Eco-engineering for clarity : clearing blue-green ponds and lakes in an urbanized area

Abstract

Small lakes and ponds are common features in urban areas and they contribute to the quality of citizens’ life. A poor water quality, however, can easily give rise to nuisance. A major cause for a poor water quality is a high concentration of plant-nourishing nutrients, eutrophication. In (semi-)standing waters, eutrophication often results in a high biomass of blue-green algae (cyanobacteria), turbid water and the disappearance of submerged aquatic plants. The cyanobacterial blooms can be accompanied with fish kills due to anoxia, the development of unpleasant surface scums and malodors. As cyanobacteria can produce potent toxins, they impose a serious risk for citizens’ health, pets and wildlife. The cyanobacterial blooms hamper the anthropogenic use of the water and can have negative economic impacts. Water managers experience that the reduction of cyanobacterial nuisance is arduous. As long-term positive effects of management interventions are not often achieved, there is need for effective approaches. The objective of this study was to investigate the efficacy of promising methods to reduce cyanobacterial nuisance in city waters, targeting the clear water state and promoting the growth of aquatic plants. Various methods were tested, in the laboratory, in small and large compartments and were eventually applied in whole ponds and lakes. It is widely accepted that the reduction of nutrient inputs is essential for long-term positive effects. This study focused on the reduction of the input and the availability of the key-nutrient phosphorus. It was shown that cyanobacterial nuisance was wide spread in urban ponds and lakes in the Dutch province of North Brabant. The phosphorus inputs of four urban lakes in this province were addressed. The study lakes differed greatly in the phosphorus sources and loads, depending on site-specific characteristics. These differences affected the selection of measures. It was shown that in-lake measures were effective in realizing the long-term abatement of the cyanobacterial nuisance, provided the external phosphorus input was limited. If the external phosphorus input could not be limited sufficiently, in-lake measures did not result in the long-term reduction of cyanobacterial nuisance. To reduce the bioavailable phosphorus stock in the lake with in-lake measures, sediment capping with a phosphorus-binding agent (lanthanum-modified bentonite, LMB) can be effective and cheaper than sediment removal by dredging. The additional use of a flocculant may have added value and suppressed cyanobacterial blooms quickly and effectively. Aquatic plants and macroinvertebrates responded positively to the achieved improvement of the water quality. Accumulation of lanthanum was shown in aquatic plants and fish, following LMB exposure. No toxic effects of lanthanum from LMB were observed. Depending on site-specific characteristics, dredging or LMB did not suffice to limit the available phosphorus stock in the lake. For this situation, the additional capping of the sediment with sand was tested and subsequently applied in a lake. Management of the fish biomass and lake reconstruction can support rehabilitation. The results of this study underpin the importance of a site-specific diagnosis (water system analysis). The diagnosis clarifies the underlying causes of cyanobacterial nuisances and is essential for a site-specific tailored set of measures. This study showed that a site-specific set of measures reduced cyanobacterial nuisance effectively for a long term. As eutrophication control is not always feasible or might be effective only in the long run, curative measures are needed for symptom relief. Several curative end-of-pipe measures that are often suggested were evaluated: effective microorganisms (EM®), golden algae, plant extracts, ultrasound and artificial mixing of non-stratifying waters. No strong support for the efficacy of these measures could be shown. Next to the above mentioned application of flocculant, the use of freshwater quagga mussels is promising. The efficacy of the mussels was experimentally tested and it was shown that the introduction of mussels in a hypertrophic urban pond reduced the phytoplankton biomass, including cyanobacteria, and induced a clear water state. The quagga mussel is an invasive alien species and new introductions should be considered carefully. Based on the results from this study, the thesis provides a road map for water managers for the reduction of cyanobacterial nuisances in urban ponds and lakes.