The role of advection and turbulent mixing in the vertical distribution of phytoplankton

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Abstract

The purpose of this study is to analyse the role of the flow field on the horizontal and vertical distributions of different phytoplankton populations thriving in the water column of a shallow coastal ecosystem. Two extreme flow conditions are illustrated. The first was a low energetic flow, under calm meteorological conditions and a stratified temperature of the water column. The second flow, coincident with the passage of a storm front, was more energetic resulting in increased mixing that homogenized the temperature in the whole water column. Although the mixing level homogenized the temperature of the water column in the high-energy period, it was not enough to homogenize the temperature in the low-energy period. In contrast, in both periods, the mixing level was enough to homogenize the vertical distribution of particles. A decrease in the concentration of particles from the calm period to the high-energy period was attributed to an advection event with warmer water of lower plankton concentration that resulted in a decrease of the total concentration of suspended particles in the water column. Data are used to test a model of plankton mixing proposed by Ruiz et al. (J. Plankton Res., 18 (1996) 1727).

Introduction

It is known that different phytoplankton groups occupy different niches in the water column depending on their sizes (Tremblay, Legendre, & Therriault, 1997). Small phytoplankton cells (<5 μm) tend to be ubiquitous in the water column, whereas large phytoplankton cells thrive in a narrow range depending on the hydrodynamic conditions. Even though it is not resolved, this difference in the phytoplankton behaviour has been attributed to several factors, such as the sinking rate of phytoplankton, their photosynthetic efficiency, the uptake of nutrients, advection and grazing, coagulation, among others. Large cells are susceptible to sinking or grazing by small herbivores that do not influence small cells. The export of larger phytoplankton from the surface to the bottom is crucial in order to explain the transport of carbon through the water column.

It is generally well accepted that there is a strong relationship between the vertical mixing of the water column and the distribution of particles within it. Usually, high levels of mixing in the water column are associated with homogeneous distribution of particles. Even though Ryther and Hulburt, 1960, Ignatiaded, 1979 showed that particle stratification can occur even when fluid properties appear well mixed, both studies had low vertical resolution of physical and biological data. Other studies, based on vertical microstructure measurements of the fluorescence (Cowles, Moum, Desiderio, and Angel, 1989, Cowles and Desiderio, 1993, Ruiz, Garcı́a, and Rodrı́guez, 1996, Ruiz, 1996), suggested the presence of microstructures in the mixed layer, which might be caused by a photo-adaptative response to the light gradient or also by the energy dissipation level over the water column.

Mixing has also been found to dominate the coagulation process of particles in a water body (Serra, Colomer, and Casamitjana, 1997, Serra and Logan, 1999). In this case, small particles with low settling velocities have been found to coagulate; attaining higher sinking rates than they had previously attained (Li & Logan, 1997). Other processes like advection can also determine the fate of suspended particles in the water column, as has been demonstrated in previous studies on phytoplankton (Kiørboe et al., 1998) and on suspended sediment particles (Colomer, Serra, Piera, Roget, & Casamitjana, 2001). Several models (Ruiz, Garcı́a, and Rodrı́guez, 1996, Ruiz, 1996) have been proposed with the aim to evaluate the mixing effect on the vertical distribution of phytoplankton. In these models, advection processes have not been considered.

Despite these studies, the role of mixing in regulating the concentration of particles of different sizes i.e. to the concentration of different populations of microorganisms, has not been adequately demonstrated for field data. In this study, the effects of the mixing and advection processes on the particle concentration of different species in the water column of a shallow coastal system will be compared by combining high-resolution data on current shear, particle size distributions and pigments. Two different energetic conditions are illustrated. A low-energy period, which represents stratified temperatures, and a high-energy period, which represents vertically homogeneous temperatures. In this study, and based on the field data obtained, we also tested the model of plankton mixing proposed by Ruiz, Garcı́a, and Rodrı́guez, 1996, Ruiz, 1996. In this study, we stressed the importance of horizontal transport in regulating plankton distributions in shallow coastal ecosystems.

Section snippets

Theory

Settling velocity of particles is described by the Stokes equation, provided laminar flow occurs, i.e. inertia forces do not overwhelm viscous forces. The ratio of these two forces is balanced by the Reynolds number, Re=wdlv where d is the particle diameter, w is its settling velocity and v is the fluid kinematic viscosity. For Re<0.1, there is very little departure from Stokes equation. The maximum speed velocity reported for algae has been found to be 6 mm s−1 (for Ethmodiscus rex), with

Methods

The study site was situated at the northeast (NE) coast of Spain (Fig. 1a), in the Fenals Point (Fig. 1b). From measurements of the light intensity made monthly during 1-year period, a minimum value of 4% of the subsuperficial light intensity was found to reach the bottom of the water column (E. Gacia, personal communication). Therefore, the photic layer coincided with the depth of the water column. Two field campaigns were carried out, the first campaign was carried out on 2 June 1998 and the

Results

During the first week of June 1998, wind and current speeds and wave heights increased and current and wave directions shifted during the passage of a storm front (Table 1). From the morning of 1 June to the evening of 2 June, current speeds were low (∼0.05 m s−1) and the maximum wave height was <0.75 m (Fig. 2). By 3 June, current speeds and wave height increased to the maximum values of ∼0.40 m s−1 and ∼1.50 m, respectively, which were related to the passage of a storm front.

The water column

Discussion

Changes in the weather conditions measured from 2 to 5 June affected the structure of the whole water column, increasing turbulent mixing in the water column with values of the eddy diffusivity one order of magnitude higher than before the passage of the storm (Fig. 3c). This change in the structure of the water column was demonstrated as a change in the temperature and in the potential density (Fig. 3a, b, respectively). When mixing excursions dominate over swimming motions, i.e. when D=Kz/hv>l

Conclusions

The change in the weather conditions during the stratified period of a coastal ecosystem determined the structure of the water column, increasing the mixing level from a calm period (with Kz∼10−3m2s−1) to a high-energy period (with a mean vertical Kz∼10−3m2s−1). The water column remained stratified during the calm-energy period, whereas in the high-energy period it mixed due to the increase of the turbulence. During both periods, mixing was found to dominate the vertical distribution of

Uncited references

Cowles, Moum, Desiderio, and Angel, 1989

Acknowledgements

We would like to thank Esperança Gacia for her interesting comments, which helped to improve the article.

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