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Dense mats of Chaetomorpha Iinum were incubated in the laboratory at low and high surface irradiance and were enriched by a simulated sediment nutrient flux. Algal activity resulted in marked diurnal variations and steep vertical gradients In O2 and NH,' concentration profiles within the mats. In the light, O2 production caused supersaturation in the surface layers, and algal assimilation significantly reduced the flux of nutrients to the water column. The depth gradients of decreasing light and increasing nutrient availability within the mat were reflected in the algal tissue composition. At h ~ g h surface irradiance, chlorophyll concentrations increased towards the bottom of the mat and C/N ratios gradually declined. This pattern suggested light limitation in the bottom of the mat and progressive N limitation towards the mat surface. 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 was high, and comparisons to communities of other benthic macrophytes, benthic m~croalgae, and phytoplankton demonstrated a general pattern of increasing volume-specific productivity at decreasing extension of the photic zone, whereas the area productivity (depth-integrated) of the different plant communities is remarkably uniform. As algal density and selfshading increases, the algal mats can switch from being net productive to a status where consumption exceeds production. Reduced irradiance and increased water temperature may also trigger this shift, and the resulting effects on 0, and nutnent balances make shallow macroalgal-dominated systems inherently unstable.

Production within dense mats of the filamentous macroalga Chaetomorpha linum in relation to light and nutrient availability