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Examination of the three‐dimensional structure of periphyton communities through time indicated that their microsuccession is analogous to higher plant succession. The development of attached diatom communities in two reservoirs was studied using artificial substrates, and the morphology of dominant organisms, patterns of spatial heterogeneity, and community interactions were documented with scanning electron microscopy. Of 93 taxa found, Gomphonema parvulum, G. olivaceum, Navicula graciloides, Nitzschia palea, and N. dissipata were dominant, depending upon season and reservoir. Comparisons of community diversity (SIMI) between reservoirs within seasons ranged from 0.023–0.843 (median = 0.254), indicating that the reservoirs were quite different with respect to diatoms present and their apportionments. Colonization was slow the first 2 wk in spring and fall, and throughout winter, but rapid during summer. Shifts in numerical dominance between certain species occurred in fall, spring, and summer. These inverse correlations of abundances and the functional dominance of overgrowth suggested competition for substrate surface area in the periphyton. The colonization sequence was often predictable—a presumably organic coating and a variety of bacteria, followed by low profile diatoms, and finally an upperstory of long‐stalked and large‐rosette diatoms and filamentous green algae. Periphyton microsuccession is similar to higher plant succession in the consistent change in vertical community structure from low to high physical stature, in the association of numerical dominance with large stature (via cell size or long mucilaginous stalks), and in the progressive slow‐down in the rate of succession. Diatom mucilage also contributed to community structure by binding particulates and entrapping other algae and serving as the mechanism for substrate attachment.

The function of diatom mucilage in the formation of spatially complex periphyton communities was investigated in McConaughy reservoir (Nebraska, U.S.A.). Seasonal biofilms on natural and artificial substrates revealed high densities of periphytic diatoms (up to 3.5 × 104 cells mm-2). Growth habits of the dominant taxa (short stalk, Achnanthes minutissima; long stalk, Cymbella affinis and Gomphonema olivaceum; rosette, Fragilaria vaucheriae and Synedra radians) depend on the mucilage morphology. The volume of stalk mucilage in late stages of community development was estimated to be more than two times the volume of the cells producing it. When the periphytic biomass exceeded the carrying capacity of its substrate, portions of the community were sloughed, resulting in the loss of the upper story of cells and mucilage. Diatom mucilage affects community structure as follows: (i) it allows cell–surface adhesion; (ii) stalks of Cym. affinis provide increased surface area for attachment by Ach. minutissima; (iii) dense canopies of long-stalked Cym. affinis and Gomphonema olivaceum trap euplanktonic algae which have settled into the periphyton; (iv) mucilage binds detrital particulates; (v) stalked diatoms enable vertical stratification in the community, as spring collections revealed the development of an upper tier composed of Gomphonema olivaceum and Cym. affinis covering a lower tier dominated by F. vaucheriae and Stephanodiscus minutula.

SUMMARY Chlorella pyrenoidosa Chick (Indiana University Number 252), Nostoc commune Vaucher (I.U. 584), and Oscillatoria prolifica (Grev.) Gomont (I.U. 1270) were grown separately on Peoria loess soil material to measure their effects on the water stability of soil aggregates. Each alga significantly (10% LSD) increased the percentage of soil aggregates after 6 weeks of incubation as compared with the soil without algae. Oscillatoria, Chlorella, and Nostoc increased water stability of aggregates >74 μ in diameter by 3.4, 1.1, and 0.6%, respectively. Nostoc and Oscillatoria produced measurable water stable aggregates in the 1000–2000 μ diameter range; Chlorella formed them in the 500–1000 μ range, while the control soil showed no aggregates >295 μ.