ABSTRACT. Nutrient enrichment experiments were conducted to assess the relationship between nutrient concentration in the tissue of the macroalga Enteromorpha sp. and nutrient history. Experimental units were outdoor microcosms containing mixed assemblages of algae representative of communities commonly found in coastal lagoons of southern California, USA. We determined the relationship between nutrient supply rates (nitrogen and phosphorus as well as N:P ratio), water-column nutrient concentrations, and nlacroalgal tissue nutrient concentration in samples taken from experimental microcosms containing macroalgae, phytoplankton, and cyanobacterial mats, and treated with 13 combinations of mtrogen and phosphorus addition. The regression coefficients that descnbed the fit of the relationships between tissue P and both P supply rate and water-column concentration were greater than for the same relationships for N. The relationship between tissue nutrients and either measure of nutrient history (water-column concentration or supply rate) was most useful when the nutrient was not limiting.

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.