Phytoplankton

Phytoplankton are microscopic plants that produce oxygen and organic matter from sunlight, carbon dioxide, and inorganic nutrients, like plants on land. Phytoplankton increase in abundance (or bloom) in the spring, and to a lesser extent, in the fall. Blooms can occur when the water column is stable, so phytoplankton can remain near the surface where light levels are high, and nutrients are available.

Phytoplankton support many marine food webs as the key food source for zooplankton, that are in turn food for many fish and marine mammals. Phytoplankton abundance is an indicator of how productive a system is. Changes in the timing of the spring bloom can have consequences for many other organisms in the ecosystem.

Through direct sampling and satellite imagery, scientists measure “chlorophyll a” in the surface ocean. Chlorophyll a is the main pigment used in photosynthesis. These measurements are used to represent the biomass and productivity of phytoplankton in the ocean. The more chlorophyll a that is detected, the more phytoplankton cells are assumed to be in the water (Figure 11). The magnitude, peak time, and duration of the blooms are assessed by looking at changes in the chlorophyll a concentrations (Figure 12a, b, c). This provides information on how the entire system changes from year to year. [See box: Crucial links between climate and marine productivity].

Microscopic images of phytoplankton. Credit: Fisheries and Oceans Canada.

• Declining nutrient and chlorophyll levels may indicate that Atlantic ecosystems have a lower production potential than in the previous decade. • The general trend has seen a gradual decline in overall phytoplankton abundance in the Atlantic. Since 2015, most parts of the region had phytoplankton levels well below average. • Generally, patterns of variation in phytoplankton abundance reflect those reported for nutrients. However, they lag behind nutrient variability by about one year. • The patterns of variation in the features of the spring phytoplankton bloom have been relatively consistent across the Northwest Atlantic. • The magnitude of the spring bloom increased from 1999 to 2011, when it reached its highest peak. It then declined to an average state by 2016. The extent to which the spring bloom amplitude varied followed a similar pattern. • The day of year at which the spring bloom starts has been variable from year to year. The shift has been from either generally early or late in relation to average over periods of 3 to 5 years. • Conditions in the 2010–2012 period were very warm and spring phytoplankton blooms were early.

However, gradual cooling since this time seems to have resulted in a general delay in the start of the bloom from 2013–2016. • Duration of the spring bloom varies greatly among the different parts of the Northwest Atlantic.

However, there was a general decline in the overall duration of the bloom from 1999 to 2011, after which conditions returned to near average from 2012 to 2015 followed by a record duration in 2016. CHLOROPHYLL a INVENTORY (0–100m) Figure 11 : Index of Chlorophyll a concentrations relative to the long-term average (1999–2010). These values are used to indicate the biomass of phytoplankton. The black trend line represents the combined anomalies for all areas (See Figure 1 for NAFO Divisions). Above average values represent higher concentrations. In most of the region, phytoplankton biomasses have been well below average since 2015.

St. Lawrence River on January 22, 2018. Credit: NOAA/NASA.

c) DURATION OF SPRING BLOOM

Figure 12 : Observations of the spring phytoplankton bloom including indexes of a) magnitude, b) peak time, and c) duration. All are relative to the long-term average (1998 to 2010). The black trend lines represent the combined anomalies for all areas (See Figure 1 for NAFO Divisions).