The interactive roles of nutrient loading and zooplankton grazing in facilitating the expansion of harmful algal blooms caused by the pelagophyte, Aureoumbra lagunensis, to the Indian River Lagoon, FL, USA

• Ammonium enrichment frequently enhanced the growth of Aureoumbra.
• Urea and ammonium accounted for 90% of N uptake within bloom populations.
• Aureoumbra-dominated plankton populations were enriched in isotopically lighter N compared to non-bloom populations.
• There was a near absence of grazing pressure on Aureoumbra but on other plankton.

Confined to Texas, USA, for more than 20 years, brown tides caused by Aureoumbra lagunensis emerged in the Indian River Lagoon and Mosquito Lagoon, Florida, USA, during 2012 and 2013, affording the opportunity to assess whether hypotheses developed regarding the occurrence of these blooms are ecosystem-specific. To examine the extent to which top-down (e.g. grazing) and bottom-up (e.g. nutrients) processes controlled the development of Aureoumbra blooms in Florida, nitrogen (N) uptake, nutrient amendment, and seawater-dilution, zooplankton grazing experiments were performed and the responses of Aureoumbra and competing phytoplankton were evaluated. During the study, Aureoumbra comprised up to 98% of total phytoplankton biomass, achieved cell densities exceeding 2 × 106 cells mL−1, and contained isotopically lighter N compared to non-bloom plankton populations, potentially reflecting the use of recycled N. Consistent with this hypothesis, N-isotope experiments revealed that urea and ammonium accounted for >90% of N uptake within bloom populations whereas nitrate was a primary N source for non-bloom populations. Low levels (10 μM) of experimental ammonium enrichment during blooms frequently enhanced the growth of Aureoumbra and resulted in the growth rates of Aureoumbra exceeding those of phycoerythrin-containing, but not phycocyanin-containing, cyanobacteria. A near absence of grazing pressure on Aureoumbra further enabled this species to out-grow other phytoplankton populations. Given this alga is generally known to resist zooplankton grazing under hypersaline conditions, these findings collectively suggest that moderate loading rates of reduced forms of nitrogenous nutrients (e.g ammonium, urea) into other subtropical, hypersaline lagoons could make them susceptible to future brown tides caused by Aureoumbra.