maerl beds in Galway Bay, Ireland

CHAPTER 12

Physical oceanographic drivers of geomorphology of rhodolith/maerl beds in Galway Bay, Ireland Siddhi Joshi and Eugene Farrell Discipline of Geography, School of Geography and Archaeology, Ryan Institute for Environment, Marine and Energy Research, National University of Ireland Galway, Galway, Ireland

Abstract Galway Bay is a semienclosed, large bay (62 by 33 km) located in the West of Ireland, Northeast Atlantic. Living maerl or rhodolith beds (nongeniculate coralline red algae) as well as dense biogenic gravel beaches (composed of fossilized dead maerl debris) and the associated subaqueous dune systems are found in both inner Galway Bay and complexes (i.e., bays in the North coast), with some deeper Aran maerl beds also found at depths up to 26
30 m. Peak combined wave
current-induced sediment mobility and residual currents were key physical surrogates for maerl, governing distribution of live maerl beds and debris beaches. Increased intensity and frequency of storm events, attributed to anthropogenic climate change, may result in future erosion of subaqueous maerl dune systems and a sharp decline in habitat condition.

Keywords: Maerl; Rhodolith; hydrodynamic modeling; wave modeling; shear stress; climate change; sediment dynamics; storminess; habitat dynamics

Introduction Maerl or rhodolith beds are free-living, nongeniculate coralline algae aggregations found living in carbonate shelf environments [see glossary for exact definitions of maerl and rhodolith and an exhaustive list of 12 terms on p. 9 of Riosmena-Rodrı´guez (2017)]. They are a globally significant carbon sink (Amado-Filho et al., 2012; Adey et al., 2015). Maerl beds are ecosystem engineers and have significant ecological significance as well as geological and geomorphological significance in the shallow marine habitat. They form dense biogenic gravel beaches and are linked to their subtidal or intertidal source of live maerl by subaqueous dune systems (Hall-Spencer et al., 2010). In Europe two maerlforming species: Lithothamnium corallioides and Phymatholithon calcareum are found in the Annex V of the EC Habitats Directive, with indirect protection under Annex I (EC Seafloor Geomorphology as Benthic Habitat. DOI: https://doi.org/10.1016/B978-0-12-814960-7.00012-9 © 2020 Elsevier Inc. All rights reserved.

231

232 Chapter 12 Council Directive 92/43/EEC). Therefore they are protected as Special Areas of Conservation (SACs) in Ireland. This case study is on maerl habitats of Galway Bay in West of Ireland, a large semienclosed bay 62 km (west to east) by 33 km (north to south), narrowing to 12 km (north to south) at the headland of Black Head. The Galway Bay seafloor geology is underlain by granites of Connemara region, County Galway, located to the north and the Carboniferous karst limestone outcrops of the Burren, County Clare, located to the south and the three Aran Islands semienclosing the bay to the west (McCabe, 2008). The three Aran islands, known as Inishmore, Inishmaan, and Inisheer (from west to east), enclose the bay and protect it from the strongest swells of the North Atlantic. In North Sound a major submarine fault (known as the “Galway Bay Fault”) was discovered in the national seabed mapping program (Irish National Seabed Survey, “INSS,” and its successor, “INFOMAR”) multibeam survey of 2007 (INFOMAR, 2007). The inner bay (east of the headland of Black head) has a depth range of 0
20 m, with 20
60 m in the outer bay and in the sounds between the Aran Islands. The depth range of living maerl habitats is largely dependent on the depth of light penetration and in Galway Bay is restricted to ,25
30 m. The main riverine inputs into Galway Bay are from the Corrib River at Galway city (Bflow rate 100 6 60 m3 s21) and the Owenboliskey River at Spiddal as well as the significant influence of submarine ground water discharge (SGD), especially from the karst region of the Burren, the most extensive carboniferous limestone region in Europe (INFOMAR, 2007). Tides are dominated by the semidiurnal tidal constituents and are macrotidal (Table 12.1). Outer bay wave data are 6-year seasonal means from a nondirectional wave buoy and inner bay wave data are 10-year seasonal means from the Spiddal wave rider buoys. Temperature and salinity ranges are based on a ROMS model Table 12.1: Oceanographic statistics in Galway Bay, based on Admiralty (2015) and MI (2016). Spring

Summer

Autumn

Winter

Tidal range (m)

4.50 m (mean spring tides) 1.90 m (mean neap tides)

Significant wave height (m) (mean) (outer bay) Significant wave height (m) (mean) (inner bay) Wave period (s) (mean) (outer bay) Wave period (s) (mean) (inner bay) Wave direction ( ) [mean (direction)] Temperature ( C) (mean 6 SD) Temperature range ( C) (min
max) Salinity (mean 6 SD) Salinity range (min
max)

2.78 0.64 7.27 3.83 219 (SW) 9.79 6 1.49 6.89
12.7 33.2 6 2.43 13.3
34.9

1.95 0.58 6.25 3.62 229 (SW) 15.7 6 0.93 14.0
17.7 34.0 6 1.65 19.7
35.1

3.08 0.81 7.38 3.90 223 (SW) 13.8 6 1.60 8.56
15.7 34.0 6 2.25 12.5
35.1

3.89 1.03 8.05 4.36 223 (SW) 9.01 6 1.25 5.49
11.2 33.0 6 3.28 10.0
35.1

Source: Reproduced with permission from Elsevier from Joshi, S., Duffy, G., Brown, C., 2017b. Critical bed shear stress and threshold of motion of maerl biogenic gravel. Estuar. Coast. Shelf Sci. 194, 128
142.

Geomorphology of Rhodolith/Maerl in Galway 233 with 6-year seasonal means (MI, 2016). An anticlockwise gyre is present in Galway Bay as the net inflow from South Sound is greater than the net outflow from the North Sound (Booth, 1975; Lei, 1995, Joshi et al., 2017a), with deposition of sediment in the center of the Bay (Booth, 1975; O’Connor et al., 1993). The moderate exposure to wave action results in the deposition of fine sand and mud (Keegan, 1972). Maerl has been studied in the west of Ireland for over 100 years, since Arthur Cotton and Madame Lemoine took part in the Clare Island survey (Rindi and Guiry, 2004). As a result the distribution of maerl in Galway Bay is known, though its morphodynamics is rapidly evolving, and these beds represent more than 65%
70% of the maerl beds in Ireland (De Grave and Whitaker, 1999). The Galway Bay maerl beds were mapped by the National Parks and Wildlife Service (NPWS) using Roxann Acoustic Ground Discriminating System (ADGS) in the 1990s and have been extensively surveyed, especially in and around the SACs. NPWS have mapped living and dead maerl dunes and beds using diver surveys in Galway Bay and surrounding complexes (NPWS, 2013). A diver survey of the Aran Islands maerl beds is still underway northeast of the island of Inishmaan (Yvonne Leahy, personal communication). Overall the authors feel that the naturalness of the Galway Bay maerl beds is very good, with little influence of invasive species (such as Crepidula fornicata—slipper limpet) in comparison with other maerl beds, for example, in Brittany where they are extensively impacted by anthropogenic activity. Intertidal maerl beds occur at Muckinish in County Clare, where five species of maerl are found together (Hernandez-Kantun et al., 2017). However, the condition of the maerl beds in Galway are altering significantly during the winter months due to increased intensity and frequency of storm events (e.g., 2013/2014 storms discussed here). In fact during the last 5 years there have been significant patch disturbing processes due to extreme storm events and it is likely that the physical oceanographic forcing factors in the region have resulted in a declining trend in habitat quality, especially during the winter months. Therefore this case study and ongoing work seeks to identify the important physical surrogates for maerl habitats, such as the peak combined wave
current-induced sediment mobility during severe storm conditions (Joshi et al., 2017a). The Atlantic coast is severely impacted by storm events driven by the North Atlantic Oscillation (NAO) anomalies (Fig. 12.1). The data used in this case study include multibeam and LiDAR bathymetry and backscatter from the INFOMAR survey of Galway Bay as part of regional coupled hydrodynamic wave
sediment transport models of Galway bay (fully discussed in Joshi et al., 2017a). These models utilize results of extensive experimental work to determine the maerl hydrodynamic and physical properties of maerl from three contrasting environments in Galway bay (beach, open marine, and intertidal). The wave model is driven using winds from Mace Head Atmospheric Research Station. These models have been validated using oceanographic measurements (Tide gauges, ADCP, Wave rider buoys) and the sediment

234 Chapter 12

Figure 12.1 Bathymetry in Galway Bay obtained using INFOMAR LiDAR and multibeam.

transport model utilized 110 grab samples (Fig. 12.2). Additionally, the fundamental hydrodynamic properties of maerl such as the critical bed shear stress and settling velocity, as well as the physical properties such as grain size, grain shape, and grain density have also been extensively studied and utilized here (see Joshi et al., 2017a,b).

Geomorphic features and habitats The geomorphology of maerl beds is an understudied area of science and research is ongoing. Maerl as a biogenic reef is an ecosystem engineer and as a structure-forming species it significantly increases the habitat complexity and rugosity of the seafloor. As a prerequisite to understanding the sediment dynamics, mobility, and geomorphology of maerl beds it is necessary to obtain laboratory measurements of the fundamental hydrodynamic properties of maerl; specifically its settling velocity and critical Shields parameter (Joshi et al., 2014, 2017b). Joshi et al. (2014) calculated the settling velocity of maerl and proposed a modification to the Ferguson and Church (2004) universal equation for settling velocity, with the inclusion of the grain size-dependent C2 parameter [Joshi et al., 2014; Eq. (12.1)]: ws 5

Rgd2 C1 v 1 ð0:75C2 Rgd3 Þ0:5

(12.1)

Geomorphology of Rhodolith/Maerl in Galway 235

Figure 12.2 Maerl bed distribution based on a combination of diver surveys commissioned by National Parks and Wildlife Service (NPWS) and RoxAnn survey at Aran Beds. Grain Size distribution (d50 of 110 grab samples) of seafloor sediment obtained between March 2009 and August 2010, with the offshore Special Areas of Conservation (SACs).

where ws is the terminal settling velocity, v is the kinematic viscosity coefficient, R is the submerged specific gravity, g is the gravitational acceleration, d is the median grain diameter, and C1 and C2 are the two parameters which are allowed to vary with grain shape in the Stokes region and the Newtonian region respectively (Ferguson and Church, 2004). By partially constraining the C1 (as 16 is the minimum possible C1 for a bluff body) and linearly varying the C2 parameter with grain diameter, it is possible to extend the theoretical model to include maerl from three different environments in Galway Bay (Joshi et al., 2014). Maerl was found to have overall a much lower setting velocity than quartz grains due to increasing drag on the maerl grain, with increasing branch density (roughness and convexity). This suggested that a larger part of the maerl grain size distribution can remain in suspension longer than quartz grains of comparable dimension (Fig. 12.3). Sediment mobility of maerl in Galway Bay A sediment mobility model was developed by Joshi et al. (2017a), utilizing the coupled hydrodynamic
wave model, as an intuitive alternative to the sediment transport model. High-resolution multibeam bathymetry and LiDAR data from the INFOMAR survey was utilized. Results from the flume experiment to determine the critical bed shear stress of maerl (Shields parameter of 0.033) were utilized as the threshold grid of spatially varying critical Shields parameter. The Soulsby (1997) formula was utilized for the quartz areas of

236 Chapter 12

Figure 12.3 Grain size distribution with maerl areas (A), spatially varying critical Shields parameter as the threshold of motion of both maerl area (Joshi et al., 2017b) and quartz areas (Shields criterion) (B). Mobilization Frequency Index (MFI) due to combined wave
currents over the spring
neap cycle in summer (C), and due to combined wave
currents over the spring
neap cycle in winter storms (D). Source: Modified from Joshi, S., Duffy, G., Brown, C., 2017a. Mobility of maerl-siliciclastic mixtures: impact of waves, currents and storm events. Estuar. Coast. Shelf Sci. 189, 173
188 with permission from Elsevier publishing.

this grid. Two sediment mobility indices were evaluated and the mobilization frequency index (MFI), specifically due to combined wave
currents during storm conditions, was identified to be the most significant hydrodynamic parameter governing maerl distribution. The residual currents were evaluated from the combined wave
current model to show that maerl beds formed in areas with high residual currents and intermediate sediment mobility (Fig. 12.4). An open research question is on whether this is due to “rhodolith factories” being present in the subtidal and open marine environment, in areas with high residual current, when maerl can grow, but not to the extent of suspension or rapid maerl bed erosion. Live maerl distribution is likely to be limited to ,26
30 m water depth due to light intensity in Galway Bay. Geomorphology of open marine maerl beds The presence of maerl subaqueous dunes in high-resolution multibeam bathymetry in the Aran open marine maerl beds has been documented in previous university research (Fiona Stapleton and Garret Duffy personal communication; Fig. 12.5). The grain size distribution of maerl beds in Galway Bay was measured as part of the critical bed shear stress of maerl study by Joshi et al. (2017b) and the grain shape parameter of convexity was found to vary linearly with grain diameter by Joshi et al. (2014). The structural integrity of maerl has been studied by Melbourne et al. (2015) who found that the magnesium
calcium ratio changes due to weakening of the maerl matrix leading to fragmentation of the fragile thalli.

Geomorphology of Rhodolith/Maerl in Galway 237

Figure 12.4 Combined wave
current-induced residual current distribution under storm conditions with maerl area in white were found to be a physical surrogate for maerl. Source: Reproduced from Joshi, S., Duffy, G., Brown, C., 2017a. Mobility of maerl-siliciclastic mixtures: impact of waves, currents and storm events. Estuar. Coast. Shelf Sci. 189, 173
188 with permission from Elsevier publishing.

The overall quality of both living and debris maerl beds in Galway Bay has been studied by the NPWS diver surveys and this is likely to be impacted significantly by the increased frequency of storm events due to climate change (Mo¨lter, 2016).

Biological communities Species of maerl/rhodolith Galway Bay is a particularly significant area for the numerous maerl species occurring together. Although the taxonomy of maerl requires complex DNA analyses and is beyond the scope of this chapter, Hernandez-Kantun et al. (2017) studied the five cooccurring species, with the sixth species Lithothamnion glaciale having its lower geographical limit north of Galway Bay. According to Hernandez-Kantun et al. (2017) the major maerlforming species in Galway Bay are P. calcareum and L. corallioides, whereas the minor maerl-forming species in Galway Bay are Lithophyllum incrustans, Phymatolithon purpureum, and Phymatolithon lusitanicum.

Biological communities associated with maerl/rhodolith Maerl/rhodolith beds are ecologically important habitats for a range of invertebrate species including sponges, bivalves, brittle stars, sea cucumbers, bryozoans, as well as nursery areas

238 Chapter 12

Figure 12.5 Maerl subaqueous dunes in the multibeam bathymetry at Inverin Bank. Source: Garret Duffy, personal communication.

for commercially important juvenile fish species and rare epiflora (and meiofauna and endophytes). In the Galway Bay complexes and the nearby Mannin Bay to the north of Galway Bay, eel grass communities are found alongside maerl beds and are a particularly rare biotope. Dead maerl also have a significant importance as a habitat where invertebrates can bury. Species endemic to maerl are also found and this is of great significance when commercial exploitation of adjacent habitat is being considered.

Geomorphology of Rhodolith/Maerl in Galway 239

Figure 12.6 Combined wave
current mobilization frequency index under storm conditions shows correlations between sediment mobility and multibeam backscatter (Fig. 12.7).

Surrogacy The key aim of this study was to identify which hydrodynamic variables were physical surrogates for maerl and it is necessary to quantitatively measure statistical relationships between physical surrogates and maerl distribution. Wave-induced currents were found to be much more significant than tidal currents alone in the Atlantic coast in governing the sediment mobility. The two key hydrodynamic variables likely to have a significant impact on maerl distribution were found to be the MFI during storm conditions and the peak combined wave
current residual currents. Furthermore, maerl was found at the periphery of the residual current gyres during storm conditions. The multibeam backscatter data also showed partial correlations with the MFI during storm conditions, with a correlation coefficient of 0.804 (Joshi et al., 2017a; Figs. 12.6 and 12.7). Though this is not above the 0.95 threshold, it is believed to be an important parameter governing the patchiness of the seafloor. Storm depositional processes are the key driver governing the combined wave
current-induced sediment mobility. Increased storminess is likely to increase patch disturbing processes and therefore the patchiness of the maerl habitat, as demonstrated by the increased patchiness of the Aran open marine maerl beds in comparison to the Muckinish intertidal beds.

Conservation significance Sediment mobility indices can be used to quantify the disturbance regimes of regions of the continental shelf and assign biogeographically representative MPAs which contain both disturbed and undisturbed patches (Harris, 2012b). This research could help assess whether

240 Chapter 12

Figure 12.7 Multibeam backscatter from the INFOMAR national seabed mapping survey of Galway Bay.

the criteria of Comprehensiveness, Adequacy, and Representativeness of MPAs (CAR principle of marine conservation) are met in Galway Bay (ANZECC TFMPA, 1999). The first principle asks if the network of MPAs is Comprehensive and does it take the full range of ecosystems into account? In Galway Bay the SACs are protecting differing ranges of maerl habitats such as duned beach systems, live branched maerl beds and more nodular forms, and dead algal gravels. In Galway Bay the SAC network is comprehensive, except for the Aran maerl areas around Inishmaan and Inisheer. The second principle requires the network to have an Adequate level of reservation. In Galway Bay one could argue that the EU Habitats Directive Annex V level protection of “being subject to management measures” is too low for pristine live maerl beds not subject to anthropogenic disturbance for over a century. Additionally, many rarer species are not protected due to difficulties in species identification. The third principle asks if the protected areas are Representative of the biotic diversity of the marine ecosystems. In Galway Bay one could also argue that preference should be given to live maerl beds which are the most productive and diverse habitats. However, it is important to conserve the entire range of ecosystems and sediment mobility may be a way of regionalizing the seafloor (e.g., Porter-Smith et al., 2004) to guarantee representation.

Acknowledgments This research has been funded by Geological Survey Ireland 2017 Short Call No. 043 (PI Siddhi Joshi). The author(s) would like to thank Diana Steller for advise; and Peter Harris for improving the quality of this

Geomorphology of Rhodolith/Maerl in Galway 241 manuscript and Colin Brown and Garret Duffy for their supervision during PhD. Acknowledgments go to the National Parks and Wildlife Service and the Department of Arts, Heritage, and the Gaeltacht for providing data on the spatial distribution of maerl in Galway Bay and acknowledge the use of data from the INFOMAR Project, a joint seabed mapping project between the Geological Survey of Ireland and the Marine Institute.

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280. INFOMAR, 2007. INFOMAR Survey Report: Leg CV07_01, Zone 1, Galway Bay, Geological Survey of Ireland. Joshi, S., Duffy, G., Brown, C., 2014. Settling velocity and grain shape of maerl biogenic gravel. J. Sediment. Res. 84, 718
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Further reading Foster, M.S., 2001. Rhodoliths: between rocks and soft places. J. Phycol. 37, 659
667.