2.1 productivity 2.2 oxygen 2.3 salinity 2.4 carbon 2.5 ... to 29.pdf · nutrients – phosphorus...

Nutrient Cycling in an Aquatic Ecosystem

2.1 Productivity2.2 Oxygen2.3 Salinity2.4 Carbon2.5 Nitrogen

2.6 Phosphorous2.7 Iron

2.8 Sulphur2.9 Silica

Phosphorous in Freshwaters• Phosphorous plays a major role in biological metabolism. In comparison to

other macronutrients required by biota, phosphorous is least abundant and commonly is the first element to limit biological productivity.

• Many quantitative data exist on the seasonal and spatial distribution of phosphorous in streams and lakes and the loading rates to recipient waters from drainage basins.

While concentration of PS(Soluble Phosphorous) and PT(Total Phosphorous) of oligotropic lakes exhibit little variation with increasing depth, eutropic lakes with strongly clinograde oxygen profiles commonly show a marked increase in phosphorous content in the lower hypolimnion.

Phosphorous in Freshwaters• Orthophosphate (PO4

3) is the only directly utilizable form of soluble inorganic phosphorous. Phosphate is extremely reactive and interacts with many cations to form relatively insoluble compounds.

• A large proportion of phosphorous in freshwaters is bound in organic phosphates.

• The range of total phosphorous in freshwaters is large, from < 5 µg/L in very unproductive waters to > 100 µg/L in highly productive waters.

• Most uncontaminated freshwaters contain between 10 and 50 µg/L total P/L.

Where does phosphorus come from?

Presenter

Presentation Notes

Although some phosphorus may derive from atmospheric deposition of dust and soil particles containing adsorbed phosphate, the great majority usually comes from the watershed in close association with the transport of soil particles. Untreated or partially treated domestic or industrial wastewater effluents are also major sources and may be point (end-of-pipe) or nonpoint (diffuse). See http://www.lakeaccess.org for information about a watershed-scale lawn fertilizer phosphorus experiment conducted in the Medicine Lake watershed in the Minneapolis, MN metropolitan area. There are also links to other sources of information on this topic.

Phosphorus – external sources

• Nonpoint sources–Watershed discharge from tributaries–Atmospheric deposition

• Point sources–Wastewater–Industrial discharges

Phosphorus – nonpoint sources• Watershed discharges from tributaries

– Strongly tied to erosion (land use management)– Stormwater runoff (urban and rural) – Agricultural and feedlot runoff– On-site domestic sewage (failing septic systems)– Sanitary sewer ex-filtration (leaky sewer lines)

• Atmospheric deposition– Often an issue in more pristine areas– Arises from dust, soil particles, waterfowl

Phosphorus – point sources

• Wastewater–Municipal treated wastewater–Combined sewer overflows (CSOs)–Sanitary sewer overflows (SSOs)

• Industrial discharges

Phosphorus – internal sources• Mixing from anoxic bottom waters with high phosphate

levels is closely tied to iron redox reactions– O2 > 1 mg/L – Insoluble ferric (+3) salts form that

precipitate and settle out, adsorbing PO4-3

– O2 < 1 mg/L (anoxic) – ferric ion reduced to soluble ferrous ion (Fe+2) – allowing sediment phosphate to diffuse up into the water

• Wind mixing (storms and fall de-stratification) can re-inject high P water to the surface, causing algal blooms

Presenter

Presentation Notes

Students should be referred to any limnology, microbial ecology, or geochemistry textbook for a more thorough discussion of the link between phosphorus and iron cycling. Not all lakes have enormous releases of phosphate from the sediment interstitial (pore) water during late summer or under-ice anoxia, because there may not be much iron in the system. For the purposes of this curriculum, i.e., training students for careers in applied lake management, it is fair to say that most stratified lakes will exhibit substantial increases in bottom water phosphate, which accumulates after the lakes become anoxic at the sediment-water interface. Note also that as redox values fall and the sediments become increasingly reduced, there will be large amounts of sulfide produced as hydrogen sulfide gas (H2S) from the energy metabolism of heterotrophic sulfate-reducing bacteria. If ferrous iron concentrations are relatively high, it will react with the sulfide to form ferrous sulfide – the black gooey muck you find in mudflats and marshes. This can also affect P-cycling in some cases, but turns out to be even more important in transformations involving heavy metals at polluted sites, where toxic metal reactions with sulfide act to bind the metals and reduce their exposure as toxicants to aquatic organisms. Texts will also point out that manganese (Mn) cycling can also influence P-cycling, much the same as iron does, because of the close similarity in redox chemistry between these two ions. They may often co-occur as well, although Mn levels are usually substantially lower than Fe. Where lakes have been historically polluted by high P-inputs, this reservoir of P can exceed annual watershed inputs (see Halsteds Bay and Shagawa data on WOW website for case studies) In summary: No redox or respiration reactions are directly involved with phosphorus chemistry. Organisms are not generating energy from P-transformations. PO4–3 ion is highly adsorptive to cationic sites (Al+3, Fe+3, Ca+2) on soil particles and in the sediments. P concentrations are strongly affected by Fe redox reactions: Ferric (+3) = insoluble floc; ferrous (+2) = soluble, unless it reacts with sulfide and FeS precipitates. Soil properties and transport associated with water movements are the major factors affecting phosphorus levels and cycling.

Phosphorus – Lake budget

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Presentation Notes

Note to instructor: Left-hand figure: Use this figure to highlight inputs and outputs of phosphorus. Right-hand figure: Discuss the relative magnitudes of the pools and sources and sinks for phosphorus. This particular cartoon shows a thermally stratified lake with increased P in the hypolimnion. It seems to show a large inflow of P relative to outflow, suggesting that a lot of phosphorus must be settling out of the epilimnion. However, if this were the case, one might expect the sediment phosphorus pool to be much larger than is indicated. Note: This is not necessarily a great schematic, but it does allow the instructor to discuss what might be going on in a real lake. It was taken from the Maine NEMO (Nonpoint [source] Education for Municipal Officials) Extension Program slideshow, but is a generic schematic of unknown original origin.

Nutrients – phosphorus cycle

• Major pools and sources of P in lakes– “Natural” inputs are

mostly associated with particles

– Wastewater is mostly dissolved phosphate

– P is rapidly removed from solution by algal-bacterial uptake or by adsorption to sediments

Presenter

Presentation Notes

Modified from Horne and Goldman. 1994. Limnology. McGraw Hill. Note to Instructor: Point out to students that phosphate is also called ortho-P, dissolved inorganic-P, soluble reactive phosphorus (SRP), etc. Step students though each “pool” in the figure to the right. Highlight that phosphate concentrations are usually very low in the euphotic zone due to rapid assimilation by algae and bacteria, even in relatively productive systems. High levels of phosphate in the upper water column usually indicate an influx of high P water, such as from a wastewater plant or agricultural drainage. It may also be a short-term result of transient mixing of high-P anoxic bottom waters in productive shallow lakes.

Phosphorus cycling – major sources

• Sewage– Dissolved

• Tributaries and deposition– Particulate

• Erosion– Particulate

• Sediments– Particulate and

dissolved

Presenter

Presentation Notes

Note – These statements are all generalizations.

Phosphorus cycling – internal recycling

• Rapid PO4-3

recycling– Bacterial uptake– Algal uptake– Adsorption to

particles– Detritus

mineralization– Zooplankton

excretion– Fish excretion

Presenter

Presentation Notes

The pool of phosphate in the oxygenated, sunlit portion of the water column is extremely dynamic. Typically, relatively low concentrations of dissolved orthophosphate are maintained by a dynamic balance between rapid uptake by P-starved algae and bacteria and regenerative processes such as the decomposition of dead organic material from plants and animals and excretion by animals (mostly zooplankton, but fish may also be important).

Phosphorus cycle – major transformations

Modified from Horne and Goldman, 1994.

Limnology. McGraw Hill.

• The whole phosphorus cycle

Iron in Fresh Waters

• The most comon form of iron in natural waters is hydrated ferric hydroxide (Fe OH3).

• Under low pH and low redox potential, ferrous diffuse from the sediments and accumulate in anaerobic hypolimnetic water of productive lakes.

Iron in Fresh Waters• Iron is an essential micronutrient to freshwater flora and

fauna• Under restricted availability of iron, photosynthetic

productivity can be limited.• Certain chemosythetic bacteria can utilize the energy of

inorganic oxidations of ferrous salts in relatively inefficient reactions involving carbon fixation.

• Other autotrophic and heterotropic iron oxidising bacteria deposit oxidised iron. These bacteria are restricted to zones of steep redox gradients between reduced metal ions and oxygenated water.

Iron Cycle in Fresh Waters

Sulphur Cycle in Aquatic Ecosystems

• Sulfur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulfur atoms. Sulfur is important for the functioning of proteins and enzymes in plants and animals that depend upon plants for sulfur. Plants absorb sulfur when it is dissolved in water. Animals will consume these plants, so that they will take up enough sulfur to maintain their health.

• Most of the earth's sulfur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments.

• Sulfur can also be found in the atmosphere. It enters the atmosphere through both natural and human sources.

• Natural resources can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms.

• When sulfur enters the atmosphere through human activity, this is mainly a consequence of industrial processes where sulfur dioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.


Vertical distribution of sulphate and hydrogen sulphide for stratified lake is depicted in the figure below.

Under oxic conditions, as in the case in many oligotrophic and mesotrophic lakes, H2S is absent and SO4 concentration change little with depth.


• When sulfur dioxide enters the atmosphere it will react with oxygen to produce sulfur trioxide gas (SO3), or with other chemicals in the atmosphere, to produce sulfur salts.

• Sulfur dioxide may also react with water to produce sulphuric acid (H2SO4). Sulphuric acid may also be produced from demethylsulphide, which is emitted to the atmosphere by plankton species.

• All these particles will settle back onto earth, or react with rain and fall back onto earth as acid deposition. The particles will than be absorbed by plants again and are released back into the atmosphere, so that the sulfur cycle will start over again.

Sulphur cycle

A schematic representation of the sulfur cycle:

Sulphur cycle in natural waters

Silica Cycle in Aquatic Ecosystems

• The silica cycle is similar to phosphorus in that there is no atmospheric reservoir as with nitrogen and carbon.

• Silica occurs in relative abundance in natural waters as dissolved silicic acid and particulate silica.

• Diatom algae assimilate large quantities of silica and markedly modify the flux rates of silica in lakes and streams.

• Utilization of silica in the trophogenic zones of lakes by diatoms often reduces the epilimnetic concentrations

2. Silica and lake trophic status.• Figure 14-16 of Wetzel presents a stylized view of Si distribution in

oligotrophic and eutrophic lakes. Lake Erie is a famous example of the consequences of phosphorus based eutrophication leading to the complete exhaustion of dissolved Si.

When the concentration of silica is reduced below about 0.5mg/L, many diatoms species cannot compete effectively with non-siliceous algae, and their growth rates decline until silica supplies are renewed, usually during autumnal circulation.

2.1 productivity 2.2 oxygen 2.3 salinity 2.4 carbon 2.5 ... to 29.pdf · nutrients – phosphorus...

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