Salinity stress, nitrogen competition, and facilitation: what controls seasonal succession of two opportunistic green macroalgae?

Differential tolerance of low salinity, competition for nitrogen (N), and facilitation by altering N supply all may act to determine the pattern of seasonal succession of Enteromorpha intestinalis (L.) Link and Ulva expansa (Setch) S. and G. in estuaries and lagoons of southern California. Low salinity negatively affected both of these algae. However, when N was in sufficient supply, salinities of 15 ppt favored E. intestinalis while oceanic salinity (35 ppt) favored U. expansa; neither alga had a clear advantage at 25 ppt. When starved of N, E. intestinalis and U. expansa competed directly for nutrients. When grown alone, they had similar N uptake and growth rates; when grown together, E. intestinalis was the superior competitor, negatively affecting growth of U. expansa. In addition, U. expansa facilitated the growth of E. intestinalis when N was in short supply; when grown together, there was a positive effect of U. expansa on E. intestinalis. The mechanism of this effect may have been the release or ‘leaking’ of DON when U. expansa no longer had sufficient tissue N to grow. Thus, E. intestinalis would be favored immediately after a rain, but would be replaced by U. expansa when N is available and tidal action reestablishes oceanic salinity. However, at the end of the rainy season when N becomes scarce, E. intestinalis would outcompete U. expansa. We hypothesize that U. expansa may facilitate the dominance of E. intestinalis by leaking N that can be assimilated by E. intestinalis.