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NOVA GOL 111 - Marine Productivity

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GOL 111: Introductory Oceanography 4/2010 Northern Virginia Community College Dr. Victor P. Zabielski Lecture Assignment II: Marine Productivity Spring 2010 Name: ____________________ Due: Thursday, April 30, 2010 As any fisherman can tell you, some parts of the ocean are better than others for catching fish. Additionally, some regions of the oceans exhibit a seasonal cycle in productivity, perhaps showing higher fishing yields in the spring or fall than in the winter or summer. This second lecture assignment is designed to help you understand what controls the cycles of marine productivity that we see in the oceans. You have been assigned several locations across the globe. For each of these regions, you will determine the seasonal cycles in temperature, solar input, and productivity, and based on this information, you will need to determine the dominant mechanism responsible for productivity in that region. Part I: Seasonal Cycles Study Sites: 1) 15°N, 60°E (Northern Arabian Sea) 2) 0°, 120°W (Eastern Equatorial Pacific) 3) 60°N, 175°W (Bering Sea) 4) 10°S, 80°W (Peruvian Coast) 5) 40°S, 40°W (South Atlantic) 6) 41°N, 67°W (Georges Bank) 7) A site of your choosing Make a single graph (template is provided in the assignment) for each of the listed locations showing the seasonal cycle of temperature, productivity and solar input. A seasonal cycle can be obtained by making a single estimate per season for each parameter. March 21 (Boreal Spring, Austral Autumn) June 21 (Boreal Summer, Austral Winter) September 21 (Boreal Autumn, Austral Spring) December 21 (Boreal Winter, Austral Summer)Temperature: You can obtain temperature estimates using the archive of global temperature maps at http://weather.unisys.com/archive/sst/. This site contains monthly global sea-surface temperature maps from 1999 to 2006. Be careful that you use the sea-surface temperature (sst) map and not the sea-surface temperature anomaly map. See values on left y-axis on graph. The seasonal extreme temperatures generally occur about one month after the solar extreme (solstices), thus you may want to use the month after the solstices to obtain values for your graphs. The temperatures values are plotted on the left y-axis on the template. Productivity: You can obtain sea-surface productivity estimates at the SeaWiFS Biosphere Globe site located at http://oceancolor.gsfc.nasa.gov/cgi/biosphere_globes.pl. You will need to input the latitude, longitude and season that you are interested in. In addition, the globes are best viewed with a black background using the largest possible pixel density (2048x2048). When you set your parameters, hit the “fetch” button and your globe should appear. The color scheme on each globe is representative of the concentration of chlorophyll-a in the surface waters which is directly related to the productivity of the water. The color scheme is as follows: Purple – very low productivity (VL) Blue – low productivity (L) Green – moderate productivity (M) Yellow - high productivity (H) Orange / Red – very high productivity (VH) For the purposes of this exercise we can limit the estimate to relative productivity rather than absolute numbers. See values on right y-axis on the template. Solar Input: Solar input is determined by the latitude of the site and the date. Solar input is greatest at 23.5°S (or any site south of 23.5°S) on December 21. It is greatest at the equator on March 21 and September 21, and it is greatest at 23.5°N (or any site north of 23.5°N) on June 21. Thus, each site can potentially have three values: Maximum sunlight, medium sunlight, and minimum sunlight (max, mid, min). For locations above or below the Tropics of Cancer and Capricorn, the maximum and minimum solar input will correspond to the Summer and Winter Solstice respectively, and the middle values will correspond to the Equinoxes. Be very careful when determining the maximum, minimum, and middle values for locations between the Tropics! The values for Solar Imput are on the right y-axis of the template, alongside the productivity values.Create a series of graphs for each site with all three factors on a single set of axes. I have provided template graphs that you can use at the end of this assignment. I have created an example of how to fill in the graphs for one site located just south of Tasmania. In this example, productivity peaks just after solar input starts to rise, and begins to decrease as sea surface warms (seasonal thermocline strengthens). This is indicative of a strong seasonal upwelling signal.Part II: Mechanisms of Marine Productivity Now that you know when each of the sites has peak productivity (or if it does at all), it is up to you to determine the specific or dominant mechanism for that productivity bloom (or lack thereof). In order for a region to be productive it must have ample sunlight and nutrients. The sunlight is provided on a seasonal cycle associated with the orbit of the earth around the sun. The nutrients are provided by the upwelling of nutrient rich deep waters. When these two mechanisms coincide, a region will “bloom” with productivity. There are five (5) basic mechanisms / types of marine productivity. These are: 1) Coastal Upwelling 2) Topographic Upwelling 3) Divergent Upwelling 4) Seasonal Upwelling 5) Continental Runoff Coastal Upwelling: Coastal upwelling is associated with the offshore Ekman transport of surface waters thus requiring a compensatory upwelling of deep water to replace the removed surface waters. This mechanism will thus be associated with a specific wind pattern blowing parallel to the coast and may be seasonal or continuous throughout the year depending on the duration of the winds. The direction of Ekman transport is to the right of the wind direction in the northern hemisphere and to the left of the wind direction in the southern hemisphere. Two global wind maps, representing January winds and July winds are available as a PowerPoint on my web page. Topographic Upwelling: Topographic Upwelling is associated with a topographic break in the seafloor, usually a seamount, guyot, or some other topographic high that rises from the seafloor. Deep currents encountering this feature will be forced up, bringing nutrient-rich waters to the surface. Regions dominated by Topographic upwelling will generally show relatively


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