High amplitude tides that result in floating mats decouple algal distribution from patterns of recruitment and nutrient sources

Abstract

Three processes that may facilitate proliferation of Ulva intestinalis blooms in eutrophic estuaries include: rapid growth in response to nutrient enrichment, the capacity for biweekly sexual recruitment, and the ability to transition from attached benthic stages to floating mats. We conducted field surveys and in situ experiments to evaluate whether these 3 processes affect spatial and temporal distribution of U. intestinalis biomass within a eutrophic California (USA) estuary. We sampled environmental variables (sediment redox potential, organic content, total nitrogen, and total phosphorus; water column nitrate, ammonium, and dissolved organic nitrogen) and algal biomass and cover at 2 to 4 mo intervals over 15 mo at 3 permanent sites at the head, middle, and mouth of the estuary. Principal component analysis demonstrated significant yet weak relationships between macroalgal abundance and water and sediment N concentrations. Recruitment was abundant throughout the estuary (76.1 ± 22.8 spores cm −2 ), with little spatial pat- tern. Thus, we hypothesized that water flow may affect distribution by causing size-dependent detachment of benthic filaments. In 2 experiments on different substrata (tiles and sediment cores), 2 lengths of U. intestinalis filaments were exposed to accelerated and ambient water flow during flood tides. On both substrata, there was greater removal at high flow compared to ambient flow and greater removal of tall filaments. Pull tests confirmed that tall filaments required less force to break. Together, these experiments demonstrated that rapid water flow during high amplitude tides caused transitions between attached and floating stages and at least partially decoupled distribution from recruitment patterns and nutrient sources.