Rhodoliths (maërl) are widely distributed in the worlds' oceans and have an excellent fossil record. Individuals are slow growing, may be long lived ( Ͼ 100 years), and are resilient to a variety of environmental disturbances. Their external morphology and internal growth bands are potential archives of environmental variation at scales of within years to tens of years. At high densities, these free-living nongeniculate coralline algae form rhodolith beds, communities of high diversity that can be severely impacted by resource extraction.It has long been known that free-living, non-geniculate (i.e. lacking uncalcified joints) coralline algae can occur at high concentrations over large areas (e.g. Weber-Van Bosse and Foslie 1904). Such observations were not oddities: In terms of area covered, maërl or rhodolith beds may be one of the Earth's "Big Four" benthic communities that are dominated by marine macrophytes, ranking with kelp beds and forests, seagrass meadows, and non-geniculate coralline reefs. Moreover, rhodolith beds provide habitat for numerous associated macroalgae and invertebrates. Difficulties with identification and confused terminology and taxonomy of the rhodoliths themselves, their occurrence in habitats not often frequented by marine scientists, and a literature fragmented between biological and geological journals have, however, inhibited interest and progress in understanding these calcareous tumbleweeds of the sea and their reefs, which rock and roll. These difficulties are being overcome and some turned into strengths. My purpose is to comment on what is known and suggest what I consider necessary and fruitful areas of future research.definitions and descriptions Nature, differences among scientific disciplines, and history have conspired to inhibit a common rhodolith language. Since the insightful reviews by Adey and Macintyre (1973) and Bosence (1983a,b), many investigators have settled on "rhodoliths" as the name for free-living structures composed mostly ( Ͼ 50%) of

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Wiley is collaborating with JSTOR to digitize, preserve and extend access to Ecology This content downloaded from 131.247.112.3 on Mon, Abstract. The subtidal (10-15 m) assemblage in the relatively sheltered giant kelp forest at Stillwater Cove in Carmel Bay, California, consists of perennial species forming three major vertical layers: a Macrocystis pyrifera surface canopy, a dense subsurface canopy of another kelp, Pterygophora californica, and an understory of articulated and encrusting coralline algae. The kelp canopies alone or in combination can reduce bottom light to <3% (usually <1%) of surface influx. The effects of light reduction by these vegetation layers on algal recruitment and subsequent growth were determined by removing various combinations of canopies over a 2-yr period, and following subsequent changes relative to appropriate controls. Removing both M. pyrifera and P. californica canopies resulted in moderate recruitment of these species as well as of the annual brown alga Desmarestia ligulata var. ligulata. None of these algae recruited into control areas where one or both canopies were left intact. Highest brown and red algal recruitment occurred when both kelp canopies plus understory coralline branches were removed. Removal of the latter alone had no significant effect. The time of year when algal canopies were removed had little effect on the composition of subsequent algal colonization, as the recruitment of noncalcareous species occurred primarily during a short period in the spring. These results indicate that the relatively low levels of both physical and biological disturbance in Stillwater Cove allow the establishment of a few perennial algal species that inhibit their own recruitment, as well as invasion of other species, by shading. This contrasts with nearby kelp forests subjected to greater and more frequent disturbance, and characterized by a diverse assemblage of annual algal species.

ABSTRACT1. Rhodolith beds, unattached coralline reefs, support a high diversity and abundance of marine species from both hard and soft benthos. We used surveys in multiple shallow (3-20 m) beds in the Gulf of California to (1) examine seasonal patterns in associated floral and faunal diversity and abundance, (2) compare differences in faunal associations between rhodolith beds and adjacent sedimentary habitats, (3) examine the importance of complexity of rhodolith structure to community structure, and (4) estimate the impact of anthropogenic disturbance on rhodoliths and associated species.2. Macroalgal richness was seasonal, and beds supported higher richness in winter (to 36 species) than summer (6-7 species), primarily due to foliose red algae. Strong seasonal variation in the abundance of dominant cover organisms was due to a shift from macroalgae and mat-forming colonial invertebrate species to microalgae.3. The community in a rhodolith bed of high-density thalli (El Coyote average $11000 thalli/ m À2 ) had higher richness (52 versus 30 species) and abundance of epibenthic and crypto-and infaunal species compared with an adjacent sand community. Species diversity and abundance was particularly high for unique cryptofaunal organisms associated with rhodolith interstices. Cryptofauna reached average densities of 14.4 organisms/ cm À3 rhodolith, the majority of which were crustaceans, polychaetes and cnidarians along with rhodolith-specific chitons.4. Results from sampling across a range of rhodolith morphs in the El Requeson bed (with lower average cryptofaunal densities of 2.3 organisms/ cm À3 ) revealed that the total organisms supported by a rhodolith significantly increased with both complexity (branching density) and space available (thallus volume). These data suggest that reducing the population size structure, structural complexity and cover of living rhodoliths could decrease species richness and abundance.5. While disturbance is a natural feature of these free-living beds, increased anthropogenic disturbance from trawling, anchoring and changes in water quality can directly impact the bed community through substrate alteration. Commercial fishing threatens rhodolith beds in the Gulf of California by decreasing rhodolith size and increasing sedimentation and burial rates. In addition to

Seaweed population biology has received far less attention than trophic dynamics, yet is critically important in establishing and maintaining algal communities. Complex life histories of habitat-forming kelps and fucoids, including spores, gametophytes, gametes, and microscopic and macroscopic benthic stages, must be considered within the context of their highly dynamic nearshore environments. We evaluate differences within and between kelps and fucoids in life histories as they affect population biology; dispersal and potential limitations in population establishment; macroscopic stages and variations in survival and longevity affecting stand structure; and microscopic stage responses to disturbance and variation in the physical environment. We suggest that the commonly made comparisons of seaweeds with terrestrial seed plants are misleading because of large differences in morphology, environments, and the ephemeral nature of propagule banks in the sea. We conclude that progress in understanding algal populations depends on better knowledge of microscopic stages and on feedback through density-dependent reproductive processes, dispersal, and settlement.

One of the most commonly predicted effects of global ocean warming on marine communities is a poleward shift in the distributional boundaries of species with an associated replacement of cold‐water species by warm‐water species. However, these types of predictions are imprecise and based largely on broad correlations in uncontrolled studies that examine changes in the distribution or abundances of species in relation to seawater temperature. Our study used an 18‐year sampling program in intertidal and subtidal habitats and before–after, control–impact analyses. We show that a 3.5°C rise in seawater temperature, induced by the thermal outfall of a power‐generating station, over 10 years along 2 km of rocky coastline in California resulted in significant community‐wide changes in 150 species of algae and invertebrates relative to adjacent control areas experiencing natural temperatures. Contrary to predictions based on current biogeographic models, there was no trend toward warmer‐water species with southern geographic affinities replacing colder‐water species with northern affinities. Instead, the communities were greatly altered in apparently cascading responses to changes in abundance of several key taxa, particularly habitat‐forming subtidal kelps and intertidal foliose red algae. Many temperature‐sensitive algae decreased greatly in abundance, whereas many invertebrate grazers increased. The responses of these benthic communities to ocean warming were mostly unpredicted and strongly coupled to direct effects of temperature on key taxa and indirect effects operating through ecological interactions.