Science – Seabed Habitats

How do marine heat waves affect cold-water corals in the deep-sea?

Ocean temperature plays an important role in governing the biophysical environment and in turn the realized ecological niche of benthic organisms and the distribution of marine habitats in the global ocean. As a result of anthropogenic-induced climate change Marine Heat Waves (MHWs) have resulted in widespread coral bleaching and mass mortality of tropical corals. Comparatively little is known about the impact of sub-surface MHWs on cold-water coral communities in the deep-sea, which lack the dependence on symbiotic zooxanthellae of their tropical cousins in surface waters. This study aims to address this deep-ocean knowledge gap utilising numerical modelling.

Previous studies have shown that prolonged and intense sub-surface MHWs and their cumulative intensity results in increasing thermal stress encountered by benthic organisms in depths down to 2000m. This study utilises hydrodynamic modelling outputs of reanalysis data to study impacts of sub-surface MHWs on cold-water corals. With intensification and duration of sub-surface MHWs predicted to rapidly increase in this century, it will increase the thermal stress experienced by cold-water corals and associated communities in the deep-sea.

It is hypothesized that through increased stratification and increased mixed layer depth due to increased storminess and deepening of the thermocline, cold-water coral distribution will be impacted physically (mechanically), however their mortality is more likely to be dependent on food availability and oxygen supply. Preliminary results support this hypothesis in the North Atlantic, indicating that although the cold-water corals may be resilient to thermal stress physiologically, the resulting physically dynamic environment will lead to secondary impacts at the local scale in the benthic boundary layer. Furthermore, oxygen saturation has been found to decrease with increasing temperature and is likely to have a secondary impact on cold-water coral health. This study demonstrates how numerical modelling can provide quantitative spatial and temporal information of biophysical interactions to inform marine habitat mapping studies and species distribution models of habitat suitability.

(Background: This is my research abstract made for the GeoHab 2024 conference – if you would like to learn more, my short oral presentation will be happening on Thursday 9th May 2024 at 1415 in Arendal, Norway. More information about the conference, including how to be a virtual participant, on the official conference website. An interesting program!) See also: https://www.marineheatwaves.org

New update: Marine Heat Waves Shiny App!!

Science

Integrating sediment dynamics into habitat mapping approaches using sediment mobility indices and seabed classification in Galway Bay, Ireland.

Integrating sediment dynamics into habitat mapping approaches using sediment mobility indices and seabed classification in Galway Bay, Ireland. from Siddhi Joshi

Sediment dynamics information provide vital insights into the important role of oceanographic forcing factors on habitat distribution; yet remains an under-utilized physical surrogate in marine habitat mapping studies. An integrated oceanographic and geophysical analyses of dynamic processes combining sediment mobility indices, obtained from coupled-hydrodynamic- wave sediment transport models; with seabed classification has been made at Galway Bay, Ireland.

Maerl or rhodolith coralline red algae beds are abundant in Galway Bay and these beds represent more than 65% – 70% of the maerl habitats in Ireland (De Grave and Whitaker, 1999). Maerl beds are particularly affected by hydrodynamics and increased storminess resulting in recurrent disturbance of the benthic habitat patch during winter storms. Live maerl beds are biodiversity rich coastal habitats and form subtidal and intertidal banks and open marine beds. Dead maerl beds of the branched maerl morphotype are considered to be biogenic sediment with form dense biogenic gravel debris beaches.

Sediment mobility modelling is of importance to a range of disciplines including sediment dynamics, marine conservation, coastal engineering, and renewable energy (Harris and Coleman, 1998; Idier et al., 2010; Li et al., 2015, Joshi et al, 2017a, Coughlan et al. 2021). It is based on the fundamental quantity of bed shear stress and the impact of pure currents, wave-only, wave-induced currents or combined wave-current flow on surficial sediments.

Multibeam backscatter from the INFOMAR national seabed mapping program of Ireland have been utilized for seabed classification using the new machine learning and deep learning libraries in ArcGIS Pro and Python.

An integrated interpretation of the dynamic processes happening at the seafloor is made as a result of the combined wave-current induced disturbance regime during storm conditions. Implications for future conservation management of maerl beds impacted by increased storminess and anthropogenic activity are discussed.

References

Coughlan, M., Guerrini, M., Creane, S., O’Shea, M., Ward, S.L., Van Landeghem, K.J.J., Murphy, J., Doherty, P., 2021. A new seabed mobility index for the Irish Sea: Modelling seabed shear stress and classifying sediment mobilisation to help predict erosion, deposition, and sediment distribution. Continental Shelf Research 229, 104574.

De Grave, S., Whitaker, A., 1999. A census of maerl beds in Irish waters. Aquatic Conservation: Marine and Freshwater Ecosystems 9,303-311.

Harris, P.T., Coleman, R., 1998. Estimating global shelf sediment mobility due to swell waves. Marine Geology 150, 171177.

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, 365–377.

Joshi, S., Duffy, G.P., Brown, C., 2017a. Mobility of maerl-siliciclastic mixtures: Impact of waves, currents and storm events. Estuarine, Coastal and Shelf Science 189, 173–188.

Li, M.Z., Hannah, C.G., Perrie, W.A., Tang, C.C.L., Prescott, R.H., Greenberg, D.A., 2015. Modelling seabed shear stress, sediment mobility, and sediment transport in the Bay of Fundy. Canadian Journal of Earth Science. 52, 757–775.

Cite as:

Joshi, Siddhi. (2022). Integrating sediment dynamics into habitat mapping approaches using sediment mobility indices and seabed classification in Galway Bay, Ireland. GeoHab 2022, Venice. https://lnkd.in/e9vHBu8v

StoryMap:

A StoryMap of this research can be found at the link

Science

Seafloor Geomorphology as Benthic Habitat- Second Edition

9780128149607

The second edition of Seafloor Geomorphology as Benthic Habitat, edited by Peter Harris and Elaine Baker has been published at the end of last year and I was pleased to get my copy of the paperback version just a few weeks ago! The book is a “GeoHab Atlas of Seafloor Geomorphic Features and Benthic Habitats,” (GeoHab International Habitat Mapping community).

Seafloor Geomorphology as Benthic Habitat: GeoHab Atlas of Seafloor Geomorphic Features and Benthic Habitats, Second Edition, provides an updated synthesis of seabed geomorphology and benthic habitats. This new edition includes new case studies from all geographic areas and habitats that were not included in the previous edition, including the Arctic, Asia, Africa and South America. Using multibeam sonar, the benthic ecology of submarine features, such as fjords, sand banks, coral reefs, seamounts, canyons, mud volcanoes and spreading ridges is revealed in unprecedented detail. This timely release offers new understanding for researchers in Marine Biodiversity, environmental managers, ecologists, and more. (Elsevier.com)

This book, as well as the first edition, contain a balance between different approaches to marine conservation, scientific theory of marine geology and geophysical mapping techniques. It is a joy to flick through the images, learn about different geomorphic features around the world and understand the state of the art approaches to surveying benthic habitats. The major part of the book contains detailed practical case studies as examples of different approaches to seafloor mapping and examination of the seafloor for marine conservation management and scientific purposes. The first edition of the book had been very useful during my PhD, with the opening Part 1 being vital guidance for me as a research student. My coauthor and I were please to be able to contribute a review chapter on the “Physical oceanographic drivers of geomorphology of rhodolith/maerl beds in Galway Bay, Ireland” (Chapter 12 case study by Joshi and Farrell) to this edition! This chapter is one of two case studies from Ireland, which form a part of the major “Case Study” portion of the book (Part 2). A synthesis chapter serves to be a useful benchmark, together with a summary of outputs of a questionnaire for authors (Part 3). A vital edition to all academic marine science libraries and for government scientists and marine biology and geological oceanography students.

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Proud author of book chapter!

Seafloor Geomorphology as Benthic Habitat – Second edition Elsevier Homepage Here

Science

International Rhodolith Workshop, Roscoff, France

Rhodolith-2018_Group

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:

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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.

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Acknowledgements

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!

Science, What's New

Modelling maerl habitat dynamics in response to increased storminess

**Figure 1.** A. Biogenic gravel composed entirely of dead maёrl (debris) at Trá an Doilin (Carraroe) beach, County Galway. B. Individual clasts of maёrl debris on beach (photo length: 0.05m). C. Living maёrl beds in approximately 5m bathymetry (photo length: c.0.60m). D & E. Concentric patterns at Trá an Doilin suggests maёrl has a higher mobility than quartz sediments.

An exciting new project involving field work has begun at NUI Galway School of Geography, which focuses on quantifying the impacts of storminess on maerl beach morphodynamics. Rhodolith (maerl) beds are unique, relatively rare, free-living, non-geniculate coralline red algae forming biodiverse habitats and dense biogenic debris beaches. These beds provide hard habitat for other marine algae on their surface and for invertebrates living on and in the rhodoliths. This one year field research project investigates the response of offshore maerl beds and maerl debris beaches to storminess. Specifically, the morpho-sedimentary evolution of maerl beaches over timescales of seconds (swash dynamics) to months (seasonal weather) will be measured using a suite of integrated, multi-disciplinary field and laboratory methods based on hydrodynamic modelling, bathymetric and topographic mapping, and groundwater fluxes. The experiments will utilise results from previous research. The impact of the Intergovernmental Panel on Climate Change (IPCC) scenarios on the regional hydrodynamic model will be made to quantify possible impacts of climate change on maerl. Using XBeach, an open-source numerical model with a domain size of kilometres, on the time scales of storms, outputs will be compared with nearshore-beach DEMs derived from UAV surveys (water and land), and supplemented with baseline INFOMAR LiDAR data from Greatman’s Bay. This project will integrate oceanographic observations (waves, currents, tide) to compliment habitat mapping. A poster of this work was presented at the Irish Geomorphology Group Meeting at the Geological Survey Ireland in Dublin. A0 Poster - Siddhi Joshi Poster Final The poster is available for download here: Siddhi Joshi Eugene Farrell Poster Final

Acknowledgements

This project is funded by the Geological Survey Ireland Short call 2017-SC-043.

Science, What's New

Critical bed shear stress of maerl experiment

Concentric patterns at Maerl Beach, Trá an Doilín in Carraroe, County Galway

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).

Live maerl thalli (Lithothamnion glaciale, Image credit to Jason Hall-Spencer, University of Plymouth)

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.

References

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)