Rhodolith/Maërl Beds: A Global Perspective

Rhodolith/maërl beds are living and dead aggregations of free-living non-geniculate coralline algae that cover extensive benthic areas in recent oceans and are common in fossil deposits. They are slow growing organisms and can be long-lived (>100 years), distributed over a wide depth range from intertidal sites to 270 m. Rhodolith/maërl beds are a common feature of modern and ancient carbonate shelves worldwide that represent a sedimentary transition from sandy/muddy areas to the rocky substrate. They are bioengineers and provide a three-dimensional habitat for associated species. It has been demonstrated that rhodolith/maërl grounds are a suitable habitat for multispecies recruitment and provide refuge for juvenile life stages of commercially important shellfish species. Rhodoliths are resilient to a variety of environmental disturbances, but can be severely impacted by harvesting these commercial species, ocean acidification or global warming. The value of rhodoliths as a unique biotope around the world is under threat from different kinds of human activities. Despite the importance of rhodolith/maërl beds in the marine environment, a major limitation for protection is the lack of a clear definition of an ecosystem. A thorough review of the literature revealed a total of 12 vernacular/scientific terms that have been applied to free-living coralline red algae and these should be treated as synonyms. The Challenger Expedition (1872–1876) was one of the first voyages that promoted the understanding of the rich flora and fauna associated with coralline deposits. During the nineteenth century additional surveys in other areas of the world have confirmed the value of this ecosystem. During twentieth and twenty-first centuries many researchers have produced a vast scientific literature, documenting the importance of rhodolith/maërl, to understand their relevance regarding biodiversity in nearshore habitats. The relevance includes the description of new species or where the distribution of poorly known species has been extended, but more importantly the high number of associated species which includes species under protection, species ecologically relevant or species which are part of a formal fisheries. As a consequence of the concern about the state of the ecosystems in Europe at the end of the twentieth century, the EU developed a network of protected areas known as Natura 2000 sites. A series of publications on the conservation status of the maërl/rhodoliths in Atlantic and Mediterranean waters, Brittany, Gulf of California, and their relationship with fisheries, stated clearly that the health of rhodolith habitats in some areas of the world is decreasing, and there is an urgent need for management strategies. The combination of the interest in developing rhodolith/maërl conservation in other countries, the decline of the French Atlantic maërl deposits, and the correlation of rhodolith/maërl presence in or near oil deposits has motivated the exploration of rhodoliths in other areas such as Brazil, México, Australia and New Zealand. Understanding is increasing about the ecological role of rhodoliths in nearshore environments worldwide, the biodiversity associated with rhodoliths, and how human activities are having an increasing impact. The recognition of the importance of rhodolith beds as biodiversity centers has increased with the number of published papers and the growth in knowledge about the taxonomic status of the associated species.

Calcifying marine organisms can be used as recorders, or proxies, of past environmental conditions if they lock physical or chemical signals within their skeletal material. Coralline algae lay down regular growth bands and the study of their structure and composition has gained increasing attention as a technique for reconstructing past environments in tropical, temperate and polar regions. Structurally, growth band width and percentage calcification have been used as records of historic light availability (e.g. due to cloud cover and sea ice extent). The chemical composition of their high Mg calcite skeleton has received significantly more attention, being used to reconstruct temperature, salinity, dissolved inorganic carbon, upwelling patterns and wider climate indices. At the ecosystem level, such reconstructions have been used to shed light on the drivers of past changes in marine productivity. Against a backdrop of projected ocean acidification coralline algae show significant potential for reconstructing historic changes in ocean acidification-driven marine carbonate chemistry. Due to their global distribution, coralline algae are becoming a regularly used tool for understanding environmental and ecosystem change, particularly in areas where other proxies are not available or instrumental records are sparse.

Coralline algae are expected to be adversely impacted by global warming and ocean acidification, although there has been no synthesis of these effects on habitat-forming species. We compiled published responses of maërl and rhodolith-forming species to ocean acidification and warming. Although the response is variable among species, their recruitment, growth, health and survival are usually negatively affected under elevated CO₂. Most studies show that coralline algal calcification is adversely affected under near-future ocean acidification scenarios and that in combination with a 1–3 °C increase in seawater temperature this has an even larger impact. Most research has involved relatively short-term experiments on single species, which makes it difficult to predict long-term effects at the ecosystem level because the impact of global changes on coralline algal habitats will depend on the direct impacts on individual species and the indirect effects of altered interspecific interactions. Studies in areas with naturally high CO₂ levels show that coralline algae are adversely affected by long-term acidification through increased competition from non-calcified competitors. Coralline algal habitats such as vermetid reefs, coralligene and beds of rhodoliths or maerl are likely to decline in the near future as higher CO₂ levels benefit fleshy algae and corrosive waters reduce calcareous habitat complexity and associated biodiversity.

Carbonate materials are important economic resources: limestones are excellent reservoirs and valuable building stones; unconsolidated sediments may be used as a viable source of calcium carbonate for soil conditioning. Since Late Cretaceous coralline algae are one of the most important shallow-water carbonate producers. Sediment production and deposition in carbonate platforms are controlled by physical, chemical and biotic factors. Chemical and biotic factors rule over sediment texture, composition, distribution and early diagenetic processes, and consequently they have a major impact over limestone properties after diagenesis, especially over porosity and permeability. Porosity and permeability in turn control limestone mechanical properties, its durability and its reservoir potential. Thus, understanding the factors controlling formation and fate of coralline-algal carbonate factories is necessary for both sustainable management of the coralline-dominated marine habitats and profitable exploitation of reservoirs and quarries.

Calcareous coralline algae (Rhodophyta; Corallinales, Hapalidiales, and Sporolithales; corallines hereafter) constitute one of the most widespread and successful groups of marine macrophytes. They occur as crusts partially coating hard or soft substrates, as laminar thalli growing directly on the seabed, or forming structures rolling freely on the substrate with an inner nucleus or without it. These latter structures are called rhodoliths. They can be one of the most abundant components in carbonate platform deposits, forming the so-called rhodalgal facies. In assessments of the rhodoliths, internal and external algal growth morphology, rhodolith external form, rhodolith inner arrangement, and assemblages of organisms forming the rhodoliths can provide valuable information for reconstructing palaeoenvironmental and palaeoclimatic conditions. Rhodoliths can occur massively concentrated in beds several meters thick. These concentrations are referred as rhodolith beds. These rhodolith beds may be the result of biotic (autochthonous rhodolith beds), abiotic (allochthonous rhodolith beds) concentrations or due to a mixture of processes (paraautochthonous rhodolith beds). Taphonomic and facies analyses, as well as faunal assemblages, can provide the information needed to confidently differentiate among them. The rock record offers unique information to envisage the founding conditions and the long-term maintenance of the rhodolith beds. In this chapter, we review and update the information on fossil rhodoliths and rhodolith beds, and discuss their value for palaeoenvironmental and palaeoclimatic reconstructions. Also, we discuss the sedimentary and the sequence stratigraphy contexts in which rhodolith beds are preferentially formed and developed.

The knowledge of re-sedimented rhodolith deposits has always lagged behind that of in situ deposits, which can be formed in shallow and deeper water carbonate and mixed siliciclastic-carbonate depositional settings. A combination of detailed outcrop analyses from three published case studies reveals a series of palaeobiological and taphonomic signals that are used to identify fossil re-sedimented rhodoliths. The re-sedimented rhodolith deposits of the middle Eocene carbonates in the Venetian area (northeast Italy), the lower Miocene carbonates from southern Sardinia (Italy), and the lower–middle Miocene carbonates from Southern Apennines (southern Italy) are described in terms of rhodolith morphology, coralline algal assemblages, inner arrangement, outer growth-forms, and taphonomic signatures. In all the cases, shallow water rhodolith beds were redeposited to feed offshore deposits through submarine channel systems. The sedimentological features, rhodolith characteristics and taphonomic signatures of the rhodolith deposits are compared from the carbonate factory, through the shelf-margin to the proximal and distal parts of the tributary belt. Within submarine channelized carbonate settings, complex relationship patterns of autochthonous/parautochthonous and allochthonous rhodolith deposits were governed by the interplay of changes in environmental factors such as water energy, light irradiance, substrate characteristics, and residence time on the sediment-water interface.

Rhodoliths are common components in Neogene shallow-water marine deposits from the Mediterranean and Paratethys regions. Rhodolith-rich deposits (rhodolith beds) appear to be most widespread in the Langhian (middle Miocene) and late Tortonian-early Messinian (late Miocene) intervals and their occurrences decline in the Piacenzian (late Pliocene). Numerous shallow-water platforms surrounded relatively small islands in the Mediterranean region from the Burdigalian to the early Messinian. In the Miocene subtropical to warm-temperate climatic context of the region, these platforms with limited siliclastic input were favourable places for carbonate production by coralline algae. Rhodoliths were major components of carbonate factories on ramps, with maximum concentrations in middle to proximal outer zones. Rhodoliths may occur among coral patches (mainly Porites) in inner-ramp deposits, but most rhodolith beds formed basinwards of corals, seagrass meadows or submarine dune fields (shoals) on the middle and proximal outer ramp. They also occur in lagoons, on reef tops, and on fore-reef slopes in rimmed platforms; and can be locally abundant in siliciclastic systems, in intervals of interrupted or reduced sediment influx. The occurrence of rhodolith concentrations is not indicative of any precise palaeodepth range within shallow-water platforms (from the shoreline to about 120 m), although most commonly formed at palaeodepths of several tens of meters. No clear patterns of variation in rhodolith shape and internal growth forms of coralline thalli can be detected along palaeodepth gradients, although loose branching growths occur mainly in the deepest concentrations. Rhodoliths formed in intermediate palaeodepths tend to comprise more complex combinations of coralline growth forms than shallower and deeper nodules. The taxonomic composition of algal assemblages in rhodoliths usually changes with palaeobathimetry. Spongites or Lithophyllum, and minor Neogoniolithon are abundant in the shallower rhodoliths, whereas they are less common or absent in deeper-water assemblages, which comprise mainly species of Mesophyllum, Lithothamnion, and Sporolithon. The number of species of Spongites (and probably of other genera) apparently decreases throughout the Neogene in the region. By the early Pliocene, components of coralline assemblages are similar to those forming rhodoliths in the present-day Mediterranean (about 90 % of species are extant ones).

During the Oligocene, extensive carbonate platforms developed in the central Mediterranean. These areas were colonized by different organisms, being the coralline algae one of the most important contributors to the carbonate production. Red algae produced sediments in shallow seagrass environments, however dominate the mesophotic and oligophotic zones where produced rhodolith beds. The diffusion of red algae during the Oligocene was favoured by reduction in atmospheric CO₂, a rise in seawater alkalinity and increasing Mg/Ca ratios. This chapter deals with these deposits analyzing the depositional models and the controlling factors accounting for the origin of rhodolith beds.

Distribution of living rhodoliths in the Macaronesian realm is limited by extensive rocky shores and narrow insular shelves that rapidly drop off beyond the 50-m isobath. Wind and wave erosion is most intense on north and northeast-facing shores due to the prevailing northeasterly trade winds over much of the region. Southern shores offer more sheltered, leeward settings. Rhodolith beds tend to thrive on eastern shores with strong long-shore currents and southeastern shores that benefit from wave refraction. Rhodoliths are not entirely absent off northern shores, but may fail to reach maximum size before being washed ashore to make berms and beaches. Islands considered in greater detail in this survey include Santiago, Maio, and Sal from the Cape Verde Islands, Fuerteventura and the related islet of Lobos in the Canary Islands, Selvagem Grande and Pequena from the Savage Islands, Porto Santo in the Madeira Islands, and Santa Maria in the Azores. This contribution expands on the concept that living rhodoliths enter the fossil record through a range of taphofacies defined by the degree of breakage and corrosion and further characterized by sedimentological criteria regarding the amount of matrix and packing among bioclasts. Rhodolith deposits in Macaronesia seldom reflect settings under natural growth conditions. Rather, rhodoliths are subject to transportation and post-mortem disintegration resulting in the accumulation of rhodolith materials captured by subtidal storm deposits, tidal pools and platform over-wash deposits, as well as beachrock, beach, berm, hurricane, tsunami, and coastal dune deposits. Some of this material is transferred farther offshore, but exposed island strata show a tendency for shoreward migration of taphofacies. Rhodolith beds provide a habitat for some species of marine invertebrates, including epifaunal and infaunal elements directly associated with whole rhodoliths and these features play a role in rhodolith biostratinomy.

Beds of coralline algal sediment form ecologically and economically important habitats in the North Atlantic. These habitats can occur from the intertidal down to 60 m depth, and they are locally abundant in several countries. Thirteen species of coralline algae form rhodoliths or maerl in this region; Lithothamnion corallioides, L. glaciale, L. tophiforme and Phymatolithon calcareum are the most widely recorded. The structure and biodiversity of these habitats is destroyed by dredging and can be degraded by towed demersal fishing gear and by mussel and salmon farming. Legislation has been passed in the European Union (EU) to protect P. calcareum and L. corallioides which should be extended to include the other maerl species from the region. Outside the EU there is a lack of baseline information concerning the importance of these habitats: a fuller understanding of their role may lead to protection in Scandinavia, Iceland and the Atlantic coasts of Canada and the United States. The design of such protected areas would need to consider the ongoing effects of invasive species, ocean warming and acidification.

The available references to rhodolith beds have been analyzed for a total of 125 locations in the Mediterranean Sea, equally distributed in the eastern and western sub-basins. Mediterranean rhodolith beds occur from 9 to 150 m of water depth, with a mean depth of about 55 m in both sub-basins. Most rhodolith beds lay within the depth range 30–75 m, while those extending deeper than 75 m are about 18 % of the total, and those shallower than about 25 m are uncommon. The deepest and the largest Mediterranean rhodolith beds are both located in the Balearic Sea. Water motion from bottom currents, waves, and tides is needed to keep rhodoliths unburied, within mesotrophic to oligotrophic water conditions. Rhodolith beds are commonly small (<0.01 km²) and multispecific, showing growth-form mixing and a much higher coralline biodiversity than Atlantic beds. They are vulnerable to physical disturbance by fishing gears and smothering, and to water pollution by organic enrichment and sewage. The existing instruments of legal protection appear ill defined, since these are based on scientific literature mostly derived from northern Europe, where specific and different environmental settings, species composition, depth distribution, and anthropogenic pressures occur. The protection of a specific habitat type cannot be effectively gained without access to geospatial and compositional data and an increased research effort is needed to improve taxonomic inventories, habitat mapping, and monitoring activities on a basin scale.

The largest continuous latitudinal distribution of rhodolith beds occur in the South Atlantic Ocean. Up to now rhodolith beds were referred exclusively to the western portion of the South Atlantic. Here we describe the recent advances in the South Atlantic taking into account latitudinal distribution, species composition, structure and ecosystem functions, threats and conservation status. Rhodolith beds have been mapped and ecologically described from extensive areas of the continental shelf (Abrolhos Bank), seamounts tops (Vitoria Trindade Chain), insular shelfs of oceanic islands (Fernando de Noronha Archipelago) and atolls (Rocas Atoll). Thirty three species of crustose coralline algae were recorded forming rhodoliths. Despite some initiatives, the richness of fauna associated with rhodoliths in SW Atlantic is still poorly known. Specific microbiome described associated with rhodoliths indicates important role in biomineralization process. The environmental services provided by the recently described rhodolith beds (Abrolhos Banks and Vitoria Trindade Seamounts) as calcium carbonate production, increase habitat complexity, benthic diversity and associated fish assemblages justify urgent actions to protect these ecosystems.

In the Eastern Pacific (EP) the only region where rhodolith beds have been well studied in terms of taxonomy, ecology, distribution and conservation status is the Gulf of California. Outside this region the knowledge of rhodolith-forming species is attributed to the initial separate floristic surveys of Dawson and Lemoine, performed more than 50 years ago. After a detailed review of the published literature and information produced during our expeditions throughout the EP, a total of 36 rhodolith-forming species have been recorded from the Aleutian Islands in Alaska to Guarello Island in Chile. Despite the research efforts developed at present, more species remain to be discovered in the EP, particularly in the Tropical Pacific of Mexico and the Pacific coast of Baja California where we found undescribed species of Sporolithon and Lithothamnion. Therefore, we contend that further studies are needed in order to better catalogue the wide rhodolith-forming species diversity that is extremely relevant for the marine realm of the EP.

The Western Pacific is one of the regions where rhodolith beds are abundant. However, the studies of the rhodolith beds and rhodoliths themselves have been conducted mainly from geological perspectives. Here we review rhodolith beds and 20 rhodolith-forming species which are distributed in both shallow and deep waters (lower intertidal zones to 135 m deep) from cold to tropical regions of the Western Pacific. The coralline species composition of each rhodolith bed has not been well studied, and the taxonomic status of each species has to be defined by modern taxonomic concepts using molecular approaches. Research on rhodoliths in the Western Pacific therefore remains a poorly studied in aspects of phycology and of coastal biology. The present chapter represents an account of the current knowledge.

Rhodoliths of the South Pacific remain largely unstudied. Twenty two rhodolith-forming species from ten genera have been reported from the southern Pacific, although of these, only nine species in six genera have been confirmed as rhodolith records in the literature. There have been few ecological or distributional studies of rhodoliths in the southern Pacific, although some data are available on the age and growth rates of rhodoliths from the region. The taxonomy and nomenclature of rhodolith-forming taxa in this region are problematic.