I2 12.1 IntroductionSome VarietiesThe apparent uniformity of the oceans has turned outof Biological to be an illusion generated by the original need forOceanography widely spaced sampling both horizontally and verti-cally. We can no longer accept concepts based on rel-atively smooth gradients in temperature or salinity. In. H. Steele his paper, "Varieties of Oceanographic Experience,"Stommel (1963) pointed out the wide range of scales,in space and time, on which variability occurred. Im-provements in technology and development of theo-retical bases (Rhines, 1977) portray the oceans as aphysical system whose structure can be as rugged asthat of the terrestrial world (see chapter 11).The spatial and temporal variability of the organismsthat inhabit the oceans has been recognized for de-cades. Without this variability, commercial fishingwould be uneconomical and sport fishing unexciting.Patches of plankton extending for tens of kilometerswere reported in the 1930s (Hardy and Gunther, 1935)and mapped in the 1960s (Cushing and Tungate, 1963).There was, however, no detailed knowledge of the pos-sible relation of these biological observations to cor-responding physical structure.In recent years there have been several attempts tointegrate the physics and biology, but on two differentlevels. The development of fluorometric techniques(Lorenzen, 1966) has permitted continuous in vivomeasurement of phytoplankton pigments. This, com-bined with continuous measurement of nutrients suchas nitrate, allows detailed portrayal of the spatial struc-ture of the first step in the production cycle. Whencombined with temperature and salinity measure-ments from a moving ship, they provide the basis (fig-ure 12.1) for attempts to determine the physical factorsdetermining horizontal phytoplankton patchiness-or,alternatively, to ascribe some aspects of this patchinessto biological mechanisms. The basic technique, spec-tral analysis, was started by Platt (1972) and developedboth theoretically and technically (Steele, 1978a).The other major area of interest in environmentalvariability relates to the study of fish populations. Theexpansion, indeed overexpansion, of commercial fish-eries leads to fishing on populations with a youngeraverage age. The fisheries, and the populations them-selves, become more and more dependent on'the yearlyrecruitment. In nearly all stocks this recruitment hasvery large year-to-year fluctuations (figure 12.2). Thestudy of these fluctuations has attracted much researchand produced many hypotheses. A large proportion ofthese hypotheses has attempted to relate variable re-cruitment to changes in the physical environment,either year-to-year differences or longer-term trends(Hill and Dickson, 1978). During this same period,however, it has been realized that individual speciescannot be treated separately, and "multispecies man-376J. H. Steele8752IA35.3I 35.235.16420TemperatureChlorophyllSalinityNitrate:- i5900'N58030'Figure I2z. Measurements made at 3 m in the northern NorthSea during the hours 0000-0500 on 16 May 1976. There areno obvious relations between variations in chlorophyll andnitrate, or between these and the physical parameters tem-perature and salinity.15%K-10zc;5*6063..69· 70· 73*71.68.62.58*67 · 6665.53.61 54.57 *64· 5576 72 59.755- 674, , i, i, I , , , , , ,5 10 15SPAWNING STOCK 8/OMASS(10-5ons}Figure z2.2 Year class strength of North Sea herring as afunction of the biomass of the spawning stock. Only in thefinal years of stock collapse, 1974-1976, do the values ofrecruitment fall outside the range of previous variability. (Ull-tang, in press.)agement" is now in vogue. This recognizes the inter-relation between species, in their food requirementsand, potentially, in their recruitment. Thus, again,there is a need to separate the physical and biologicalfactors acting to produce the observed distribution andabundance of fish populations.For these two extremes of the food web, phytoplank-ton and fisheries, there is an extensive literature, whichI shall review very briefly. For both, it is apparent thatthe biological factors limiting our understanding lie inthe intermediate components of the food web-thezooplankton, which graze on the plants and which, inturn, are the source of food for the fish populations.But these interactions must be placed in the context ofthe variability of the physical environment at a widerange of space and time scales.These problems are applicable to all regions of thesea, but, scientifically and economically, are mostacute in areas of the continental shelf. Certain parts ofthe open ocean, such as the centers of gyres (Eppley,Renger, Venrick, and Mullin, 1973), may be consideredrelatively uniform horizontally, but the shelf is domi-nated by changes in all significant physical, chemical,and biological parameters-depth composition of thebottom, temperature, salinity, nutrients, and the quan-tity and quality of living organisms-at all the possiblehorizontal scales. Moreover, there is an equally greatvariability in the vertical structure of the water col-umn, and this variability is conditioned by, and relatedto, horizontal changes. An example of vertical changesis shown (figure 12.3) in the close correspondence be-tween temperature and fluorescence during passage ofan internal wave packet produced by variable bottomtopography in Massachusetts Bay. For these reasons,there is an emphasis in this chapter on variability incoastal areas.12.2 Space and Time Scales of Variation12.2.1 Physical VariationAs a point of departure, it is necessary to start withcertain observed regularities that relate to patterns ofhorizontal variability. Experiments on dye dispersionby Okubo (1971) and others show a consistent relationbetween the variance of concentration across a patchand the time from release of the dye. This relationdemonstrates the expected dependence of horizontaldiffusivity on spatial scale. Using the standard devia-tion o derived from this variance, the relation withtime t is, approximately,Or = t1.17(12.1)where the units are kilometers and days (Steele, 1978b).This almost linear
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