Indicator Details

Forecast

This summer it is predicted that Prorocentrum blooms will occur in the lower reaches of the Patuxent River in June. Blooms are also predicted to occur in June in Baltimore Harbor. At this stage, no blooms are predicted to occur in the late summer and fall period in the forecast regions.

Blooms of the harmful algae Prorocentrum are forecast for six regions of the Chesapeake Bay in Maryland. Forecasts are provided for each month of the year. Once field survey data becomes available, the observed conditions will also be provided. Forecasts will be updated at regular intervals as additional data becomes available to improve the forecast certainty.


Prorocentrum forecast provided by Jon Anderson.

Data

Methodology

The Prorocentrum forecast is based on a model that relates river flow and water quality (temperature, salinity, and clarity) to Prorocentrum bloom conditions. Seven routine water quality monitoring stations were chosen to represent distinct regions in the Maryland section of Chesapeake Bay that typically see high concentrations of Prorocentrum (Figure 1). Water quality data for the model was supplied by the Chesapeake Bay Program and Maryland Department of Natural Resources, and river loads data was supplied by USGS. Data from January 1985 to May 2009 was used in the model.

The predictive model uses a classification and regression tree method to partition "bloom" and "no bloom" conditions based on above-pycnocline water temperature, salinity, water clarity, and mean river flow. A fuzzy logic membership curve was implemented that incorporates the uncertainty of how to define a bloom concentration of Prorocentrum (Figure 2). The membership curve defines how to classify a particular concentration as "bloom" or "no bloom". A lower boundary is set for defining a "bloom" that reflects the certainty among regional harmful algal bloom experts that concentrations below 1000 cells·mL^-1 are not indicative of "bloom" conditions. Conversely, it is generally agreed that concentrations greater than 5000 cells·mL^-1 are indicative of "bloom" conditions. Concentrations between 1000 and 5000 cells·mL^-1 are allowed to have membership in both "no bloom" and "bloom" designations, depending on their proximity to either the lower or upper thresholds. For example, a Prorocentrum concentration of 4000 cells·mL^-1 would be classified as 75% "bloom" and 25% "no bloom". Fuzzy memberships are used to weight the classification and regression tree decision-making, such that higher weights are given to data that are closer to either extreme (i.e., weights are proportional to the certainty of the bloom designation).

Since the objective is to create a predictive model of Prorocentrum blooms, a temporal offset was imposed, where water quality data from prior dates were fit to observations of Prorocentrum in the future. Twelve different offsets were applied, ranging from 0.5 months to 6 months prior, resulting in twelve distinct predictions for a particular date. Figure 3 shows the model with temporal offset of 0.5 months for station CB3.3C.

Each model's performance was assessed by fitting the model's predictions of 2005-2007 to observations of that same time period. Relative performances of each of the twelve models were calculated from Akaike Information Criteria (AIC) and evidence ratios calculated from equations defined in Burnham and Anderson (2002). For each prediction, the model with the highest evidence ratio was chosen. However, if water quality data was not available at the "best" model's designated offset, the second best model was used.



 

Figure 1. Map of Chesapeake Bay Program monitoring stations (red dots) used to develop the predictive model. Shaded regions represent the areas in which predictions apply. Green dots represent USGS river discharge gauges used in the model.




Figure 2. Fuzzy membership curve describing how a Prorocentrum "bloom" is defined. Cell concentrations below 1000 cells·mL^-1 are generally not indicative of bloom conditions. Conversely, cell concentrations above 5000 cells·mL^-1 are considered a reasonable "bloom". The overlap between 1000 and 5000 cells·mL^-1 conveys the uncertainty of defining a bloom.




Figure 3. A sample decision tree for water quality station CB3.3C ("Bay Bridge" region) using water quality conditions that occur 0.5 months prior (a).

Literature Cited:

Burnham, K.P., and D.R. Anderson. (2002) Model selection and multimodel inference. 2nd Edition, Springer-Verlag, New York, NY. 488pp.

Background

What are harmful algal blooms (HABs)?
A harmful algal bloom (HAB) is defined as the proliferation of algae, causing negative impacts to other organisms. Harmful algal blooms may be composed of microscopic cyanobacteria or phytoplankton that, when the conditions are right, multiply and build up into bloom levels, often creating thick mats and or surface water scum. These blooms may become harmful to other organisms, leading to fish kills or human health problems, due to the natural toxins they may contain, and/or reduction in dissolved oxygen levels during their decay.

What is Prorocentrum minimum?
Prorocentrum is a heart shaped dinoflagellate that tends to occur in waters of mid salinity. Blooms of Prorocentrum typically cause reddish-brown tides, often referred to as mahogany tides. Mahogany tides associated with Prorocentrum blooms have the potential to severely reduce the oxygen in the water and have caused significant depletions in shellfish, submerged aquatic vegetation (SAV), and fish populations historically. Prorocentrum has been found to cause human health problems globally when shellfish affected by the dinoflagellate are consumed.


Prorocentrum minimum. (Source: Maryland Department of Natural Resources.)

What factors encourage harmful algal blooms?
The major factors needed for an algal bloom to occur include water temperature, salinity, and the amount of nutrients in the water. Excess nutrients such as those contributed by human activities can increase the intensity of the bloom (see Methodology tab).


Mahogany tide in February 2008 in the Lafayette River. (Source: The Virginia-Pilot.)

What are some local areas that are prone to Prorocentrum minimum blooms?
Blooms are typically found in mid-salinity waters and have occurred in the mid reaches of Chesapeake Bay's mainstem, and in many tributaries including Potomac, Choptank, Wicomico, and Patuxent Rivers.

Additional Information

Relevant Web Sites

MD DNR HAB website
MD DNR HAB current blooms map
NOAA Karlodinium forecast
NOAA HAB forecast