Lecture 8a.
Primary Production in Water
-water cycle is central to the functioning of terrestrial ecosystems,
-but almost irrelevant to aquatic ecosystems, in the direct sense
-since water is so important to terrestrial production,
but by definition abundant aquatically,
-we may expect to find similar differences in production dynamics
between aquatic and terrestrial ecosystems
** here, aquatic means pelagic
-open water, off shore, too deep for plants on the bottom to contribute
-open-water zone of lakes and the open water of the ocean, neritic or pelagic
-in this region, primary production is carried out by phytoplankton
-free-floating algal cells, sometimes filamentous or colonial but more often single-cells,
-often very small (2-200 ìm), can be very numerous
[A concentrated assemblage of phytoplankton, in case you have forgotten what they look like]
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-no vascular system like land plants:
-nutrients and CO2 absorbed over entire cell surface
-growth mostly by production of new cells by binary fission
-in the oceans, phytoplankton are responsible for 95% of all primary production
-lesser amount in freshwater lakes, which have shallow littoral zones
Measuring Production
-same principles of photosynthesis apply to water as to land
-can still estimate GPP, NPP, Respiration, and standing crop
** Production is much easier to measure in aquatic ecosystems than on land
-because the producers are so small, we can take samples of entire communities
-standard means of measuring aquatic production is light-dark bottle technique
-ground glass jars are filled with water from the site, containing phytoplankton
-one jar is clear, to let in sunlight, the other is opaque
-photosynthesis is measured as change in dissolved oxygen concentration per unit time
-recall that photosynthesis evolves oxygen
-in the light bottle, both photosynthesis and respiration are occurring
-in the dark bottle, only respiration occurs
-therefore, net primary production is the difference between the light and dark bottles
-we can back calculate from the amount of oxygen produced to the rate of C fixation
-nowadays, more precise methods are used, based on uptake of radiolabelled 14CO2
-commonly a set of paired jars, each one suspended at a different depth
-as in stoogled diagram on next page
** production in both terrestrial and aquatic ecosystems is expressed on an areal basis
-rates are given as mg C m-2 h-1 or similar for an entire water column
-includes seaweeds or rooted plants if the water is shallow
-need to integrate over the entire water column is why multiple bottles are often used
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-we can also measure the standing crop of phytoplankton very accurately
-by extracting chlorophyll and measuring the concentration by fluorescence
-a long-standing technique that is widely used everywhere
-fortunately, algal cells contain a relatively constant amount of chlorophyll,
-so we can translate concentrations to total cell mass with good accuracy
-again, that technique has been replaced by direct flourometery
-which measures the chlorophyll concentration without extraction
-according to the intensity and wavelength of light that it reflects
-for large areas of ocean, satellite imagery may be used,
-with corrections from ship samples
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Recall from Ecology:
-generalizations about production in aquatic environments
1. Of all NPP on the planet, about 1/3 comes from the oceans, 2/3 from land
-therefore, oceans tend to be less productive on an areal basis than land
2. Coastal zones and upwellings are far more productive than the open ocean
3. Coral reefs and estuaries are amazingly productive
-as productive as rainforests, but they get much less attention
4. Fresh waters only moderately productive compared with dry land ecosystems
-exception is nutrient-enriched lakes that can be very productive
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-coral reefs and estuaries are clearly special cases, and not pelagic
-otherwise, both fresh and marine waters have relatively low productivity
compared with terrestrial ecosystems
-note especially the extremely low production in the mid-ocean gyres
-most freshwater pelagic production would be comparable with these numbers
-except much greater in shallow, nutrient-rich lakes
-this figure from the first edition of the text illustrates this (now Fig 6.5, p. 163)
-note extremely low production in freshwater except where nutrient rich
What Controls Aquatic Primary Production?
When discussing primary production on land, we identified four main controlling factors:
Light
Temperature
Nutrients
Water
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-the first three of these apply to aquatic ecosystems just as they do to terrestrial systems
-clearly, water is not a limiting factor aquatically
-but presence of water instead of air in aquatic ecosystems is the reason they are so different
three features stand out:
(1) concentrations of oxygen and carbon dioxide are typically much lower in water than in air
-more importantly, rate of diffusion of gases into water is relatively slow
-nevertheless, CO2 concentrations rarely limit photosynthesis
-it can happen in some very productive lakes
(2) water is a dense, viscous fluid, much different from light, neutral air
-viscosity refers to “stickiness” of a fluid,
-how much the molecules tend to cling to objects and each other
-water is very viscous because of hydrogen bonds
-density and viscosity create buoyancy
(3) water has an extremely high heat capacity, more commonly called the specific heat
-amount of energy needed to raise the temperature of a substance by 1oC
-water has a very high specific heat because of hydrogen bonds
-thus large bodies of water tend to dampen heat fluctuations on adjacent land
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-a fourth feature that table doesn’t mention:
(4) water absorbs sunlight very quickly
-most visible light is absorbed in the first few metres of water
-even faster when substances suspended or dissolved in it
-different wavelengths absorbed at different rates
**light intensity declines quickly with depth
-by contrast, almost no light absorbance takes place in air
-except for a little bit lost to water vapour
What does all this mean for primary production in the pelagic environment?
(1) aquatic GPP is restricted to the top few metres of the ocean or lake
-production tapers off quickly with depth because of light absorption
-phytoplankton are scattered throughout the top mixed layer, because of mixing,
-but active only in the top layer
-in terms of light sensitivity, phytoplankton are equivalent to shaded land plants
(2) irresistible turbulent mixing constantly carries cells out of optimal depth in euphotic zone
-to maintain populations requires small cells, rapid cell division and sometimes swimming
-picoplankton, the smallest phytos, (bacteria size) are responsible for 90% of GPP in open ocean
-which constitutes 90% of all ocean area
(3) water-based vascular system is neither necessary nor possible for transport of nutrients
-impossible because no evaporation to drive water movement
-unnecessary because no soil or sediment nearby as a source of nutrients
-instead, nutrients are taken up directly from the surrounding water
-requires that individual plants remain small, to maximize surface area
-production therefore limited by the dissolved nutrient concentration
-competition for nutrients maintains space between cells
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(4) physical support (buoyancy) is much greater in water than in air
Recall in Ecology I said that aquatic organisms live in a world without gravity
(I would amend that a little: gravity is a pervasive but weak force in aquatic ecosystems)
-with relatively little effort, any marine organism can become buoyant and move up in the water
-therefore, physical structures that dryland organisms must construct to combat gravity
are unnecessary in water
-this physical difference leads to a much greater efficiency in energy use,
-because support structures are not needed
-without stems, branches, roots, entire plant is photosynthetic tissue
-therefore, efficiency of production per unit of standing stock is very high
Production/Biomass Ratios
Recall from Intro Ecology:
-ratio of production to biomass is the P/B ratio
-Figure 14.4 in Krohne, General Ecology, shows average ratios of P/B for different ecosystems
-low P/B of forests result of large accumulation of biomass in woody stems and roots
-grassland ecosystems invest less energy in tissues so they have a higher P/B ratio
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-remarkable difference is between aquatic and terrestrial ecosystems
-aquatic ecosystems tend to be less productive than dryland systems,
-but they do so on a base of far less biomass,
-leading to enormous P/B ratios
-note scale difference on X-axis:
P/B ratios of aquatic ecosystems two orders of magnitude greater than in terrestrial systems
-because aquatic production is based mainly on phytoplankton
-these species invest very little in support tissues, like stems and roots,
-therefore can devote themselves purely to photosynthesis
The Importance of Soil
-final difference twixt water, air:
-recall that plant roots can only take up material out of solution
-but substances are constantly dissolving from solid phase (soil particles)
to replace the removed material
-a process augmented by decomposer organisms
-relatively little loss of material by gravity
-because roots are already below ground level, there is
-most elements bind to soil particles to a greater or lesser degree, hindering their movement
**in water, everything falls to the bottom: gravity is slow, but omnipresent
-in open-water ecosystems, nutrients can limit production as severely as water on land
-pelagic ocean is extremely unproductive because biota cannot prevent the inevitable loss
of organic matter containing nutrients
-open oceans are nutritional deserts
-much better production at upwellings and around continents where nutrient limitation is relieved
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-where do we find the most productive ecosystems?
-naturally, where both nutrient and water limitations are least
(1) tropical rainforest (2) wetlands (3) estuaries (4) coral reefs
-note that all but one of these ecosystems is dominated by vascular plants
-in wetlands and estuaries, plants have overcome water limitation,
-because they have access to flooded soil, they have little nutrient limitation either
** not only presence of water, but lack of sediments differentiates pelagic ecosystems
Evolution of Structure
-why do land plants really need support structures?
-algae growing on top of the soil can and do survive perfectly well
-but they will be overgrown by vascular plants that can put up stems
-so over evolutionary time, competition for light has created physical structure
of terrestrial plant communities
-probably same thing goes on belowground,
-where competition for nutrients favours development of roots
** in terrestrial ecosystems, multiple layers of vegetation are responsible for light absorption
-canopy, subcanopy, shrubs, herbs, ground layer
-these multiple layers are possible because NPP is based on large, fixed, physical structures,
-the largest being trees
-no large physical structure in the oceans
-constant turbulence prevents the creation of layers of vegetation
** therefore aquatic ecosystems always structurally simple compared with terrestrial ones
-some depth preferences among algae
-based on adaptations to light intensities and wavelengths
-but much less developed than on land
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Controls on Aquatic NPP
-what do we find if we try to model control of aquatic NPP
-according to the state variables and interactive controls we saw earlier?
-how well do Jenny’s five state factors apply to an aquatic ecosystem?
-to take them out of order:
Parent Material, by definition, is completely irrelevant in a pelagic ecosystem
-that’s like trying to compare penguins in terms of flight speed
–so models designed for terrestrial ecosystems simply do not work in truly aquatic ecosystems
-maybe this would work if we replaced soil resources with mixing depth of the ocean
-closer to shore the water is shallower and soil resources become progressively more important
-Biota are still important, but concept of plant functional types has no currency in the ocean
-phytoplankton differ mostly with respect to cell size,
-which influences their ability to compete for nutrients
Climate is a major controller
-ocean circulation is equivalent to atmospheric circulation,
-not only distributes heat, it mixes the water of the oceans
-the latitudinal gradient of primary production is reversed in the oceans
-production is least at the equator, increases toward the poles
-cold polar water is less firmly stratified,
therefore allows upwelling, which brings up nutrients
-and mixing by strong west winds and storms in the Ferrell Cell is more effective
-what about season length – so important on land?
-temperature gradient is more than offset by the difference in nutrient concentrations
-a seasonal effect on light intensity is very important
-because of the angle of the sun, light intensity is least in winter, which slows photosynthesis;
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-algal compensation mechanisms are less successful than on land
-strong seasonal cycles of algal production, where nutrients permit
-such cycles are least at the equator, greater at higher latitudes
-on land, season length in the north defined by freezing temperatures, formation of ice
-in the oceans, ice is only a problem in extreme high latitudes, and only because it blocks light
-even under ice flows, water is available and NPP carries on
Time matters only seasonally: there is a succession of production every year
-once again the marine situation is reverse from that on land
-at high latitudes especially, nutrient availability is greatest in winter,
-when winds are strongest and vertical stratification is least
-greatest production at high latitudes occurs in spring,
-when temperatures rise and nutrients from winter are readily available
**difficulty of applying Jenny’s state factors supports supposition that terrestrial
and marine ecosystems are fundamentally different
Nutrient Control of Aquatic Production
-of the four main controls on NPP on land (water, light, temperature, nutrients)
nutrients are the most important in water
-at least, within the photic zone
-other factors important mostly as they affect the supply of nutrients
-terrestrial ecosystems commonly limited by nitrogen
-nitrogen limitation also commonly found in estuaries and coastal zones
-in open ocean, limitation by phosphorus is severe
-phosphorus is very reactive with oxygen
-tends to form a variety of insoluble compounds in oxygenated water
-so it rapidly settles out of surface waters
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** large parts of subtropical and Antarctic oceans are limited by iron
-these oceans may not respond to additions of N or P,
-but turn green when given extra Fe
-large-scale iron-seeding experiments in these waters led to blooms visible from satellites
-iron is only sparingly soluble in well-oxygenated water,
-so again it settles out in time
-some scientists have suggested adding massive amounts of Fe to southern oceans,
-as a way of reducing atmospheric concentration of CO2
-extra C would be fixed into algal cells, then transported to the bottom of the ocean by gravity
-but the stimulation would only last until something else became limiting
-in coastal waters, nutrient supply from nearest land mass mostly limits production
-these systems are affected by terrestrial parent material, land use,
and water mixing by tidal currents
** many coastal systems have been greatly enriched by human pollution
-remember everything that we put into a river eventually ends up in the ocean
-best (worst?) example is the Dead Zone in the Gulf of Mexico caused by excess production
Seaweeds
-greater productivity of coastal areas arises from proximity to land as a nutrient source,
-combined with the nearness of sediments, which allows efficient nutrient recycling
** in water shallow enough that some light penetrates to the bottom,
phytoplankton production is complemented or replaced by seaweeds
-in seaweed beds, same dynamic of competition for light creates 3D structure as in a forest
-a water vascular system still does not work
-but by holding onto the substratum, the seaweed can stay in one place, resist currents,
and remain in the photic zone
-plant can then optimize photosynthesis to one particular depth (or a small gradient)
-large size and therefore small SA/Volume ratio can be maintained because nutrient
concentrations are relatively high
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-buoyancy of water hold them up so they still don’t need any support tissue
-therefore, entire plant can be photosynthetic, greatly increasing productivity
Fresh Waters
>> watch the video!
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