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Plankton: Building Blocks and Bio-Indicators - June 14, 2015

    Even though many are microscopic, plankton are some of the most diverse and vital organisms on the planet. Plankton are pelagic organisms; free-floating and carried by wave and current action throughout the worlds oceans. There are thousands of known plankton species, and possibly thousands more that we have yet to discover. 
 
    Planktonic organisms can come in all shapes and sizes, from simple unicellular organisms to complex multicellular colonies. Because of the plethora of planktonic organisms, scientists have created numerous sub groups within the larger planktonic group in order to aid us in our understanding of these organisms. 
 
    The two biggest subgroups are the phytoplankton and zooplankton groups. The simplest way to remember the difference between these two groups is that phytoplankton are autotrophs and zooplankton are heterotrophs. What this means is that phytoplankton can undergo photosynthesis to create complex organic compounds which they can then metabolize for energy, while zooplankton lack the ability to undergo photosynthesis and therefore must obtain their energy source from somewhere else.
 
    This difference in energy sources explains their different trophic levels; phytoplankton are at the lowest level of the food web, the base-level producers for the aquatic ecosystem, and zooplankton are at the second lowest level, the base-level consumers for the aquatic ecosystem. These divisions are man-made to assist scientists and researchers and as such, some plankton species can fall into both divisions simultaneously or switch between autotrophic or heterotrophic behaviors seemingly at will. 
 
    Because of their different energy needs and relative buoyancy, plankton species can be found throughout the worlds oceans, bays and rivers. Phytoplankton species that rely on photosynthesis are generally found in the euphotic zone, the top 200 meters of the aquatic ecosystem that receives sunlight. If they fail to remain buoyant, the phytoplankton will drop in the water column to the twilight zone or below and are subsequently unable to undergo photosynthesis to produce energy. 
 
    Zooplankton can be found in a much larger area and do not need to stay within the euphotic zone. However, since phytoplankton generally tend to stay in the euphotic zone, most zooplankton will go in and out of these zones to find food.   
 
    Over the past few years, scientists and researchers have seen changes in planktonic populations, distribution and production in different parts of the worlds oceans believed to be caused by climate change and other anthropogenic sources. So in order to better understand the effects of climate change, scientists and researchers often use both zooplankton and phytoplankton as bioindicators for a number of reasons. 
 
    First, Zooplankton are poikilothermic, which means that the physical processes they undergo, such as reproduction, ingestion or respiration, are very sensitive to temperature. Some plankton species physiological process rates double or even triple with just a 10°C temperature rise, which combined with warming surface waters, could result in massive population spikes and more.
 
    Secondly, because of the free-floating nature of plankton, the distribution of zooplankton within our oceans and water column can accurately reflect temperature and ocean currents better than other methods, giving us more complete data.
 
    And finally, because of the fact that many marine animals have at some point in the life cycle a zooplanktonic life stage, we can see how climate change affects distribution and population trends for a wide variety of aquatic organisms and what effects it could have on the complex and dynamic aquatic food web. 
 
    Phytoplankton will also be seriously effected by climate change and the warming ocean, maybe more so than zooplankton, so they are excellent indicators as well. Phytoplankton are essential for numerous ecological and biogeochemical cycles in the aquatic system, such as atmospheric CO2 regulation, so understanding the effects of climate change on these organisms is essential. 
 
    Research has shown that marine phytoplankton biomass and productivity decreases in response to the rising temperature of surface water, in part due to the fact that the top layers of the ocean are becoming isolated from cooler, nutrient rich waters due to temperature-driven stratification. With this temperature-driven stratification, we expect to see an alteration in phytoplankton community composition, due to the reduction in nutrients being added to the euphotic zone from more nutrient-rich colder waters. 
 
    Researchers have also seen a direct correlation between warmer water temperature on phytoplankton populations, specifically that there is a significant increase in the proportion of smaller-sized plankton species in warmer waters in both freshwater and marine systems. As surface waters continue to increase in temperature, we expect to see a gradual shift in phytoplankton populations where the smaller sized species will be present in higher proportions than larger species compared to previous years, which could lead to shifts in food web dynamics and more. 
 
    Plankton are the building blocks of the marine ecosystem, and without them there would be no aquatic life and most likely no terrestrial life either. These microscopic organisms play a major role in some of the most complex and essential biological, geochemical and other cycles necessary for life to exist on this planet. 
 
    As climate changes continues to alter our planet, it is vital that we understand the changes that plankton communities go through and will go through in the future. By understanding what different trends we might see because of climate change in plankton species, we can better prepare ourselves for our changing global climate.
 
Harrison Jackson is the Coastal Stewards Coordinator for the Maryland Coastal Bays Program. 
 


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