Dynamics of Phytoplankton and Zooplankton Communities on the Continental Shelf of Northern Portugal

chierchiae) seemed to be capable of important grazing on D. acuta, although restricted to 
locations with more than 9x10
3
 cells L
-1
. Since the three smaller copepods accounted for 
57% of all mesozooplankton, we speculate that the bloom of D. acuta was not being top-
down controlled by copepods.  
 6
 Abstract 
 
In order to study simultaneously the communities of phytoplankton and zooplankton on 
the NW coast of Portugal, four cruises were carried out covering a 80km cross-shelf transect 
in 2002 (spring and summer) and 2003 (winter and late summer). Fifty-one stations were 
sampled for hydrographic information, fluorescence, phytoplankton (six depth levels) and 
zooplankton (five depth strata of 25m). The oceanographic conditions observed in each 
cruise were considerably different, however the vertical structure of the water column was 
always dependent on temperature. Temporal variability of phytoplankton presented a 
seasonal pattern characteristic of temperate regions (minimum in winter and maximum 
concentrations in spring and late summer). Spatial distribution of phytoplankton (namely 
across the shelf) seemed to be forced by winds stress over the water surface layer, with 
possible offshore transport due to north winds and onshore transport related to wind 
relaxation or south winds. Temporal variability of zooplankton followed the seasonal pattern 
of phytoplankton close to shore, confirming a direct trophic relationship, while offshore (shelf 
edge) the same was not observed. Zooplankton was always concentrated close to the 
phytoplankton patches (especially at inner-mid-shelf) but was less influenced by wind forcing 
than phytoplankton. Very low concentrations of phytoplankton and zooplankton were 
repeatedly observed in the outer-shelf, possibly caused by an upwelling-associated feature of 
surface water deepening that may have forced the sinking and loss of organisms. 
Phytoplankton showed a seasonal succession of species that is common for temperate 
regions, although affected by upwelling, with diatoms (e.g. Guinardia delicatula, 
Leptocylindrus danicus) dominating mixed and upwelled waters whereas dinoflagellates (e.g. 
Ceratium fusus, Dinophysis spp.) prevailed in thermally stratified conditions. The seasonal 
succession of zooplankton was less clear than for phytoplankton due to an important inter-
annual variability in the community structure, with replacement of dominant species. 
Copepods dominated the zooplankton, with seven taxa accounting for nearly half the 
community. No differences on the relative composition of zooplankton were found with depth, 
while across the section two clusters of taxa were identified: a nearshore group (e.g. Temora 
longicornis, Podon spp.) and a widespread group that included some offshore taxa (e.g. 
Clausocalanus spp., Paracalanus spp., Ctenocalanus vanus). The temporal/spatial variability 
of interactions between zooplankton taxa assemblages and phytoplankton groups was 
considered and several grazing “hot spots” were detected in thermally stratified conditions. 
 14
 Introduction 
 
The Portuguese coast is located on the northern limit of the Eastern North Atlantic 
Upwelling Region (Wooster et al., 1976; Haynes et al., 1993). Several authors refer that 
the summer atmospheric circulation in the region is dominated by upwelling-favourable 
north winds (e.g. Fiúza et al., 1982; Jorge da Silva, 1982), while outside this season the 
variability of this process is high (Huthnance et al., 2002). The continental shelf of NW 
Portugal is relatively wide, flat and with a pronounced shelf-break, which can induce a 2-
cell upwelling system, similar to the reports of other regions like NW Africa (Jacqes & 
Tréguer, 1986). Because of these environmental characteristics, the NW Portuguese 
coastal area presents high planktonic productions (Cunha, 2001), supporting the most 
important fishing activities of the country (ICES, 2005). In order to contribute to the 
development of better models for the assessment and management of pelagic fisheries it 
is fundamental to understand the seasonal cycles of phytoplankton/zooplankton 
production, as well as their spatial variability linked to upwelling events. 
The understanding of the planktonic environment should be multidisciplinary, 
however, the majority of the works in this field have followed separate ways (phytoplankton 
or zooplankton), frequently independent of each other. The phytoplankton community 
structure and seasonal/spatial variability has been described for the NW Portuguese 
coastal area by Mendes (1999) and Moita (2001) and the annual succession cycle was 
further studied by Palma (2003). The seasonal/spatial distribution patterns of the 
mesozooplankton community for this region have been described by Massapina (1990), 
Santos (1992) and Cunha (2001). Queiroga et al. (2005) studied zooplankton variability in 
relation to upwelling but used samples obtained with a 500 µm mesh size. None of these 
studies described the vertical variability of zooplankton considering different 
oceanographic conditions. Regarding the NW Iberian upwelling system, several 
zooplankton studies have integrated phytoplankton information but were generally limited 
to biomass estimations and/or to high-level taxonomic groups (e.g. Valdés et al., 1991; 
Barquero et al., 1998; Cunha, 2001; Bode & Alvarez-Ossorio, 2004).  
The present study intends to describe, for the first time, the relationship between the 
phytoplankton and zooplankton communities (at genus or species level) in the NW Iberian 
shelf, covering the water column in several depth strata. The objectives of the work were: 
i) to examine the extent to which physical factors affect the distribution of the 
 15
phytoplankton and zooplankton communities, as well as the meso-scale interactions 
between them; ii) to describe and compare the community structures of phytoplankton and 
zooplankton with attention to the seasonal succession iii) to study the vertical variability of 
zooplankton taxa assemblages, iv) to examine the extent to which the upwelling process 
determines the spatial distribution patterns of zooplankton with depth. 
 
 
 
Material and methods 
 
Sample collection and treatment 
The data came from four cruises carried out during spring and summer of 2002 (May 
and July onboard RV “Mestre Costeiro”) and during the winter and late summer of 2003 
(February and September onboard RV “Noruega” and “Capricornio” respectively), which 
covered a section offshore Figueira da Foz (NW Portugal) (Fig. 1). The sampling took 
place from near the coast to 80km offshore, covering the entire continental shelf, without 
regard for time of day. Temperature and conductivity in the water column were measured 
with a SeaBird CTD.  
39.8
40.0
40.2
40.4
40.6
40.8
10.0 9.5 9.0 8.5
F1F2F3F4F5F6F7F8F9F10F11F12F13F14F15F16
3
0
 
m
2
0
0
 
m
1
0
0
0
 
m
3
0
0
0
 
m
 CTD
Plankton
sampling
1
0
0
 
m
Figueira da Foz
1º10º9º8º7º
Longitude W
36º
37º
38º
39º
40º
41º
42º
L
a
t
i
t
u
d
e
 
N
PORTUGAL
L
i
s
b
o
a
Porto
Faro
2
0
0
0
m
2
0
0
m
Figure 1 – Regional location of study area (left) and cruise sampling
stations offshore Figueira da Foz (right). The shelf-break is indicated by
the 200m isobath. 
 16
Abstract 
 
Dinophysis acuta, responsible for DSP, reached particularly high concentrations on 
the NW coast of Portugal in 2003. In the Ria de Aveiro (40º41’N) the species reached a 
maximum concentration of 50x10
3
cells L
-1
 on the 8
th
 September, the highest value in a 17-
year record of monitoring. The bloom followed a brief period of upwelling favourable winds, 
at the end of an extremely hot summer marked by weak upwelling, thereby favouring the 
development of highly stratified conditions. In mid-September, during a cruise transecting 
the shelf 30km south of Aveiro, subsurface maxima of D. acuta were identified by 
fluorescence and the population reached >24x10
3
 cells L
-1
. The species was restricted in a 
relatively thin layer of 5m (maxima between 18-20m) within the pycnocline extending 20km 
offshore. Cross-shelf distributions revealed the presence of two smaller forms of D. acuta 
the smallest of which was identified as Dinophysis dens. Their coincident distribution with 
that of D. acuta, reinforced the supposition that these smaller forms correspond to different 
life cycle stages of D. acuta, D. dens representing a gamete of D. acuta. The high cell 
concentrations in the thin layer are thought to embody a species strategy to ensure 
successful gamete mating during sexual reproduction. 
 74
Introduction 
 
HABs dynamics appear in some systems to be dominated by the interactions 
between the species life cycle/behaviour and the physical processes governing those 
systems. Some motile species can form highly concentrated layers in the pycnocline that 
range in thickness from a few decimetres to a few meters, but can extend horizontally for 
tens to hundreds of kilometres (Donaghay and Osborn 1997). Most studies of mixotrophic 
Dinophysis spp., associate their high abundance with thermal and haline stratification, with 
little evidence of blooms induced by elevated nutrient concentrations (Delmas et al. 1992, 
Reguera et al. 1995).  An increasing number of studies are showing that these organisms 
can exist in very high numbers in subsurface layers of the water column (Gentien et al. 
1995, Carpenter et al. 1995). These layers can be very thin and reach significant depths 
(Gisselson et al. 2002, Kononen et al. 2003). In the case of Dinophysis spp., high 
concentrations seem to be achieved not only through simple asexual growth in the upper 
mixed layer, but also as a result of other mechanisms which create accumulations of cells 
in restricted bodies of water, such as the increase of the density gradient or the diel 
vertical migrations of autotrophs (Setälä et al. 2005).  
Dinophysis acuta and Dinophysis acuminata both responsible for diarrhetic shellfish 
poisoning (DSP), have extreme negative economic impact on shellfish catches on the 
Portuguese coast owing to their persistence in the Iberian upwelling system. These species 
are most abundant from the end of spring until early autumn, when the water column is 
most stratified. Both species often coexist, but blooms seldom coincide in space or in time. In 
Iberian waters, maximum concentrations of D. acuta are normally located in the pycnocline. In 
1995, maximum concentrations reaching 5 x10
4
 cells L
-1
, were observed inside Ria de Aveiro 
and the Galician Rias (Anonymous 1996). This paper reports on the physical-biological 
interactions (with emphasis on the life cycle of D. acuta) responsible for the very high 
concentrations of this species on the NW coast of Portugal in 2003. 
 75