Five southern California estuaries were investigated to evaluate whether they were supporting the key function of nutrient cycling and retention by (1) quantifying spatial and temporal patterns of water column and sediment nutrients, sediment characteristics and macroalgal biomass; (2) investigating the effects of water flow on size-dependent removal of macroalgal filaments in order to assess an ontogenetic shift of habitat usage that may contribute to estuarine function; and (3) quantifying nutrient uptake rates and storage capacities in two dominant green macroalgae.

Comparisons were made within and between, Carpinteria Salt Marsh Reserve, two arms of Mugu Lagoon, Upper Newport Bay and Tijuana River Estuary. We sampled environmental variables (sediment redox potential, organic content, total nitrogen and total phosphorus, and water column nitrate, ammonium and total Kjeldahl nitrogen) and macroalgal biomass over fourteen months at the head, middle and mouth of each estuary. Our results confirmed that three systems were not processing available N, and two systems retained nutrient cycling capabilities. All five estuaries were nutrient enriched and eutrophic with macroalgal biomass not restricted to any season or site. Biomass was not related to water column or sediment nutrients, except in Carpinteria. In situ experiments indicated that recruitment was abundant throughout the estuary and not related to adult biomass. Accelerated water flow caused juvenile filaments to be removed from the substrate, facilitating an ontogenetic habitat expansion from intertidal to include subtidal regions of the estuary.

In a 24-hour experiment, we quantified the effects of initial macroalgal tissue nitrogen status and varying pulses of nitrate concentration on uptake and storage of nitrogen in Ulva intestinalis (L.) and Ulva expansa (L.). Both species of algae exhibited a high affinity for NO₃^- across all N-pulses and initial tissue contents. Maximum tissue NO₃^- concentration for U. expansa was five times that of U. intestinalis, suggesting that U. expansa has a higher storage capacity. These results showed opportunistic algae are highly adapted to estuaries with high levels of pulsed nutrients and are key players in supporting estuarine functioning. These systems have been severely impacted by anthropogenic nutrient enrichment and some are missing key functions of nutrient cycling and retention.