Interactions between neighboring algae and snail grazing in structuring microdistribution patterns of periphyton

The micro-distribution of periphyton (filamentous algae) on homogeneous sub strates was examined in experimental tanks with and without the pressure of grazing snails. The growth of periphyton attached to artificial substrate was estimated at a small spatial scale (9.3 mm x 9.3 mm cells) by varying t he number of grazers (0, 5, or 10 snails per tank), using image processing analysis without removing the periphyton. The results suggest that periphyt on growth within a cell was negatively affected by the biomass of periphyto n in the cell but was positively affected by the biomass of periphyton in n eighboring cells. A semivariogram analysis indicated that spatial heterogen eity increased with increasing grazing pressure. The size of patches was no t clearly related to the number of snails, but there was a tendency for rel ative patch size to increase with snail density. Computer simulations were also conducted to examine factors affecting the degree of spatial heterogen eity. The simulation studies indicated that snails should graze a site that was previously grazed in order to produce the observed spatial heterogenei ty of periphyton. The results also indicated that the positive effects of n eighboring periphyton on the growth of algae might create patches. The inte ractions among neighboring algae and snail grazing might be an important fa ctor creating the spatial heterogeneity of periphyton even on homogeneous s ubstrates.