PRODUCTION WITHIN DENSE MATS OF THE FILAMENTOUS MACROALGA CHAETOMORPHA-LINUM IN RELATION TO LIGHT AND NUTRIENT AVAILABILITY

Dense mats of Chaetomorpha linum were incubated in the laboratory at l ow and high surface irradiance and were enriched by a simulated sedime nt nutrient flux. Algal activity resulted in marked diurnal variations and steep vertical gradients in O-2 and NH4+ concentration profiles w ithin the mats. In the light, O-2 production caused supersaturation in the surface layers, and algal assimilation significantly reduced the flux of nutrients to the water column. The depth gradients of decreasi ng light and increasing nutrient availability within the mat were refl ected in the algal tissue composition. At high surface irradiance, chl orophyll concentrations increased towards the bottom of the mat and C/ N ratios gradually declined. This pattern suggested light Limitation i n the bottom of the mat and progressive N limitation towards the mat s urface. Algal productivity declined with depth in the mats, reflecting a pronounced self-shading, and the photic zone (i.e. the depth of 1% surface irradiance) was only 8 cm deep. Productivity per unit volume w as high, and comparisons to communities of other benthic macrophytes, benthic microalgae, and phytoplankton demonstrated a general pattern o f increasing volume-specific productivity at decreasing extension of t he photic zone, whereas the area productivity (depth-integrated) of th e different plant communities is remarkably uniform. As algal density and self-shading increases, the algal mats can switch from being net p roductive to a status where consumption exceeds production. Reduced ir radiance and increased water temperature may also trigger this shift, and the resulting effects on O-2 and nutrient balances make shallow ma croalgal-dominated systems inherently unstable.