Effects of green macroalgal blooms on intertidal sediments: net metabolism and carbon and nitrogen contents

Abstract

Eutrophication in coastal areas promotes blooms of green macroalgae that accumulate on the sediment, affecting the exchange of mass and energy at the sediment–water interface. The effects of macroalgal blooms on the microbenthic net metabolism and on the carbon and nitrogen contents of the sediment were studied during an in situ experiment. Two sediment enclosures (with and without macroalgae) of 1.5 × 1.5 m2 were installed and maintained 2 to 3 wk during every season on an intertidal sediment flat in the Sancti Petri Channel (Bay of Cádiz, Spain) from summer 2002 to spring 2003. Biomass of the macroalgae changed seasonally, with a minimum in winter and a maximum in summer (annual mean: 188.3 ± 97.3 g dry wt m–2). This relatively low macroalgal biomass always suppressed the photosynthetic activity of microphytobenthos, decreased the oxygen availability for the sediment and reduced the oxygen penetration depth in the light and in the dark. The microbenthic community metabolism clearly shifted to heterotrophic, while the photoautotrophic activity was located in the macroalgal canopy. This macroalgal mat was net autotrophic, and more productive than the microphytobenthic community inhabiting the bare sediment. Part of the macroalgal production was buried in the sediment, increasing its carbon and nitrogen contents with respect to bare sediment during all seasons except spring. Concentrations of inorganic nutrients in the sediment were always higher below the macroalgal mat than in bare sediment, likely due to a direct input of inorganic nitrogen and phosphate associated with macroalgal debris.