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Upper Newport Bay estuary, Orange County, California, US, was added to the Clean Water Act 303(d) list for nutrients, due in part to the excessive growth of macroalgae during certain seasons. In 1998, a Total Maximum Daily Load for nitrogen and phosphorus was adopted for San Diego Creek, the largest tributary to the estuary. As part of the implementation of the TMDL, the N and P water quality objectives were evaluated. Insufficient data were available to determine whether they were appropriate; therefore, mechanistic studies investigating the processes in UNB that control macroalgal biomass were needed. We 1) investigated the contribution of nutrients from estuarine sediments to macroalgal growth and tissue nutrient content; 2) determined if N or P is the nutrient most limiting to macroalgae; 3) measured rates of N and P uptake by Enteromorpha intestinalis and Ulva expansa, the dominant, green, bloomforming macroalgal species; and 4) investigated the effect!

s of variation in the frequency and concentration of nutrient pulses on macroalgal growth and tissue nutrient content. Sediments were an important source of nutrients to macroalgae, particularly where water column nutrient availability was low. N was the nutrient most limiting growth of macroalgae from UNB and P was the next most limiting nutrient after N. Rates of nutrient uptake varied with external substrate concentration, initial tissue nutrient status, and phase of nutrient uptake. E. intestinalis grew most with daily additions of nutrients but also took up and stored nutrients from the episodic pulses to maintain growth in a low nutrient environment for up to 28 d. This work furthers our understanding of nutrientmacroalgal dynamics in a southern California estuary.