CLASS NOTES Oct. 9
(01/11:628:200) Fall 2003

Estuaries/coastal ocean/productivity/organic matter

  1. Estuaries: hugely important to human life and commerce

Estuary: A semi-enclose bay, sea or river mouth that has a free, open connection to the ocean but is diluted with freshwater such that it’s salinity is less than that of the ocean.

  1. Mixing
    • salt wedge: results from high river flow
    • well-mixed: strong tidal mixing, low river flow
    • Mixed: slanted isohalines: moderate situation
    • Note mixing style depends on river flow, tidal amplitude, and shape of river mouth.
    • Flow proceeds from unidirectional in river to reversing in estuaries
  1. Chemical gradients in estuaries

- ionic strength: rivers are low, ocean is high

- 1% by volume of seawater into river water gives resulting estuarine water with a ratio of ions very close to that of seawater:
    • pH: surface ocean is ~ 7.8-8.1(slightly basic), rivers vary 5-8.

- Small changes in salinity and pH have large effects on dissolved substances, localized in the landward (riverine) end of the estuary.
    • Example: Iron (Fe)
    • Fe is high concentration in rivers, low in ocean
    • "dissolved" Fe in rives exists mostly as colloids, which are inorganic clays and organic matter stuck together in particles too small to sink or be caught by standard filters in the lab.
    • In upper estuary, cations from seawater (even at salinity 1-2) neutralize the naturally negative charges on these colloids, colloids no longer repel each other, so they aggregate ("flocculate") into large particles and sink to the river bed. This process removes dissolved Fe within the estuary, and lots of pollutants as well.
    • See mixing plot for nonconservative elements in estuaries.
    • Counterexample: Barium (Ba)
    • much of the Ba in river water is contained in suspended particles. When these particles encounter the high cations of a little seawater mixing in, the Ba is desorbed and transferred from particles to dissolved phase in estuary
    • therefore mixing plot for Ba is opposite that of Fe.
  1. Other processes in estuaries

- Productivity: especially important in estuaries with slow flushing time. Estuaries in general are very productive environments.
    • phytoplankton use nutrients, but release them again after death, so estuary is a "leaky" filter for nutrients.
    • Estuaries can therefore act as "nutrient traps"
    • Estimates are that 80% of nutrients escape the estuary to get the the coastal ocean in an unpolluted estuary – not so clear in a polluted estuary where eutrophication occurs.

-Nutrients in Estuaries…

Effects of Nutrient Over-Enrichment--eutrophication

    1. Increased Primary production
    2. Increased O2 demand and HYPOXIA
    3. Harmful algal blooms
    4. Shifts in community structure
    5. Degradation of seagrass and agal beds
    6. Formation of nuisance algal mats
    7. Coral reef destruction
    8. Disease and pathogen increase
    9. Economic impacts

- Sedimentation:

    • decreased flow velocity and increased water residence time from river to estuary means particles can settle by gravity
    • estuaries are effective sediment traps: . 95% of fine grained suspended sediment that enters estuaries (clay and organic material) sinks and is deposited in estuaries.

-Resuspension:

    • Remixing of surface sediments up into estuary’s water column, gives increased chances for interaction between dissolved and particulate phases. If locally intense, can cause a turbidity maximum.

- Pollutants:
- Urban sources tend to be on estuaries, and pollutants can be trapped in sediments, exchange with water and take a long time to flush out.
  1. Productivity
    • Primary Productivity means production of living organic matter (biomass) using carbon (from CO2), nutrients, and an energy source (sun).
    • Phytosynthesis: makes plant biomass, and is the most important type of primary production in the ocean.

Basic photosynthesis equation: CO2 + H2O Þ (CH2O) + O2 (energy required)

Organic matter

Respiration: Opposite process from photosynthesis (above equation backward):

(CH2O) + O2 Þ CO2 + H2O (energy release)

    • this reaction is carried out by animals (you and me!) and bacteria, who derive evergy by "burning" organic matter, using oxygen.

Now let’s incorporate the necessary nutrients N and P:

 

Life’s Stoichiometry (= ratios of elements):

A. The Redfield-Ketchum-Richards (RKR) Equation (written for photosynthesis), was "fleshed out" by Richards in 1965, based on the C:N:P ratios in the 1963 RKR paper:

 

106 CO2 + 16 HNO3 + PO4 + 122 H2O Þ (CH2O)106(NH3)16PO4 + 138 O2 (1)

organic matter

1. Things to note:

a. C:N:P = 106:16:1 for phytoplankton- the average composition - this is called the REDFIELD RATIO (remember this!!!!)

b. this is a nonequilibrium reaction whose slow kinetics are enzymatically facilitated. Plants are specially designed to speed this reaction.

c. (CH2O)106(NH3)16PO4 represents the elemental composition of "average plankton", and simplifies organic matter by leaving out S, metals, etc.

 

Where does the carbon in organic matter come from? Dissolved CO2 in water!

But CO2 is an unusual gas! It reacts with water to form ions.

 
  1. Introduction to the carbonate system:

- CO2 in the atmosphere is in relatively low concentration: ~375ppm or 0.0375% (compare to oxygen at ~21%).

Absorption of CO2 from the atmosphereby the ocean involves 3 equilibria

1 CO2(aq) + H20 ß à H2CO3(aq) (carbonic acid—so we are adding acidity to the ocean)

2 H2CO3(aq) ß à H+(aq)+ HCO3-(aq)(bicarbonate ion)
    1. HCO3-(aq) ß à H+(aq)+ CO3-2(aq) (carbonate ion)

So dissolving this gas in water makes other ions, allowing more CO2 to dissolve. This is the reason that the ocean contains far more CO2 than the atmosphere – it can "hide" in different chemical forms in the ocean (more on this in climate lecture). At equilibrium in the ocean, HCO3-(aq)(bicarbonate ion) is by far the most abundant form of dissolved CO2 . Therefore we can simplify the dissolution of CO2 gas as follows:

CO2(aq) + H20 ß à HCO3-(aq) +H+(aq)

Upshot: plants make organic matter using dissolved CO2, but they have a huge store of this nutrient in the form of bicarbonate ion, which dissociates to form more CO2 as it is used. Therefore, unlike N and P, phytoplankton in the ocean never run short of C.

Lets think about what these equations really tell us about what the biology is doing.

A simple Ocean: just surface and deep!