Two mesocosm experiments investigating the control of summer phytoplankton growth in a small shallow lake

Abstract

1. Mesocosm experiments were carried out to examine the relative importance of top down (fish predation) and bottom up (nutrient addition) controls on phytoplankton abundance in a small shallow lake, Little Mere, U. K., in 1998 and 1999. These experiments were part of a series at six sites across Europe. 2. In the 1998 experiment, top-down processes (through grazing of large Cladocera) were important in determining phytoplankton biomass. The lack of plant refugia for zooplankton was probably important in causing an increasing chlorophyll a concentration even at intermediate fish density. Little Mere normally has abundant macrophytes but they failed to develop substantially during both years. Bottom-up control was not important in 1998, most probably because of high background nutrient concentrations, as a result of nutrient release from the sediments. 3. In 1999 neither top-down nor bottom-up processes were significant in determining phytoplankton biomass. Large cladoceran grazers were absent even in the fish-free enclosures, probably because dominance of cyanobacteria and high phytoplankton biomass made feeding conditions unsuitable. As in 1998, bottom-up control of phytoplankton was not important, owing to background nutrient concentrations that were even higher in 1999 than in 1998, perhaps because of the warmer, sunnier weather. 4. The differing outcomes of the two experiments in the same lake with similar experimental designs highlight the importance of starting conditions. These conditions in turn depended on overall weather conditions prior to the experiments.

Effects of nutrients and fish on periphyton and plant biomass across a European latitudinal gradient

“…The relationship between nutrient concentration and macrophyte biomass, expressed as percentage reduction in biomass at each nutrient level with respect to the nutrient control (N0), was similar in almost all countries except the United Kingdom (Fig 2). The low plant density in the UK showed a clear inhibition effect with regard to nitrate but a positive effect with phosphorus (Stephen et al 2004b). With the exception of the UK and The Netherlands, in which different responses were obtained for nitrates depending on the year of experiment, nitrates and SRP generally overlapped in their effect in relation to plant decline.…”

“…For phosphorus, Henricsson (1976) and Blindow (1988) did not observe toxicity effects on Chara up to a concentration of 2 mg P L -1 and 1 mg P L -1 respectively. In the UK experiment in 1999, plant growth was still observed at mean SRP concentrations in excess of 8 mg P L -1 (Stephen et al 2004b). Nitrogen can also be toxic to plants.…”

“…Physicochemical and biological analyses were performed weekly on each of the mesocosms before addition of nutrients. Detailed information about the fish used and data for planktonic chlorophyll-a (chl-a) and nutrient concentrations in each of the countries are presented elsewhere Fernandez-Aláez et al 2004;Hansson et al 2004;Hietala et al 2004;Romo et al 2004;Stephen et al 2004b;.…”

“…We conducted a series of mesocosm experiments (Fernandez-Aláez et al 2004;Hansson et al 2004;Hietala et al 2004;Romo et al 2004;Stephen et al 2004b; whose main aim was to examine the relative importance of top-down and bottom-up mechanisms in trophic webs of shallow lakes and how this balance would be affected by the latitudinal gradient in climatic factors. Detailed results on phyto and zooplankton and complete data for each of the experiments have been published in the aforementioned papers.…”

Interactive effects of omnivorous fish and nutrient loading on net productivity regulation of phytoplankton and periphyton

“…0.8 m) for the periphyton assemblage at Cabiúnas Lagoon (Guariento et al 2009) and thus does not specifically favor autotrophic nor heterotrophic processes. We mixed the water column in the enclosures twice a week to homogenize nutrients, oxygen and temperature and to minimize seston settlement (Stephen et al 2004). Per enclosure, 1 tile was sampled every week.…”