Physical and Biological Forcing

Chairperson: 	Gustav-Adolf Paffenhöfer
Rapporteur:	Anthony Michaels



Text of Chair/Rapporteur's Report:

Physical Factors and Variables

Meteorological forcing from wind stress results in the displacement of surface waters, turbulence and mixing. Depending on the strength of the winds, these effects can reach to depths of tens of meters, not only affecting the vertical and horizontal distributions of zooplankton, but also their population birth, death and growth rates (Wroblewski 1980, Paffenhöfer et al. 1994).

Temperature variability has direct and indirect effects. Diurnal cycles of insolation can warm the surface waters during the day resulting in surface advection sometimes in excess of 15 cm s-1, as well as vertical current shear (Weller et al. 1985). Seasonality in ocean heating and wind stress result in deep convective overturn of the water column in some temperate regions like the North Atlantic, but not in others, like the Subarctic Pacific, which has a permanent halocline. Macrozooplankton assemblages in these open ocean regions are dominated by species adapted to the local physical forcing -- e.g., Calanus finmarchicus in the North Atlantic versus Neocalanus spp. in the Subarctic Pacific.

The formation and displacement of eddies with widely ranging signatures also occurs in the open ocean, thus creating temporary subprovinces which change over time with horizontal advection (e.g., Gulf Stream rings). Differential advection with depth, accompanied by diel vertical migration can result in wide dispersal of populations and assemblages of zooplankton (Riley 1976).

Richard Feynman called turbulence the subject of top research priority in physics. In the ocean, there is good evidence of large-scale turbulence at the surface due to wind forcing and current shear, but little empirical evidence on the occurrence of mm to cm scale turbulence. Semi-quantitative observations, however, indicate that feeding schools of juvenile and adult fish create short-term (< sec) bursts of turbulence resulting in heavy predation mortality on copepods (Kils 1992). It may be that significant small-scale turbulence in the ocean originates from biological activity.

Bottom topography (ridges) seem indirectly to cause aggregations of vertebrate predators in the euphotic zone; the effects of these rises on zooplankton have been documented (McGillivary 1988 thesis, and others) but thorough studies are lacking.

Internal waves occur in most parts of the oceans and should at least temporarily effect vertical distributions of zooplankton directly and possibly indirectly by redistribution of potential food organisms.

Biological Factors and Variables

The main biological factors affecting population dynamics of zooplankton are thought to be food quantity and quality (Lampert 1986, Kleppel 1993) and predators (Landry 1976). Not so much is known about the significance of diseases and parasites. There are, however, reports on widespread effects of parasites on salps (hyperiid amphipods, e.g., Laval 1980) and on copepods (Ianora et al. 1987).

Aggregations are of great significance for the dynamics of populations, whether the aggregations are the animals themselves, or their predators or prey (e.g., Omori and Hamner 1982). Anywhere in the ocean zooplankton form some type of loose or tight aggregation, be it early juveniles in an upper mixed layer (e.g., Paffenhöfer 1983) or older stages in deep layers, tight patches of calanoid or cyclopoid copepods behind obstacles (e.g., Hamner and Carleton 1979), or near the seafloor (Ueda et al. 1985). Tight aggregations of cells occur over extended periods of time in the open ocean (Cowles et al. 1993). These features are often less than 1 m thick and represent environments in which zooplankton would not be food limited. Migrating zooplankton which are otherwise in a nutritionally dilute environment, particularly in the open ocean, need to locate and exploit such discontinuities. Aggregations are also thought to reduce predation or enhance feeding success (schools versus individual fish, Kils 1992). Traditional sampling will not locate such aggregations of potential food or predators. As shown by Kils (1990) for fish schools and their ciliate prey in nearshore regions, quantification of predator-prey interactions in space and time are essential for understanding how and where physical and biological forcing affect the population dynamics of zooplankton in the open oceans.


Long-term time series studies are needed for open-ocean populations. Target species should be selected with an emphasis on those which co-occur in at least two different open ocean provinces. Population rate processes of target species must be assessed at an adequate frequency, coupled with the measurement of relevant physical and biological variables. Study options include:

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