Drivers of phytoplankton dynamics in old Tampa Bay, FL (USA), a subestuary lagging in ecosystem recovery

- Seasonal runoff explains ∼24% of the variance in Pyrodinium bahamense abundance.
- Nutrient reduction strategies may mitigate seasonal phytoplankton blooms, but not necessarily those of P. bahamense.
- Phytoplankton community assemblages group on N-S and W-E axes within Old Tampa Bay.

In the past four decades, consistent and coordinated management actions led to the recovery of Tampa Bay, FL (USA) - an estuary that was declared dead in the 1970s. An exception to this success story is Old Tampa Bay, the northernmost subestuary of the system. Compared to the other bay segments, Old Tampa Bay is characterized by poorer water quality and spring and summer blooms of cyanobacteria, picoplankton, diatoms, and the saxitoxin-producing dinoflagellate Pyrodinium bahamense. Together, these blooms contribute to light attenuation and lagging recovery of seagrass beds. Yet, studies of phytoplankton dynamics within Old Tampa Bay have been limited - both in number and in their spatiotemporal resolution. In this study, we used field sampling and continuous monitoring to (1) characterize temporal and spatial variability in phytoplankton biomass and community composition and (2) identify key drivers of the different phytoplankton blooms in Old Tampa Bay. Overall, temporal variability in phytoplankton biomass (using chlorophyll a as a proxy) and community composition surpassed spatial variability of these parameters. We found a base community of small diatoms and flagellates, as well as certain dinoflagellates, that persisted year round in the system. Seasonally, freshwater runoff stimulated phytoplankton growth, specifically that of chlorophytes, cyanobacteria and other dinoflagellates -- consistent with predictions based on ecological theory. On shorter time scales, salinity, visibility, and freshwater inflows were important predictors of phytoplankton biomass. With respect to P. bahamense, environmental drivers including salinity, temperature and dissolved nutrient concentrations explained ∼24% of the variability in cell abundance, indicating missing explanatory parameters in our study for this taxon, such as cyst density and location of cyst beds. Spatially, we found differences in community trajectories across north-south and west-east gradients, with the northernmost sampling station being the most unique in the region. This work contributes to the knowledge of phytoplankton biomass and community composition in Tampa Bay by generating spatially and temporally rich phytoplankton community and environmental data for the Old Tampa Bay subestuary. Moreover, it enhances our understanding of bloom drivers and provides recommendations for ecosystem management. Specifically, our findings support continued nutrient reduction measures as a way to mitigate seasonal blooms of diatoms, cyanobacteria and chlorophytes, but not necessarily blooms of P. bahamense. Prediction and mitigation of P. bahamnese blooms should incorporate first order drivers such as cyst location and abundance.

Results of the canonical correspondence analysis (CCA) conducted using environmental and phytoplankton community composition data. Groupings show (1) a base community of small diatoms and flagellates as well as certain dinoflagellates (center, grey); (2) taxa associated with runoff (right, green); and (3) larger diatoms associated with greater salinities and lower nutrient concentrations (left, blue). Taxa abbreviations are as follows: Bid, Biddulphiineae; Cer, Ceratium spp.; Cha, Chaetocerotaceae; Chlor, chlorophytes; Chr, chrysophytes; Coc, coccolithophores; Cryp, cryptophytes; Cyan, cyanobacteria; Cyl, Cylindrotheca spp.; Dinp, Dinophysis spp.; Eug, euglenoids; Flag, unidentified flagellates; Fra, Fragilariophyceae; Gony, Gonyaulax spp.; Gym, gymnodinioids; Hap, haptophytes; Karl, Karlodinium veneficum; Lep, Leptocylindraeceae; Mes, Mesodinium rubrum; OthDinf, other unidentified dinoflagellates; Per, peridinioids; Polyk, polykrikoids; Pras, prasinophytes; Pro, prorocentroids; Psn, Pseudo-nitzschia spp.; Pyro, Pyrodinium bahamense; Rhi, Rhizosoleniineae; and Thl, Thalassiosiraceae. Nutrient abbreviations are as follows: P, total dissolved phosphorus; Si, dissolved silica; N, total nitrogen; N:P total nitrogen to total dissolved phosphorus; Si:N, dissolved silica to total nitrogen.173