Ll Introduction

This chapter considers the sinks and, more particularly, the dynamic rates of loss of formed cells from phytoplankton populations. Several processes are involved -- hydromechanical transport, passive settlement and destruction by herbivores and parasites -- which, separately or in concert, may greatly influence the structuring of communities and the outcome of competitive interactions among phytoplankton. Moreover, these same processes may contribute powerfully to the biogeochemical importance of pelagic communities, through their role in translocating bioproducts from one point of the planet's surface to another.

Before expanding upon these processes, however, the opportunity is taken to emphasise that the losses considered in this chapter are those that affect the dynamics of populations. The (sometimes very large) loss of photosynthate produced in excess of the cell's ability to incorporate in biomass is not considered here. The topic is covered in a different context in Chapter 3 (see especially Section 3.5.4). The emphasis is necessary as the term 'loss rates' was applied collectively to the dynamics of almost all measurable photosynthetic production that did not find its way into increased producer biomass (Jassby and Goldman, 1974a). It had been supposed by many workers at the time that the realised shortfall was attributable to grazing and sedimentation of biomass. However, with the demonstration that, very often, production in some systems was almost wholly and precisely compensated by simultaneous bulk loss rates (Forsberg, 1985), when the rates of grazing or sedimentation might only rarely explain the disappearance of the equivalent of the day's new product, it became clear that some further separation of the 'losses' was necessary, together with some refinement of the terminology. Here, adjustments to the photosynthate content that the cell is unable to deploy in new growth or to store intracellu-larly and which must be dispersed through accelerated respiration, or photorespiration, or secretion as glycollate or other extracellular product, are considered to be 'physiological'. The adjustments are as much to protect the intracellular homeostasis as to supply any other component of the pelagic system. On the other hand, successfully replicated cells in growing populations are continuously but variably subject to physical or biological processes that deplete the pelagic concentrations in which they are produced. Detracting from the numbers of new cells added to the population, these losses are 'demographic' and, as such, are the proper focus of the present chapter. Its objective is to establish the quantitative basis for estimating the drain on the potential rate of recruitment, provided by cell replication, that is represented by the counteracting processes which, effectively, dilute the recruitment of phytoplankton biomass. In the sense of Eq. (5.3), the task is to quantify the magnitude and variability in the rates of dilution of finished cells (rL).

As already suggested, the principal loss processes are hydomechanical dispersion (wash-out from lakes, downstream transport in rivers, patch dilution at sea), sedimentation and consumption by grazers. Attention is also accorded to mortalities through parasitism (a specialised consumption) and physiological death and wastage. Although highly disparate in their causes, each process has the effect of diluting the locally randomised survivors. Hence, each is describable by an exponent, summable with other loss and growth terms. Just as Eq. (5.1) explains the rate of population change, SN/St, by reference to the first-order multiplier, eTnt, and where, from Eq. (5.3), it may be asserted that rn = r' — rL, it may now be proposed that:

where rW, rS, rG... are the respective exponents for the instantaneous rates of biomass loss due to wash-out, sedimentation, grazing, etc. It is accepted that these terms, either individually or in aggregate, may raise rL > r, in which case, rn is negative and symptomatic of a declining population.

The following sections will consider the magnitudes and variabilities of the loss terms.

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