Temporal variability of plankton in the north and northwest Iberian shelf : understanding plankton dynamics from monitoring time-series

Planktonic organisms play a crucial role in pelagic food webs. Half of the global primary production is due to phytoplankton activity and an important part is transferred to higher trophic levels by zooplankton organisms. Because of their short life-span and sensitivity, planktonic organisms have been proposed to be good sentinels of environmental changes and furthermore constitute an appropriate biological model to study population and community dynamics. The general objectives of this theses are: to explore the patterns of variability of plankton along the northern Atlantic coast of Spain by analyzing the time-series obtained within the RADIALES monitoring program (http://www.seriestemporales-ieo.com/) (Chapter 1); to investigate the connections between environment variable and plankton fluctuations at multiple temporal scales (Chapter 2 and 3); and finally, to infer how environmental variability and community interactions can shape community assembly of plankton populations (Chapter 4). The first chapter (Seasonal and long-term variability of mesozooplankton along the Northern Iberian Atlantic shelf), aim to describe and compare the patterns of variability at 9 oceanographic stations distributed in 4 across-shelf sections along the North-western shelf of the Iberian Peninsula. According to the periodicities observed in biomass time-series at all scales, three main spatial domains could be distinguished: oceanic (stations: RSt6 and RGi3), coastal Cantabrian (RSt2, RGi1 and RGi2) and coastal Galician (RVi1, RVi3, RCo2 and RSt4). Seasonality accounted for the main proportion of variability in all the stations of the study area. However, in the southern part of the Galician shelf the seasonal cycle presented only one main wider peak from late spring to autumn, while a semi-annual component was observed in the Cantabrian shelf, reflecting the occurrence of peaks in spring and occasionally in autumn. At the decadal scale, the Santander section presented a positive trend in annually averaged biomass and a negative trend in abundance (significant in the mid-shelf), resulting in an increase in the average individual weight significant in coastal and shelf sites. A Coruna station presented positive and significant trends in biomass and abundance and a decrease in the average individual weight through time. The observed trends in average individual weight may be indicative of shifts in zooplankton community structure. The more conspicuous increase in biomass and abundance was observed in the section off Vigo but without apparent changes in individual weight. In the second chapter (Long-term and seasonal zooplankton dynamics in the northwest Iberian shelf and its relationship with meteo-climatic and hydrographic variability), attention was given to the previously detected abrupt increase observed in the Vigo section. Long-term and seasonal dynamics of zooplankton, in terms of abundance and taxonomic composition, and its relationship with meteo-climatic and hydrographic factors were investigated at two locations, within and off the Ria of Vigo (Station RVi1 and RVi3, respectively). Total abundance of zooplankton varied annually following on average a unimodal cycle. In the long term, zooplankton abundance exhibited three contrasting periods: A) 1995–2001, characterized by low abundance and low amplitude seasonality, with a stepped increase towards 2001; B) 2001–2006, of high abundance and marked seasonality enclosing the maximum values of the time series; and C) 2006–2010, of intermediate abundance and amplitude of the seasonal cycle. Principal component analysis revealed that the shift in zooplankton dynamics from 2001 onwards affected annual averages abundances of all zooplankton taxa. This shift was concomitant with sustained trends for upwelling intensity (increasing), precipitation (decreasing) and Gulf Stream North Wall position (equatorward displacement) between 2000 and 2005. The results stress the importance of hydrodynamics, driven by meteo-climatic conditions, in the control of the abundance levels of zooplankton at seasonal and long-term scales. In order to explore in depth the processes that have driven the abrupt changes in the Vigo stations, the temporal structure of the zooplankton community has been studied, given rise to the third chapter (“Environmental multi-scale effects on zooplankton inter-specific synchrony”). Monthly time-series of marine zooplankton (taxonomic composition, total abundance, and biomass) and their relationship with upwelling index and river outflow have been analyzed using wavelet methods. The annual oscillation of biomass and abundance increased in 2000 corresponding to the highest amplitudes of environmental forcing. Concomitantly, enhanced synchrony was observed among the main taxonomic groups of zooplankton and among copepod species, the most relevant group in terms of occurrence and abundance. The degree of synchrony appeared to be correlated with the upwelling index and, more closely, with the duration of the upwelling events. The results suggest that amplified seasonality of the environmental variables between 2000 and 2004, combined with a reduction of off-shore exportation by shortening of upwelling events, favoured retention in winter, and primary production in summer. These changes modulated community aggregated properties and affected the stability of the zooplankton community through an increase in inter-specific synchrony, allowing the community to shift to another state and likely a reorganization of the community size structure. Within the fourth chapter (Critical transition towards synchronicity: How environmental forcing can synchronize population fluctuations?), we explore the effect of environmental fluctuations, such as nutrient availability, on phytoplankton community temporal structure by combining observations and simulation approaches. Observations correspond to the monthly time-series of diatom composition collected off A Coruna (Station RCo2) where the availability of nutrients has been described to rely principally on the upwelling dynamics. The level of synchrony, derived from the wavelet decomposition of the 12 most abundant species of diatoms, varied in time, and the lowest values were observed from 1998 to 2002. During this period, the upwelling index series presented its highest amplitude for various periodic components: 2-3 weeks, 1.5 months and 1.5 years. To better understand the mechanisms and test if changes in the amplitude of nutrient inputs can impact the temporal association between competitors, we employ a simple chemostat model in which two species compete for two fluctuating essential resources. Synchrony as been estimated between the two populations along a gradient of nutrient input amplitude. For each possible stoichiometric ratio requirement, the synchrony increase together with the amplitude of nutrient input in a non-linear fashion. Indeed, inter-specific competition seems to buffer the effect of nutrient fluctuations until a certain threshold where it becomes the main force controlling the temporal association of species.