The availability of “ingredients” that phytoplankton need to grow — carbon, nutrients, light — is directly tied to the ocean’s physical environment.
Oceans can be layered with upper waters that are distinctly less dense than deeper waters. This causes a physical “barrier” to form, separating the surface from the nutrient-rich colder waters below. In this example, the thermocline — a thin layer where temperature changes abruptly — separates shallow from deep waters.
This plot shows how low salinity water can “float” above high salinity water.
Under mixed conditions, temperature and salinity DO NOT vary much from shallow to deep. During high mixing, phytoplankton are suspended in a turbulent ocean, spending time in both the sunlit upper ocean and the deeper dark zone.
Determine whether marine waters are layered or mixed AND if these conditions are tied to phytoplankton diversity. You will be recording data from these marine stations:
Print this data sheet and use it to record data from the stations.
#1) Discover whether stations had layered or mixed conditions when sampled.
STEP A: Read here for background about temperature, depth, and mixing. Look the temperature profiles for each station for autumn, spring, and summer. Keep track of the following:
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Is the station layered for the listed season (i.e., large temperature contrast between shallow and deep water)?
- OR is the station well-mixed for the listed season (i.e., temperature is relatively consistent)?
- For stations with layered conditions, find the depth where there is a distinct decrease in the temperature, like this example:
- Note this depth on your data sheet under “Top of Thermocline.”
STEP B: Read this background page about salinity. Look at the salinity for each station during each season. Record whether the salinity is mixed or layered.
- At these stations, is the salinity profile consistent with the temperature profile for that season? In other words, do stations with layered temperature profiles also have layered salinity profiles? And do stations with “well-mixed” temperature profiles also have “well-mixed” salinity profiles?
#2) Compare the top of the thermocline with the depth of fluorescence.
Read this first three sections of this page about light loss and fluorescence measurements. Stop after the Chlorophyl section. Record the depth at which the fluorescence measurement drops below the value of 1.
- Compare the recorded depths for the tops of the thermoclines with those you found here where fluorescence has dramatically diminished.
- Is the top of the thermocline at the depth where fluorescence declines sharply? Can you guess why or why not?
- Quickly examine a few (6 or so) fluorescence profiles for mixed conditions. Are their traces different from those of layered stations? Why or why not?
#3) Examine whether phytoplankton species diversity is tied to layered versus mixed ocean conditions.
Read the last section of this page about phytoplankton. Look at the plankton net samples and descriptions for every station during each season.
- For each station/season, record whether the plankton assemblage is mostly diatoms (write DIA), mostly dinoflagellates (write DIN) or about equal amounts (write MIXED).
- When possible, compare these findings with whether each station is “layered” or “mixed” during that season.
One view of the relationship between physical conditions and phytoplankton is shown (below). According to your Gulf of Maine research, do certain types of phytoplankton thrive better under layered conditions? How about mixed conditions?
