SALINITY STRESS, NITROGEN COMPETITION, AND FACILITATION WHAT CONTROLSSEASONAL SUCCESSION OF 2 OPPORTUNISTIC GREEN MACROALGAE

Differential tolerance of low salinity, competition for nitrogen (N), and facilitation by altering N supply all may act to determine the pat tern of seasonal succession of Enteromorpha intestinalis (L.) Link and Ulva expansa (Setch) S. and G. in estuaries and lagoons of southern C alifornia. Low salinity negatively affected both of these algae. Howev er, when N was in sufficient supply, salinities of 15 ppt favored E. i ntestinalis while oceanic salinity (35 ppt) favored U. expansa; neithe r alga had a clear advantage at 25 ppt. When starved of N, E. intestin alis and U. expansa competed directly for nutrients. When grown alone, they had similar N uptake and growth rates; when grown together, E. i ntestinalis was the superior competitor, negatively affecting growth o f U. expansa. In addition, U. expansa facilitated the growth of E. int estinalis when N was in short supply; when grown together, there was a positive effect of U. expansa on E. intestinalis. The mechanism of th is effect may have been the release or 'leaking' of DON when U. expans a no longer had sufficient tissue N to grow. Thus, E. intestinalis wou ld be favored immediately after a rain, but would be replaced by U. ex pansa when N is available and tidal action reestablishes oceanic salin ity. However, at the end of the rainy season when N becomes scarce, E. intestinalis would outcompete U. expansa. We hypothesize that U. expa nsa may facilitate the dominance of E. intestinalis by leaking N that can be assimilated by E. intestinalis.