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Temperature and residence time controls on an estuarine harmful algal bloom: Modeling hydrodynamics and Alexandrium fundyense in Nauset estuary - PubMed

  • ️Thu Jan 01 2015

Temperature and residence time controls on an estuarine harmful algal bloom: Modeling hydrodynamics and Alexandrium fundyense in Nauset estuary

David K Ralston et al. Estuaries Coast. 2015.

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.

Keywords: Alexandrium fundyense; cyst germination; growing degree day; harmful algal bloom; hydrodynamic-biological model; residence time.

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Figures

Figure 1
Figure 1

Nauset estuary location, bathymetry, and model grid detail. Sampling stations from the marsh-wide surveys are shown on the main map (gray circles), with the larger dots showing the locations of thetime series in Fig. 7. A detail of Salt Pond illustrates the model grid resolution, and also shows thelocations of transects A-A’ and B-B’ for field observations (casts at gray circles) and model results (redline) shown in Figs. 5 and 8. Note that the model bathymetry extends above mean higher high water toallow wetting and drying of the marsh, while plots of interpolated observations (Fig. 11) are shown withthe coastline as the mean sea level contour to demark the navigable channels.

Figure 2
Figure 2

Comparison of A. fundyense concentrations derived from bottle samples and from chlorophyll fluorescence. Data are from large-scale, weekly surveys in 2011 and 2012 and from high-resolution, tidalcycle surveys in Mill Pond and Salt Pond in 2011. In-situ fluorometer profiles were converted to cell concentrations using a laboratory calibration to cultured A. fundyesne, and extracted at depths corresponding with the sample bottles. The vertical dashed line is the concentration at which the fluorometer saturated on the CTD used for the weekly surveys, and lines with slopes of 1, 0.5, and 0.1 areshown for reference.

Figure 3
Figure 3

Observations and model results from spring 2011. (a) Water level in Salt Pond. (b) Near surfaceand near bottom temperature in Salt Pond and (c) Mill Pond, (d) Town Cove, and (e) near-bottom temperature at Hemenway in the central marsh.

Figure 4
Figure 4

Maps of surface temperature from model results for spring 2011. Times shown are extracted to correspond with marsh-wide CTD surveys that occurred around high water, as in Fig. 3 of Ralston et al.(2014).

Figure 5
Figure 5

Temperature sections from Salt Pond observations (left column) and model results (right) for 9May 2011. Sections are from the pond through the channel into the central marsh (A-A’, larger panels),and across the pond (B-B’, inset panels); section locations are shown on the Salt Pond inset of Fig. 1. Time of day is shown in the lower left of each panel, and tidal phases for the 4 sections are shown in the lower left panel with the water levels in Salt Pond (gray) and at Nauset Inlet (black). Salinity contours(0.2 psu interval) are overlaid on the model results.

Figure 6
Figure 6

Maps of mean A. fundyense concentration from model results for spring 2011. Times shown are extracted to correspond with marsh-wide CTD surveys that occurred around high water, as in Fig. 2 of Ralston et al. (2014).

Figure 7
Figure 7

Depth-averaged A. fundyense cell concentrations stations from weekly surveys and model results in spring 2011: (a) Town Cove, (b) Mill Pond, (c) Salt Pond, and (d) Hemmenway. Station locations are shown in Fig. 1. Vertical lines indicate the concentration range observed in samples collected at multiple depths

Figure 8
Figure 8

Observed chlorophyll-a fluorescence (left column) and modeled A. fundyense concentration (right) from Salt Pond for May 9, 2011. Sections are the same as in Fig. 5. Time of day is shown in thelower left of each panel, and tidal phases for the 4 sections are shown in the lower left panel with the water levels in Salt Pond (gray) and at Nauset Inlet (black).

Figure 9
Figure 9

Observed (upper panels) and modeled (lower panels) profiles of temperature and chlorophyll-a (observed) or A. fundyense concentration (model) in the center of Salt Pond on May 9-10, 2011. Time of day is in the upper right of each panel. Irradiance at the time of each profile is noted at the bottom of theupper panels, and tidal phase is shown with the red circle in the lower panels.

Figure 10
Figure 10

Residence time calculations from model results for different swimming and forcing cases. (a) Concentration of A. fundyense remaining in Salt Pond for spring tide cases: diel vertical migration up to 1/kw depth (swim), no vertical migration (don't swim), diel migration to the surface (swim to surface), anddiel vertical migration to 1/kw with barotropic physics and barotropic physics and no wind forcing. Exponential fits are shown for each case. (b) Residence time calculated from exponential fits for springtide (shown in (a)) and neap tide cases. For reference, the residence time for tidal exchange with a well-mixed pond (Vpond/Qtide) is shown for spring and neap tide cases.

Figure 11
Figure 11

Maps of observed cyst distributions (0-1 cm sediment depth) in the falls of 2008, 2009, and 2011. Sample locations are shown with circles, and interpolated maps were used as bottom boundary conditions for the Alexandrium model. The coastline shown here is the mean sea level contour.

Figure 12
Figure 12

Sensitivity to cyst distribution, comparing model results using the observed cyst distribution with a spatially uniform cyst distribution equal to the average cyst concentration and with cyst densities reduced by factors of 10 and 100. Cell concentrations from spring 2011 are shown for (a) Town Cove, (b) Mill Pond, (c), Salt Pond, and (d) Hemenway. Horizontal gray bar represents approximate range of concentrations in the ponds at the times that weekly toxicity sampling exceeded the regulatory threshold of 80 μg toxin per 100 g mussel tissue (MA Division of Marine Fisheries).

Figure 13
Figure 13

Sensitivity of model results to cyst emergence rate, comparing the base case of temperature-dependent cyst emergence with a case with cyst emergence at the maximum rate from lab data, independent of temperature. Cell concentrations in the water column and the total number of germinatedcysts are shown for each case; both are averaged over the entire Nauset marsh to remove effects of spatial variability in cyst concentration.

Figure 14
Figure 14

A. fundyense concentration from observations and model results from Salt Pond for (a) 2009, (b) 2011, and (c) 2012. Observations are from weekly surveys each year, and vertical bars represent therange of concentrations from samples at multiple depths. Model results are shown using two different mortality formulations: based on temperature (Q10) and on growing degree days.

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