Clean Coastal Waters Understanding and Reducing the Effects of Nutrient Pollution (2000) / Chapter Skim
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3 Which Nutrients Matter?
3 Which Nutrients Matter?
Pages 63-83
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From page 63... ...
Understanding the Problem
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From page 64... ...
Clean Coastal Waters: Understanding and Reducing the Effects of Nutrient Pollution 8000 7000 6000 5000 4000 3000 2000 1 000 o ~ Nitrogen · Phosphorus _~ An, .~ -- Exceptional Q plankton blooms I Filamentous green algae ,, Early indications _^ of eutrophication /,, I_ 1 950 1 960 1 970 Year 1 980 Figure 3-3. Transport of nutrients to L`aholm Bay, Sweden.
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From page 65... ...
· A variety of ecological and biogeochemical mechanisms lead to these differences between freshwater and coastal marine ecosystems, including the relative inputs of nutrients from adjoining systems, the preferential storage or recycling of nitrogen versus phosphorus within the ecosystem, and the extent to which nitrogen fixation can alleviate nitrogen shortages. · Eutrophication of coastal systems is often accompanied by decreased silica availability and increased iron availability, both of which may promote the formation of harmful algal blooms.
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From page 66... ...
nitrogen generally is more limiting to primary production by phytoplankton, and nitrogen inputs are more likely to accelerate eutrophication (Howarth 1988; Vitousek and Howarth 1991; Nixon 1995; Paerl 1997~. Note that the concept of "nutrient limitation" is often poorly defined and used rather loosely; the committee follows the definition of control of the potential rate of primary production, allowing for potential changes in the composition of the ecosystem (Howarth 1988~.
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From page 67... ...
. EVIDENCE FOR NITROGEN LIMITATION IN COASTAL MARINE ECOSYSTEMS Most of those who in the 1980s disagreed with the assertion that nitrogen is the key to regulating marine eutrophication in coastal marine systems instead argued that phosphorus is the critical nutrient, as in lakes.
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From page 68... ...
. A decade ago, there were no comparable experiments testing the relative importance of nitrogen and phosphorus as regulators of eutrophication in coastal marine ecosystems (Howarth 1988~.
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From page 69... ...
_ Cat lo 4 E In In o Cat z E . _ ~ O 2 50 40 An' 30 Q o o 20 10 69 C1 C2 N1 N2 P1 P2 N+P1 N+P2 Individual Enclosures C1 C2 N1 N2 P1 Individual Enclosures P 2 N+P 1 N+P 2 FIGURE 3-1 Response of estuarine mesocosms in Narragansett, Rhode Island, to experimental nutrient additions, clearing showing nutrient limitation by nitrogen but not phosphorus.
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From page 70... ...
This is strong evidence that nitrogen was the element most controlling eutrophication in this estuary. During the year that phosphorus loadings were experimentally increased, there was no effect on primary production; however, there was an unusually large bloom the following spring, probably due both to some residual high levels of phosphorus and to an unusually high input of nitrogen from spring floods (Elmgren and Larsson 1997~.
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From page 71... ...
(modified from Elmgren and Larsson 1997~. Note that the major period of eutrophication in this estuary coincided with a period in which nitrogen inputs were increasing yet phosphorus inputs were decreasing.
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From page 72... ...
MECHANISMS THAT LEAD TO NITROGEN LIMITATION IN COASTAL MARINE ECOSYSTEMS What ecological or biogeochemical mechanisms can lead to nitrogen control of eutrophication in most coastal marine systems and to phosphorus control in so many freshwater lakes? This question was reviewed by Howarth (1988)
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From page 73... ...
Margarets Bay, Nova Scotia Central Kaneohe Bay, Hawaii Vostok Bay, former USSR LAGOONS Beaufort Sound, North Carolina Chincoteague Bay, Maryland Peconic Bay, NewYork High Venice Lagoon, Italy FJORDS Baltic Sea Loch Etive, Scotland 73 in- 120 53 225 61 - 62 l it' L ~ ~ 240 - * 190 —~ 125 0 10 20 30 40 FIGURE 3-4 Summary of nitrogen:phosphorus ratios in 28 sample estuarine ecosystems.
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From page 74... ...
For instance, lakes receive nutrient inputs from upstream terrestrial ecosystems and from the atmosphere, while estuaries and coastal marine systems receive nutrients from these sources as well as from neighboring oceanic water masses. For estuaries such as those along the northeastern coast of the United States, the ocean-water inputs of nutrients tend to have a nitrogen:phosphorus ratio well below the Redfield ratio due to denitrification on the continental shelves (Nixon et al.
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From page 75... ...
the ratio of nitrogen:phosphorus in external inputs to the ecosystem; 2) the relative rates of recycling of nitrogen and phosphorus in the water column, with organic phosphorus usually cycling faster than organic nitrogen; 3)
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From page 76... ...
1996~. That is, available evidence indicates that denitrification tends to drive both coastal marine and freshwater ecosystems toward nitrogen limitation, with no greater tendency in estuaries.
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From page 77... ...
These changes result in an increase in phosphorus availability in eutrophic systems, intensifying nitrogen limitation and encouraging the growth of algae and other organisms (including some heterotrophic organisms, such as the heterotrophic life stages of Pfiesteria) with high phosphorus requirements.
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From page 78... ...
Much research has been directed at the question of why nitrogen fixation by planktonic organisms differs between lakes and coastal marine ecosystems, with much of this focused on single-factor controls, such as short residence times, turbulence, limitation by iron, limitation by molybdenum, or limitation by phosphorus (Howarth and Cole 1985; Paerl 1985; Howarth et al. 1988a; Howarth et al.
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From page 79... ...
concluded that coastal eutrophication is largely a phosphorus problem, and that "removal of nitrates in the river supply should lead to increased nitrogen fixation, no significant effects on final nitrate concentrations, and no significant effect on eutrophication." The Committee on Causes and Management of Coastal Eutrophication disagrees strongly. While nitrogen fixation in oceanic waters may alleviate nitrogen deficits over tens of thousands of years, nitrogen fixation simply does not occur in most estuaries and coastal seas and does not alleviate nitrogen shortages.
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From page 80... ...
under identical conditions except for a lower availability of molybdenum as one would find in an estuary. Molybdenum is required for nitrogen fixation, and its lower abundance leads to slower growth rates by cyanobacteria.
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From page 81... ...
Nitrogen fixation does occur in the Baltic Sea, yet even there the water residence time is on the scale of a few decades, thousandsfold shorter than the time scale of response by nitrogen fixation in Tyrrell's model. While debate continues as to whether or not nitrogen fixation completely alleviates nitrogen shortages in the Baltic, much evidence shows that it does not and that much of the Baltic Sea remains nitrogen limited (Graneli et al.
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From page 82... ...
1996~. Decreasing silica availability and the consequent lower abundances of diatoms also lowers organic matter sedimentation and thereby have a partially mitigating influence on low-oxygen events associated with eutrophication.
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From page 83... ...
1996~. However, there is no evidence that iron limits primary production in estuaries and coastal seas (although it may partially limit nitrogen-fixing cyanobacteria in estuaries)
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