Dimension effects of enclosures on ecological processes in pelagic systems

Several characteristics inherent to experimental ecosystems were examined for their influence on ecological processes in five cylindrical indoor benthic-pelagic enclosures of different size and shape. Ecosystem development diverged significantly among the mesocosm dimensions even though environmental parameters such as surface photosynthetically available radiation (400-700 nm), turbulence intensity, water exchange rate, and nutrient input were held constant across systems. Here we show that factors that lead to the development of different plankton assemblages can be related to mesocosm geometry. The ratio of light-receiving surface area-to-water column volume (A(s) : V) was shown to control both the rate of NO₃/^- consumption and gross primary productivity. The attenuation of water column irradiance was positively correlated with the wall area-to-volume ratio (A(w) : V). This was manifested in greater light attenuation in systems with a high A(w) : V ratio. In addition, notably greater microalgal biomass developed on the walls in systems with a high A(w) : V ratio. Finally, the total surface area-to-volume ratio (A(t) : V) of the mesocosms influenced the rate of energy gain and dissipation and water column temperature. The differences in temperature among dimensions possibly affected biological parameters such as bacterial biomass. Although the influence of area-to-volume effects on biological components in artificial systems may be substantial, our analysis indicates that some of these effects may be predictable and that future experiments can be explicitly designed to minimize artifacts of enclosure.