Drivers of phytoplankton dynamics in a subestuary exhibiting lagging ecosystem recovery
Increased nutrient loading to aquatic systems, a pervasive phenomenon resulting from human activities, has triggered or exacerbated phytoplankton blooms globally, leading to cascading negative effects including hypoxia/anoxia, habitat loss, and changes in the structure and function of food webs. Tampa Bay, FL (USA), an estuary that was declared “dead” in the 1970’s, provides a notable success story of the reversal of eutrophication. With decreases in nutrient loading and increases in habitat, the bay now provides ecosystem and economic services estimated at 55 billion dollars annually. However, the recovery of the northwestern segment of the estuary - Old Tampa Bay - has notably lagged that of other bay segments. Compared to other regions, Old Tampa Bay is characterized by poorer water quality, a more heavily urbanized watershed, and longer residence times, in part due to bisecting causeways that restrict water exchange. In this region, spring and summer blooms of diatoms, picoplankton, and the toxic dinoflagellate Pyrodinium bahamense commonly occur. In this study, we couple discrete and continuous sampling with laboratory experiments focused on the cyst dynamics of P. bahamense to characterize the spatiotemporal variability of phytoplankton dynamics and identify drivers of blooms that may be preventing bay-wide recovery.
Between January 2012 and October 2014, there were predictable peaks in chlorophyll concentrations between the spring and fall of each year, consistent with the timing of Pyrodinium bahamense blooms. There were also recurring patterns of community succession characterized by taxa changes preceding and succeeding blooms of P. bahamense. Overall, freshwater pulses resulted in increases in phytoplankton biomass on both short- (i.e., days to weeks) and long-term (i.e., seasonal) scales. Multivariate analyses showed that distinct phytoplankton communities were associated with decreases in salinity and increases in nutrient concentrations. Laboratory experiments revealed that P. bahamense cysts are able to germinate over a wide range of conditions, but that lag time to germination is affected by changes in environmental parameters, including temperature and light. Moreover, field observations revealed a strong link between cyst distribution in surface sediments and location of bloom initiation, highlighting the role of benthic-water column interactions. Our results suggest restoration strategies that consider cyst dynamics (e.g., the development and expansion of cyst beds) and continue to target nutrient loading associated with freshwater inflows may improve recovery of Old Tampa Bay through mitigation of nuisance phytoplankton blooms.