Optical Constituents of the Ocean - Ocean and Surface Effects

Background Theory

2.4 Ocean and Surface Effects

2.4.3 Optical Constituents of the Ocean

The sun glint and whitecap radiances are surface dependent and do not hold any infor-mation about the oceanic constituents. The two most desired properties in ocean color remote sensing are the water-leaving radiance (L_w(λ)) and the remote sensing reflectance (R_rs(λ)), which both are water-leaving signals and properties of the ocean. When an-alyzing these properties in detail, it will be observed that several factors are influencing its shape. Direct and diffuse sunlight penetrating the water surface may not only be ab-sorbed or scattered by the water molecules, but also by different suspended and dissolved materials present in the water [5]. The remote sensing reflectance is a property that can give valuable information about the constituents of the water and depends naturally on the constituents of the ocean which traditionally are divided into the following groups [13]: :

1. Sea water (water + inorganic dissolved materials) 2. Phytoplankton

3. Colored dissolved organic material (CDOM, also referred to as gelbstoff)

4. Organic (algae) particles and suspended inorganic (mineral) particles (also referred to as detritus)

It is important to mention that for shallow, clear waters, the remote sensing reflectance can also be affected by sunlight backscattered from the bottom of the ocean. Nevertheless, the sun rays rarely propagate the whole way down to the ocean bottom and back out again, but should be considered for very shallow clear waters. The shape of the remote sensing reflectance is therefore defined by these other constituents. The two different types of wa-ters often addressed in ocean color, namely the Case 1 and Case 2 wawa-ters, are dependent

on how the optical properties are determined by the different constituents. The definition of these cases commonly used today [41] are that Case 1 water are waters whose optical properties are determined primarily by the phytoplankton and related CDOM and detritus degradation products. In Case 2 waters the optical properties are heavily dependent on other constituents than chlorophyll such as mineral particles, CDOM and microbubbles, and where the concentrations do not covary with the phytoplankton concentration.

Fig. 2.16 shows a plot of the absorption coefficients for clear water (aw), phytoplank-ton (aph), coloured dissolved organic matter (acdom), detritus/minerals (adm) and the sum of all of them (atot) for two different oceanic situations. The clear open ocean (upper plot) and coastal water (lower plot) are to illustrate the Case 1 and Case 2 waters, respectively.

The plot of the Coastal water case shows how the particulate and dissolved organic matter dominates the blue and green parts of the spectrum in comparison to the clear open ocean.

To date, remote sensing has focused a lot on the Case 1 waters and the major AC and IOP retrieval algorithms in use are built for Case 1 waters, and tend to fail on the complex Case 2 waters [5, 11, 33].

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0.0 0.1 0.2 0.3 0.4 0.5 0.6

a( )[ m

]

Clear open ocean^a^w^{( )} aph( )

acdom( ) adm( ) atot( )

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Wavelength [nm]

0.0 0.2 0.4 0.6

a( )[ m

]

Coastal water

Figure 2.16:aw(λ),aph(λ),acdom(λ)adm(λ)andatot(λ)as a function of wavelength for clear open ocean (top) and coastal water (bottom). The absorption coefficients are extracted from the synthetic dataset from the IOCCG Report 5 [12].

2.4 Ocean and Surface Effects Water

The water itself contributes to both absorption and scattering. When talking about water in this context, it is referred to as pure water indicating a hypothetical medium consisting of no other substances than water molecules and inorganic salts dissolved in the water [5]. In clear open oceans, the water effect on the ocean color in VIS must be taken into account, whereas for NIR it might not be present due to high absorption. The absorption of water is both temperature and salinity dependent, which must be considered when the optical properties of the water are calculated [13].

Phytoplankton

The phytoplankton category also includes other microscopic organisms, but the major in-fluence on the remote sensing reflectance is inin-fluenced by phytoplankton. Therefore, it is more convenient to address the other microscopic organisms to this category since they would not affect the optical properties appreciable. Phytoplankton are single cell, free-floating organisms, and posses a pigment called chlorophyll that allows them to produce rich organic material by harvest sunlight through the process of photosynthesis [13]. Due to this, phytoplankton is an important component of global carbon cycle and forms the base of the aquatic food web and is often the primary reason to study ocean color. There exist thousands of phytoplankton species with different sizes, shapes, and physiologies and contains a large number of different pigments, not only chlorophyll. However, the main pigment, chlorophyll-a often serves as an index of the phytoplankton biomass and is often desired to measure. Nevertheless, phytoplankton may covary with the other substances leaving it to a complex problem to distinguish them from each other. The phytoplankton component in remote sensing context therefore also includes other microscopic organ-isms. This can be done as the highly pigmented phytoplankton normally dominate the signal from microscopic organisms [5]. The concentrations and species composition may change rapidly over time and space, leaving it to a difficult task to monitor the changes with small scale measurements. Satellites, however, capture pictures on a synoptic scale with comparable timescales, and therefore suits ocean color monitoring well. Chlorophyll absorbs light in the blue and red region and waters with high concentrations of phyto-plankton would therefore shift towards green, accordingly, which can be observed by a satellite. However, a limitation with ocean color is that the satellites only can measure near-surface chlorophyll concentrations, which potentially could underestimate the total amount of phytoplankton present at all water depths [42].

Colored Dissolved Organic Matter

CDOM is a group of organic, dissolved substances consisting of humic and fulvic acids and is often referred to as yellow substances or gelbstoff in addition to CDOM. CDOM comes from various sources like degradation of phytoplankton cells and other organic particles [5]. In addition, they may be transported from distant regions. Rivers flowing through organic-rich soils or heavily wooded regions accumulate a load of CDOM trans-porting it into oceans. Human activities such as logging, agriculture, effluent discharge, and wetland drainage can also increase the amount of CDOM [43]. This property is

there-(a)Satellite picture of polluted rivers spilling into the Atlantic Ocean.

(b)Using picture from (a) to highlight CDOM with dark brown colors.

Figure 2.17:Landsat 8 CDOM imagery after Hurricane Florence’s destruction. NASA scientists use this image to inform state and local agencies on water quality post-Hurricane Florence [44].

fore heavily dependent on the location, where for instance rural areas may lead to much higher concentrations of CDOM. In addition, locally sources of CDOM such as degrada-tion of phytoplankton tend to accumulate more at depth than in the surface layers. CDOM transported from distant areas are more likely to be found in the upper layers, and leading to a higher impact on the ocean color measured by satellites. Due to this, it is more likely to find high, unpredictable concentrations of yellow substances in coastal areas. For open oceans, the yellow substances can also be predicted from the chlorophyll concentrations, as they tend to covary there because the major CDOM comes from degradation of phyto-plankton cells. Pure water on the other hand absorbs longer wavelengths as red light, and will therefore in regions with low concentrations of CDOM appear blue. The absorption of CDOM is present in the short wavelength regions near the blue bands in VIS. Therefore areas with high amounts of CDOM will appear more brown and yellow. Fig. 2.17 shows a Landsat-8 imagery (a) capturing CDOM after Hurricane Florence’s destruction. NASA scientists use this imagery to help inform state and local agencies on water quality post-Hurricane Florence [44]. The CDOM is highlighted as darker brown colors in Fig. 2.17 (b).

Inorganic Material and Minerals

Inorganic materials are also referred to as suspended materials, but refers to the inorganic suspended materials since phytoplankton and other microscopic organisms also are sus-pended materials. The category is a bit loose but can be summarized to include all inor-ganic particulate material that is not included in the phytoplankton category [5]. Examples of such are when waves and current brings up bottom sediments into suspension in shallow inland and coastal waters. These materials can then affect the ocean colors significantly.

Regions with muddy rivers and estuaries, areas of large tidal excursions, and waters influ-enced by the outflow from rivers are regions where inorganic materials play an important role in ocean-colors. This category also includes minerals or detritus which are particles of rock as a result of erosion and weathering.

2.4 Ocean and Surface Effects Water-leaving Radiance and Remote Sensing Reflectance

The spectral shape of both the water-leaving radiance and the remote sensing reflectance is defined by the composition of the different constituents of the water, like CDOM, minerals, chlorophyll, and water itself. These properties, therefore, consists of valuable information regarding the state of the water and are often the desired product of AC. Recall remote sensing reflectance given asLw(0⁺, λ, θ0, θ,∆φ)/Ed(0⁺, λ). The water-leaving radiance depends on the solar irradiance reaching the ocean, whereas Rrs(λ) will be somewhat unaffected by this due to the division of the incoming irradiance. The amount of irradiance reaching the surface is highly affected by the solar zenith angle as this decides how long the solar rays are propagating through the atmosphere before reaching the water surface. The remote sensing reflectance will therefore to some extend remove this angular dependency.

This is shown in Fig. 2.18 where the three different chlorophyll concentrations shown shapes with small variations. Rrs(λ)is shown to be a good AOP due to that external environmental conditions affect the shape, while it still is very sensitive to the different IOPs [13].Rrs(λ)still refers to a particular viewing direction (θ,∆φ) which could further be removed by converting the remote sensing reflectance into the normalized water-leaving reflectance ([ρ_w]_N). This is, however, a bit more complicated to calculate, but for RT models like Hydrolight, this can be calculated as [13]:

[ρ_w]_N =πR_rs(HydroLight;θ₀= 0, θ= 0) (2.36)

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Wavelength [nm]

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Rrs()

CHL = 3.2 mg/m³, 0 = 0°

CHL = 5.2 mg/m³, 0 = 0°

CHL = 15.3 mg/m³, 0 = 0°

CHL = 3.2 mg/m³, 0 = 40°

CHL = 5.2 mg/m³, 0 = 40°

CHL = 15.3 mg/m³, 0 = 40°

CHL = 3.2 mg/m³, 0 = 60°

CHL = 5.2 mg/m³, 0 = 60°

CHL = 15.3 mg/m³, 0 = 60°