorption. Since SPS might be affected by pH and redox status, we also tested these materials at pH values of 6, 7, 8 and 9 and under anoxic condition. All tested materials experienced decreased adsorption capacities under anoxic condition, with maximum adsorptions of 5.3 mg P g-1 for LMB, 5.9 mg P g-1 for AMZ, 0.2 mg P g-1 for RS and 0.2 mg P g-1 for BAU. All materials were able to adsorb P across the range of pH values that were tested. The maximum adsorption capacities of LMB and RS were highest at pH 6, AMZ was higher at a pH of 9 and BAU at a pH of 8. Thus, pH influenced P adsorption differently. Given the effects of pH and anoxia, other abiotic variables should also be considered. Considering the criteria that classify a useful SPS (i.e., effective, easy to produce, cheap and safe), only the two modified materials that were tested seem to be suitable for upscaling to enclosure studies with anoxic sediments.

Beating the blues by floc & lock

“…Lake Rauwbraken shifted from a eutrophic to a predominantly mesotrophic status and water quality met the WFD and BWD demands (C Ch ha ap pt te er r 5 5). This study demonstrates the strong point of a lake system analysis (Cooke et al 2005;Lürling et al 2016;Mucci et al 2019), which pinpointed the mitigation effort to reducing the internal P-load, rather than the paradigmatic reduction of external nutrient loads only.…”

“…Despite the long history of studies regarding the mitigation of nuisance blooms of cyanobacteria, there are still several interesting ongoing debates. The overall debate is on single phosphorus control versus nitrogen control or dual reduction (Mucci et al 2019;Paerl 2008;Schindler et al 2008). Among the hottest of the topics are: phosphorus inputs versus internal loading, safe bloom removal, effectiveness and longevity of sediment capping using a SPS, and the fate of lanthanum as we chose the LMB.…”

“…After the treatment in the course of 2008, Lake Rauwbraken was void of A. flos-aquae, indicating that the bloom burying goal was achieved. Later observations with Planktothrix rubescens in Lake de Kuil (Lürling et al 2020c;Mucci et al 2019;Waajen et al 2016a) indicated that successful sinking a bloom depends on the species of cyanobacteria present, its buoyancy, the flocculent and the amount of sinking weight applied.…”

“…was done during a bloom of Aphanizomenon (Waajen et al 2016a), with iron(III)chloride as flocculant (Waajen et al, 2016a) and as ongoing diffuse P inputs led to new nuisance blooms the treatment was repeated in 2017 (Mucci et al 2019), now during a bloom of Planktothrix with PAC as flocculant. While in both cases, the treatment resulted in good water quality (Mucci et al 2019;Waajen et al 2016a) and the first observations indicated that Planktothrix was sunk to the bottom, it reappeared in the water column after two weeks, albeit in a much lower concentration. It seems as if some of the entrapped filaments had escaped the flocs (Lürling et al 2020b).…”

“…The route to prevent human exposure to cyanobacteria may thus vary from providing clean drinking water to putting up a warning sign. Having established that the water quality problem to mitigate is a high density of toxic cyanobacteria, a lake system analysis should be done before deciding on mitigating efforts (Cooke et al 2005;Lürling et al 2016;Mucci et al 2019).…”