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 Oceanography Lecture
  Upper Ocean Ecosystems
From parts of Chapters 13 and 16



Upper Ocean (above 200 meters depth)
Recall that euphotic zone is only 2% of the oceans volume, but accounts for an estimated 40% of the Earth's total photosynthesis.

Most of the biomass in the ocean lies within or just below this zone.  Why?
Relatively little of this biomass is thought to be primary producer.  Does this contradict the 2nd law of thermodynamics?



The traditional oceanic food chain:
               
         
where photosynthetic single-cell eukaryotes (unicellular algae) such as diatoms, dinoflagellates, coccolithophores and silicoflagellates  are the primary producers consumed by multicellular zooplankton and pelagic animals.


In oceans, primary production (the standing crop of phytoplankton) is typically estimated directly by counting and measuring cells or indirectly by determining chlorophyll concentration.  The rate of primary production is estimated by measuring the uptake by cells of radioactive C14 incubated at various light levels.

The limiting nutrient (the nutrient in shortest supply relative to demand that as a result limits metabolic processes) tends to be nitrogen in open ocean systems.  However,  freshwater systems tend to be phosphorus limited, whereas coastal and estuarine nutrients may be either.  Silicon and iron also can limit marine phytoplankton growth (and may be useful in developing a strategy to address global warming)

In temperate oceans, phytoplankton productivity tends to be high in the spring.  Why would mixing of the water column be greater in the winter, and how would this lead to lower productivity?   Extended periods of stratification of the water column (formation of a stronger thermocline) leads to reduction of nutrient exchange from deeper depth (why is net movement of nutrients downward?), so that by summer nutrients are depleted from the ephotic zone, and there is a shift to smaller size phytoplankton cells and smaller-bodied zooplankton (Why?).


Primary production of phytoplankton in open oceans can be higher than expected given in situ nutrient concentrations, suggesting that regeneration of nutrients by zooplankton may be important.


Much of the consumer biomass moves upward to shallower depths during the night and downward during the day.  Why?









The concentration of zooplankton from this vertical migration is so dramatic that it reflects sonar echos, leading navy ships in WWII to believe that this deep scattering layer (DSL) was a (false) bottom.









Zooplankton species differ over horizontal gradients across the ocean.  For example,  in neritic regions where phytoplankton densities are high though variable, the copepod genera Paracalanus, Centropages and Temora usually dominate and these three genera produces a feeding current and releases eggs.   The genera Oithona, Oncaea and Clausocalanus dominate in open tropical oceans where food concentration is low, and these genera feed actively and brood eggs.  Why would these types of life history strategies be advantageous?


Fish predators may also be important
in structuring open ocean ecosystems
(top-down regulation marine.rutgers.edu/courses/expl_oceans/07Cury.PDF)




Commercial fishing may be dramatically changing oceanic food webs.   A study based on data representing all major fisheries in the world over the past 47 years "determined that within the first 15 years of operation, commercial fisheries reduced fish populations by 80 percent on average".  Fisheries have focused on large predatory fish, resulting in a reduction in food chain complexity.


Emerging views oceanic food chain:

Recall that bacteria may make up to 90% of all biomass in the ocean, with heterotrophic bacteria making up 40-70% of all biomass in the ocean, and cyanobacteria making up to 70% of all photosynthetic activity in the ocean.


In fact, in some aquatic areas, secondary production of heterotrophic bacteria can exceed primary production phytoplankton due to recycling of organic carbon within the detrital loop before passing through the food web.

Bacteria are too small for most multicellular zooplankton (e.g. the ever abundant copepod) to consume.Where is this  production going  and what drives this production?


Recall, that non-photosynthetic single-cell eukaryotes (protozoans), whose importance by oceanographers has been traditionally under-recognized, graze bacterial cells.  Energy is hence transferred to higher trophic level through the detrital (microbial) loop.

  In addition, larvaceans may also be extremely important in consuming dissolved organic matter.


Viruses, also much more common than previously believed, may also be important in transferring (shunting) energy through the detrital loop by lysing cells.




For a good review of these emerging ideas, download "Viruses and Nutrient Cycles in the Sea" (Wilhelm S. W.; Suttle C. A. 1999)

And to make matters even more difficult in understanding oceanic food chains, food chains may be changing in response to global warming.  Phytoplankton have declined substantially in many areas of open water in Northern oceans since the early 1980's.  How has warming resulted in this trend (hint: how does warming affect temperature stratification and hence vertical nutrient transfer)?

Phytoplankton currently account for nearly half the transfer of carbon dioxide from the atmosphere back into the biosphere by photosynthesis.  How does CO2 influence global warming?

In addition, changes in global warming may be effecting patterns of ocean circulation, which not only redistribute heat globally (see Current Lecture), but also affect transfer of limiting nutrients from deeper water.

Another complicating factor is that phytoplankton absorb significant heat as sunlight pentrates the ocean surface.

So how would fertilizing the ocean change climate?




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