The indirect effect of sediment nutrient enrichment on the epiphytic algal community

Lake littoral zones occupied by aquatic plants are very productive. Submerged macrophytes are an important component of lake littoral zones forming a complex three-dimensional structure, which results in a large surface area available for colonization by epiphytes. Nutrients are one of the important factors affecting the growth of both macrophytes and algae. Most of the studies investigating the effect of nutrients on epiphytes focus on nutrients in the water column. However, water column nutrient levels in lakes where submerged macrophytes occur are usually low. This indicates that sediments can be an important source of nutrients affecting the growth of macrophytes and, indirectly, epiphytic algae. Macrophytes are efficient nutrient pumps, transporting and releasing nutrients from the sediments to the epiphytes and water.; In this study the effects of low and high concentrations of the sediment nutrients phosphorus and nitrogen on epiphytic algal communities was investigated: (1) on Potamogeton richardsonii in the littoral zone of Douglas Lake, Michigan, and (2) on Myriophyllum exalbescens in a mesocosm experiment in the presence and absence of snails, Amnicola limosus. The high nutrient sediments resulted in a greater biomass of macrophytes and a larger surface area available for algal colonization. Also, a higher biomass of loosely attached epiphytes per unit of leaf area was observed on old leaves of plants growing in high nutrient sediments. However, the presence of snail grazers controlled the epiphytic algal biomass. In the presence of snails tightly attached adnate taxa like Cocconeis placentula and Geitleribactron sp. were more common. In addition, snails facilitated nutrient recycling in the epiphyton and delayed plant senescence. Sediment nutrients affected the epiphytic algal community composition and structure on old leaves. Proportionately, more diatoms (e.g. Cocconeis placentula) were observed on the plants growing in low nutrient sediments and more green algae (e.g. Coleochaete spp. and Asterococcus sp.) and bluegreen algae (Aphanothece spp.) were observed on the plants growing in high nutrient sediments.; With the application of a multivariate ordination method, 60% of the variance within species data was explained by leaf age and by sediment nutrient concentrations. An increased epiphytic algal biomass, indirectly stimulated by sediment nutrients, potentially affects higher trophic levels in lake littoral zone food webs.