Phytoplankton

Figure 1. Thalassiosira are a type of diatom and one of many phytoplankton species that can be found in the spring bloom in coastal B.C. waters. Their size is usually in the range of 5 to 30 microns across the cell. Diatoms usually dominate the phytoplankton biomass in the spring on the continental shelf. (Photo: Moira Galbraith, DFO)

Phytoplankton are the microscopic plants in the ocean at the base of the marine food web (Figure 1). Just like plants on land, they need nutrients, carbon dioxide, and light to grow. They also contain chlorophyll, which helps convert light into chemical energy through photosynthesis. Under the right ocean conditions phytoplankton abundance can explode or ‘bloom’, so that concentrations double in several hours to days. The spring bloom is the first rapid increase of phytoplankton after months of low winter light levels.

Monitoring the timing and magnitude of the spring bloom can be a challenge because of high variability in space and time; bloom timing can vary by several weeks each year, or by several weeks in the same year for different areas. For example, the spring bloom in the northern Strait of Georgia started on February 21 in 2015 and on March 28 in 2016. On the Central Coast, the spring bloom was observed on April 15 in 2016.

Ocean colour satellites are often used to monitor the spring bloom because they provide a snapshot of the spatial extent and concentration (Figure 2), but clouds can prevent a clear image for weeks at a time on the B.C. coast. Furthermore, the standard satellite chlorophyll (an index of phytoplankton biomass) images can return bad data in nearshore waters (e.g., the Strait of Georgia) so additional in situ observations (i.e.,measurements in the water) are needed to validate the data. Alternatives to the standard satellite chlorophyll images include regional chlorophyll products such as those being developed by the University of Victoria Spectral Lab, or the fluorescence line height (FLH) shown in Figure 2. Other monitoring methods include sensors deployed in the water which give bloom timing, but only at one location unless a network of instruments is used. Periodic ship-based monitoring can accurately measure phytoplankton biomass, but ships are usually expensive to operate and have spatial and temporal limitations as well. The best option for monitoring the spring phytoplankton bloom in B.C. is to use a combination of approaches.

Figure 2. The true colour (left) and fluorescence line height (FLH; right) satellite images for March 19, 2017, at about the peak of the spring bloom in waters around Vancouver Island. FLH is a proxy for chlorophyll and tends to give better results than the standard satellite chlorophyll in nearshore waters. Colours represent low chlorophyll (blue; <1 mg/m³) to high chlorophyll (red; ~20 mg/m³). These are the 300 metre spatial resolution data from the European Space Agency’s OLCI sensor which was launched in early 2016. Source data: European Space Agency.

The timing and magnitude of the spring bloom depends on a number of environmental factors and has implications for marine organisms that directly or indirectly depend on phytoplankton for food. If the timing is too early or too late, or the concentrations are too low, there may not be enough food for the zooplankton that eat the phytoplankton, and in turn for the fish or seabirds that eat the zooplankton. For example, a study in the Strait of Georgia found that the lowest juvenile herring abundance occurred in years when there was a mismatch between the timing of the herring spawn and the timing of the spring bloom.

The most complete time series of spring bloom timing for the B.C. coast can be derived using the satellite data from several sensors starting in 1997. Figure 3 shows the bloom timing and magnitude in recent years using satellite chlorophyll from NASA’s MODIS sensor for areas off the west coast of Vancouver Island and in Hecate Strait. For both areas, the bloom was earlier in 2015 compared to 2016 and 2017, and the bloom is usually earlier off the west coast of Vancouver Island compared to in Hecate Strait.

Figure 3. Satellite chlorophyll time series in spring averaged for 1-degree squares off the west coast of Vancouver Island (top plot; 48–49° N, 126–127° W) and in Hecate Strait (lower plot; 51–52° N, 129–130° W). The most recent three years are shown with the average for the MODIS time series (2003–2017). Source data: NOAA ERDDAP.

In the Strait of Georgia, time series of higher temporal resolution (i.e., hourly or daily, rather than weekly or monthly measurements) from a sensor deployed in the central Strait demonstrates another method for monitoring blooms (Figure 4). In 2017, there was a slow start to the spring bloom with chlorophyll concentrations gradually increasing from the beginning to the middle of March. The satellite image for March 19, 2017 shows that the bloom covered most of the Strait with the highest concentrations in the central Strait (Figure 2). The bloom timing in the central Strait of Georgia was similar to the bloom on the west coast of Vancouver Island in recent years (Figure 3 and Figure 4).

Figure 4. Chlorophyll fluorescence data from the Halibut Bank buoy in the central Strait of Georgia for the three most recent springs (2015, 2016, and 2017) and the average for the buoy data time series (2011–2017). Source data: Strait of Georgia Data Centre.

Individual and Organization Actions:

• Interested individuals can monitor the timing of the spring bloom using satellite imagery.
• Become involved in citizen scientist programs. For example, the Pacific Salmon Foundation organized a three-year sampling program conducted primarily by citizen scientists.

Government Actions and Policy:

• Continue in situ monitoring in areas that have ongoing time series (e.g., Strait of Georgia, Central Coast).
• Implement monitoring for other areas that currently have little or no data on spring bloom timing (e.g., the west coast of Vancouver Island, North Coast, and Haida Gwaii).