Comparative ecophysiology of bloom-forming macroalgae in the Indian River Lagoon, Florida: Ulva lactuca, Hypnea musciformis, and Gracilaria tikvahiae

- Macroalgal blooms in the Indian River Lagoon (IRL) are influenced by nutrients.
- Differences among U. lactuca, H. musciformis, and G. tikvahiae were studied.
- Rapid light curves (RLCs) are short-term assessments of macroalgal health.
- Nutrient concentrations affect growth and photosynthesis more than N:P ratios.
- Opportunistic Ulva spp. often have the competitive edge under high nutrients.

Macroalgal blooms are ecological responses to nutrient enrichment in shallow seagrass-dominated estuaries. For decades the Indian River Lagoon (IRL) a biodiverse estuary in east-central Florida, has experienced persistent blooms of red drift macroalgae, including Gracilaria and Hypnea spp. Since 2013, extensive blooms of green macroalgae, such as Chaetomorpha and Ulva spp., have developed. To better understand IRL nutrient effects on bloom-forming macroalgae, field and laboratory studies (2012) assessed nitrogen (N) versus phosphorus (P) limitation and morphological/physiological characteristics in relatively urbanized (Titusville, FL) versus rural (Fort Pierce, FL) IRL segments. Field studies indicated Ulva lactuca, Hypnea musciformis, and Gracilaria tikvahiae all grew fastest in Titusville (average ± SD; 0.49 ± 0.07, 0.35 ± 0.03, and 0.14 ± 0.05 doublings d− 1, respectively). However, U. lactuca had the most rapid biomass doubling time (2 days). Laboratory nutrient enrichment assays revealed 3-fold increases in rapid light curve (RLC) maximum values and 2-fold faster growth at high concentrations of N and P for U. lactuca. This superior growth and photosynthesis was attributed to higher surface area:volume ratios averaging (± coefficients of variation, %) 565.2 ± 2.15 cm2 g dry wt.− 1 compared to lower ratios for H. musciformis (110.7 ± 3.97 cm2 g dry wt.− 1) and G. tikvahiae (91.1 ± 1.81 cm2 g dry wt.− 1). Finely- and coarsely-branched H. musciformis and G. tikvahiae were similar photosynthetically but not morphologically based on a functional/form model. These data provide a physiological basis explaining bloom distributions and the recent success of green macroalgae in the increasingly eutrophic IRL.