Investigating nutrient dynamics and macroalgal community structure in an eutrophic southern California estuary: Results of field monitoring and microcosm experiments

Macroalgal blooms caused by anthropogenic eutrophication are a pervasive problem impacting southern California estuaries. The objective of this dissertation research was to evaluate selected factors influencing nutrient dynamics and their interaction with macroalgal communities in southern California estuaries. Chapter 1 presents the results of quarterly field monitoring of spatial and temporal patterns in water column and sediment nutrients in Upper Newport Bay (UNB), Orange County, California, USA between December 1996 and March 1998. These data are compared to concurrently collected data on abundance and species diversity of the macroalgal community of UNB to examine temporal interactions of nutrient pools and macroalgal blooms in the estuary. Chapters 2 and 3 describe microcosm experiments testing the responses of the two dominant macroalgae in southern California estuaries Enteromorpha intestinalis (L.) Link and Ulva expansa (Setch) S. and G. to nutrient limitation and interspecific nutrient competition, respectively. Field monitoring data demonstrated that UNB is a highly eutrophic estuary in which nutrient dynamics vary both seasonally and between tidal creek and main channel habitats. Riverine influences dominated nutrient supplies during the winter and spring wet seasons, while internal nutrient recycling appears to affect nutrient availability to primary producers during summer and fall, particularly in tidal creeks. Such seasonal shifts may be an important factor influencing macroalgal blooms in this region. The nutrient limitation experiment indicated that summer algal blooms in UNB are N-limited, and mirrored patterns of increasing macroalgal biomass and corresponding declines of sediment TKN observed during field sampling. These data suggest that sediment nutrient flux is the likely mechanism allowing summer macroalgal blooms to persist when water column nitrogen is low. The microcosm experiment evaluating interspecific competition between E. intestinalis and U. expansa under low nitrogen loads suggests that pre-eutrophic conditions in southern California estuaries were characterized by low macroalgal biomass and intense N competition between algal species in summer, with E. intestinalis emerging as the competitive dominant. Comparing final macroalgal assemblages in experiment 1 and 2 indicates that U. expansa dominates competition between the two species under higher nitrogen loads, while E. intestinalis is the better competitor under lower N loading. Our results suggest that eutrophication of southern California estuaries has resulted in increases in macroalgal biomass and may have caused a shift in summer algal community structure from dominance by E. intestinalis to dominance by U. expansa. The management implications of our findings are also discussed.