Nutrient effects on producers and consumers in the littoral plankton and periphyton of a subtropical lake

An in situ mesocosm experiment was conducted to evaluate the effects of increased nutrient loading on both producers and micro-consumers (protozoa, rotifers, and micro-crustacea) in the nutrient-deficient littoral zone of a subtropical lake. Areas of the natural community were enclosed in replicate 1.2 m 2 transparent tubes and treated with nitrogen, phosphorus, or nitrogen plus phosphorus over 28 days. Responses of six distinct community components (floating surface periphyton mats, phytoplankton, epiphyton associated with Utricularia, epiphyton on live and dead stems of Eleocharis, and a benthic algal mat) were evaluated and compared to non-enriched controls. All treatments were carried out in triplicate and nutrient inputs simulated those measured in the lake's more eutrophic pelagic region. Nutrient additions stimulated growth in a consistent manner across both habitats and trophic levels. Although there was a limited number of responses to addition of nitrogen or phosphorus alone, the overwhelming response was increased biomass when the two nutrients were added together. One community component that did not display an increase in algal biomass in response to nutrient addition was the benthic algal mat. Nevertheless, benthic consumers increased significantly in all nutrient-addition treatments. This may have been a response to the rain of particulate material stimulated by nutrients higher up in the water column. To our knowledge, this is the first study to evaluate nutrient effects on littoral micro-consumers in a subtropical setting; results generally were consistent with those obtained in temperate lakes.