Just by going to the beach, I had been fascinated by how maerl was freely moving, carried, mobilised and transported by almost every wave. The beach, composed almost entirely of “coral” is actually made of branched free-living coralline algal gravels (maerl). I was intrigued to see these concentric patterns, almost like “beach cusps,” observed at Trá an Doilín maerl beach in Carraroe, County Galway. Furthermore, large maerl megaripples (or sub-aqueous dunes) had been observed subtidally, such as those in Northern Ireland (video). The flow strength required for initiation of motion is a classical problem in fluid dynamics and we found very little work had been done on maerl and the conditions under which it is mobilised and transported.
Our new study entitled “Critical bed shear stress and threshold of motion of maerl biogenic gravel” has just been published in Estuarine, Coastal and Shelf Science (in press). The critical bed shear stress is a fundamental sediment dynamics quantity – a measure of the threshold of motion of sediment. When we began our study on modelling the sediment mobility of maerl in Galway Bay, we found that this quantity for maerl coralline alga was an unknown which had largely been overlooked in classical sediment transport experiments. Its knowledge was a prerequisite for quantifying maerl mobility, rate of erosion and deposition in conservation management. Through as series of lab (flume) experiments on biogenic free-living maerl beds, our study determines the critical Shields parameter for maerl in three contrasting environments (open marine, intertidal and beach) in Galway Bay, west of Ireland.
The bed shear stress was determined using two methods, Law of the Wall and Turbulent Kinetic Energy, in a rotating annular flume and in a linear flume. The velocity profile of flowing water above a bed of natural maerl grains was measured in four runs of progressively increasing flow velocity until the flow exceeded the critical shear stress of grains on the bed (from Abstract, Joshi et.al 2017b).
The critical Shields parameter and the mobility number are estimated and compared with the equivalent curves for natural quartz sand. The critical Shields parameters for the maerl particles from all three environments fall below the Shields curve. Along with a previously reported correlation between maerl grain shape and settling velocity, these results suggest that the highly irregular shapes also allow maerl grains to be mobilised more easily than quartz grains with the same sieve diameter (from Abstract, Joshi et.al 2017b).
The intertidal beds with the roughest particles exhibit the greatest critical shear stress because the particle thalli interlock and resist entrainment. In samples with a high percentage of maerl and low percentage of siliciclastic sand, the lower density, lower settling velocity and lower critical bed shear stress of maerl results in its preferential transport over the siliciclastic sediment. At velocities ∼10 cm s−1 higher than the threshold velocity of grain motion, rarely-documented subaqueous maerl dunes formed in the annular flume (from Abstract, Joshi et.al 2017b).
The full research paper can be found here, as well as the related papers in the full study below.
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: https://doi.org/10.2110/jsr.2014.51 (Paper 1)
Joshi, S., Duffy, G., & Brown, C. (2017a). Mobility of maerl-siliciclastic mixtures: Impact of waves, currents and storm events Estuarine, Coastal and Shelf Science DOI: https://doi.org/10.1016/j.ecss.2017.03.018 (Paper 3)
Joshi, S., Duffy, G., & Brown, C. (2017b), Critical bed shear stress and threshold of motion of maerl biogenic gravel, Estuarine, Coastal and Shelf Science, https://doi.org/10.1016/j.ecss.2017.06.010 (Paper 2)
Our new study on “Mobility of maerl-siliciclastic mixtures: impact of waves, currents and storm events,” has just been published (in press) in Estuarine, Coastal and Shelf Science. This is the final part of my PhD in maerl sediment dynamics. Sediment mobility in its simplest form is the percentage of time grains of a particular size are mobile during a tidal cycle (Idier et.al., 2010). This study focuses on the sediment mobility of maerl in particular, utilising coupled hydrodynamic-wave-sediment transport models to model the oceanography during calm and storm conditions and the resulting sediment transport. Sediment mobility models are another way of quantifying the disturbance of the seafloor as a result of currents, waves and combined wave-currents. This study calculates two sediment mobility indices, the Mobilization Frequency Index (MFI) and the Sediment Mobility Index (SMI), related to the magnitude and frequency of disturbance events (Li et.al, 2015). The residual currents, which are the part of the current remaining after removing the oscillatory tidal component, show that maerl prefers intermediate mobility environments and is often found at the periphery of the residual current gyres. Sediment mobility maps can be used to inform marine spatial planning for the management of both live and dead (fossil) maerl beds, as a result of climate change or anthropogenic activity.
The full research paper, Joshi et.al. 2017, can be found here.
Idier, D., Romieu, E., Pedreros, R., & Oliveros, C. (2010). A simple method to analyse non-cohesive sediment mobility in coastal environment Continental Shelf Research, 30(3-4), 365-377 DOI: 10.1016/j.csr.2009.12.006
Joshi, S., Duffy, G., & Brown, C. (2017). Mobility of maerl-siliciclastic mixtures: Impact of waves, currents and storm events Estuarine, Coastal and Shelf Science DOI: 10.1016/j.ecss.2017.03.018
Li, M., Hannah, C., Perrie, W., Tang, C., Prescott, R., Greenberg, D., & Rygel, M. (2015). Modelling seabed shear stress, sediment mobility, and sediment transport in the Bay of Fundy Canadian Journal of Earth Sciences, 52 (9), 757-775 DOI:10.1139/cjes-2014-0211
It had been a dream of mine to see the mangroves since learning about their importance in coastal defence in my lectures and blogging about them. This winter my family and I had the brilliant opportunity to go to the Tamil Nadu Backwaters to see the mangroves of the east coast of south India- at Pichavaram.
“Pichavaram is known for its unique mangrove ecosystem, found in areas such as the Sunder bans in West Bengal and in Australia. The mangroves are trees rooted in a few feet of water and the whole area stretches to over 3,000 acres comprising more than 1,700 islets. Pichavaram consists of a number of islands interspersing a vast expanse of water and covered with green trees. The area is about 2800 acres and is separated from the sea by a sand bar, which is a patch of extraordinary loveliness. To a botanist, rare species like Avicennia and Rhizophora will present a special attraction; to a zoologist, no doubt, the sight of numerous birds like Water snipes, Cormorants, Egrets, Storks, Herons, Spoonbills and Pelicans holds great interest. Of around eighty species worldwide, Pichavaram is home to 14 species, chief among them being Avicennia and Rhizophoro. The Pichavaram swamps are one of the healthiest occurrences in the world, and act as a nursery for a variety of finfish and shell fish. They also effectively demonstrated their role as a bio-shield in the recent Indian Ocean Tsunami; there was no loss of life in communities living next to the mangroves.” (Tamil Nadu Tourism)
We also took the opportunity to visit the Centre of Advanced Study in Marine Biology of Annamalai University, where they very kindly offered us a tour of their centre. The centre has a fascinating zoology museum and a long history of training over 600 PhD students as well as study of the mangroves and also deep sea organisms off the Indian coast.
A wonderful experience- a special thank you to my family and the Annamalai university for the tour!
This new book on Rhodolith/Maërl Beds* has been much anticipated by the rhodolith research community. With over four years in the making, it is a volume tributed to Prof. Rafael Riosmena-Rodríguez, who dedicated 25 years particularly to the study of the taxonony of coralline algae and sadly passed away earlier this year. Prof. Riosmena-Rodríguez will be very much missed by the rhodolith research community and this book is an important tribute.
The book begins with the role of rhodolith/maërl beds in modern oceans, with chapters on natural history and biodiversity around maërl beds, rhodoliths as climatic archives, modern day threats of ocean acidification on maërl and economic importance. The role of rhodolith in historical oceans and the geological significance is explored by the following section, with a particular focus on the Mediterranean area as well as the North Atlantic sedimentary dynamics. The final part of the book covers the conservation status of rhodoliths globally and serves to be an important summary of current state of regional knowledge of rhodoliths in the major geographic areas.
“These marine beds occur worldwide, from the tropics to the poles, ranging in depth from intertidal to deep subtidal habitats and they are also represented in extensive fossil deposits.”
Overall, this is a much needed edition for marine biology libraries around the world and highly recommended for students of one of the four macrophyte dominated benthic communities. I made a blog post about attending the International Rhodolith Workshop in Granada and one of the key conclusions of the 2015 workshop in Costa Rica was that international recognition is needed for rhodolith habitats to ensure their protection. This book is an important step required to make this possible. It serves to be a useful and comprehensive introduction summarizing the multidisciplinary study of global rhodoliths/maërl beds.
*The term maerl originally refers to the branched growth form of Lemoine (1910) and the term rhodolith is sedimentalogical or genetic term for both the nodular and branched growth forms (Basso et. al, 2015).
Basso (2015) Monitoring deep Mediterranean rhodolith beds. Aquatic Conservation Marine and Freshwater Ecosystems. 26:3. doi:10.1002/aqc.2586.
Lemoine (1910) Répartition et mode de vie du maërl (Lithothamnium calcareum) aux environs de Concarneau (Finistère). Annales de l’Institut Océanographique. 1: 1–29.
Riosmena-Rodríguez, R., Nelson, W., and Aguirre, J. (Editors) (2016) Rhodolith/Maërl Beds: A Global Perspective, Coastal Research Library 15, VIII, 368pp.DOI: 10.1007/978-3-319-29315-8
“Before The Flood is the product of an incredible three-year journey that took place with my co-creator and director Fisher Stevens. We went to every corner of the globe to document the devastating impacts of climate change and questioned humanity’s ability to reverse what may be the most catastrophic problem mankind has ever faced. There was a lot to take on. All that we witnessed on this journey shows us that our world’s climate is incredibly interconnected and that it is at urgent breaking point.”
-Leonardo DiCaprio, UN Messenger of Peace on Climate Change (Source: Wikipedia)
Watch the full film “Before the Flood” on YouTube till 6th November.
We would like to invite you to a seminar on Seafloor Geomorphology in Galway on Friday 23rd September at 12 noon at the MRI Annex lecture theatre, National University of Ireland, Galway. All welcome!