Abstract
Fragmentation is a reproductive and dispersal strategy used by macroalgae and can facilitate the dispersal and establishment of species. Many factors can affect fragmentation, including water movement, which can create large drag forces causing tension or bending stresses, leading to fragmentation. This may be influenced by the amount of force occurring in flow and the ability of an alga to resist those forces. The morphological traits of algae, particularly total thallus length, and their relation to physical properties, such as tensile strength and flexibility, are fundamental aspects of plant-flow interactions, yet are understudied in estuarine algae. Thus to investigate algal length and its relationship to tensile strength, flexibility, and fragmentation in flow, I conducted experimental biomechanical and flume tests using three estuarine species from San Francisco Bay: native Gracilariopsis andersonii, cryptogenic Ulva spp., and nonnative Sargassum muticum. I predicted that if shorter algae were less prone to fragmentation than longer specimens, shorter algae would have increased tensile strength, more flexibility, and fragment less in flow. I tested the relationship between thallus length and tensile force required for fragmentation in each species using a tensile testing rig. Each alga was attached to a floating spring balance and pulled laterally until the specimen broke. To test the relationship between algal size and flexibility, each alga was bent around rods of decreasing diameter until breakage occurred. Finally, to test fragmentation in flow as it relates to algal size, individual algae were held parallel in flow inside a flume. Flow was then increased incrementally until breakage occurred or the maximum velocity was reached. Contrary to previous studies on marine algae, biomechanical and flume tests in this study did not generally support my hypothesis that shorter algae would have a higher tensile strength and flexibility, and incur less fragmentation in flow. Only G. andersonii exhibited a significant negative relationship between thallus length and tensile strength (not Ulva spp. or S. muticum). Similarly, algal length had no effect on fragmentation when algae were exposed to different water velocities. My results reveal all three algae were highly flexible and appeared resistant to flume water velocities, which were higher than those typically experienced in the estuary. However, I found some differences between species, namely G. andersonii had significantly less tensile strength than either Ulva spp. or S. muticum and tended to fragment more often in flow (although not significantly more). The propensity of G. andersonii to fragment may facilitate its dispersal and is consistent with its main reproductive strategy in estuaries. Gracilariopsis andersonii relies primarily on vegetative fragmentation within estuaries, and sexual reproduction on the higher flow outer coast. Regardless of morphology, high velocity of flow did not cause much fragmentation, despite the velocities used in this study being generally higher than those in the field. These results suggest flow-related drag forces alone are not driving fragmentation of these species. Other morphological factors and environmental mechanisms potentially affecting fragmentation should be investigated further to better understand the dispersal abilities of both native and nonnative fragmenting algal species in San Francisco estuaries.