At the end of last month, the International Rhodolith Workshop took place in Roscoff, Brittany, France and around 50-60 international scientists came from the far reaches to present their work on maerl or rhodoliths. In the geology session, had the brilliant opportunity to present some of our work on the habitat dynamics and the impact of storminess on maerl:
We went on a boat trip in the Bay of Brest and sampled some of the maerl from an unfished and a fished site. Here are some photos of our trip to collect some specimens from the Bay of Brest.
This trip was funded by the Marine Insitute Travel and Networking Award, Ireland and we would like to thank the organisers of the conference and the Marine Institute for making this trip possible!
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)
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
Finally I can bring to you the trailer for the maerl documentary! This trailer gives you a small taster of the final hour long documentary film. As a PhD student studying maerl I encountered many researchers with diverse and in-depth knowledge about maerl beds in Ireland and worldwide and felt quite compelled to make this documentary. It includes interviews about marine botany, zoology, ecology, geology and marine geophysics, as well as the threat of anthropogenic impacts on maerl, climate change and possible solutions. Having been busy editing to sew together nine interviews, breathtaking scenery and diving footage. I am now consulting with my team and friends for suggestions of how to improve the near-final cut. Please tell your friends about this film and we hope it will help the next generation of scientists, educators and policy makers to conserve, protect and manage this vulnerable benthic habitat.
Our 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.
Using integrated novel 3D techniques for complex flow field modelling around coralline algae. Sebastian Hennige is a NERC Independent Research Fellow at Centre for Marine Biodiversity and Biotechnology at Heriot-Watt University, Edinburgh.