The microplankton succession in the Ría de Vigo revisited: species assemblages and the role of weather-induced, hydrodynamic variability
Introduction
The species composition of the microplankton community affects key aspects of the functioning of aquatic ecosystems, such as those relative to energetics (e.g., linear or web-like food chains), homeostasis (e.g., low or high diversity), or nutrient cycling (e.g., open or closed mineral cycles) (e.g., Odum, 1969, Reynolds, 1997). However, despite numerous observational (e.g., Karentz and Smayda, 1984), experimental (e.g., Flöder and Sommer, 1999), and modelling studies (e.g., Moloney and Field, 1991), the factors and mechanisms that shape the structure of planktonic communities are still under debate (Reynolds, 1997). Although it is generally accepted that weather-induced, hydrodynamic variability plays a major role (Smayda, 1998) (i.e., allogenic control sensu Reynolds, 1997), it is difficult to demonstrate and quantify its effect on the structure of the microplankton community due to the wide range of temporal scales involved (Kononen, 2001), the complex interplay between factors (Padisák et al., 1993), or the likely lagged response of the organisms to forcing (Duarte, 1990). Besides, it is uncertain and difficult to analyse in natural populations the relevance of self-regulation (i.e., autogenic control sensu Reynolds, 1997), acting through mechanisms such as differential grazing (Cowlishaw, 2000) or allelopathic interactions (Rengefors and Legrand, 2001).
The research conducted by Margalef et al. (1955) and Durán et al. (1956) in the Ría de Vigo established the basic features of succession phenomena in marine phytoplankton communities and the relationship of characteristic phytoplankton assemblages with environmental factors. Form this pioneer research, this enclosed coastal ecosystem has been the focus of numerous investigations dealing with phytoplankton dynamics and its relationship with environmental factors. However, as it happens with similar studies in other marine ecosystems (see, for instance, Cebrián and Valiela, 1999, for the North Atlantic), these investigations either cover long-term periods (≥1 year) at low resolution (i.e., biweekly to monthly sampling) (e.g., Figueiras and Ríos, 1993), or are conducted during brief periods (2–20 days) at high sampling frequency (≥2 samples per week) (e.g., Tilstone et al., 2000) and, thus, disregard, respectively, the effect of short-term variability or the seasonal and long-term scenarios. Recently, Nogueira et al. (2000) analysed high-resolution time series of meteorological, hydrographic, and microplankton species abundance data for the period September 1990–April 1992, showing the relevance of meteorological and hydrographic short-term disturbances on the structural changes of the community. In the present investigation, we followed a similar approach to investigate the dynamics of microplankton community structure in the year 1987, which differed markedly from the previously studied period (centred in the year 1991) in terms of the climatological and meteorological conditions. The analysis involved: (1) the characterisation of the meteorological and hydrographic variability at seasonal and short-term scales; (2) the definition of characteristic microplankton assemblages and their ecological interpretation; and (3) the identification of temporal changes in the structure of the community and its plausible relationship with meteorological and hydrographic variability. The results obtained here were compared with those exposed in Nogueira et al. (2000), offering the opportunity to evaluate the consistency of the conclusions articulated in the aforesaid paper and to assess the significance of interannual variability. We have also considered the succession at the species level, considering the processes of coexistence and dominance.
Section snippets
Materials and methods
The study site, type of time series, and protocols of numerical analysis were, in general terms, similar to those in Nogueira et al. (2000), making the comparison of results straightforward.
Meteorological and hydrographic variability
The meteorological time series were significantly correlated among them (absolute values of the Spearman rank order correlations, ∣rs∣, range between 0.4 and 0.6 (p<0.001), partly due to their strong and coupled seasonality (Fig. 2a and b) (Nogueira et al., 1997). The year 1987 split in two contrasting terms: the ‘winter’ period, from January to mid April and from November to December, characterised by the prevalence of SW winds, high runoff, and low irradiance; and the ‘summer’ period, from
Discussion
We have related the structural changes of the microplankton community with meteorological variability. The significance of allogenic, weather-induced control operating through the modification of the hydrodynamic conditions, mainly in relation to the z- and y-axes in this enclosed coastal ecosystem (i.e., vertical mixing/stratification and washout/dilution following the main axis of the Ría), has been pointed out before (e.g., Figueiras and Pazos, 1991, Figueiras et al., 1995, Figueiras et al.,
Acknowledgements
The authors thank the members of the Grupo de Oceanoloxía of the Instituto de Investigacións Mariñas who collaborated in the sampling, and two anonymous referees whose suggestions considerably improved the manuscript. E. Nogueira acknowledges the receipt of Marie Curie postdoctoral fellowships MAS3-CT97-5049 and MCFI-2000-01958 from the EU. F.G. Figueiras acknowledges the receipt of the EU project HABILE (EVK3-CT-2001-00063).
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