Ceratophyllum demersum -: phosphorus interactions in nutrient enriched aquaria

High macrophyte density in shallow lakes is often associated with clear water, especially when the non-rooted, submerged angiosperm Ceratophyllum demersum is dominant. Lack of true roots and high surface area: volume ratio suggest that nutrient uptake from the water column by C. demersum may be high. Therefore, possible competition for nutrients, including phosphorus (P), could contribute to phytoplankton inhibition. C. demersum ability to absorb and store P at four nutrient levels (unenriched + three enrichment treatments) was investigated in a 34-day laboratory experiment using agar-based nutrient diffusing substrates (NDSs). P uptake rates and abatement potential by C. demersum were assessed from total phosphorus concentration (TP) patterns in the water column. Changes in C. demersum biomass (wet weight) also were determined. C. demersum took up P quickly. Some P release occurred during the experiment, especially under high nutrient conditions. Initial net P uptake by C. demersum was high, but medium-term (five weeks) average uptake was relatively low. Projected long-term net P uptake approached zero. Plant biomass loss and production of macrodetritus (plant fragments > 1 mm) were highest in unenriched aquaria. Biomass loss in the lower enriched treatments was equally divided between loss as macrodetritus and as dissolved organic matter (DOM), but loss as DOM was four times higher than loss as macrodetritus in the highest nutrient treatment. The results suggest that medium- and long-term low phytoplankton biomass in C. demersum-rich lakes is achieved via mechanisms other than direct competition for nutrients from the water column.