The ecology of intertidal algal crusts: variation within a functional group

Abstract

Encrusting marine algae are found at all depths in the photic zone from polar to tropical seas worldwide, yet little is known about their ecology. Crusts (including red, brown, and green algae as well as lichens and Cyanobacteria) tend to predominate in areas of high disturbance (e.g. from herbivores or sand scour) or high stress ( = low potential productivity, e.g. high on the shore). Patterns of crust and herbivore distribution and abundance were quantified in the intertidal zone in Washington state, and then the two environmental parameters predicted to be critical to among-crust variation in survival were manipulated: disturbance potential by varying abundances of herbivores, or by eliminating herbivores and artificially disturbing crusts with nylon and steel brushes; productivity potential by transplanting crusts along an environmental gradient from the high to the low intertidal zone and onto a submerged raft. Within-functional-group variation among crusts was studied by transplanting eight species (varying in calcification, thickness, tightness of construction, and other characters), and measuring their survival and growth over 2–3 yr in the different environments. Crusts varied widely in their responses to both disturbance and productivity potential. In general, thick, calcified, or very tightly constructed crusts withstood being steel-brushed at regular intervals. Nylon brushing sometimes actually benefitted these crusts relative to the controls (unbrushed but cleaned monthly), probably because of the reduction in fouling. In contrast, thinner or more loosely-constructed species showed poor survival when steel-brushed and often suffered significant losses even when nylon-brushed. Most crusts tended to do best in habitats having higher productivity potential (lower intertidal zone), showing mortality or reduced growth when transplanted into more stressful areas, although two species were healthiest under “stressful” conditions, i.e. desiccated or submerged in high pools. Experiments in the laboratory with herbivores (littorinid snails, limpets, and a chiton) demonstrated that they did not always consume the species that are mechanically easiest to remove (as indicated by the steel brushing experiments). Three species were avoided, including a brown known to contain high concentrations of phenolics, a blue-green crust, and a lichen with a blue-green phycobiont. All crusts grew very slowly, even in their optimal habitats; the fastest species grew laterally at < 20 mm/yr, and the slowest measurable at < 0.5 mm/yr. Some species showed no lateral growth over 2 yr. This low ability to sequester space, and their two-dimensional growth form, means that crusts often rely on grazers or other disturbance to keep from being overgrown. In other cases, they grow in areas where desiccation or low light levels apparently eliminate other algal forms. Crusts survive disturbance by either resisting it (through calcification or toughness), tolerating it (by being thick or regenerating rapidly), or avoiding it (by having an opportunistic life history, living in refugia such as crevices or high tidepools, or being unpalatable). Their life histories are highly variable, allowing them to exploit habitats in different ways. For example, in the mid intertidal zone in Washington the three dominant crust species include: (1) a fast-growing, frequently-recruiting brown ( Ralfsia pacifica Hollenberg) that senesces after several years and sloughs away; (2) a fast-growing but rarely-recruiting red (“Petrocelis”) that lives for decades; and (3) a very slow-growing and rarely-recruiting red [ Hildenbrandia rubra (Sommerfelt) Meneghini] that persists even when overgrown, and regenerates following disturbance. These diverse morphologies and life history strategies help make this morphology an ecological success.