GEOS 110 Lab 10 Fall 2020 Mystery of the dying crabs

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1 LAB 10 – SOLVE THE MYSTERY OF THE DYING CRABS Acknowledgement: This lab is adapted and slightly modified from the online laboratory manual: Ocean data labs: Exploring the Ocean with OOI data – Lab 8 Solve the Mystery of the Dying Crabs, by Freeman, R. Smalley, G. and Lichtenwalner, S. (2020). In Bristol, D.L. and Pfeiffer-Herbert, A. (Eds.), Ocean Data Labs: Exploring the Ocean with OOI Data – Online Laboratory Manual . Rutgers, The State University of New Jersey. Accessed 11/16/2020 https://datalab.marine.rutgers.edu/ooi-lab-exercises/ Data Collection Data used in this lab have been collected by Ocean Observatories Initiative ( https://oceanobservatories.org/ ) using the Coastal Endurance Array off the Coast of Oregon. More information about the Coastal Endurance Array can be found here: https://oceanobservatories.org/array/coastal-endurance/ Key terms dissolved oxygen, hypoxia, anoxia, coastal upwelling, Ekman transport, wind, Learning outcomes • Recognize how changes in physical and chemical composition of the ocean affects life • Describe patterns in individual data sets and correlations between the different data types presented. • Interpret the provided dissolved oxygen, temperature and wind speed data. • Explain the relationship between wind direction and anoxic events on the Oregon Coast using evidence and relevant scientific concepts to support your conclusions. 10.1. Background: Hypoxia and Dead Zones 10.1.1. General Hypoxia and dead zones develop when the oxygen concentration in the water – dissolved oxygen (DO) drops below 2 mg/L. When the DO concentration drops that low, higher organisms including crabs cannot live anymore and die. In order to study such events, we will use data from the OOI Inshore mooring at the coast of Oregon in 2017. We use these data because crab fishing off the coast of Oregon was impacted in that year. Normally fisherman would find lively and tasty crabs in their traps, but in 2017 they only found dead and dying crabs in their traps. Using the following dataset you will learn how natural processes lead to this negative impact on crabs. 10.1.2.Ingredients If and when hypoxia and subsequent dead from in the ocean depends on the following ingredients: a) the direction of the wind b) the direction of the Ekman transport c) is upwelling or downwelling occurring d) is the surface of ocean fertilized with nutrients Point a directly influences b to d. Depending on the location of high and low pressure zones winds along the Oregon coast blow either from north to south or from south to north. In general, high and low pressure zones change with seasons as we have discussed previously. However, the location of high and low pressure
2 zones can also change within short periods of time, and therefore the wind can also change its direction in these short time periods. 10.1.3. Ekman transport and ocean fertilization Ekman transport does not only affect currents, but also areas of coastal upwelling and downwelling (Figure 1). In general, if downwelling or upwelling occurs depends on the wind direction which then subsequently affects the Ekman transport. In areas where water is removed from through Ekman transport, deeper water reaches the surface. The opposite is true for areas where water piles up. Such areas of higher sea level experience downwelling. Particular regions where the direction of the Ekman transport changes are coastal setting (See Figure 1). The effect of an Ekman transport towards the cost is downwelling, yet when Ekman transport is moving water from the coast towards to open ocean upwelling is the result. Upwelling is of particular interest because it leads to the fertilization of ocean surface waters with nutrients. Such fertilization event may instigate the growth of microscopic (phytoplankton) or macroscopic algae (e.g. seaweed). This can have positive effects as algae are on the bottom of the food chain and allow higher organisms to flourish. However, too much fertilization has very negative consequences. Fertilization of the ocean in upwelling occurs because the deep water that moves to the surface in upwelling regions is cold and nutrient rich. Hence, such zones of upwelling are very productive. In such productive areas, a lot of phytoplankton can grow. Phytoplankton carry out photosynthesis: 𝐶𝑂 2 + 𝐻 2 𝑂 𝑙𝑖𝑔ℎ𝑡 → 𝐶𝐻 2 𝑂 + 𝑂 2 So in general, upwelling allows for a thriving food chain. However, too much fertilization can have the opposite effect and the food chain will be crashin. 10.1.4 Oxygen formation (photosynthesis) and oxygen depletion (decomposition) Deep water is not only nutrient rich, but also has less oxygen than surface water. The reason for this difference is that during photosynthesis oxygen is produced according to the formula: Figure 1. Upwelling and downwelling in the ocean because of Ekman transport
3 𝐶𝑂 2 + 𝐻 2 𝑂 𝑙𝑖𝑔ℎ𝑡 → 𝐶𝐻 2 𝑂 + 𝑂 2 Yet when phytoplankton dies and sinks, the organic material (CH 2 O) oxidizes (reacts with oxygen) and is converted back to CO 2 and water: 𝐶𝐻 2 𝑂 + 𝑂 2 → 𝐶𝑂 2 + 𝐻 2 𝑂 Therefore, the deep water that is brought to the surface is oxygen poor. These reduced oxygen levels do not affect phytoplankton as they (phytoplankton) produce oxygen, but too low oxygen concentration affect higher organism who are depending on oxygen in the water. The extent to which deep water is oxygen depleted depends also on how many phytoplankton grow. The more phytoplankton grow, the more organic material is produced. Therefore, the more phytoplankton grows in the first place, the more phytoplankton dies. The more organic material of the dead phytoplankton reacts with oxygen in the water, the more oxygen depleted the water becomes. Hence, the lower the oxygen concentration in the water the less likely it is for crabs to survive. 10.1.4 Blocking of sunlight Another aspect of too much fertilization may lead to blocking of sunlight. As we learn before, particles in the ocean and adsorb or reflect sunlight and therefore decrease the depth to which sunlight penetrates the ocean. We learned that light penetration is larger in open ocean waters than in coastal waters. Fertilization will lead to phytoplankton blooms and therefore to more particles in the water column. Once the particles reach a certain density, light will only be available to the most shallow waters. Hence, no light penetrates to slightly deeper layers and phytoplankton in those slightly deeper layers will die because the light necessary to perform photosynthesis is blocked. 10.1.5. Development of a dead zone The development of a dead zone in the ocean or any other waterbody follows the same steps: a) (over-)fertilization of the water b) phytoplankton or algae bloom c) blocking of sunlight d) dying of phytoplankton e) decomposition of dead phytoplankton f) oxygen depletion g) dying of higher organisms once dissolved oxygen drops below 2 mg/L. Upwelling (as discussed in the previous chapters) is one way of creating a dead zone. However, human activities on land can also create a dead zone. One of the best examples is the dead zone that develops each year in the Gulf of Mexico. As humans fertilize their fields in areas that drain into the Mississippi river in spring, excess nutrients are flushed into the Mississippi river and
4 transported to the Gulf of Mexico. Once the nutrients reach the Gulf of Mexico, the development of the dead zone follows the same points listed from a) to g). 10.1.6. Which organism are most affected by hypoxia and anoxic conditions In general, there are 3 groups of organism in the ocean: • Nekton: aquatic animals that are able to swim and move independently of water currents. • Benthos: sessile organism on the seafloor, organism that are fixed to the seafloor and or have only a limited ability to move long distances. • Plankton: floating animals Of these three groups, benthos is probably most affected by hypoxia and anoxia, because the a live the furthest from the ocean atmosphere boundary where they would have access to ocean diffusing into the water column, and secondly their limited movability does not allow them to escape when hypoxia or anoxic conditions develop. Nektonic organisms like fish may be able to leave the zone of hypoxia, because they are mobile. And plankton is either not dependent on oxygen (phytoplankton produces oxygen) or they live closer to the air-ocean interface because this is the zone where they find their food (like zooplankton who eat phytoplankton). 10.2. Initial questions 10.2.1. Watch videos Before you start to answer the questions listed below, please watch the following 3 short movies: Dead zones in the Gulf of Mexico: https://youtu.be/ovl_XbgmCbw Upwelling and hypoxia: https://youtu.be/_IdFXLS9msM Crabs and hypoxia: https://youtu.be/lPOzAaWHq_A 10.2.2. Answer questions Please answer (make bold the correct answer) the questions below after you read the background information and have watched the videos listed above. i. Which of these are reasons that Dead Zones might form? Check all that apply. a. Excess fertilizer run-off b. Improperly disposed-of animal waste c. Increased run-off during storms d. Upwelling of deep ocean water into shallow areas e. Downwelling of shallow water into deep areas ii. Compared to the shallow ocean, water in deeper parts of the ocean is often _____________ and contains _____________ dissolved oxygen. a. Warmer, more b. Warmer, less c. Colder, more d. Colder, less
5 iii. In the northern hemisphere, if the wind is blowing to the north just west of a coastline that runs north/south, shallow water currents will flow to the __________________, and _________________ will occur. a. East, upwelling b. East, downwelling c. West, upwelling d. West, downwelling 10.3 – What trends or patterns can you observe in dissolved oxygen levels in the ocean at this location? Please go to the following webpage: Oxygen concentration at the Oregon Inshore Mooring . Please do not get confused as the activity on this webpage is called lab 8. For us, this is lab 10. Once you open the webpage listed above, you see the following graph (Figure 2): Figure 2. Dissolved oxygen concentration at the Oregon Inshore mooring from May to June 2017 10.3.1. Warm-up questions Please answer the following questions directly to the graph you see on the webpage: i. What does the x-axis represent? The x axis represents the threshold line. ii. What does the y-axis represent? The y axis represents the dissolved oxygen. iii. What are the units on the x-axis? Mg/L. iv. What are the units on y-axis? Mg/L. v. How much time, in days, does the dataset cover? The dataset covers 23 days. vi. On June 1, what was the DO level at 10am? To answer this question accurately, you will need to use the slider bar to zoom in on the data. The DO level was at 2.74.
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