The Functional Morphology of Turf‐Forming Seaweeds: Persistence in Stressful Marine Habitats

Many seaweeds that occur in physically stressful habitats or habitats subject to moderate herbivory grow as colonial turfs rather than as spatially separated individuals. The turf growth form is energetically expensive (the net production per gram ash free dry mass of turfs being 33—61% lower than that of individuals), but turfs suffer less physiological damage during desiccating low tides and lose less biomass to herbivores. The upper portions of turf—forming species show significantly greater rates of apparent photosynthesis and dark respiration than do the lower portions. This spatial partitioning of photosynthetic activity decreases the energetic cost of the turf arrangement and may allow basal portions to function as persistent resting stages during periods of adverse conditions when uprights cannot be maintained. Turf—forming species are specialized for areas that are subject to moderate grazing pressure and physical stresses. They are dependent upon these factors to prevent their competitive exclusion by more productive, but less resistant, seaweeds. Damage to apical portions causes increased branching that results in a more tightly compacted turf. Algae that regenerate in this way can adjust their growth form in accordance with varying levels of disturbance encountered in different habitats and thus incur the minimal cost consistent with survival in that area. For seaweeds occurring in stressful habitats, selection has favored characteristics that increase persistence in space and time even though these involve considerable losses in competitive ability and productivity.