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

Competition for nutrients among estuarine phytoplankton and algal mats (a combination of floating and attached green macroalgae and attached cyanobacterial mats) was studied using replicate expenmental microcosms. At high nutnent loading (nitrate-N = 77 F M d -l ) , the growth of phytoplankton was reduced by a factor of 10 in the presence of the algal mats. Without the algal mats the phytoplankton was very abundant (> 5 X 106 cells rill-l) 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 = 1.2 pM d-l), the presence of the algal mats shifted the phytoplankton from flagellates to blue-green algae, but did not affect total biomass of the phytoplankton. We conclude that the attached forms of macroalgae as well as the cyanobacterial mats were better competitors for high levels of nutrients than the phytoplankton. This resource competition may explain the negative correlation found in field studes between phytoplankton and macroalgae growing in shallow nutrient-enriched estuaries.