Modelling tidally induced larval dispersal over Anton Dohrn Seamount
Massachusetts Institute of Technology general circulation model is used for the analysis of larval dispersal over Anton Dohrn Seamount (ADS), North Atlantic. The model output validated against the in situ data collected during the 136th cruise of the RRS ‘James Cook’ in May–June 2016 allowed reconstruction of the details of the baroclinic tidal dynamics over ADS. The obtained velocities were used as input data for a Lagrangian-type passive particle tracking model to reproduce the larval dispersal of generic deep-sea water invertebrate species. It was found that the residual tidal flow over ADS has a form of a pair of dipoles and cyclonic and anti-cyclonic eddies located at the seamount periphery. In the vertical direction, tides form upward motions above the seamount summit. These currents control local larval dispersal and their escape from ADS. The model experiment with a large number of particles (7500) evenly seeded on the ADS surface has shown that the trajectory of every individual particle is sensitive to the initial position and the tidal phase where and when it is released. The vast majority of the particles released above 1000 m depth remain seated in the same depth band where they were initially released. Only 8% of passive larvae were able to remain in suspension until competent to settle (maximise dispersal capability) and settle (make contact with the bottom) within the specified limits for this model. It was found that every tenth larval particle could leave the seamount and had a chance to be advected to any other remotely located seamount.
KeywordsLarva dispersion Tidal residual currents Baroclinic tides
Cold-water corals are typical habitats for all oceanic banks and seamounts. The reef-forming species attract the interest of marine biologists over last decades. Globally, they occur within a wide depth range (40–3500 m) in well-defined depth zones parallel to the shelf break, or the rim of offshore banks and seamounts (Buhl-Mortensen et al. 2015). The highest population density of one of them, the Lophelia pertusa, known so far has been found along the Norwegian coast and in the eastern North Atlantic (Buhl-Mortensen et al. 2017).
In the marine environment, the adult corals are immobile; although at a larval stage, they live in the water column for a certain period of time, moving with currents before settling down in a new area. It is larval dispersal that keeps distant populations connected.
Anton Dohrn Seamount is a guyot with its summit at nearly 600 m depth situated in the central part of the Rockall Trough. It is topographically complex and harbours diverse biological assemblages, including communities dominated by cold water corals and sponges (Davies et al. 2015).
The strongest current in the Rockall Trough area is the Slope Current (SC) schematically shown in Fig. 1a. It resembles a jet stream transporting Atlantic waters along the edge of the continental slope northward with maximum velocities 0.15–0.3 m s−1 (Sherwin et al. 2015). Its core is confined to the slope above the 400–500-m isobath.
Note that the reported SC is situated at a considerable distance from ADS (see Fig. 1a) and hardly contributes to the dynamics around ADS. All other currents in the area are either much weaker or located in the surface layer. In fact, the summit of ADS is below 600 m deep, so it is not expected that any surface current like wind-driven flows (their penetration depth is less than 300 m) can influence the water circulation around ADS. The only dynamical process that can significantly affect the whole water column is the tide. Tidal currents interacting with rough bottom topography generate internal waves.
Henry et al. (2014) conducted investigations of the influence of internal tidal waves on the megabenthic communities (below 1000 m water depth) in the area of the Hebrides Terrace Seamount. They found that the internal tides may significantly enhance the biological diversity on considered and adjacent seamounts in the Rockall Trough. The authors also assumed that coral populations at bathyal depths have higher tendency to become isolated over a given distance due to the currents decreasing with depth (i.e. the larvae might not be able to travel far away from the place of its origin). Note, however, that this conclusion is not valid for the areas with a substantial internal tidal activity which produces strong currents in the deep, as well.
The observational data were used for further validation of numerical reconstruction of the internal tidal currents in the ADS area (Vlasenko et al. 2018). The model-predicted velocities were the background fields for a Lagrange-type model that predicts the process of larval dispersal considering larvae as floating passive particles that move with internal tidal currents.
A similar study was conducted by Bartsch and Coombs (1997) who made predictions for blue whiting larval transport by the Shelf-Edge Current along the European coast. Thiem et al. (2006) reported another modelling effort with the focus on the influence of an along slope jet current on the position of Lophelia pertusa coral reefs outside the Norwegian coast. Their model results suggest that the majority of the Lophelia pertusa reefs are concentrated in the areas along the shelf edges where slope currents provide a good supply of food.
In the present paper, we use a similar approach for investigation of the larvae transport over ADS. The paper is organised as follows. Section 2 presents the details of the setting of the hydrodynamic model and description of the Lagrangian model. Section 3 discusses an experiment on larvae dispersal. Finally, conclusions are summarised in Section 4.
2.1 Hydrodynamical model
The Massachusetts Institute of Technology general circulation model (Marshall et al. 1997) was used for simulations of internal tides in the ADS area. The model was forced by the principal tidal harmonic M2 added to the right-hand side of the momentum balance equations as a tidal potential. Stashchuk et al. (2014) presents the details of the procedure of tidal implementation into the MITgcm.
The parameters for the tidal forcing were taken from the inverse tidal model TPXO8.1 (Egbert and Erofeeva 2002); specifically, the maximum tidal discharges for eastward and northward direction were 96.3 and 50 m2 s−1, respectively, and the phase shift between the two equals π/4.1.
The model domain included a 768×794 mesh grid in which only a central part of 512×538 grid points with the horizontal resolution of Δx = Δy= 115 m was used for the analysis. The rest of the model domain was an “ad hoc” addition, i.e. the lateral boundary layers, with a two-step telescopically increased grid: (i) 118 grid points with the increasing periphery-ward grid step from 115 to 5500 m and (ii) last ten grid points where the grid step was increased up to 2⋅ 108 m. The addition of such an ad hoc area to the domain with seamount allows providing propagation of generated internal and barotropic waves to the boundaries during a long time without reflection from them.
2.2 Lagrangian model
One of the methods for investigation of the larvae dispersion could be the addition of an extra passive tracer transport equation into the governing system considering the evolution of the tracer. The MITgcm has such an option, and we applied this method for modelling of the in situ experiment conducted in the Jones Bank area (Celtic Sea) (Stashchuk et al. 2014). It was found there that after 4 days of the in situ and model experiments, the Rhodamine concentration fell down below the threshold of its detection both in observation and in the numerical fields. That is the reason why we chose here a Lagrangian-type model for tracing the larvae. It is shortly outlined below.
Procedures (3)–(6) allow calculation of a new position of the particle and its velocity every 5 min using the model output. The described algorithm is repeated again and again until the whole 40-day particle trajectory is calculated.
Concerning the time of model prediction, Larsson et al. (2014) in their laboratory investigations of embryogenesis and larval development of cold-water coral Lophelia pertusa have shown that nematocysts appear when larvae are 30 days old. After this time, they can settle and give rise to a new coral colony. We have used a planktonic larval duration of 40 days. That is close to 43 days reported by Hilário et al. (2015) as the mean minimum duration for eurybathic species. In our methodology, we followed an assumption that the larvae can be considered as particles with a neutral buoyancy that are unable to swim by themselves.
3 Residual currents
Theory wise, a weak tidal flow interacting with nearly flat bottom topography generates systems of linear internal waves that do not produce any residual water transport. Trajectories of fluid particles in such waves are circular so that all particles return to their initial positions after one tidal cycle. However, the situation is getting more complicated with a moderate tidal forcing and rough topography. Strong nonlinear advection accompanied by bottom friction introduces an asymmetry in the particle trajectories which ultimately leads to the generation of residual tidal currents.
It is clear that the larva trajectories depend on the spatial structure and intensity of the possible residual currents. Pingree and Maddock (1980) showed that the tidally induced frictional stresses over sloping ideal seamount result in the generation of four eddies located at its periphery.
Figures 5b and 5c show two transects with residual vertical currents. Here, a number of local vertical circulation cells are seen. The fluxes above the summit are directed mostly upward and restricted with the depth of 400 m.
4 Experiment on larvae dispersion
It should be noted here that in the described experiment, the initial position of the particles was 5 m above the bottom. To understand how sensitive the results of particle tracking from the released depth could be, an extra experiment with 1-m initial particle height above the bottom was performed. It was found for the tidal phase Δt = 0 that the total amount of the settled on the seamount particles was only 0.6% larger than that in the previous experiment.
Analysis of the particle trajectories has shown that they were settled at different moments of time. The question whether the settled larvae can give rise to a new coral colony depends on the time of deposition. According to the investigation of Larsson et al. (2014), the nematocysts that are needed to make larvae settle appear when they are 30 days old. Thus, if the larva particle sinks to the bottom before 30 days after its release and becomes motionless, it would be unable to develop into a future coral. So, we consider the particles that settled before 30 days from the beginning of the experiment as dead larvae.
5 Summary and conclusions
Connectivity of seamount populations remains an area of active study. However, the role of oceanographic processes as a potential isolating mechanism, and in determining observed patterns of poor connectivity over the depth gradient, remains unknown. According to Sherwin et al. (2015), the strongest currents in the surface 400 m layer in this area do not exceed ∼20 cm s−1. The currents are even weaker below this level. Under such conditions, the water circulation at those banks below 600 m depth is mostly controlled by tides.
Vlasenko et al. (2018) conducted a detailed analysis of baroclinic tidal activity over Anton Dohrn Seamount. The MITgcm was used for investigation of the interaction of the semi-diurnal tidal flow with ADS. A consistency of the model output with the in situ collected data was a starting point for the present study of quantification of larva dispersion near ADS, i.e. use of the model-predicted fine-resolution velocity fields (115-m horizontal and 10-m vertical resolutions) as an input data for a Lagrange-type passive tracer tracking model. A series of 5-min model outputs of the velocity components were used for a three-linear interpolation of larva evolution evenly seeded initially at the ADS surface (7500 sites) and simultaneously released from the bottom.
Conducted numerical experiments have shown that the larvae that escape from ADS were captured by tidally generated residual currents that exist at the periphery of ADS in the form of four eddies (two cyclonic and two anti-cyclonic vortexes). However, statistical wise, the probability of such an escape is not very high. It accounts for only 9–12% of all released particles. Thus, only every tenth larva particle leaves the topography and has a chance to be transported to any other remotely located seamount. The vast majority of the particles started their motion above the 1000-m isobath remains seated in the same depth band where they were initially released.
The conclusions formulated above are purely based on the hydrodynamical processes developing around ADS. It was found here that only 6–9% of particles can undergo maxi- mum dispersal with successful recruitment to the benthos.
Note that different sites of ADS do not contribute equally to a potential distant larva travel. Vlasenko et al. (2018) found that the main places of internal wave activity are the steep flanks of the ADS topography. As a result, the larva particles released here (below 1000 m depth) are the most mobile. They have a higher probability to escape from ADS or relocate to its deeper or shallower parts.
In general, the principal question on cold-water coral reef survival and sustainability is a good food supply to feed them. According to Frederiksen et al. (1992), the highest abundance of Lophelia pertusa corals around the Faroe Islands tends to be at depths where the bottom slope is critical to internal waves of semi-diurnal frequency. The casual link behind this is suggested to be an increase of food availability either through higher primary production at the surface or by a redistribution of suspended particles in the bottom mixed layer.
The authors would like to thank the Captan, Crew and Scientific Parties, especially the ROV ISIS team working during the JC136 cruise. We thank two anonymous reviewers and Associate Editor Prof. Jarle Berntsen for their useful comments.
This work was supported by the UK NERC grant NE/K011855/1.
- Davies JS, Stewart HA, Narayanaswamy BE, Jacobs C, Spicer J, Golding N, Howell KL (2015) Benthic assemblages of the Anton Dohrn Seamount (NE Atlantic): defining deep-sea biotopes to support habitat mapping and management efforts with a focus on vulnerable marine ecosystems. PloS one 10 (5):pe0124815CrossRefGoogle Scholar
- Henry L-A, Vad J, Findlay HS, Murillo J, Milligan R, Roberts JM (2014) Environmental variability and biodiversity of megabentos on the Hebrides Terrace Seamount (North Atlantic). Sci Rep 4(5589):1–10Google Scholar
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