Ecosystems in the Sea: A Review

What makes the ocean ecosystem unique?

It is highly three dimensional and it flows. In comparison to the terrestrial
environment it is nearly 250 X thicker (5000 m vs 250 m at most). While
there are organisms such as birds and insects that occupy the atmosphere,
few, if any, spend their entire existence airborne. One might call them
mero-aeolian organisms.
 
 
Distribution of life in the sea:

The vertical zonation of life in the sea can be coursely divided into the photic zone where light fuels photosynthesis given the proper nutrient supply, and a deep layer that is dependent on a flux of carbon from above in the form of detritus or vertically migrating organisms. The upper layer forms a delicate balance between the upward mixing of nutrients that accumulate in the deeper layers and the light supply.

The intermediate layer just below the epipelagic or photic zone is the mesopelagic zone. This layer is characterised by various migrators that spend the night in the surface layer either grazing on phytoplankton or predation on these grazers, but move into the deeper dark and cooler depths in the daytime. This offers a refuge from visual predators and a lower metabolic rate because of lower temperatures.

The lower zones, the bathypelagic, abyssal and hadal zones are dependent on the organic flux from above. Most of the material reaching these depths is in the form of fecal material from the near surface zooplankton. This occurs because the phytoplankton, being very small, sink extremely slowly. Since the organic flux is typically low most of these deep layers have low biomass.

Finally, in areas with high surface productivity or where there is significant bottom chemosynthesis, there is an increase in biomass in the as one enters the benthic environment.

An important concept used to quantify the rate of exchange between these different vertical layers that make up the ocean ecosystem is residence time. The residence time for a system is defined as

Residence time = Amount in the system / Rate of input
or
Residence time = Amount in the system / Rate of output

For these to definitions to be the same the concept of residence time relies on an assumption that the system of interest is a equilibrum. In terms of the equations derived for the plankton ecosystem in Lecture 6 this means that the derivatives are all equal to zero, i.e. the different components (N, P, Z, D) are not changing in time.

The thin (200 m or less) photic zone has a very fast residence time, \tau, of only a few weeks and very low nutrients, NO3 = 0 - 5 micro-moles/kg. By comparison the deep ocean (2000-5000 m) have high nutrients,
NO3 = 10 to 40 micro-moles/kg and very long residence times, 1000 years. The intermediate layers have variations between these values.

Nutrient concentrations also vary greatly in the horizontal. In the photic zone there are broad regions in the center of the mid-latitude oceans where nutrients are almost undetectable. These low NO3 environments have very low biomass and are term oligotrophic. Coastal and some high latitude environments have significant NO3 in the surface layers and are defined as eutrophic.

The balance of input and output of nutrients in the photic zone involves the upward flux of "new" NO3 from the deep ocean versus the sinking of organic matter with nitrogen in it (mostly in protiens) out of the surface layers. This input and export of nitrogen from the photic zone only accounts for 10% of the nitrogen involved in the plankton system described in Lecture 6. The other 90% is recycled with in the photic zone. This nitrogen is referred to as "old" production. A similar flux balance exists in the deep benthic communities that depend on chemosynthesis. Unfortunatley, we have little information on the rates in these deep regions.

Together these transfers of chemicals and the role that the living components of the system have in them are at the heart of the Gaia hypothesis of Lovelock and Margulis, i.e. that the earth itself is a living system. The ransfer of chemicals in the ocean and atmosphere associated with this intricate combination of physical and biological fluxes is denoted as biogeochemical cycles. These will be discussed further later in the course.

This lecture concludes with an introduction to various ocean ecosystems and how they vary in time. First, consider a slightly more complicated model of the plankton (but still NPZD) over an annual cycle at 26 N in the Atlantic (slightly north of Miami and in mid-ocean). All of the components are displayed in terms of their nitrogen content. Notice the spring bloom of phytoplankton as the mixing in the region decreases allowing phytoplankton to get enough light. In summer there is not enough NO3 in the surface layer to support much biomass in either P or Z. The system has become oligotrophic. Almost all of the phytoplankton are concentrated in a deep chlorophyll maxima that is found in the thin region at the bottom of the photic zone where there is both a little light and some NO3.
 

The final portion of the lecture involves looking at phytoplankton biomass from space. Link to the NASA CZCS (Coastal Zone Color Scanner)
images at ........ 

Oligotrophic areas are in purple. The biomass observed from space increases to eutrophic waters in red. Note, the sensor on the satellite can not detect the deep chlorophyll maximum layer in much of the ocean. We, therefore, can not really measure these areas very well from space. From ship based observations in these regions there are up to 100 species of phytoplankton present. The question is how? In the lab, if one starts with a mixture of plankton at constant conditions in N, and several species of plankton, the end result is dominance of a single species. Therefore, we have the PARADOX of the PLANKTON, i.e. why are there so many types of organisms in one sample? ....too be continued.