Temperature and Residence Time Controls on an Estuarine Harmful Algal Bloom: Modeling Hydrodynamics and Alexandrium fundyense in Nauset Estuary

Abstract

A highly resolved, 3-d model of hydrodynamics and Alexandrium fundyense in 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. The A. fundyense model 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 the A. fundyense bloom 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.

Ocean warming since 1982 has expanded the niche of toxic algal blooms in the North Atlantic and North Pacific oceans

“…Observational studies in the NW Atlantic have described shifts in A. fundyense bloom dynamics and duration that are related to interannual temperature variability (6,7,22). A compilation of A. fundyense bloom dynamics from the Bay of Fundy indicated that, consistent with our findings, the first appearance of A. fundyense cells was, on average, 3 wk earlier in the first decade of this century compared with the final 12 y of the 20th century and that maximum cell densities during blooms increased more than threefold over the same time period (23).…”

“…Further, this significant correlation provides quantitative evidence that the modeled bloom season presented by this study accurately tracked the dynamics of Alexandrium populations in this region. Beyond the effects on the growth of vegetative cells, warming temperatures will also promote an earlier emergence of cysts that provide the inoculum for A. fundyense blooms (22,24).…”

Zooplankton community grazing impact on a toxic bloom of Alexandrium fundyense in the Nauset Marsh System, Cape Cod, Massachusetts, USA

“…However, high-frequency, in situ observations using the IFCB in Salt Pond (Brosnahan et al, submitted) show net population growth rates that were in excess of those expected from culture-derived growth rates. This discrepancy is under active investigation, but highlights the difficulties and uncertainties in calculating growth rates from a limited number of samples (weekly) during a bloom in a tidal embayment, even ones as retentive as Salt Pond and Mill Pond (Ralston et al, in press). …”

“…Dinoflagellates form subsurface aggregations and are selectively retained relative to the tidal exchange in the NMS salt ponds due to avoidance of highly illuminated surface waters by diel vertical migration (Anderson and Stolzenbach, 1985), and the bathymetric constraints on flow and suppression of mixing by stratification (Ralston et al, in press). Thus, the NMS provided an opportunity to study population dynamics over the course of a natural A. fundyense bloom in a single location where physical advection of dinoflagellate populations was relatively minimal.…”

Residence Time of a Highly Urbanized Estuary: Jamaica Bay, New York

“…Transport time scales, such as residence time-the time taken by a water parcel or its constituents to leave a defined region of interest through its outlets to the sea [1,2], or flushing time-a bulk or integrative parameter that describes the general exchange characteristics of a waterbody without its spatial distribution [3]-are usually used as first-order metrics of transport to assess the health and integrity of estuarine and coastal ecosystems. For example, the concept of residence time has been used to study pollutant concentrations [4], distributions of plankton [5,6], harmful algal blooms [7,8], pelagic bacteria [9], and dissolved nutrients [10,11].…”