title: Temperature and residence time controls on an estuarine harmful algal bloom: Modeling hydrodynamics andAlexandrium fundyensein Nauset estuary.
abstract: A highly resolved, 3-d model of hydrodynamics andAlexandrium fundyensein an estuarine embayment has been developed to investigate the physical and biological controls on a recurrent harmful algal bloom. Nauset estuary on Cape Cod (MA, USA) consists of three salt ponds connected to the ocean through a shallow marsh and network of tidal channels. The model is evaluated using quantitative skill metrics against observations of physical and biological conditions during three spring blooms. TheA. fundyensemodel is based on prior model applications for the nearby Gulf of Maine, but notable modifications were made to be consistent with the Nauset observations. The dominant factors controlling theA. fundyensebloom in Nauset were the water temperature, which regulates organism growth rates, and the efficient retention of cells due to bathymetric constraints, stratification, and cell behavior (diel vertical migration). Spring-neap variability in exchange altered residence times, but for cell retention to be substantially longer than the cell doubling time required both active vertical migration and stratification that inhibits mixing of cells into the surface layer by wind and tidal currents. Unlike in the Gulf of Maine, the model results were relatively insensitive to cyst distributions or germination rates. Instead, in Nauset, high apparent rates of vegetative cell division by retained populations dictated bloom development. Cyst germination occurred earlier in the year than in the Gulf of Maine, suggesting that Nauset cysts have different controls on germination timing. The model results were relatively insensitive to nutrient concentrations, due to eutrophic conditions in the highly impacted estuary or due to limitations in the spatial and temporal resolution of nutrient sampling. Cell loss rates were inferred to be extremely low during the growth phase of the bloom, but increased rapidly during the final phase due to processes that remain uncertain. The validated model allows a quantitative assessment of the factors that contribute to the development of a recurrent harmful algal bloom and provides a framework for assessing similarly impacted coastal systems.
authors.affiliation: Woods Hole Oceanographic Institution, Applied Ocean Physics and Engineering Department, Woods Hole, Massachusetts, USA, 02543.
authors.forename: David K
authors.initials: DK
authors.lastname: Ralston
authors.affiliation: Woods Hole Oceanographic Institution, Biology Department, Woods Hole, Massachusetts.
authors.forename: Michael L
authors.initials: ML
authors.lastname: Brosnahan
authors.affiliation: National Park Service, Cape Cod National Seashore, Wellfleet, Massachusetts.
authors.forename: Sophia E
authors.initials: SE
authors.lastname: Fox
authors.affiliation: National Park Service, Cape Cod National Seashore, Wellfleet, Massachusetts.
authors.forename: Krista
authors.initials: K
authors.lastname: Lee
authors.affiliation: Woods Hole Oceanographic Institution, Biology Department, Woods Hole, Massachusetts.
authors.forename: Donald M
authors.initials: DM
authors.lastname: Anderson
keywords: Alexandrium fundyense; cyst germination; growing degree day; harmful algal bloom; hydrodynamic-biological model; residence time