Seasonal succession of diatom epiphyton on dense mats of Lemna minor

Annual sequences of biomass, species diversity, and community composition of epiphytic diatoms on a dense mat of Lemna minor L. growing in a freshwater marsh are described. In 2 successive years, maximum biomass occurred in early spring prior to occlusion of the water surface by the mat, while diversity reached relative maxima in the early summer (during a period of declining total biomass) and late fall. Ambient air temperature (possibly through a relationship with in situ metabolic activity and nutrient availability) was significantly correlated with community ordination scores on a temporal basis. Depth profile maxima of diatom abundance on vertically positioned artificial substrata occurred below the mat, suggesting that light limitation of algal growth within dense mats may be of lesser importance. Dominant diatom species in the spring and fall were generally ubiquitous in the surrounding marsh, while the major taxa in midsummer, Achnanthes hungarica, Amphora veneta, and Navicula twymaniana, may be specific to the Lemna microhabitat. The development of algal community specificity within a defined seasonal range is discussed in terms of the relative importance of host exudates versus exogenous nutrients as the basis for algal-substratum associations.