The lives of the plants, animals and bacteria of the plankton are ruled by the diffusive nature of their fluid medium, in which vertical turbulence is sometimes checked by density gradients caused by surface heating or inflow of freshwater. Seasonal and spatial changes in the production and species composition of phytoplankton and associated microheterotrophs result from interactions amongst light penetration, supply of mineral nutrients, and vertical mixing; many species are capable of rapid increase when multiplication exceeds losses by dispersion and predation. Water depth, currents, dispersion and food supplies control mesozooplankton.
Although Scottish coastal waters include a variety of pelagic environments, there is little evidence of site-specific flora or fauna. Planktonic assemblages are, however, locally more diverse than expected. If this excess diversity is a result of ever-changing conditions, the composition of microplanktonic associations may be largely decided by chance. Explanations based on niche theory seem to apply convincingly only to the larger, semi-nektonic, pelagic crustaceans. Thus, consideration of issues relating to conservation raises fundamental and difficult questions about the biology of plankton.
Conservation of species is impractical if accident plays a large part in recruitment and replacement. Conversely, the dispersive nature of the pelagic environment and the weakness of interspecific relations may lessen the sensitivity of plankton to anthropogenic perturbations. Nevertheless, eutrophication and climatic change can disturb pelagic communities, especially in enclosed waters.