Spatial and temporal patterns in sediment and water column nutrients in a eutrophic southern California estuary

Quarterly field sampling was conducted to characterize variations in water column and sediment nutrients in a eutrophic southern California estuary with a history of frequent macroalgal blooms. Water column and sediment nutrient measures demonstrated that Upper Newport Bay (UNB) is a highly enriched estuary. High nitrate (NO3-) loads from the river entered the estuary at all sampling times with a rainy season (winter) maximum estimated at 2,419 mol h(-1). This resulted in water NO3- concentration in the estuary near the river mouth at least one order of magnitude above all other sampling locations during every season; maximum mean water NO3- concentration was 800 muM during spring 1997. Phosphorus (P)-loading was high year round (5.7-90.4 mol h(-1)) with no seasonal pattern. Sediment nitrogen (N)-content showed a seasonal pattern with a spring maximum declining through fall. Sediment and water nutrients, as well as percent cover of three dominant macroalgae, varied between the main channel and tidal creeks. During all seasons, water column NO3- concentrations were higher in the main channel than in tidal creeks while tidal creeks had higher levels of sediment total Kjeldahl nitrogen (TKN) and P. During each of the four sampling periods, percent cover of Enteromorpha intestinalis and Ceramium spp. was higher in tidal creeks than in the main channel, while percent cover of Ulva expansa was always higher in the main channel. Decreases in sediment N in both creek and channel habitats were concurrent with increases in macroalgal cover, possibly reflecting use of stored sediment TKN by macroalgae. Our data suggest a shift in primary nutrient sources for macroalgae in UNB from riverine input during winter and spring to recycling from sediments during summer and fall.