I2 Some Varieties of Biological Oceanography H Steele 12 1 Introduction The apparent uniformity of the oceans has turned out to be an illusion generated by the original need for widely spaced sampling both horizontally and vertically We can no longer accept concepts based on relatively smooth gradients in temperature or salinity In his paper Varieties of Oceanographic Experience Stommel 1963 pointed out the wide range of scales in space and time on which variability occurred Improvements in technology and development of theoretical bases Rhines 1977 portray the oceans as a physical system whose structure can be as rugged as that of the terrestrial world see chapter 11 The spatial and temporal variability of the organisms that inhabit the oceans has been recognized for decades Without this variability commercial fishing would be uneconomical and sport fishing unexciting Patches of plankton extending for tens of kilometers were 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 possible relation of these biological observations to corresponding physical structure In recent years there have been several attempts to integrate the physics and biology but on two different levels The development of fluorometric techniques Lorenzen 1966 has permitted continuous in vivo measurement of phytoplankton pigments This combined with continuous measurement of nutrients such as nitrate allows detailed portrayal of the spatial structure of the first step in the production cycle When combined with temperature and salinity measurements from a moving ship they provide the basis figure 12 1 for attempts to determine the physical factors determining horizontal phytoplankton patchiness or alternatively to ascribe some aspects of this patchiness to biological mechanisms The basic technique spectral analysis was started by Platt 1972 and developed both theoretically and technically Steele 1978a The other major area of interest in environmental variability relates to the study of fish populations The expansion indeed overexpansion of commercial fisheries leads to fishing on populations with a younger average age The fisheries and the populations themselves become more and more dependent on the yearly recruitment In nearly all stocks this recruitment has very large year to year fluctuations figure 12 2 The study of these fluctuations has attracted much research and produced many hypotheses A large proportion of these hypotheses has attempted to relate variable recruitment 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 species cannot be treated separately and multispecies man376 J H Steele 8 Temperature 7 tl 5 Chlorophyll 2 AI J V 35 3 Salinity I 35 2 35 1 r 6 4 N Nitrate 2 i 0 5900 N 58030 Figure I2z Measurements made at 3 m in the northern North Sea during the hours 0000 0500 on 16 May 1976 There are no obvious relations between variations in chlorophyll and nitrate or between these and the physical parameters temperature and salinity 60 15 K10z 63 69 62 c 5 67 70 66 73 71 58 53 54 61 65 57 64 55 68 76 72 59 75 5674 agement is now in vogue This recognizes the interrelation between species in their food requirements and potentially in their recruitment Thus again there is a need to separate the physical and biological factors acting to produce the observed distribution and abundance of fish populations For these two extremes of the food web phytoplankton and fisheries there is an extensive literature which I shall review very briefly For both it is apparent that the biological factors limiting our understanding lie in the intermediate components of the food web the zooplankton which graze on the plants and which in turn are the source of food for the fish populations But these interactions must be placed in the context of the variability of the physical environment at a wide range of space and time scales These problems are applicable to all regions of the sea but scientifically and economically are most acute in areas of the continental shelf Certain parts of the open ocean such as the centers of gyres Eppley Renger Venrick and Mullin 1973 may be considered relatively uniform horizontally but the shelf is dominated by changes in all significant physical chemical and biological parameters depth composition of the bottom temperature salinity nutrients and the quantity and quality of living organisms at all the possible horizontal scales Moreover there is an equally great variability in the vertical structure of the water column and this variability is conditioned by and related to horizontal changes An example of vertical changes is shown figure 12 3 in the close correspondence between temperature and fluorescence during passage of an internal wave packet produced by variable bottom topography in Massachusetts Bay For these reasons there is an emphasis in this chapter on variability in coastal areas 5 i Ii 10 15 12 2 Space and Time Scales of Variation 8 OMASS 10 5ons SPAWNING STOCK Figure z2 2 Year class strength of North Sea herring as a function of the biomass of the spawning stock Only in the final years of stock collapse 1974 1976 do the values of recruitment fall outside the range of previous variability Ulltang in press 12 2 1 Physical Variation As a point of departure it is necessary to start with certain observed regularities that relate to patterns of horizontal variability Experiments on dye dispersion by Okubo 1971 and others show a consistent relation between the variance of concentration across a patch and the time from release of the dye This relation demonstrates the expected dependence of horizontal diffusivity on spatial scale Using the standard deviation o derived from this variance the relation with time t is approximately Or t 1 17 12 1 where the units are kilometers and days Steele 1978b This almost linear relation suggests that populations spreading from some initially small area should have a patch size in kilometers numerically similar to 377 Some Varieties of Biological Oceanography TEMPERATURE I I f tl l 10 20 30 40 10 20 30 40 50 6 0 iK it ql It 20 30 40 50 WL cr FLUORESCENCE ZWEIM 1100 m m L 1130 TIME 3 175 10 20 30 40 50 I t I I I I N 1200 30 AUGUST p977 Figure I2 3 The passage of an internal wave packet past a drifting ship in Massachusetts Bay produces
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