Short communication Species, thallus size and substrate determine macroalgal break force and break location in a low-energy soft-bottom lagoon

Biomechanical pull-tests were used to determine if thallus size and substrate type affect break force and break location of six macroalgal species (Ulva curvata, Gracilaria verrucosa, Gracilaria foliifera, Agardhiella subulata, Fucus vesiculosus and the alien Codium fragile) from a low-energy soft-bottom lagoon in Virginia, USA. Both small and large individuals of all six species incorporated into polychaeta (Diopatra cuprea) tube caps were weakly attached (<0.5 N), compared to individuals attached to bivalve shells (0.6–12.2 N). For algae attached to bivalve shells, U. curvata had the weakest attachment (small individuals = 0.6 N, large = 1.2 N), followed by G. verrucosa (2.5, 4.5 N), A. subulata (2.9, 4.4 N), F. vesiculosus (3.5, 8.4 N), G. foliifera (5.0, 7.6 N) and C. fragile (6.4, 12.2 N). Most species on shells had dislodgment (substrate-holdfast breakage) to pruning (algae breakage) ratios of ca. 50:50, compared to 90:10 on tube caps, suggesting that recovery following disturbances may be more important for individuals attached to shells. From break force and size data, mean break velocities were calculated for large individuals incorporated onto tube caps (the group most susceptible to hydrodynamic forces) to 0.4–1.2 m s . Because currents often reach 1.5 m s 1 in tidal lagoons, breakage may be common in these systems, limiting algal size, particularly when attached to soft substrates. # 2004 Published by Elsevier B.V.