ICES CM 2010/N:02 Not to be cited without prior reference to the author MULTIDISCIPLINARY AND MULTISCALE APPROACH TO UNDERSTAND (HARMFUL) PHYTOPLANKTON DYNAMICS IN A NW MEDITERRANEAN BAY Elisa Berdalet 1,* , Oliver N. Ross 2 , Jordi Solé, Mireia L. Artigas 1 , Gisela Llaveria 1 , Clara Llebot 1 , Rubén Quesada 2 , Jaume Piera 2 , Marta Estrada 1 1 Institut de Ciències del Mar, CMIMA (CSIC), Pg. Marítim de la Barceloneta 37-49, E- 08003 Barcelona, Catalunya, Spain 2 Unitat de Tecnologia Marina, CMIMA (CSIC), Pg. Marítim de la Barceloneta 37-49, E- 08003 Barcelona, Catalunya, Spain * Corresponding author: [email protected], phone: 34 93 230 9595; FAX: 34 93 230 9555 ABSTRACT The mechanisms underlying the population dynamics of species causing Harmful Algal Blooms (HABs) are complex because they result from the interplay of a spectrum of physico-chemical and biological factors, to which the organisms respond with a variety of strategies. Still, it is not clear whether the responses of harmful species are different from other phytoplankters. In order to resolve some of these questions, we started a multidisciplinary study in 2007 in Alfacs Bay (Ebre Delta), an active aquaculture site in the NW Mediterranean that is exposed to recurrent HAB events. Through a series of meteorological and hydrographical observations combined with modelling exercises, we try to understand how the circulation in the Bay affects the retention, dispersion, and thus the net development of (harmful) phytoplankton populations. The small-scale characterization of the physical water column properties is performed using a high- resolution acoustic Doppler current profiler and a SCAMP (temperature microstructure profiler to deliver information about turbulence). With this approach, we aim to explain the observed preferential vertical concentration of the target organisms (harmful or not). The field studies are complemented by physiological research in the laboratory which has already shown a particular sensitivity of dinoflagellates to small-scale turbulence. For hypothesis testing, we combine these field and laboratory observations with an individual based (Lagrangian) turbulence model. Here, we present some of our progress and highlight our future goals which includes the deployment of a real-time automated physico-optical observation system to provide a better understanding of the in situ biological (growth and grazing rate) dynamics of (harmful) phytoplankton. INTRODUCTION In general, coastal embayments constitute the paradigm of a coastal ecosystem and represent a test bench to improve our capacity for detection and prediction of HABs. Our studies are conducted in Alfacs Bay (Figure 1) since 2007. This is a 12 Km long and 4 Km wide lagoon, with a maximum depth of 6 m and an average depth of 3 m, partially separated from the Mediterranean Sea by a sand barrier that leaves an opening of about 3 Km. The study of HABs dynamics is specially relevant in this area, given its high ecological interest and aquaculture activity. Our studies cover different processes that operate at different spatio-temporal scales and that may be involved in the dynamics of phytoplankton in general and, eventually, of harmful species.
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ICES CM 2010/N:02
Not to be cited without prior reference to the author MULTIDISCIPLINARY AND MULTISCALE APPROACH TO UNDERSTAND (HARMFUL) PHYTOPLANKTON DYNAMICS IN A NW MEDITERRANEAN BAY Elisa Berdalet1,*, Oliver N. Ross2, Jordi Solé, Mireia L. Artigas1, Gisela Llaveria1, Clara Llebot1, Rubén Quesada2, Jaume Piera2, Marta Estrada1
1Institut de Ciències del Mar, CMIMA (CSIC), Pg. Marítim de la Barceloneta 37-49, E-08003 Barcelona, Catalunya, Spain 2Unitat de Tecnologia Marina, CMIMA (CSIC), Pg. Marítim de la Barceloneta 37-49, E-08003 Barcelona, Catalunya, Spain * Corresponding author: [email protected], phone: 34 93 230 9595; FAX: 34 93 230 9555 ABSTRACT The mechanisms underlying the population dynamics of species causing Harmful Algal Blooms (HABs) are complex because they result from the interplay of a spectrum of physico-chemical and biological factors, to which the organisms respond with a variety of strategies. Still, it is not clear whether the responses of harmful species are different from other phytoplankters. In order to resolve some of these questions, we started a multidisciplinary study in 2007 in Alfacs Bay (Ebre Delta), an active aquaculture site in the NW Mediterranean that is exposed to recurrent HAB events. Through a series of meteorological and hydrographical observations combined with modelling exercises, we try to understand how the circulation in the Bay affects the retention, dispersion, and thus the net development of (harmful) phytoplankton populations. The small-scale characterization of the physical water column properties is performed using a high-resolution acoustic Doppler current profiler and a SCAMP (temperature microstructure profiler to deliver information about turbulence). With this approach, we aim to explain the observed preferential vertical concentration of the target organisms (harmful or not). The field studies are complemented by physiological research in the laboratory which has already shown a particular sensitivity of dinoflagellates to small-scale turbulence. For hypothesis testing, we combine these field and laboratory observations with an individual based (Lagrangian) turbulence model. Here, we present some of our progress and highlight our future goals which includes the deployment of a real-time automated physico-optical observation system to provide a better understanding of the in situ biological (growth and grazing rate) dynamics of (harmful) phytoplankton. INTRODUCTION In general, coastal embayments constitute the paradigm of a coastal ecosystem and represent a test bench to improve our capacity for detection and prediction of HABs. Our studies are conducted in Alfacs Bay (Figure 1) since 2007. This is a 12 Km long and 4 Km wide lagoon, with a maximum depth of 6 m and an average depth of 3 m, partially separated from the Mediterranean Sea by a sand barrier that leaves an opening of about 3 Km. The study of HABs dynamics is specially relevant in this area, given its high ecological interest and aquaculture activity. Our studies cover different processes that operate at different spatio-temporal scales and that may be involved in the dynamics of phytoplankton in general and, eventually, of harmful species.
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Estuarine circulation. In the Bay, tidal amplitude is small (15-20 cm) but over the year the water shows wide oscillations in both temperature (7 to 29 ºC range) and salinity (30 to 37.7 range). Its circulation dynamics is influenced by natural and anthropogenic physical processes, involving mainly temperature-driven stratification, easterly wind pulses and NW wind events and salinity-driven stratification caused by freshwater inflow from irrigation channels in the Northern side of the Bay, used in rice cultivation and that open mainly between April and November. The circulation is typical of a positive stratified estuary, with a flow oriented predominantly seaward in the low salinity surface layer and landward in the saltier, deeper layer.
Figure 1. Study site. Left: the Alfacs Bay (in the Southern side of the Ebro Delta, 200 km South of Barcelona). Right: location of the main sampling points including four freshwater channels (C1 to C4). Coordinates values x105. One important step in the characterization of a water body is the study of the relationship between forcing factors and water circulation patterns. Our approach has been twofold. First, the Huang’s Empirical mode decomposition (EMD), a method of time-series analysis that produces a linear decomposition of a series in a number of non-linear modes of time-depending frequency, called Intrinsic Mode Functions (IMF), was used to assess the temporal variability of the main meteorological and hydrographical variables in Alfacs Bay and to test the effect of meteorological forcing
on the hydrographic properties of the bay.
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Figure 2. Joint plots of the seasonal intrinsic model functions of air temperature (T4) and surface water temperature (TS4). Redrawn from Figure 10 in Solé et al. 2009. The main findings of this study (Solé et al., 2009) were that the annual modes of water temperature were correlated with wind speed and with the corresponding modes of air temperature and air pressure (Figure 2), and that water column stratification correlated with opening and closing of freshwater drainage channels, but not with rainfall (not shown). A second approach has been the implementation of a three-dimensional numerical model to explore the effect of potential forcing factors such as tides, wind stress, earth rotation and density gradients on the spatial and temporal dynamics of the estuarine circulation. The chosen model, Si3D (Semi-Implicit, Three-Dimensional Model for Estuarine Circulation) is a 3-D free surface hydrodynamic model that has been successfully applied to studies of estuaries and also lakes (Rueda and Cowen, 2005). Our main goal is to determine which of the forcing sources has major effect on the stratification or mixing of the bay, and to characterize it numerically. We also aim to understand better the water column structure of the bay during the closed channels (December-March) and open channels periods. Our basic hypothesis is that wind stress and density differences due to salinity are key parameters in Alfacs Bay circulation. The relative effect of earth rotation, wind speed and tide and freshwater input have been tested, respectively, by means of the Rossby radius of deformation, the Richardson Number and the Estuarine Richardson Number. We conclude that the bay circulation is affected by Earth rotation, but it is not strongly influenced by tides, which are very small in the region. The wind is one of the key forcing sources. When the wind velocity is lower than 4.5 m·s-1 the bay is weakly mixed, but when winds are stronger the mixing is intense. The input of freshwater is essential to have stratification. Comparing model results with field observations in literature allows us to conclude that there must exist other sources of freshwater (underground water) to explain the stratification during the closed channels period as it has been suggested by others (Bayó et al. 1997). Currently, the model is being validated with field observations using ADCP (e.g. Figure 3) and temperature and salinity records. Future work includes a study of the distribution of water residence times in the bay under selected physical forcing scenarios. Three different physical scenarios for different HAB species. At present, we have a preliminary characterization of three physical scenarios (Artigas et al. 2008) where three main groups of phytoplankton and associated harmful species can be found:
Scenario A: "the Karlodinium-Dinophysis spp. bloom period". These organisms tend to proliferate in spring-early summer, encompasing a period of high backscatter values (likely related to freshwater runoff) recorded by the ADCP, high temperatures (23 up to 26 ºC) and relatively low values of turbulent intensity (ε ≈ 10-9.5 cm2s-3).
Scenario B: "the winter-Alexandrium minutum bloom period". In this time of the year, this species can proliferate following relatively intense windy and stormy periods.
Scenario C: "the autumn-Pseudo-nitzschia spp. bloom period". The group is present along the whole year in the Bay, with capacity to reach high numbers under a wide range of temperature and physical conditions. However, great proliferations tend to occur in autumn, coinciding with complete mixing of the water column.
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Figure 3. The three components of the water velocity and associated shear estimated in the Central point of the Bay (Figure 1) in summer (from Julian day 207 to 215) coinciding with the Scenario A of the Karlodinium-Dinophysis spp. bloom period. Vertical distribution of some harmful species. In our surveys covering the whole bay, the ichtiotoxic Karlodinium spp. was mainly located at 4-5 m depth (Figure 4) throughout the diel cycle while the rest of phytoplankton species varied their position in the water column following the dynamics of water bodies in the Bay. At present we are investigating whether this position was the response to selected factors (e.g. photoacclimation, light:dark cycle, presence of prey as the organism appears to be mixotrophic) on the migratory behaviour. A series of laboratory experiments are combined with individual based (Lagrangian) turbulence model. In addition, the caracterization of the water circulation in the bay can help us to identify particular retention areas (specially towards the shallower and calmer inside part of the bay) where Karlodinium spp. and other phytoplankters can specifically thrive.
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Figure 4. Distribution of Karlodinium spp. in summer 2007. Along a transect from the mouth to the inside of the bay, parallel to the main coast line (see Figure 1). Future studies. The understanding of HABs dynamics faces two major challenges: 1) the early and fast detection of harmful organisms, in particular of those able to cause toxicity events when present at low biomass; and 2) the measurement of relevant biological parameters, especially growth, encystment, mortality, grazing, and swimming speed, with a spatio-temporal resolution matching that available for the physico-chemical variables. Our long term goal is to implement a multidisciplinary observatory and early warning system in the bay (Figure 5). We are specially interested in the development of hyperspectral optical sensors for the detection of phytoplankton optical functional groups and to progress in the study of the physical-biological interaccions related to HABs.
Figure 5. Scheme of a multidisciplinary observatory, with continuous power supply that would facilitate the obtention of physical and biological data on real-time.
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References Artigas, M.L., R. Quesada, Berdalet, E., Piera, J., Fernández-Tejedor, M., Diogène, J., Estrada, M. (2008). Exploring the link between small-scale hydrodynamic properties and harmful phytoplankton blooms in the Ebro Delta (NW Mediterranean). General Assembly of the European Geosciences, Vien, Austria. Bayó, A., E. Custodio, et al. (1997). Las aguas subterráneas en el delta del Ebro. Revista de Obras Públicas 3.368: 47-65. Rueda, F. J. and Cowen, E. A. (2005). The residence time of a freshwater embayment connected to a large lake. Limnology and Oceanography, 50(5):1638–1653. Solé, J., Turiel, A., Estrada, M., Llebot, C., Blasco, D., Camp, J., Delgado, M., Fernández-Tejedor, M., Diogène, J.- 2009. Climatic forcing on hydrography of a Mediterranean bay (Alfacs Bay). Continental Shelf Research, 29: 1786–1800.
