e = natural lo g k = light attenuation coefficient z = distance from the light source. In this exercise this is light intensity at the surface of the ocean. In this exercise z is depth in the ocean. To determine k we use a formula that relates light intensity at the light source, light intensity at some distance from the source, and k I z = I 0 e − k z Where 2 d x = light intensity at a distance z from the source. Clear oceanic waters have very low k values while turbid coastal waters have high values. k is a measure of the amount of light that is absorbed and/or scattered as light passes through the water. Determining k The graph can be used to determine k, the diffuse attenuation coefficient for nondirectional light. In this figure, the compensation depth in Oceanic water is ∼ 120 m and in coastal waters ∼ 10 m. The light intensity is equal to 1% of the light intensity at the surface. The bottom of the photic zone is also called the compensation depth, approximately the depth at which The photic zone is defined as the surface layer where light intensity is strong enough so that photosynthesis by marine algae exceeds respiration. For this example, we use light intensity at the surface is about 500 W / m 2. Light attenuation in oceanic and coastal waters. Becoming familiar with Semi-log graph Figure 1. The measurement of light attenuation in your local ocean you would likely collect data that would result in a different line on this graph. Figure 1 (below) shows two examples of light attenuation versus depth. The importance of some of these depends on the biological residents of the water and the proximity to a source of suspended sediment. Light Sunlight penetrating the ocean surface can be attenuated by the water, organisms in the water, dissolved organic molecules from organism's wastes and decay, and suspended particles. Evaluate how the depth of the photic zone and the surface mixed layer varies seasonally at temperate latitudes and how this relates to seasonal phytoplankton productivity dynamics. Gain an understanding of k, the attenuation coefficient for non directional light. Goals In this exercise we will work with light, temperature, and phytoplankton biomass data to - Practice unit conversions. In this exercise we examine seasonal cycles in sunlight intensity and mixed layer depth to gain a deeper understanding of linkages between phytoplankton productivity and physical oceanographic processes. In summer, as the waters stratify, mixing decreases and light intensity increases alleviating the light limitation, though driving nutrient limitation. During the winter lower light intensity and deeper mixing result in light limiting conditions and hence low primary productivity. Recall the seasonality in thermocline and mixed layer depth in the mid-latitudes. Seasonal variation in light, mixing depth, and primary productivity in temperate Northern Hemisphere waters Photosynthesis by algae in the ocean represents approximately half of all global primary production and supplies ∼ 50% of the oxygen that we breathe.
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