Bioenergetics of Vertical Migration

The distribution of light-orienting populations of consumers can be analyzed quantitatively by determining the distribution of food energy within the frame of reference used for orientation and comparing this with the observed distribution of the foraging population. The amount of daily gross productivity is partitioned into different light ranges, the amount available for consumption in each range being dependent upon the range's daily excursion. Curves of potential food energy can then be constructed for each light range in the vertical column of water. Regions of the orienting stimulus possessing greater potential food would have greater selective value. The method, pathway analysis, was then applied in the analysis of two natural situations in the Gulf of Maine. In addition to further substantiating the belief that vertical migration represents an unavoidable correlate of the use of light as a frame of reference (Harris 1953), this method produced some interesting new conclusions: 1. If light-orienting consumers predominate in an area and are adapted to different light ranges in a vertical column of water, if the ecosystem is at equilibrium, and if a stable age structure exists within each portion of a population adapted to different light ranges, then the biomass or number of consumers found within each light range is roughly proportional to the daily energy input, in productivity, associated with that range. 2. Although productivity may decrease regularly with light intensity, the energy available to consumers adapted to different light levels exhibits a distinct bimodality, peaks (about 5 x 103 and 10-3 μ W/square centimeter) occurring at the positions of isolumes which spend the maximum amount of time within the region of highest productivity. 3. Three modes of exploitation of productivity in the photic zone may be utilized by a light-orienting consumer population: balanced, diurnal, or nocturnal. 4. In a comparison between two stations in the Gulf of Maine, a shallow bottom at one station was found to force a change from nocturnal exploitation to a balanced mode of exploitation in a foraging population of Calanus finmarchicus, a marine copepod. 5. A suggestion is also offered that light orientation is commonly used as a frame of reference for consumer populations in the higher latitudes of the sea (above 40⚬N and 40⚬S) because of the seasonal constancy of food input associated with light orientation and nocturnal exploitation. Although total productivity decreases during winter, nighttime increases and thereby allows nocturnal exploiters more time to feed, stabilizing their annual food input. This competitive advantage would explain the high frequency of pronounced vertical migration observed among marine species of this region.