Interpretation of Plankton Observations
In the interpretation of field observations on plankton, it must be remembered that the different requirements of separate species lead to a more or less complete change of the elements in the population when external factors, especially temperature and nutrients, become altered in the water mass inhabited. When such a biological change takes place within a rather well-defined water mass, whether moving or stationary, we may speak of it as an individual population succession. This must not be confused with a change of population resulting from a sequence of distinct water masses flowing with their distinct populations into a given geographical position where successive series of observations are being made. This type of change may be termed a local sequence. It is frequently not possible to distinguish between these two important types of changes that may occur in the population of an area under investigation, though hydrographic data accompanying biological sampling will aid materially in the interpretation of the biological data by providing information on the nature of residual movements of water. Local sequences in populations are likely to be more sudden than individual population successions, since the latter depend upon biological development rather than upon a simple physical shift of water masses. The former may also at times be slow when neighboring populations become mixed or scattered only through the process of advection or lateral mixing.
An example embracing both local sequence and individual population succession in a population is illustrated by the recent observations of Redfield (1939) on the history of populations of the pteropod Limacina retroversa, its entrance and sojourn in the Gulf of Maine (see also p. 864). A population of small individuals appears in the Gulf in December with inflowing water from the east. Caught in the cyclonic circulation of the Gulf, they gradually decrease in number through mortality or some are
The changes occurring in a population may involve a succession of development stages of a given species. In view of this it should be noted that any biological succession observed may result from two causes. There may be (1) a change in composition of species, owing to different biological responses to physical or chemical changes (that is, rise or fall in temperature or nutrient state of the water) that have occurred within the individual water mass, or (2) a change in the relative maturity of the population, owing simply to the passage of time and to chronologically developed stages in the life history of the individuals of one or more species. A change in the phytoplankton involving a succession of species is well illustrated in boreal water, where in late spring or summer there is commonly a drop in the concentration of diatoms in a predominantly diatom plankton and an accompanying or following increase in dinoflagellates. Here two factors, in particular, are operative in the individual water mass: (1) an increase in temperature (owing to advance of the season), which favors the warmth-loving dinoflagellates, and (2) depletion of plant nutrients by the diatoms to a point suboptimal for their abundant proliferation, but still sufficient for the dinoflagellates, which are able to reduce the nutrients further and through their motility to adjust themselves in some degree with respect to favorable light conditions (see p. 765). The phosphate and nitrate requirements of certain dinoflagellates (Ceratium sp., Peridinium sp., Prorocentrum micans) have been found experimentally to be exceedingly low (Barker, 1935). The maximum rate of division has already been reached with 0.1 parts per million of nitrogen, and this element is probably a limiting factor only at dilutions of 0.01 to 0.001 parts per million.
A biological succession involving the percentage composition of developmental stages of a single dominant species associated with lapse of time is illustrated in studies of the life cycle of Calanus finmarchicus in the relatively slowly flushed waters of the Clyde Sea area (p. 323) where in Loch Striven it was possible (Marshall, Nicholls, and Orr, 1934) to trace the successive developmental stages and broods of Calanus that occur during the seasons.