THE SIGNIFICANCE OF OPTICAL-PROPERTIES IN COMPETITION AMONG VISUAL AND TACTILE PLANKTIVORES - A THEORETICAL-STUDY

A mathematical model describing competition for a common food resource among visual and tactile planktivores is developed. Sensitivity of th e competitive regime to environmental changes are studied by analysing their effects on foraging rates and niche availability of both plankt ivore types. Depth, spatial, daily and seasonal variations in factors of biotic and abiotic origin may result in great changes in the compet itive regime. Optical properties of the water column is of greatest im portance for competition. In addition the abundance and size of prey, small scale turbulence, and the seasonal and daily variations in sun i rradiance are of importance. Inherent contrast of prey organisms is li kely to modify the effect from the above factors under certain circums tances only, but may together with prey size, also enhance niche diffe rentiation among the competitors. By studying the zero growth rate iso clines for populations of tactile and visual planktivores at different depths, we show that the water column is a habitat divided into diffe rent competitive regimes. These can be thought of as relatively stable niches separated by an ecotone of rather unpredictable and ever chang ing competitive regime, due to changes in environmental factors. In mo st cases it is optimal for visual planktivores to stay at shallow dept hs, while tactile planktivores compete better at greater depths. Howev er, by simulations we show that high predation pressure can make near surface waters hostile to many visual planktivores such as fish, there by making this highly productive niche available to tactile planktivor es such as jellyfish. Results are discussed in relation to naturally o ccurring environmental variations and the impact of anthropogenic fact ors. Changes in optical properties due to eutrophication might have dr astic effects on the outcome of competition, and thus on the structure of marine food chains. While too little is known about the physiology , life history and behaviour of most pelagic organisms to make specifi c predictions, this work indicates a potential for strong competitive interactions among different functional types of planktivores. (C) 199 7 Elsevier Science B.V.