Settling Velocity and Grain Shape of Maerl

ResearchBlogging.orgOur recent study on maerl sediment dynamics has found that the settling velocity of maerl is primarily governed by the grain shape properties of maerl. A grain shape parameter known as the convexity has been linked to the settling velocity via the Ferguson and Church model (Ferguson and Church, 2004). Due to the grain shape of maerl and roughness, it experiences a greater drag than the natural quartz grain. Detailed measurements of maërl grain shape using microscopic image analysis confirm this link.

Maërl tends to form beach deposits with a low percentage of sand and it is hypothesised that the lower settling velocity of maerl results in this preferential transport of biogenic maerl sediments compared to quartz sands and gravels. Maërl samples found in open marine, intertidal, and beach environments show a different linear relationship between roughness and grain size, due to different degrees of abrasion. A combination of different wave climates and transport histories result in this increased spatial variability of grain textures.

The paper and study then goes on to discuss to what extent a general equation for maërl settling velocity is possible or not and to whether the sediment mobility of maerl can be predicted using the settling velocity as an input parameter.

The apparatus used to determine the settling velocity of maerl
Microscopic image analysis of the maerl grain
Lithophyllum fasciculatum
Maerl grains were found to be more convex, with a high grain roughness
Carraroe Maerl Beach in County Galway shows a higher maerl to sand ratio, with a high percentage maerl- an occurance explained here to be due to the lower settling velocity of maerl.

Ferguson, R., & Church, M. (2004). A Simple Universal Equation for Grain Settling Velocity Journal of Sedimentary Research, 74 (6), 933-937 DOI: 10.1306/051204740933

Joshi, S., Duffy, G., & Brown, C. (2014). Settling Velocity and Grain Shape of Maerl Biogenic Gravel Journal of Sedimentary Research, 84 (8), 718-727 DOI: 10.2110/jsr.2014.51

International Rhodolith Workshop

The IV International Rhodolith Workshop took place in Granada, Spain in September. Meeting every three years, delegates were from Brazil, Spain, United Kingdom, USA, Ireland, Mexico, New Zealand and Australia, as well as other countries. Rhodolith is a term largely used interchangeably to “maerl,” as free living non-geniculate coralline algae. Researchers came to share the latest research about one of the big four macrophyte dominated benthic communities (others being kelp beds, seagrass meadows and biogenic reefs) (Foster, 2001). Topics included taxonomy, ecology,  management and conservation biology, genetics, geochemistry, evolution, palaeoecology, climate change studies and sediment dynamics.

Two excursion took place; the Granada coast to look at living rhodoliths and a two-day excursion to Almería-Cabo de Gata to observe both fossil and living rhodolith beds. The first excursion involved diving off the Granada coast or shorkelling to explore the small sea-caves along the coast. The second excursion involved exploring the processes responsible for deposition of rhodolith debris as cliff-deposits and how they have been preserved across geological time.

Further information can be found on the conference website. My poster presented to the conference can be found on the Griffith NUIG Biogeosciences website


Foster M, 2001, Rhodoliths: Between rocks and soft places, Journal of Phycology, Vol 37 Issue 5, 659-667