Rhodoliths (maerl) are widely distributed in the worlds' oceans and have an excellent fossil record. Individuals are slow growing, may be long lived (>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 non-geniculate coralline algae form rhodolith beds, communities of high diversity that can be severely impacted by resource extraction.

Rhodoliths: between rocks and soft places

Calcified macroalgae are distributed in marine habitats from polar to tropical latitudes and from intertidal shores to the deepest reaches of the euphotic zone. These algae play critical ecological roles including being key to a range of invertebrate recruitment processes, functioning as autogenic ecosystem engineers through provision of three-dimensional habitat structure, as well as contributing critical structural strength in coral reef ecosystems. Calcified macroalgae contribute significantly to the deposition of carbonates in coastal environments. These organisms are vulnerable to human-induced changes resulting from land and coastal development, such as altered patterns of sedimentation, nutrient enrichment through sewage and agricultural run-off, and are affected by coastal dredging and aquaculture. The consequences of increasing sea surface temperatures and fundamental changes in the carbon chemistry of seawater due to CO2 emissions from anthropogenic activities will have serious impacts on calcifying macroalgae. It is not yet understood how interactions between a range of variables acting at local and global scales will influence the viability of calcifying macroalgae and associated ecosystems. Research is urgently needed on all aspects of the taxonomy, biology and functional ecology of calcifying macroalgae. Without an understanding of the species present, measurement of change and understanding species-specific responses will not be possible.

Calcified macroalgae - critical to coastal ecosystems and vulnerable to change: a review

Free living, nodular aggregates of non-geniculate coralline algae (rhodoliths) have occurred since the Cenozoic in diverse marine environments around the world. Fossil rhodolith morphology and distribution have been widely used as paleoecological indicators, particularly of water motion. However, few studies have verified these relationships in living beds. The relationship between water motion and rhodolith movement was examined in three subtidal rhodolith beds off the southwestern coast of the Gulf of California, one dominated by wave action and two dominated by tidal currents. Field experiments and simultaneous video and current measurements taken during winter 1996 showed that rhodoliths in the shallow margins (4.5 m depths) of wavedominated beds moved frequently due to threshold-level velocities from wind-propagated waves. Rhodoliths did not move in the middle and at deep margins of the bed due to attenuation of wave energy. Historical wind records and waveforcasting analysis indicate that shallow rhodolith movement is frequent only in the winter. In deep tidally dominated beds, maximum yearly tidal currents were not sufficient to move rhodoliths. Video and SCUBA surveys showed that bioturbation is an important mechanism for rhodolith movement in all beds. Rhodoliths in 12-m-deep tidally dominated beds and in the deep margins of wave-dominated beds appear to move only occasionally due to bioturbation and severe storms. Results imply that rhodolith morphology and distribution are dependent on a combination of factors. These factors, especially bioturbation, should be considered when using rhodoliths as paleoecological indicators.

The relationship between water motion and living rhodolith beds in the southwestern Gulf of California, Mexico

Environmental factors influencing distribution and morphology of rhodoliths in Bahia Concepcion, B.C.S., Mexico

Coralline algal assemblages in upper Neogene reef and temperate carbonates in Southern Spain

Although differences in growth-form have been widely used in delimiting taxa of non-geniculate coralline red algae (Corallinales, Rhodophyta), there has been no consistent application of the more than 100 terms employed to describe the growth-forms present, and considerable confusion has resulted. This study of over 5000 populations of non-geniculate corallines from all parts of the world has shown that an intergrading network of growth-forms with 10 focal points is present: unconsolidated, encrusting, warty, lumpy, fruticose, discoid, layered, foliose, ribbon-like and arborescent. This focal point terminology can be used to describe any specimen or species of non-geniculate coralline in a consistent, easily interpretable manner. Details of the system are provided, the relationships of the system to past proposals are discussed, and the extent to which differences in growth-forms can be used as taxonomic characters in the non-geniculate Corallinales is reviewed.

LM Irvine

Papers

Growth-forms in non-geniculate coralline red algae (Corallinales, Rhodophyta)