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Woods Hole Coastal and Marine Science Center > Coastal Model Applications and Field Measurements > Research Activities > Sediment resuspensions and harmful algal blooms

Coastal Model Applications and Field Measurements

Sediment resuspension and the dynamics of harmful algal blooms in the Gulf of Maine

This project investigates the role of sediment resuspension and transport in the fate of A. Fundyense, the organism that is responsible for outbreaks of harmful algal blooms (HABs) along the New England coast in spring. After the spring bloom, the organisms form cysts that sink to the sea floor and are sequestered in the bottom sediments over the winter. These cells germinate in spring and, depending on oceanographic conditions, can create a new bloom of cells that in sufficient concentration can make shellfish toxic to humans and require that the shell fishery be closed. Present modeling efforts carried out by investigators at North Carolina University and the Woods Hole Oceanographic Institution (WHOI) use cyst distributions measured in the sediments in the fall (the fall cyst survey) as initial input to a coupled hydrodynamic and biological model to predict the blooms in the spring. The models do not incorporate sediment resuspension or redistribution.

Project objectives

  • Understand the role of sediment resuspension and transport in dynamics of harmful algal bloom along the New England coast;
  • Improve and verify sediment transport model code for prediction in a fine-grained cohesive sediment environment;
  • Improve of maps of sediment texture in the western Gulf of Maine;
  • Carry out long-term observations of sediment resuspension in the Gulf of Maine to document frequency of cyst resuspension, and compare to model predictions.

The project addresses two questions about the role of sediment resuspension in the life history of A. fundyenese:

(1) Are stress events in spring (April and May), when cysts are viable, sufficient to resuspend them from the bottom sediment and mix them into the water column?

(2) Can wave- and current-induced resuspension and transport redistribute the dormant cyst population during the winter (October through March), thus altering the distribution of cysts mapped the previous autumn?

Importance

Sediment resuspension and transport has implications for forecasting HABs. For example, if mixing of cysts into bottom water is influenced by stress events, germination in spring might occur episodically rather than at a more constant, gradual rate. Resuspended cysts will germinate more easily in the water column due to the presence of oxygen and possibly light, compared to those in the sediments. If redistribution of the cysts by resuspension occurs after the autumn cyst map data are collected, forecasts might be improved by including this redistribution.

Approach

  • Measure sediment erodibility using University of Maryland Gust Erosion Microcosm System (UGEMS) chamber at selected stations in the Gulf of Maine (Figure 1, Figure 2);
  • Model bottom stress using wave and archived model data (Figure 3);
  • Estimate sediment mobility with ROMS/CSTMS models;
  • Estimate interannual variability of bottom stress using wave data from NDBC buoys;
  • Estimate trajectories of resuspended particles;
  • Sample and photograph surficial sediments at locations occupied by the WHOI fall cyst survey;
  • Install instruments that measure temperature, conductivity, and light transmission 10 mab on NERACOOS moorings (see link to Resuspension in the Gulf of Maine).

Field work and analysis are carried out in collaboration with WHOI and University of Maine colleagues.

Principal results (see Butman and others, 2014; Aretxabaleta and others, 2014)

  • Observations of sediment erodibility and modeling of sediment resuspension suggest that about 1 millimeter of the surface sediment can be resuspended by bottom stress.
  • The simulated spatial distribution and variation of bottom shear stress is controlled by the strength of the semi-diurnal tidal currents, which decrease from east to west along the Maine coast, and oscillatory wave-induced currents, which are strongest in shallow water (Figure 2).
  • In some years, resuspension events could episodically introduce cysts into the water column in spring, where germination is likely to be facilitated.
  • Down-welling currents associated with northeasters (winds from the north and northeast) have been shown to concentrate Alexandrium blooms nearshore and carry them southwestward along the coast. This shoreward and alongshore transport of established populations of cells has typically been the role attributed to these large storms, but the effect of cyst resuspension by strong wind events in the spring as a means to inoculate or augment motile populations has not been considered. The largest wave stress events in the western Gulf of Maine are associated with northeasters, and thus in spring these storms might introduce cysts into the water column that quickly germinate to produce motile cells, thereby contributing to the bloom inoculum.
  • The effect of resuspension on the autumn surface distribution over the winter depends on the concentration of cysts in the mobile layer; mixing in the sediment column; the number, intensity, and timing of resuspension events; and the distance the cysts are transported before re-depositing on the sea floor. The effect of these variables on the cyst concentration in the upper cm, those most likely to contribute to the spring bloom, require additional modeling and field observations. However, resuspension and transport are unlikely to change the total inventory of cysts that extends to 15 cm or more below the sediment surface.
  • Field observations of sediment resuspension are needed to further assess model predictions
Map of the Gulf of Maine showing stations sampled for erosion analysis

Stations sampled for erosion analysis (black squares) and concentration of A. fundyense cysts in 0-1 cm interval of surface sediment for 2011 (pink circles). Grand Manan (GM), eastern Maine shelf (EMS), central Maine shelf (CMS), western Maine shelf (WMS), central Maine seed bed (CMSB), Jordan Basin (JB), and Wilkinson Basin (WB). Samples obtained at the cyst-map stations were analyzed for cyst concentration (0-1 cm and 1-3 cm intervals) and sediment texture (0-3 cm interval). Multiple samples with the Craib corer were obtained at stations 43, CMSB, and GM. The largest cyst concentrations are in the Bay of Fundy east of Grand Manan Island (GM); south of Penobscot Bay (CMSB); and southwest along the western Maine and New Hampshire coasts.

 

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