Nutrient Addition Bioassay Filters - Lake Titicaca, Peru

After a 5-day in situ nutrient addition bioassay, the filtered water from the experiment shows the responses of the phytoplankton to different nutrients. Nitrogen additions clearly stimulated chlorophyll production and likely algal biomass. Phosphorus additions did not stimulate the phytoplankton. The orange color in the iron treatments is not due to algae, but rather to the precipitation of iron hydroxides during the experiment.

Respective final measured chlorophyll levels in the 4 treatments were 3.1, 12.5, 3.0, 2.2 and 2.9 ug/L (Wurtsbaugh et al. (1985) Freshwater Biology 15:185-195).

In other experiments, both phosphorus and iron additions stimulated nitrogen fixation.