Blooms of Margalefidinium (formerly Cochlodinium) polykrikoides and Alexandrium monilatum occur most years in the late summer in lower Chesapeake Bay . Annual variations in rainfall, temperature, wind, salinity, nutrient levels, cyst densities and other environmental parameters determine whether there will be blooms and affect the timing, density and distribution of blooms when they occur. Reported i mpacts of M. polykrikoides and A. monilatum blooms in the lower Chesapeake Bay have varied with year and location. L ocal shellfish aquaculturists have reported oyster mortalities during and immediately following these blooms during some years and this prompted us to conduct field studies with juvenile oysters examining the impacts of blooms and other stressors using different grow-out strategies at aquaculture sites with differing physical characteristics with an aim toward identifying mitigation strategies. Generally, m ortality was higher for oysters grown intertidally than for those grown subtidally and higher at bloom-impacted low energy sites compared to sites with little or no bloom activity . In addition, during two bloom seasons growth rates slowed during the blooms (Figure).
Ecosystem level impacts of these late summer blooms were also observed. Water samples were collected weekly in and outside of bloom patches to examine impacts on nutrient cycling and microbial community composition. The bloom of M. polykrikoides was accompanied by high production of dissolved organic carbon (DOC) , while DOC was drawn down to low concentrations during the A. monilatum bloom. The DOC produced by M. polykrikoides may have supported the A. monilatum bloom, which occurred after the M. polykrikoides bloom, either via osmotrophy or by release of nutrients via microbial decomposition. Bloom samples of both species had altered microbiomes compared to non-blooms samples. In addition, There was a selective group of prokaryotes found in the particle-associated portion of the microbiome that was only found when A. monilatum was present. Blooms of these two algal species impacted the estuarine microbiome in different ways, likely leading to shifts in estuarine carbon and nutrient cycling, with M. polykrikoides having a greater potential impact on the overall functioning of the estuarine ecosystem than A. monilatum.