Part 1—Learn the Causes of a Phytoplankton Bloom
Step 1 Explore the Factors that Trigger a Phytoplankton Bloom
Scientists consider many factors that influence where and when phytoplankton blooms occur. Some of the most important factors include water temperature, density, salinity, hydrography of the region, availability of nutrients, the species of phytoplankton and the amount of biomass that is present, what types of zooplankton are grazing on the phytoplankton, and available sunlight levels. There are very few "permanent" parameters (i.e., bathymetry/hydrography) in this system; the constantly changing values of most parameters make blooms somewhat difficult to study. This primer is intended to focus only on the oceanographic principles that influence phytoplankton blooms that can be monitored by instruments on buoys and satellites.
Step 2 Discover What Makes the Gulf of Maine Unique
Scientists have been studying microscopic organisms in the oceans for decades using simple collection nets and jars. Today, scientists study phytoplankton using research vessels, long-term moored buoys, and satellite instruments. Based on oceanographic studies that utilize all of these methods, we now know that phytoplankton blooms occur around the world in regions of upwelling or along convergent currents known as fronts. In the Gulf of Maine, the dynamics are very unique. In studying the Gulf of Maine ecosystem, it is essential to understand how the bathymetry (depth and shape of the ocean floor) affects water movement in the region.
The Gulf of Maine is almost entirely enclosed by land. Three Canadian Provinces and three U.S. states surround the Gulf of Maine creating a semi-circle shaped body of water that is still connected to the Atlantic Ocean.
Step 3 Learn More about the Physical Factors that Influence Bloom Timing
The tilt of Earth's axis and its revolution about the sun cause seasons to change. We know the difference between summer and winter weather on land, but we are less familiar with how the weather changes underwater. The ocean experiences changes in seasons too! We can track how ocean "weather" changes by studying changes in the temperature, salinity, and density of columns of water from the ocean bottom to the surface. The concept of such a vertical sample of water is referred to as the water column.a layer in which the density increases significantly with an increase in depth, and the deep zone (Garrison, 2005.) The depth of each zone varies with location.
In vertical profiles of water from the Gulf of Maine (above), cool, dense water is on the bottom and warmer, less dense water floats on top. This situation is most obvious in the spring and summer profiles. Both the color and the black lines in each graphic indicate water temperature at various depths. In winter, you can see that the water is generally uniform in temperature throughout the water column due to heavy mixing. In summer, water near the surface is warmer and therefore less dense than deeper water.
Just as storms mix the atmosphere, heavy winds, strong currents, and tidal forces mix water in the ocean. The mixing churns up relatively stable stratified water, bringing dense water up and forcing less dense water down. This brings nutrients that were confined to the deep zone to the surface zone. In the winter months, the water column is fully mixed due to the prevalence of winter storms. Even if storms are absent, the cold winter atmospheric temperatures will chill the surface layers of the ocean. The result is that surface waters becomes too dense to be supported and they sink to the bottom, displacing the less dense, nutrient-rich water below, keeping the water mixed. Once spring comes, the higher atmospheric temperatures warm the surface waters, decreasing their density, and stratified layers develop again. This annual cycle of mixing and stratification repeats each year. The status of the water column is monitored by instruments on buoys; by analyzing these data, we can interpret if the water is mixed or stratified when blooms occur.
Nutrients play an essential role in supporting a phytoplankton bloom. Phytoplankton rely on sunlight and available nutrients for energy and growth. Carbon dioxide, water, and light are necessary for photosynthesis. Nitrate, phosphate, silicate, and carbonate are all important in the production of plant matter. Silicon, phosphorus, and nitrogen also play a key role in growing plants. Because their concentrations fluctuate, nitrogen and phosphate influence the rates of phytoplankton production in the ocean. Concentrations of nutrients in the water column vary depending on the time of year, density of water, how they entered the ocean, and how much mixing has taken place during that season. In addition, each of these constituents have different residence times, or time remaining in the water column. This factor has a direct influence on the intensity of the phytoplankton bloom. In summary, nutrient availability is influenced by a range of complex factors in the Gulf of Maine.
How do we know what nutrients are available at different depths in the ocean? Currently, nutrient data is collected from water samples gathered on research vessels. Long-term oceanographic buoys are not regularly equipped with the technology to measure nutrient levels, although research is currently being done to outfit buoys with real-time nutrient sensors. What the water samples do tell us is that nutrient availability in the water column changes with the seasons. Winter mixing brings nutrients up from below, concentrating them near the surface. Spring warming creates a surface layer that floats on top, halting the supply of nutrients brought to the surface. As spring turns to summer, nutrients in the surface layer are consumed by phytoplankton, reducing nutrient availability at the surface. As summer sets in, phytoplankton die and drift to the bottom, taking the nutrients they ingested with them. Surface waters are now left with few nutrients available. Through the summer, this situation is reinforced as the surface waters are warmed and the stable situation of stratification sets in. Once autumn sets in with cooler days, a limited amount of vertical mixing brings nutrients up from below. In winter, heavy winds and plummeting temperatures cause strong mixing again. In general, when nutrients are found near the surface, they are not plentiful at deeper levels and vice versa. A more in-depth exploration of nutrient dynamics and how they influence phytoplankton blooms on a seasonal basis can be explored by studying ocean chemistry.
Step 4 Understand What "Blooms"
Major Spring Bloom Species
The community structure of a phytoplankton bloom depends on the geographic location of the bloom as well as its timing and duration.
Phytoplankton blooms are created by an array of complex factors and influences that can combine to form conditions that cause a bloom, or a high concentration of phytoplankton in an area. Stratification of the water column with an influx of nutrients and sunlight can create an exciting display of blooming life. With an introductory level of information and instructions on how to find and graph data, you are now better equipped to make educated guesses about when the spring bloom might happen. A few years ago, we did not have access to the amount of data that is now available because the necessary tools were not in place. With long-term data sets collected from moored buoys such as the NERACOOS buoy system, and an extensive library of satellite images, teachers and their students are now able to ask similar questions that can lead to conclusions that are useful to scientists. Understanding the ocean is difficult and complex, yet with these new tools it is now more approachable than ever. Enjoy investigating the mystery of the spring bloom!
Garrison, Tom, 2005: Oceanography: An Invitation to Marine Science. Belmont, CA.
Spokes, L. Phytoplankton and nutrients in the oceans. ESPERE. (16-October 2003).
Miller, Charles, 2004: Biological Oceanography. Oxford, UK.