Selection of substrata by seaweeds: Optimal surface relief

Artificial substrata allow one to test a single factor in a complex environment. We wanted to determine whether surface particle size could regulate settlement and growth in marine macroalgae. Three grades of discrete monolayers of hard particles differing only in diameter (0.1 to 0.5 mm, 0.5 to 1.0 mm, 1.0 to 2.0 mm) were cemented to the surfaces of three quadrants on acrylic discs while the fourth was left smooth. All surfaces were painted with dissolved plastic to ensure chemical uniformity. At different times of the year, 25 plates were bolted directly to basement rock in the low intertidal zone on an exposed coast at the mouth of Narragansett Bay in Rhode Island (Fig. 1). Thirteen conspicuous species of macroalgae colonized the coated acrylic plates during the course of this investigation. Of these, the most abundant were Chondrus crispus, Corallina officinalis, Polysiphonia harveyi and Ulva lactuca, which were observed throughout the program. Initial settlement did not differ significantly among the experimental surfaces, but with time patterns in the distribution of these algal populations correlated with the surfaces beneath them. These differences were visually striking and not the same for each species. Most of the Chondrus crispus (79.5%) and U. lactuca (85.2%) appeared on the two largest particles, but only for C. crispus was the difference in populations within these grades significant (P<0.05). Populations were considerably smaller on the smallest grade (20.1 and 13.8% respectively), and on the smooth quadrants only a few individuals of these two species appeared (0.5 and 1.0%). The number of plants of P. harveyi, on the other hand, did not differ on the three particle sizes tested, but on the smooth surfaces only 2.9% appeared. Corallina officinalis showed optimal development on the smallest grade (44.8%). On smooth surfaces, where 8.4% of its population was observed, no upright articulated portions developed. The season during which the substrata were introduced altered the patterns of development. C. officinalis was the first macroalga to be seen in late spring, but on plates set out in the fall this species did not appear until the following late spring. P. harveyi covered some discs in fall, spring and summer, and on these surfaces Chondrus crispus was delayed up to 1 year. By the end of the experimental period (15 months), C. crispus was the most conspicuous macroalgal species on most of the plates.