Turbinaria ornata (Phaeophyceae) varies size and strength to maintain environmental safety factor across flow regimes.

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Water motion in coastal areas can produce hydrodynamic forces that damage or dislodge benthic macroalgae if the tissues of macroalgae are not sufficiently strong. Some macroalgae vary their morphology and strength in response to ambient water motion, but little is known of how morphology and strength of macroalgae change relative to one another across flow regimes. Here, we use Turbinaria ornata, an ecologically important macroalga, to study how both the morphology and strength of macroalgae vary with ambient water motion. Typically, T. ornata exhibits weakening of its stipe when sexually mature, leading to breakage from the substratum and dispersal, which is beneficial for reproduction. Across three flow regimes, adult T. ornata increased its size but decreased its strength as water motion decreased. However, the strength of T. ornata relative to the maximum hydrodynamic forces it is expected to encounter (the environmental safety factor) did not differ between flow regimes. Our results showed that T. ornata can conform to its local flow habitat by varying both size and strength, similar to other macroalgae. Varying multiple traits between flow regimes suggested that T. ornata is capable of surviving a wide range of flow conditions, which may permit more control overthe timing of its weakening, breakage from thesubstratum, and dispersal, even with future increasesin flow velocities (e.g., large waves from storms) that are expected to occur frequently with climate change.