Advective relief of CO2 limitation in microphytobenthos in highly productive sandy sediments

Following field observations of increased photosynthesis at increased rates of sediment flushing in sandy sediments, we conducted a series of laboratory experiments to elucidate the mechanism behind these observations. Column experiments in which water was pumped though sand at rates ranging from 0 to 613 L m^-2 d^-1 showed that carbon (C) fixation, as measured using carbon-14 (14C) incorporation, increased from 6.4 to 8.6 mmol m^-2 h^-1 with increasing rates of flushing. Bottle incubations showed that the addition of inorganic nutrients [ammonium ion (NH4+), inorganic phosphate (HPO4-), silicic acid Si (OH)4] did not stimulate C fixation over short-term incubations. Microprofiles of pH showed that the pH within the photic zone increased to 8.9, reducing free carbon dioxide (CO2) concentrations to ~0.5 mol L^-1. Further bottle incubations, where pH and total inorganic carbon (TCO2) were manipulated, showed that high pH (9.6) did not affect photosynthesis if free CO2 was present at concentrations of 10 (mu)mol L^-1, suggesting a direct effect of low free CO2 concentrations. 14C fixation profiles at a resolution of 100 (mu)m recorded by (beta)-radiation imaging showed that while the depth specific maximum rates of C fixation were the same under both diffusive and advective (flushed) conditions, the integrated rates of photosynthesis were highest under flushed conditions because of a thickening of the photosynthetic zone. We conclude that advective pore-water transport can enhance benthic photosynthesis in shallow permeable sand sediments by counteracting CO2 limitation.