Mechanisms of stability of rhodolith beds: sedimentological aspects

Abstract

Rhodolith beds are highly diverse benthic communities organized around the physical structure and primary productivity of red coralline algae. Despite a worldwide distribution and growing recognition that rhodolith beds are important calcium carbonate (CaCO3) biofactories, little is known of the factors and processes that regulate their structure, function, and stability. One prevalent and largely untested paradigm is that beds develop in environments where water motion is strong enough to prevent burial by sediments.Observations over seven months and three weeks in the centre and near the upper and lower margins of a Newfoundland rhodolith (Lithothamnion glaciale) bed, as well as a laboratory mesocosm experiment with rhodoliths and dominant macrofauna from the bed, were used to characterize, parse, and model spatial and temporal variation in rhodolith sediment load (RSL) and movement among presumably important abiotic and biotic factors. RSL and movement were largely mediated by a few dominant benthic invertebrates. Hydrodynamic forces were insufficient to displace rhodoliths. Daisy brittle stars (Ophiopholis aculeata) and small common sea stars (Asterias rubens) contributed to dislodgement of sediment from rhodoliths. Large green sea urchins (Strongylocentrotus droebachiensis) easily displaced rhodoliths in mesocosms. Results provide the first quantitative demonstration that rhodolith beds need not be exposed to threshold hydrodynamic conditions to avoid burial. Beds can simply occur in areas where burial is unlikely because of low sedimentation rates. In such cases, select resident bioturbators operating simultaneously at different spatial scales (within and outside rhodoliths) appear to suffice to maintain RSL below lethal quantities, contributing to stability of beds.iii ACKNOWLEDGEMENTS

The Punta de la Mona Rhodolith Bed: Shallow-Water Mediterranean Rhodoliths (Almuñecar, Granada, Southern Spain)

“…The optimum conditions for rhodolith growth might be due to a flatter seafloor between 15 and 20 m (Figure 1). Despite the weak current perceived, the highest proportion of very fine sand and mud occurs at 18 m. This can be due to the well-known baffling effect of dense concentrations of branching rhodoliths, which stabilize fine-grained sediment below them (Bassi et al, 2009;Millar and Gagnon, 2018;O'Connell et al, 2021).…”

Morphostructural Characterization of the Heterogeneous Rhodolith Bed at the Marine Protected Area “Capo Carbonara” (Italy) and Hydrodynamics

“…Water motion, such as waves and currents, has been indicated as the main responsible for maintaining the rhodoliths unburied [16][17][18][19], and rhodoliths are often associated with sedimentary structures, such as ripple marks and dunes [19][20][21][22]. Some authors demonstrated that rhodoliths do not need to be exposed to threshold hydrodynamic forces to avoid burial [23,24], suggesting a primary role of bioturbation [23,25]. Despite this, rhodoliths are often associated with sedimentary structures, such as ripple marks and dunes [19][20][21][22].…”

“…The relation between rhodolith outer/inner structure and water energy/water depth is debated in the literature, and no straightforward interpretation can be drawn. Moreover, only qualitative indications on velocity and frequency of current related to specific shape or morphotype have been provided thus far, included both field measurements, experimental models, and tanks [6,23,24,[26][27][28].…”

Multiple Trophic Tracer Analyses of Subarctic Rhodolith (Lithothamnion glaciale) Bed Trophodynamics Uncover Bottom-Up Forcing and Benthic-Pelagic Coupling

“…The relatively complex morphology of rhodoliths creates suitable habitats for attachment (Kamenos et al, 2004a;Steller and Cáceres-Martínez, 2009;Riosmena-Rodríguez and Medina-López, 2010), reproduction (Kamenos et al, 2004b;Steller and Cáceres-Martínez, 2009;Gagnon et al, 2012), and feeding (Steneck, 1986;Gagnon et al, 2012;Riosmena-Rodríguez et al, 2017) of highly diverse algal and faunal assemblages. The important contribution of rhodolith beds to marine biodiversity (Steller et al, 2003;Gagnon et al, 2012;Riosmena-Rodríguez et al, 2017) and global calcium carbonate (CaCO 3 ) production (Amado-Filho et al, 2012;Harvey et al, 2017;Teed et al, 2020) has, in part, triggered the recent increase in studies of factors and processes regulating their structure and function (Marrack, 1999;Hinojosa-Arango et al, 2009;Millar and Gagnon, 2018) and growth resilience to natural and anthropogenic stressors (Bélanger and Gagnon, 2020;Bélanger and Gagnon, 2021;Arnold et al, 2021) Knowledge about trophodynamics in rhodolith beds is limited to only a couple of studies in northeastern Atlantic (Grall et al, 2006) and eastern Pacific (Gabara, 2014) systems, that together suggest suspended particulate organic matter (SPOM), sediment organic matter (SOM), and macroalgae are important components of rhodolith bed food webs. Both studies' findings are based on use of bulk stable isotope analysis, in particular consideration of organisms' carbon (δ 13 C) and nitrogen (δ 15 N) isotopic signatures (DeNiro and Epstein, 1978;DeNiro and Epstein, 1981;Minagawa and Wada, 1984) to identify primary producers (Peterson and Fry, 1987;Post, 2002;Bouillon et al, 2011) and trophic levels of consumers…”