Patterns of ammonium uptake within dense mats of the filamentous macroalga Chaetomorpha linum

The effect of macroalgal uptake on the flux of ammonium across the sediment-water interface was tested in laboratory experiments in which dense mats of Chaetomorpha linum were incubated at high and low surface irradiances and were exposed to a high simulated sediment nutrient flux. Depth profiles of NH4+ concentrations within the 15-cm deep mars and the timing and magnitude of NH4+ efflux through the mats to the overlying water reflected differences in macroalgal uptake between the two light treatments. Patterns of algal productivity and NH4+ uptake with depth in the mats were determined from the accumulation of C-13 and N-15 in the algal tissue. Nitrogen-saturated macroalgae incubated at low irradiance exhibited a strong diel periodicity in NH4+ uptake that was not present in the N-limited macroalgae incubated at high irradiance. Assimilation by the macroalgal mat at high irradiance was approximately 900 NH4+ mu mol m(-2) h(-1), and was sufficient to prevent NH4+ diffusion from the benthic nutrient source into the overlying water during both the light and dark periods. Uptake of NH4+ in excess of the N growth demand in the lower half of the high-light mat resulted in a spatial separation of nutrient and light resources; NH4+ did not diffuse into the upper layers and the most photosynthetically-active macroalgae remained N-deficient. Reduced irradiance decreased the total uptake of the mat by more than 50% (400 NH4+ mu mol m(-2) h(-1)), and an efflux of NH4+ into the overlying water occurred in the dark and early part of the light period. Ammonium diffused through the unproductive bottom layers of the low-light mat and was incorporated primarily in the photic zone in the upper 4 cm of the mat where photosynthesis provided the carbon required for N uptake and assimilation. These results support the hypothesis that actively-growing macroalgal mats efficiently sequester benthic nutrient inputs to the overlying water and reduce nutrient availability to a level that may limit pelagic production. Factors that reduce irradiance within the mat, such as self-shading or decreased insolation, limit macroalgal uptake of benthic flux and result in a release of nutrients into the overlying water. (C) 1997 Elsevier Science B.V.