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 experimental microco sms. At high nutrient loading (nitrate-N = 77 muM d-1), the growth of phytoplankton was reduced by a factor of 10 in the presence of the alg al mats. Without the algal mats the phytoplankton was very abundant (> 5 x 10(6) cells ml-1) and dominated by small flagellates, while in th e presence of the algal mats the phytoplankton assemblage was sparse a nd diatoms, flagellates, and unicellular blue-greens were common. The competition hierarchy was cyanobacterial mats much greater than attach ed green macroalgae > floating green macroalgae > phytoplankton. When nutrient supply rate was low (nitrate-N = 1.2 muM d-1), the presence o f the algal mats shifted the phytoplankton from flagellates to blue-gr een algae, but did not affect total biomass of the phytoplankton. We c onclude that the attached forms of macroalgae as well as the cyanobact erial mats were better competitors for high levels of nutrients than t he phytoplankton. This resource competition may explain the negative c orrelation found in field studies between phytoplankton and macroalgae growing in shallow nutrient-enriched estuaries.