Download Aquatic Ecosystems Consist Of Entire Drainage

Chapter 2.1
Structure & Productivity of Aquatic Ecosystems
• Aquatic Ecosystems consist of entire drainage
basins.
• The nutrient and organic-matter content of
drainage from the catchment area is modified in
each of the terrestrial, stream, and wetlandlittoral components, as well as in the lake or
reservoir per se.
• Productivity is generally low to intermediate in
the terrestrial components, highest in the
wetland interface region between land and
water, and lowest in the open-water portion of
the lake.
When we talk about "production" we are talking about
either primary production, secondary production, or
tertiary production.
Primary production processes deal with
photosynthesis, that is the creation of plant biomass
from carbon dioxide, water, and radiant energy.
Secondary production is the production of small
animals that feed on the primary producers.
Tertiary production is the production of larger fish and
also animals that feed on the lower trophic level
(secondary producers).
Primary Production
•
amount of plant tissue built up by process of photosynthesis over time is
primary production
•
majority of primary production is accomplished by small, dispersed pelagic
phytoplankton rather than plants (plants only account for 5-10% of total marine
productivity)
•
the total amount of organic material produced by photosynthesis represents
gross primary production
•
as a portion of the organic material produced by photosynthesis is utilized in
cellular respiration, any excess production is referred to as net primary
production; represents amount of organic material available to support
consumers, nonphotosynthetic protists, and decomposers
•
standing stock refers to the number of organisms per unit area or per unit
volume of water at the moment of sampling.
•
biomass is defined as the total weight (total numbers X average weight) of all
organisms in a given area or volume; this measure is used in preference to
standing stock because phytoplankton vary in size and total numbers of all
organisms are not as ecologically meaningful as estimates of their biomass
Measurement of Primary Production: The Light and Dark Bottle Method
•
•
A sample of water containing plankton is placed in a
transparent (clear) bottle and placed in the site for one
day; oxygen is produced by the plants and is also
consumed by the plants and animals
A similar sample of water is also placed in a dark bottle
(so no sunlight can get in, usually a bottle painted black);
if the bottle is black no sunlight can enter the bottle and
thus any change in oxygen is due to respiration (due to
plants and animals).
Transparent Bottle
Opaque Bottle
Initial, 6AM
0.288
0.288
Final, 9AM
0.292
0.282
Difference
+0.004
-0.006
Gross Productivity
0.010 millimoles O2/liter
Net Productivity
0.004 millimoles O2/liter
The clear bottom picked up +0.004 (photosynthesis + respiration) but the dark bottom (respiration only) lost 0.006.
We can calculate the "Gross Primary Productivity" as the difference between the final bottle and black bottle at 9 PM = 0.010
millimoles O2/liter/day.
And, we can calculate the "Net Primary Productivity" as the difference between the final bottle at 9 PM and the black bottle at 6
AM= 0.004 millimoles O2/liter/day.
Dissolved Oxygen (DO) profiles
In addition to using light and
dark bottles to characterize
aquatic metabolism, we can use
DO probes to characterize
changes in oxygen with depth.
In lakes that are deep, this could
be interesting for detecting the
zone where the photosynthesis :
respiration ratio is low, and the
lake flips from a net autotrophic
to net heterotrophic system (See
figure)
Factors affecting Primary Productivity:
Light
• Amount of light varies with latitude; it decreases
from the equator towards the poles
– polar regions: a single pulse of phytoplankton
abundance occurs during the summer when light
becomes sufficient for a net increase in primary
productivity
– temperate latitudes: primary productivity is generally
maximal in spring and autumn when the combination
of available light and high nutrient concentrations
allow plankton blooms to occur
– tropics: intense surface heating produces a
permanent thermocline so phytoplankton are nutrientlimited year round and there are only small
fluctuations in primary productivity
Factors affecting Primary Productivity :
•
Nutrients
nutrients: major inorganic nutrients that are required
by phytoplankton are nitrogen and phosphorus
(diatoms and dinoflagellates also required silica); all of
these nutrients occur in small amounts and are thus
limiting factors for primary productivity; each species of
phytoplankton has a particular response to different
concentrations of limiting nutrients and each has a
maximum growth rate
–
–
–
oligotrophic regions have low concentrations of essential
nutrients and therefore low productivity
eutrophic waters contain high nutrients and support high
numbers of phytoplankton
mesotrophic waters have nutrient levels between those of
the two extremes
Conclusions
Aquatic ecosystems provide a wide range of benefits to people. As efforts
to improve water productivity intensify, there is growing recognition that
future investments in water management need to consider how to sustain
these ecosystems and the benefits they provide.
In order to maintain the productivity of aquatic ecosystems and improve
dependent livelihoods, policies, institutions and governance
arrangements that foster sustainable and equitable use of these
resources need to be developed.
In particular, the full value of the range of aquatic ecosystems within
different river basins, the resources they provide, and the trade-offs
among different uses, need to be better understood; the environmental
flows required to sustain aquatic ecosystems and their values need to be
quantified; and ways to improve water productivity by incorporating
aquatic ecosystems need to be identified.