The influence of nutrient loading, dissolved inorganic carbon and higher trophic levels on the interaction between submerged plants and periphyton

1. We studied the growth of a submerged aquatic plant in relation to periphytic and planktonic algae over a range of nutrient and dissolved inorganic carbon (DIC) availabilities. 2. In consecutive years two factorial experiments were conducted in 48 artificial ponds (each 3.14 m3), comprising four concentrations of DIC (1.5, 2.5, 3.5 or 4.5 mM) each crossed with three separate nutrient loadings (10 microgram L(-1) P and 0.2 mg L N, 50 microgram L(-1) P and 1 mg L N, or 200 microgram L(-1) P and 4 mg L N). The second experiment differed by the inclusion of fish in the ponds. 3. In the first year DIC had no effect on plant growth, but nutrient loading did. Plants failed to grow in treatments where phytoplankton density was high (> 100 microgram L(-1)). Where phytoplankton was low, high numbers of invertebrates colonized the ponds, and periphyton abundance on the plants was low. In the second year, where phytoplankton never achieved the densities of the previous year, there was a significant effect of DIC concentration on plant growth but not of nutrients. Invertebrate abundance was lower and periphyton on the plants correspondingly higher. 4. In both years increased nutrient loading had no effect on the abundance of periphyton growing on the surface of the plants. Periphyton abundance was determined by the density of grazing invertebrates in the ponds. 5. There was a negative relationship between periphyton density and final plant density, which became significantly less steep with increasing DIC, indicating that periphyton and plants were competing for carbon. 6. DIC concentration has the potential to influence community structure in shallow lakes, altering competitive interactions between periphyton and plants and rendering low DIC lakes more prone to loss of plants when nutrient loading increases. However, the expression of this competition between periphyton and plants will depend on the density of grazing invertebrates present, which is itself influenced by the intensity of fish predation on those invertebrates.