UMR CNRS 7138, Syst´ematique, Adaptation et Evolution
Univ. Pierre et Marie Curie, 7 quai Saint Bernard, 75252 Paris Cedex 05, France (mkersz@ccr.jussieu.fr, francoise.gaill@snv.jussieu.fr, florence.pradillon@snv.jussieu.fr)
1 Present address: Institut Pasteur, Unit´e G´en´etique in silico, 25 rue du Docteur Roux 75724 Paris Cedex 15, France (mbailly@pasteur.fr)
2 Corresponding author. Tel: +33-1-44-27-35-02. Fax: +33-1-44-27-58-01.
Running title: Larval dispersal at bio-hydrodynamical scale
Key words: Larval dispersal, colonization, hydrothermal vents, agent-based 3D model, bio-hydrodynamical scale
Abstract
Larval dispersal is key in understanding how deep-sea hydrothermal vent communities function and are maintained. To date, numerical approaches developed to simulate larval dispersal have been conducted at ocean ridge scales. However, hydrothermal vents have complex and dynamic local physico-chemical environments. These smaller-scale, but significant variations may influence larval fate in its early stages after release, and hence have a knock-on effect knock-on both dispersal and colknock-onizatiknock-on processes. Here we present a new numerical approach to the study of larval dispersal, considering a “bio-hydrodynamical” scale ranging from a few centimeters to a few meters around hydrothermal sources. We use a physical model for the vent based on jet the-ory and compute the turbulent velocity field around the smoker. Larvae are considered as passive particles whose trajectories are affected by hydrody-namics, topography of the vent chimney and their biological properties. Our model predicts that bottom currents often dominate all other factors by en-training all larvae away from the vent. When bottom currents are very slow (< 1 mm.s−1), general larvae motion is upwards due to entrainment by the main smoker jet. In this context, smokers with vertical slopes favor retention of larvae. Additionally, larval retention rates increase with velocity of the main smoker jet. This is because entrainment in this high-speed plume is pre-ceded by a phase when larvae are attracted towards the smoker wall, where they can eventually settle. Finally, the buoyancy rate of the larvae, measured to be in the range of 0.01 mm.s−1, is generally irrelevant unless hydrodynamic conditions are in equilibrium, i.e if the buoyancy rate is comparable to both the bottom current speed and the local water velocity due to entrainment by close smokers. Overall, our model evidences the strong effect of the re-lease point of larvae on their future entrainment within local fluxes. Larvae released from smoker walls might have an entirely different fate than those released further away in the water column, which are not, or less, affected by near-chimney hydrodynamics.
1 Introduction
Species inhabiting isolated or unstable environments rely on their dispersal capabil-ities to colonize new habitats and maintain their populations. Deep-sea hydrother-mal vent ecosystems are islands spread along oceanic ridges, with short, decade-long lifespans (Haymon et al., 1993; MacDonald, 1982). Therefore, dispersal and colo-nization are a critical phase of the life cycle of species endemic to vents (Tyler and Young, 1999, 2003). Since most vent species are sessile as adults, they must disperse predominantly in their larval stages through the water column (Lutz et al., 1984). In order to understand how some of the vent species have persisted through geological time and over such a wide geographical range, dispersal processes were examined through a number of different approaches. In situ collections using net tows (Kim et al., 1994; Mullineaux and France, 1995), water pumps (Mullineaux et al., 2005), traps (Khripounoff et al., 2000; Metaxas, 2004) or colonization devices (Berg and Van Dover, 1987; Mullineaux et al., 2003) allowed the collection of larval and post-larval stages of vent species and the estimation of their distribution both through the water column and on the bottom. Previous genetic studies have confirmed migrant fluxes between populations inhabiting distant vent sites (Jollivet et al., 1995; Vrijen-hoek, 1997; Young et al., 2003). Development studies have provided information on larval life spans and potential dispersal phase duration (Marsh et al., 2001; Pradillon et al., 2001). Finally, recently published measurements were used to estimate the potential distance over which larvae might travel (Chevaldonn´e et al., 1997; Kim and Mullineaux, 1998; Mullineaux et al., 2002; Thomson et al., 2003). Larvae might be transported between vents either in bottom currents channeled within the axial valley of the ridge (Kim and Mullineaux, 1998; Thomson et al., 2003), or in currents present at 200 to 300 meters above the ridge crest after they have been entrained to this level by rising buoyant hydrothermal plumes (Kim et al., 1994; Mullineaux and France, 1995). Data gathered from these experiments provided input parameters for computational approaches developed to predict the dispersal potential of vent larvae. Taking into account measured bottom current, observed spatial vent dis-tribution along ridges and known reproductive characteristics, Chevaldonn´e et al. (1997) and Jollivet et al. (1999) modeled propagule fluxes between vent sites for polychaete species of the Alvinellid family. Dispersal models based on current data and using Lagrangian approaches were used to predict the sorts of distances larvae would be able to travel along ridges (Marsh et al., 2001; Mullineaux et al., 2002). To date all approaches have been conducted at the ridge segments scale, i.e. over tens to hundreds of kilometers, with the exception of the model developed by Kim and Mullineaux (1998), where vertical entrainment of larvae present in the water
column was considered at the vent chimney scale.
Organisms living at hydrothermal vents are exposed to a complex physico-chemical environment due to the mixing between sea water and hydrothermal fluid (Le Bris et al., 2003, 2005; Sarradin et al., 1998). For species living directly on the chimney wall, local fluxes may have a strong influence on larval fate early af-ter their release, while they are still in the vicinity of the smoker, hence affecting dispersal and colonization processes. So far, all modeling approaches assumed that larvae released from vent species were floatting in the water column around smok-ers, from which point they could be entrained by currents or by the rising smoker plume. However, when released from a smoker wall, a larvae might be trapped by topographic features of the chimney, and would therefore not be able to disperse. Here, we develop a model to qualitatively study how local physical constraints may influence the structure of the vent smoker community through their effects on larval dispersal. We work at and around the smoker chimney scale, i.e metre scale. Larvae in this “bio-hydrodynamic” range can still be considered as being in the vicinity of the smoker. To our knowledge, this is the first attempt at modeling phenomena on this intermediate scale. Our physical model is based on jet theory, and we use nu-merical methods to describe the hydrodynamic velocity fields around smokers. The passive larvae are entrained by the turbulent fluid, and may be deposited on the mineral surfaces. Using this modeling approach, we aim to identify which factors significantly affect larval fate in the early stages after release. It is what happens during this phase that can influence larger scale dispersal processes or colonization patterns. Factors tested here include hydrodynamic features such as smoker jet velocities and temperatures, smoker topography, and larval characteristics (e. g. buoyancy rates).