Biological Physical Interactions

C S Reynolds, Centre of Ecology and Hydrology and Freshwater Biological Association, Cumbria, UK © 2009 Elsevier Inc. All rights reserved.

Introduction

As formal branches of science, limnology and ecology are each around a century in age. Both disciplines feature prominently in the evolving understanding of inland waters, where they are invoked to explain observable phenomena and their role in shaping the abundance, structure, and variability of the biotic communities. It is also true that, from the pioneer studies onwards, much of the scientific investigation has been focused on the mutual relationships among the biota and their chemical environments, most especially with regard to the important nutrient elements. Physical factors were not overlooked altogether: it is well known how the specific heat of water, its curiously variable coefficient of thermal expansion, and its transparency each influence the stability and duration of seasonal thermal stratification or to the underwater distribution of macrophytic plants and photosynthetic algae. The benefit to aquatic organisms of the mechanical support contingent upon the high density of water is also generally understood. On the other hand, the mathematics of fluid motion in lakes proved to be less amenable to solution, so its impact on the evolutionary ecology of the pelagic biota - those of the open water, mostly living independently of shores or the bottom oozes - for a long time remained wholly intuitive. Although broad patterns of wind- and gravity-generated currents could be described and modeled, the smaller scales of water movement and the quantitative description of turbulence were, until the last twenty years or so, relatively intransigent to solutions relevant to the function, selection, and evolutionary ecology of aquatic biota, or even to such matters as the transport and dispersal of organisms, particles, and solutes. This chapter reviews briefly the physical properties of fluid motion at the macro and microscale in inland water bodies, seeking to establish the spatial and temporal scales that impinge directly and indirectly on the organisms that live there, as well as on the functional adaptations that enable them to do so.

Water Movements - The Concept of Scale

From the major oceanic circulations down to the (Brownian) behavior of the finest colloidal particles and the diffusion of solutes, water characteristically comprises molecules in motion. Curiously, the propensity of water molecules to polymerize into larger 'aquo complexes,' which is responsible for the relatively high density and viscosity of liquid water, makes them resistant to movement, so there is a constant battle between, on the one hand, the energy sources driving motion (the Earth's rotation, gravitational flow, convectional displacement due to thermal expansion and contraction and, especially, the work of wind forcing applied at the water surface of lakes; and, on the other, the resistance of internal viscous forces. Thus, the energy of external forcing is dissipated through a cascade of turbulent eddies of diminishing size and velocity, to the point that it is overwhelmed at the molecular level.

Turbulence in the upper water column affects pelagic organisms through several mechanisms operating at a range of temporal and spatial scales. Mixing influences the distribution of regeneration and recycling of dissolved nutrients, the dispersion of zooplankton, the location of rewarding feeding grounds for fish, and the resuspension of detrital particles, including biotic propagules. In relation to the depth of light penetration, the penetration of mixing may constrain the exposure of entrained photosynthetic algae and bacteria to light and to regulate their primary production. At the microscale, turbulence is relevant to individual organisms, conditioning their suspension in the mixed layer, the interaction with their own intrinsic motility and the fluxes of dissolved nutrients and gases to their cells. It is to the latter influences that this article is particularly addressed.

Small-Scale Turbulence

The measurement of turbulence or its convenient components, such as the shear or friction velocity (symbolised as u*), is not the concern of the present chapter. Turbulence in the upper water column is induced by wind, heat loss, and wave breaking. When wind is the predominant cause of turbulence, the turbulent velocity can be approximated from the shear stress at the air-water interface; if convective heat loss is the dominant driver, then a similar turbulent velocity scale, w*, comes from the velocity of the resultant convectional plumes; if several processes are operating simultaneously, all are included in the calculation, and the resultant turbulent velocity scale is sometimes called the turbulent intensity u. Deeper in the water column, turbulence is often caused by breaking internal waves. Technically, the turbulent intensity is the root mean square velocity of the velocity fluctuations in a turbulent flow field. This measurement has only recently been applied in lim-nological studies. More commonly, turbulence is obtained from microstructure profiling as the rate of dissipation of the turbulent kinetic energy e with the assumption that turbulence production and dissipation are in balance. When turbulence is induced by wind in the surface layer, the turbulent velocity scale is roughly proportional to the square root of the quotient of the applied force per unit area (t, in kg m"1 s"2) and the density of the water (pw, in kg m"3). Then

The units are in ms"1. The rate of dissipation of turbulent kinetic through the spectrum of eddy sizes (e) is correlated to the dimensions of the largest eddies in the turbulence field (1e) and their velocities (u) through eqn. [2]:

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