Meeting Abstract
Macroalgae provide habitat for myriad epiphytes. Hydrodynamic forces can break or dislodge algae and epibionts, both of which depend on flowing water for nutrient transport. How do the mechanical interactions between algae and their epibionts affect their performance in ambient water flow? To address this question, we used the blade-like red alga, Mazzaella splendens, and the encrusting byrozoan, Membranipora membranacea, to investigate the biomechanical and hydrodynamic consequences for algae bearing epiphytes and for the epiphytes of living on flexible algae. We found that algal blades with encrusting bryozoan colonies had a higher flexural stiffness than blades without colonies. While flexible, unfouled algae reconfigured in ambient flow into more streamlined shapes, stiffer encrusted algae did not, and thus experienced higher hydrodynamic forces. Blade tissues had lower elastic moduli and broke at higher extension ratios than did bryozoan colonies, so when a blade was stretched, ebibiotic colonies fractured and popped off the blade. Algae in habitats exposed to rapid flows had few epibionts, whereas those in slow-flow habitats were heavily fouled. Fouled algae transplanted from a protected habitat to one exposed to rapid flow lost their epibionts. We measured mass transport rates at algal surfaces in the field and showed that fouled and unfouled algae experienced greater transport rates near their attached ends than at their distal ends. Field dye studies showed that algal flapping increased the rate at which water near the substratum was replaced by new water. Bryozan cover was greatest on the older, high-transport basal regions of blades. Thus, flapping by algae enhances transport to the algae and their epibionts, while the epibionts increase the drag on the host, but the extensibility of algal tissue leads to removal of epibionts in rapid-flow habitats.
