Macroalgal bloom dynamics in a highly eutrophic southern California estuary

A 16-mo long monitoring study was carried out in Upper Newport Bay estuary (UNB), Orange County, California, to quantify the macroalgal community of a southern California estuary. Quarterly sampling began December 1996 at 8 stations along the main channel and tidal creeks ranging from the head to the lower end of UNB. At each station, two strata (one at high and one at low elevation) were surveyed. Macroalgal species abundance (% cover and biomass) and algal tissue nitrogen (N) and phosphorus (P) were measured. The algal community changed from sparse macroalgal cover during winter 1996 to larger patches dominated by Enteromorpha intestinalis in spring 1997. The community was characterized by a thick cover of macroalgae comprised of E. intestinalis and Ulva expansa in summer 1997 and U. expansa and Ceramium spp. in fall 1997. UNB returned to sparse macroalgal cover by spring 1998. In summer and fall 1997, biomass of E. intestinalis and Ceramium reached over 1,000 g wet wt m(-2) each, and U. expansa biomass exceeded 700 g wet Wt m(-2). Tissue N was high in E. intestinalis and U. expansa collected from UNB (similar to3% dry wt) and higher in Ceramium (similar to3.5% dry wt). Tissue P in all three algae ranged from 0.24-0.28% dry wt. Tissue N:P (molar) ratios in E. intestinalis and U. expansa ranged from 16.4 to 30.0 and in Ceramium from 21.8 to 40.1. A field experiment was conducted in which E. intestinalis was used as a bioassay of N and P availability. Algal tissue was cultured under known conditions and samples were deployed throughout the estuary and left for 24 h. Tissue N of algae from these bags showed a nominal increase in N with proximity to the primary nutrient input to the system, San Diego Creek (p=0.0251; r(2)=0.200). Our data indicate that UNB is already a highly eutrophic estuary, but macroalgal blooms in UNB may increase if more N is added to the system.