The role of the Azores Archipelago in capturing and retaining incoming particles
Introduction
The Azores Archipelago (AZ), located in the middle of the North Atlantic Ocean (37 ∘–40 ∘N, 25 ∘–31 ∘W), is the most isolated of the Macaronesian Islands (Azores, Madeira and Canary Archipelagos): 580 km north east of Madeira, 1400 km west of Europe, and 2000 km east of North America. It consists of nine volcanic islands disposed in three separate groups (EG—Eastern Group; CG—Central Group; WG—Western Group) (Fig. 1) and several small islets, distributed along a ~ 600 km tectonic zone, rising from the ocean basin ~ 4000 m below. As oceanic islands, the AZ are characterized by a narrow or absent island-shelf and its deep waters are punctuated by 461 seamounts. This uneven topography together with its geographic isolation, geologic youth (thought to be of Miocene origin; Briggs, 1970, Schmincke, 1973), small size, temperate climate and the presence of extreme environments associated with hydrothermal vents, allows the existence of different marine ecosystems with habitats where complex marine food webs are assembled, making the AZ a hot-spot of biodiversity in the Atlantic Ocean (Aboim, 2005). However, the dynamics of these microecosystems are poorly understood (Pinho and Menezes, 2009).
Of the 500 fish species identified in this region, around 50–60 are currently exploited by a multi-segmented fishing fleet (Morato et al., 2001), and these marine resources are central to the local economy of the archipelago. These fishing activities, mostly artisanal (Rodrigues, 2008) and small scale (Carvalho et al., 2011), are concentrated in areas less than 600 m deep, restricted to the narrow coastal fringe around the islands and to the nearby banks and seamounts (Menezes, 2003, Silva and Pinho, 2007, Morato et al., 2008, Neilson et al., 2012). These areas represent less than 1% (~ 7000 km2) of the nearly one million km2 (948,439 km2) corresponding to the Azores part of the Portuguese EEZ (Menezes, 2003). The natal recruitment supplying these fisheries depends greatly on the retention capacity of the islands and seamounts.
The regional oceanography of the Azores Archipelago is governed by the North Atlantic Subtropical Gyre, under the direct influence of the Gulf Stream (GS). Southeast of the Grand Banks, the GS breaks into two branches (Fig. 1b): the North Atlantic Current (NAC), which flows in direction of Northern Europe, and the Azores Current (AzC), which drifts south-westward and crosses the Mid-Atlantic Ridge south of the AZ (Käse and Siedler, 1982, Gould, 1985, Onken, 1993). The AzC is the northern border of the subtropical gyre, and near Madeira (around 16 ∘W) it splits again in one branch that penetrates the Gulf of Cádiz, and the second one that veers southward feeding the Canary Current (CaC) down the African coast (Johnson and Stevens, 2000). This south branch eventually connects to the westward North Equatorial Current, closing the gyre when it merges with the GS further west (Siedler and Onken, 1996, Tychensky et al., 1998). The AzC is characterized by a complex mesoscale eddy system, generating on its eastward drift eddies and meanders that propagate westward (Le Traon et al., 1990, Stammer, 1997, Richardson, 1983, Fratantoni, 2001, Reverdin et al., 2003, Volkov, 2005). Using merged altimeter data Sangrà et al. (2009) identified two westward propagating corridors of eddies north and south of the Azores Front. Eddies with a life span of more than six months had a zonal dispersive range between 33 ∘ and 38 ∘N, some of which originated from the Azores Front.
Several studies have emphasized the impact of the water circulation around small islands and archipelagos on population connectivity, larval dispersal and species assemblages (Jones et al., 1999, Jones et al., 2005, Almany et al., 2007, Burgess et al., 2007, Paris et al., 2007, Hamann et al., 2011, Andutta et al., 2012, Sala et al., 2013); however until now, no study was focused on the Azores region. For this reason, in order to understand the colonization, dispersal, fisheries recruitment and speciation processes, it is necessary to comprehend the oceanographic phenomena that affect the Archipelago. Considering the oceanographic context and location of the Azores, the aim of the present study is to analyze the capacity of these islands to capture and retain incoming particles and organisms that drift with the oceanic currents. There are three questions we address: (i) what is the origin of the majority of the particles that reach the islands? (ii) how do the different island sub-groups contribute to the retention process? and (iii) which oceanographic phenomena affect the transport and retention mechanisms? To answer these questions a series of numerical experiments were performed using the Connectivity Modeling System (CMS), an off-line Lagrangian tool, attached to an ocean circulation model, as described in detail in Section 2. The results of the different numerical experiments are presented in Section 3, and the general discussion and main conclusions are in Section 4.
Section snippets
Methodology
For this study the HYbrid Coordinate Ocean circulation Model (HYCOM; http://hycom.org/) was used. HYCOM evolved from the Miami Isopycnic-Coordinate Ocean Model (MICOM) (Bleck and Boudra, 1981, Bleck and Smith, 1990), after the implementation of the hybrid vertical coordinate system (Bleck and Benjamin, 1993). HYCOM solves the hydrostatic primitive equations in a free surface mode. Detailed information about the governing equations, numerical algorithms and the available vertical mixing schemes
Backward Experiment: where do particles come from?
The Backward Experiment provided information about the origin of the particles that reached the Azores. Table 1 shows the percentage of particles that arrived at each island group from the boundaries of the source box. These results were separated by sink box and divided into depth bins.
General discussion and conclusions
Our results highlight the role of the GS, the NAC and the AzC, as well as the role of their associated eddies, meanders and filaments in the transport, dispersion and retention of particles in the AZ archipelagic region. In the upper mixed layer, the GS and its associated eddies are a predominant regional oceanographic feature injecting particles from the north and west boundaries onto the islands. Also, the analysis of the FSLE and EKE suggests the strong influence of eddies and filaments
Acknowledgments
We thank Rui Vieira for his technical support with the computational procedures, and Fernando Tempera for the insightful discussions. RAIA (0313-RAIA-1-E) and RAIA.co (0520-RAIA-CO-1-E) projects provided funds for Iria Sala. Rui Caldeira was supported by funds from the ECORISK project (NORTE-07-0124-FEDER-000054). The RAIA Coastal Observatory has been funded by the Programa Operativo de Cooperación Transfronteriza España–Portugal (POCTEP 2007–2013). Cheryl Harrison was supported by funds from
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