Phytoplankton Primary Production in the Central Pacific

 

The Hawaii Ocean Time-series (HOT) program studies a region of the central Pacific ocean (the North Pacific Subtropical Gyre) near Hawaii.  This region is part of one of the great ocean deserts: low-nutrient regions with very low food web productivity and low abundances of most organisms.  The HOT program has been going to this site almost every month since 1988.  Among other measurements, they have been measuring net primary productivity (NPP) to determine the total phytoplankton (algae) production in the euphotic zone (the sunlit upper region of the ocean).  These scientists measure NPP using a technique known as C-14 bicarbonate uptake.  Using this method, the scientists collect a sample of water from a specific depth in the ocean.  This water sample (including the natural phytoplankton and zooplankton communities) is put in a clear bottle and the scientists add some carbon-14 labeled bicarbonate (HCO3-).  Bicarbonate is the form of dissolved inorganic carbon that is most common in the ocean.  In other words, it is basically the form of carbon dioxide that is most available to phytoplankton.  So when the phytoplankton conduct photosynethesis, they must take up bicarbonate from their environment.  After the scientists have added the carbon-14 labeled bicarbonate they will place their samples in either incubators or back in the ocean at natural light levels.  The phytoplankton will take up the carbon-14 labeled bicarbonate at the same time that they take up natural bicarbonate.  After 24 hours the scientists will end their experiment and filter all of the phytoplankton from the seawater.  They can then measure how much carbon-14 was taken up by the phytoplankton, because carbon-14 is radioactive.  This allows them to measure the NPP of the entire phytoplankton community.  The scientists make this data freely available and if you want to download it for yourself you can access it at the Biological and Chemical Oceanography Data Management Office website.

 

The HOT scientists measure NPP at multiple depths (and hence different light levels) in the ocean every month.  They also make a suite of other measurements, such as phytoplankton pigments, nutrient concentrations, temperature and phytoplankton abundances.  I have compiled some of these other data in the excel spreadsheet below.  These measurements might be useful in trying to understand what drives variability in primary production in this great ocean desert.

 

Note that the HOT scientists typically conduct two or three of these NPP measurements at a single time and depth to act as "replicates" in the light so that they can assess uncertainty in their measurements.  They also conduct similar (and simultaneous) experiments in the dark to determine whether any organisms are taking up bicarbonate without doing photosynthesis.  One of your first tasks should be to take the average of all of the "light" NPP measurements and subtract off the mean of all of the "dark" NPP measurements, so that you can find out the true amount of bicarbonate uptake that is due to photosynthesis.  Also please note that in this dataset the letters "NaN" means that a measurement was not taken.  Time-series evolve over time and logistical issues (e.g., 20-foot waves and 40-knot winds) can make it impossible to make every measurement every time.

 

HOT Primary Productivity Data.xlsx

 

Data Columns:

 Cruise = the number of the HOT cruise from which the data came

 Day = day of the month

 Month = month of the year

 Year

 Depth = depth from which the sample was collected (unit = meters)

 Pressure = the water pressure at which the sample was collected (unit = decibars)

 Temperature = temperature at which the sample was collected (unit = degrees Celsius)

 Salinity = salinity at which the sample was collected.  Note that because of the way salinity was measured it is technically unitless, although the measurement is essentially equivalent to a unit of kilograms of salt per meter cubed of seawater

 Oxygen = oxygen concentration (unit = micromoles oxygen per kilogram seawater)

 Nitrate = the most abundant nitrogen-containing nutrient available to phytoplankton (unit = micromoles nitrate per kilogram seawater)

 Phosphate = the most abundant phosphorus-containing nutrient available to phytoplankton (unit = micromoles phosphate per kilogram seawater)

 Silicic acid = the most abundant silicon-containing nutrient available to phytoplankton (unit = micromoles silicon per kilogram seawater).  Note that only some phytoplankton (including diatoms) need silicon.

 Dissolved inorganic carbon = carbon dioxide + carbonic acid + bicarbonate + carbonate.  These are basically the forms that carbon dioxide takes when it dissolves in seawater.  Unit = micromoles of dissolved inorganic carbon per kilogram of seawater

 Chlorophyll a = the most important photosynthetic pigment found in nearly all plants and phytoplankton (unit = micrograms of chlorophyll per liter of seawater)

 Prochlorococcus = the most common cyanobacteria in the ocean (unit = cells per milliliter)

 Synechococcus = another very common cyanobacteria in the ocean (unit = cells per milliliter).  Synechococcus cells are slightly larger than Prochlorococcus cells

 Small Eukaryotic Phytoplankton = Also known as picoeukaryotes, these are tiny eukaryotic phytoplankton (typically less than 2 µm in diameter, although the definition can vary based on methodology), that are nevertheless larger than Synechococcus and Prochlorococcus.  Units are cells per milliliter.

 Heterotrophic Bacteria = non-photosynthetic bacteria that play important role as recyclers (units = cells per milliliter).

 Fucoxanthin = An "accessory" pigment that helps phytoplankton use wavelengths of light that are unavailable to Chlorophyll a.  Fucoxanthin is most commonly found in diatoms.  Unit = nanograms fucoxanthin per liter of seawater.

 Particulate silicate = silicon contained in particles (unit = nanomoles of silicon per kg of seawater).  Most of this silicon is likely contained in either live or dead diatoms, although some other taxa (especially rhizarians) can produce silicon as well.

 Carbon assimilation in the light = bicarbonate uptake by phytoplankton in samples incubated at natural light levels (units = micromoles of carbon per liter of seawater per day)

 Carbon assimilation in the night = bicarbonate uptake by phytoplankton in samples incubated in the dark (units = micromoles of carbon per liter of seawater per day)