Competition with macroalgae and benthic cyanobacterial mats limits phytoplankton abundance in experimental microcosms

Abstract

At high nutrient loading (nitrate-N = 77 μM d-1), growth of phytoplankton was reduced by a factor of 10 in the presence of algal mats. Without the algal mats phytoplankton was very abundant (>5×106 cells ml-1) and dominated by small flagellates, while in the presence of the algal mats the phytoplankton assemblage was sparse and diatoms, flagellates, and unicellular blue-greens were common. The competition hierarchy was cyanobacterial mats >> attached green macroalgae > floating green macroalgae > phytoplankton. When nutrient supply rate was low (nitrate-N = 1.2μM d-1), presence of the algal mats shifted the phytoplankton from flagellates to blue-green algae, but did not affect total biomass of the phytoplankton. The attached forms of macroalgae as well as the cyanobacterial mats were thus better competitors for high levels of nutrients than the phytoplankton. This resource competition may explain the negative correlation found in field studies between phytoplankton and macroalgae growing in shallow nutrient-enriched estuaries. -from Authors