On the occurrence and implications of seasonal phytoplankton blooms
The timing of species occurrence in the environment is termed phenology. Just as we can estimate the arrival and departure of seasons by the migratory patterns of birds or the appearance of buds and flowers on trees, a similar pattern is present in the ocean. Due to their small nature, the recurrent appearance and disappearance of phytoplankton and corresponding zooplankton populations goes largely un-noticed unless it is a bloom of significance such as a toxic red tide producing organism for example: Alexandrium fundyense These microscopic organisms have a dramatic impact on food availability to higher trophic level organisms and regulate the carbon and nutrient cycling in the global oceans. In short, the frequency and timing of blooms is an important aspect of global ocean health. When these regular cycles of production shift rapidly, scientists look for clues such as fluctuations in the regular water properties. When phenology shifts over longer time periods, scientists look to larger shifts in climate patterns as a possible mechanism.
A recent finding by SIO researchers indicates that for more than a decade, the timing of spring phytoplankton blooms in the Arctic is occurring earlier each year. Using satellite ocean color data which typically provides an estimate of surface chlorophyll-a levels, the researchers found the blooms were not just earlier but shifted northwards towards the pole. The regions where blooms occur also correlate with areas of decreasing ice and earlier spring melting. Why is this of concern to scientists? The zooplankton population that relies upon the phytoplankton community as a food source may not be able to adjust to the altered timing of the spring bloom event. If this is true, it result in what is commonly called a “trophic mis-match” which is exactly as it sounds. On the bottom of the food-web (trophic level), phytoplankton blooms occur earlier, then the corresponding zooplankton population may not respond to the peak food availability and a lapse in production and consumption occurs. Shorter blooms of phytoplankton that are missing the corresponding zooplankton population may result in a greater carbon flux and lowering of available oxygen levels in the water column.
The recently published paper in Global Change biology illustrates the difficulties with applying global satellite data to address long term trends. For example, I commonly think of using SeaWIFS data sets, but in this case the data sets did not have adequate temporal coverage and also include error introduced by cloud cover. Therefore, when considering long term changes, it is valuable to have corresponding in-situ data to verfiy patterns that may not be representative.
“Alexandrium fundyense Balech”. Encyclopedia of Life, available from http://www.eol.org/data_objects/475511.Accessed March 17, 2011.
University of California – San Diego (2011, March 3). Arctic blooms occurring earlier: Phytoplankton peak arising 50 days early, with unknown impacts on marine food chain and carbon cycling. ScienceDaily. Retrieved March 17, 2011, from http://www.sciencedaily.com /releases/2011/03/110302171320.htm
KAHRU, M., BROTAS, V., MANZANO-SARABIA, M., & MITCHELL, B. (2011). Are phytoplankton blooms occurring earlier in the Arctic? Global Change Biology, 17 (4), 1733-1739 DOI: 10.1111/j.1365-2486.2010.02312.x