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Short-term meso-scale variability of mesozooplankton communities in a coastal

upwelling system (NW Spain)

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lvaro Rouraa*, Xos A. lvarez-Salgadoa, ngel F. Gonzleza, Mara Gregoria,

Gabriel Rosnb, ngel Guerraa a IIM-CSIC, Instituto de Investigaciones Marinas, 36208 Vigo, Spain TEL: (+34) 986 231 930. FAX:

(+34) 986 292762

b GOFUVI, Facultad de Ciencias del Mar, Universidad de Vigo, 36200 Vigo, Pontevedra, Spain

*Corresponding author: aroura@iim.csic.es, IIM-CSIC, Instituto de Investigaciones Marinas, 36208

Vigo, Spain TEL: (+34) 986 231 930. FAX: (+34) 986 292762.

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Abstract

The short-term, meso-scale variability of the mesozooplankton community present in

the coastal upwelling system of the Ra de Vigo (NW Spain) has been analysed. Three

well-defined communities were identified: coastal, frontal and oceanic, according to

their holoplankton-meroplankton ratio, richness, and total abundance. These

communities changed from summer to autumn due to a shift from downwelling to

upwelling-favourable conditions coupled with taxa dependent changes in life

strategies. Relationships between the resemblance matrix of mesozooplankton and the

resemblance matrices of meteorologic, hydrographic and community-derived biotic

variables were determined with distance-based linear models (DistLM, 18 variables),

showing an increasing amount of explained variability of 6%, 16.1% and 54.5%,

respectively. A simplified model revealed that the variability found in the resemblance

matrix of mesozooplankton was mainly described by the holoplankton-meroplankton

ratio, the total abundance, the influence of lunar cycles, the upwelling index and the

richness; altogether accounting for 64% of the total variability. The largest variability

of the mesozooplankton resemblance matrix (39.6%) is accounted by the holoplankton-

meroplankton ratio, a simple index that describes appropriately the coastal-ocean

gradient. The communities described herein kept their integrity in the studied

upwelling and downwelling episodes in spite of the highly advective environment off

the Ra de Vigo, presumably due to behavioural changes in the vertical position of the

zooplankton.

Key words: Mesozooplankton communities, resemblance matrices, coastal upwelling,

holoplankton, meroplankton, moon, Ra de Vigo, NW Spain.

mailto:aroura@iim.csic.es

1. Introduction 33

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Mesozooplankton (0.220 mm) are key components in coastal ecosystems; they

link the microbial food web to the classic food chain by feeding on microzooplankton

(20-200 m), which are considered the top predators of microbial food webs (Sherr

and Sherr, 2002; Calbet and Saiz, 2005). The importance of mesozooplankton is more

remarkable in coastal upwelling areas, where primary production is increased by wind-

driven currents that bring nutrient-rich subsurface water up into the photic layer (Bode

et al. 2003a). Specifically, averaged daily grazing impact on the chlorophyll standing

stock by mesozooplankton grazers has been estimated to be 11.7% in the California

upwelling system (range 6-18%, Landry et al., 1994), and 6% of primary production in

the Galician upwelling (range 2-39%, Bode et al., 2003a).

Galicia (NW Iberian Peninsula, Fig. 1) is at the northern limit of one of the four

major eastern boundary upwelling systems of the world ocean (Arstegui et al., 2006).

From March-April to September-October, north-easterly winds predominate in the

Iberian basin producing coastal upwelling. The rest of the year, the prevailing south-

westerly winds produce coastal downwelling. This seasonal cycle explains only about

10% of the variability of the wind regime, whereas >70% of the variability

concentrates on periods of 10-20 days (Blanton et al., 1987; lvarez-Salgado et al.,

2002). The hydrographic variability during the upwelling season is coupled with

changes in bacteria, phytoplankton, and zooplankton biomasses delayed on the order of

a day, days, and weeks, respectively (Tenore et al., 1995).

All physical and biological processes operate at some preferential spatial and

temporal scales, generating a multiscale variability in zooplankton communities

(Levin, 1992; Clarke and Ainsworth, 1993). In this context, several works have dealt

with zooplankton variability in N and NW Spain. Short-term (less than one month)

scale changes were studied during upwelling or downwelling events at fixed stations

(Valds et al., 1990; Fust and Gili, 1991; Tenore et al., 1995; Morgado et al., 2003;

Blanco-Bercial et al., 2006; Marques et al., 2006), as well as following the upwelled

water through lagrangian experiments (Batten et al., 2001; Halvorsen et al., 2001; Isla

and Anadn, 2004). Stable isotopes in mesozooplankton were used to infer the pelagic

food web in the Galician coast during spring (Bode et al., 2003b). Interannual

variability in mesozooplankton abundance and biomass has been determined at two

fixed stations off A Corua (Bode et al., 1998; 2003a; 2004). Finally, surveys carried

out monthly by the Instituto Espaol de Oceanografa (IEO) since 1987 allowed

studying long-term trends in the zooplankton communities off NW and N Spain

addressing their link with global warming (Valds et al., 2007; Bode et al., 2009).

However, these studies dealt mainly with zooplankton biomass and abundance, and did

not consider the community structure.

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The Ra de Vigo is a highly dynamic area, which is among the most productive

oceanic regions in the world (Blanton et al., 1984). The main driving forces

modulating the residual circulation of the Ra de Vigo are the local and shelf winds

(Souto et al., 2003), affecting the composition and abundance of phytoplankton

(Nogueira et al., 2000; Cermeo et al., 2006; Crespo et al., 2006), microzooplankton

(Teixeira et al., 2011) and ichthyoplankton (Ferreiro and Labarta, 1988; Riveiro et al.,

2004). However, most of the studies dealing with mesozooplankton were centred in the

adjacent shelf waters (Valds et al., 1990; Fust and Gili, 1991; Tenore et al., 1995;

Isla and Anadn, 2004; Blanco-Bercial et al., 2006; Bode et al., 2009) except Valds et

al. (2007), who studied long-term trends of zooplankton abundance and biomass east

and west of the Cies Islands, at the mouth of the Ra de Vigo (Fig. 1). Nonetheless,

there is a lack of studies that characterise mesozooplankton communities inside and

outside the Ra de Vigo. So, the aim of this work is to analyse the mesozooplankton

variability characterising spatially and temporally the community structure in the Ra

de Vigo. Furthermore, we aimed to understand how the physical forcing and

environmental variables constrain the integrity of these communities.

2. Material and Methods

Ten surveys to collect zooplankton and hydrographic data were undertaken in the

Ra de Vigo (NW Spain, Fig. 1) onboard RV Mytilus, in the summer (2, 4, 9 and 11

July) and autumn (26 September, 1, 3, 9, 10 and 14 October) of 2008. We focused the

sampling effort on these periods because they match with the maximum in

mesozooplankton biomass (Otero et al., 2008). Each survey was carried out at night in

four transects (T2, T3, T4 and T5) parallel to the coast following an onshore-offshore

depth gradient with average water depths of 26, 68, 85 and 110 m, respectively. A

Seabird 9/11 CTD equipped with a WetLabs ECOFL fluorometer and a Seatech

transmissometer, was deployed at the southern part of each transect to obtain vertical

profiles of temperature (T), salinity (Salt), chlorophyll-a fluorescence (Chl-a),

dissolved oxygen and stability of the water column (Stab), calculated as the square of

the Brunt Visl frequency. Dissolved oxygen was subtracted from oxygen

saturation to obtain the apparent oxygen utilization (AOU), a proxy for the trophic

status of the column: positive values indicate net heterotrophy and negative values net

autotrophy.

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