Acoustical and optical measurements provide evidence of ecological interactions in planktonic thin layers.

Abstract

Coincident physical, optical, and acoustical data were obtained in Monterey Bay, CA during an extensive study of planktonic layers. These concurrent data describing the distributions of phytoplankton, zooplankton, and small fish as well as gradients in the physical habitat were used to examine the interactions between vertically compressed plankton structures (thin layers), their consumers, and the local physical forces they experience. The steepness of the vertical gradient on the top versus the bottom of the plankton layer was correlated to the difference in the relative abundance of consumers above and below the layer. Phytoplankton layer gradients were steeper when more zooplankton were present on one side of the layer versus the other while zooplankton layers were more diffuse with a greater number of fish adjacent to one side of the layer than the other. Both layer types showed nearly symmetrical gradients when predators were in low abundance or absent. Differences in phytoplankton and zooplankton layer “shape” were not correlated with vertical gradients in shear or mixing potential surrounding layers. The consequences of trophic interactions within aggregations can be detected and assessed, and in the absence of strong physical gradients, grazers can play an important role in structuring plankton thin layers.