HOW DOES WATER COLUMN STRUCTURE INFLUENCE COPEPOD POPULATIONS IN COASTAL MARINE SYSTEMS

The influence of water column structure on zooplankton in coastal mari ne ecosystems was experimentally examined using enclosures (5 m deep; 13 . m3 volume) containing both water column and benthos. The test inv olved a comparison of well-mixed versus stratified water columns. In t he stratified system a salinity difference between surface and bottom waters and chilling of the lower layer maintained a very stable pycnoc line at 2.5-m depth. Total turbulent energy input into each system was the same and temperature in the upper mixed layer of the stratified s ystem matched that in the well-mixed system. The manipulation resulted in dramatic changes in the zooplankton community: copepods became muc h more abundant in the stratified system, steadily increasing their nu mbers despite a relatively low biomass of phytoplankton. Increases wer e seen for all species but dominance switched from Acartia tonsa in th e well-mixed system to Oithona colcarva in the stratified. The interes ting question is to determine what factors favored copepods in the str atified water column. Phytoplankton, while less abundant in the strati fied system, may have been either more nutritious or more available th an in the well-mixed system. The pycnocline also acted to isolate the water column from any predators in the benthos, possibly reducing mort ality rates in the stratified system.