Hydroponic gardens as a mitigation strategy for nutrient pollution in urban lakes
Many urban lakes are degraded due to excess nutrient inputs. Nutrient management strategies have been focused on reducing external nutrient loading, but these strategies do not remove phosphorus (P) stored within the lake ecosystem (including sediment-bound P), which can be the primary contributor to eutrophication in some lakes. In order to achieve improved water quality in these urban lakes, active removal of P from lake ecosystems must be combined with reducing P loading in the watershed. Hydroponic gardens have been used successfully to remove produce crops while removing nutrients from wastewater and aquaculture facilities. We investigated the feasibility of using lakeside hydroponic gardens to remove excess P from urban lakes, recycling this nutrient pollution back into the human food system. One particular challenge of this approach is that, in some urban lakes, nitrogen (N) becomes limiting during the growing season as phytoplankton consumes available N. In a series of mesocsom experiments using water from a local urban lake, we assessed nutrient limitation in six crops growing hydroponically in lake water, compared to lake water amended with N and with N+P. We assessed whether legumes were able to compensate for low N availability through N fixation, and measured actual and potential P sequestration from lake water. Chlorophyll was measured weekly in each plant using a hand-held chlorophyll meter, as an indicator of N-limitation. We also measured change in biomass over the 9-week experiment, and final P concentrations in plant tissue.
Chlorophyll levels decreased throughout the 9-week experiment in lake water treatments, although the decrease was significantly lower for legumes compared to non-legumes, indicating partial compensation by N-fixation. There was a slight increase in chlorophyll for +N+P treatments compared to +N treatments. However, biomass increase was more than four-fold greater for the +N+P treatment compared to lake water and +N treatments, and P sequestration was more than ten-fold greater for the +N+P treatment. These results indicate that the dissolved nutrient levels in this urban lake were insufficient to support hydroponic vegetable production. Our ongoing research is investigating whether supplemental nutrients can be delivered to hydroponic crops growing in lake water to enhance uptake of dissolved N and P.